energies

Article Current-Limiting Soft Starting Method for a High-Voltage and High-Power Motor

Yifei Wang 1,2, Kaiyang Yin 1,*, Youxin Yuan 1 and Jing Chen 1

1 School of Automation, Wuhan University of Technology, Wuhan 430070, China 2 Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA * Correspondence: [email protected]



Received: 17 July 2019; Accepted: 8 August 2019; Published: 9 August 2019


Abstract: Focusing on the starting problems of a high-voltage and high-power motor, such as large starting current, low power factor, waste of resources, and lack of harmonic control, this paper proposes a current-limiting soft starting method for a high-voltage and high-power motor. The method integrates functions like autotransformer voltage reduction–current limiting starting, magnetron voltage regulation–current limiting starting, and reactive power compensation during starting, and then the power filtering subsystem is turned on to filter out harmonics in power system as the starting process terminates. According to the current-limiting starting characteristic curve, the topological structure of the integrated device is established and then the functional logic switching strategy is put forward. Afterwards, the mechanisms of current-limiting starting, reactive compensation and dynamic harmonic filtering are analyzed, and the simulation and experimental evaluation are completed. In particular, the direct starting and the current-limiting are performed by developing a simulation system. In addition, a 10 kV/19,000 kW fan-loaded motor of a steel plant is chosen as the subject to verify the performance of the current-limiting soft starting method. As shown by the experimental results, the motor’s starting current is about 2 times that of its rated current, the power factor is raised to over 0.9 after the reactive power compensation, and the harmonic filter can effectively eliminate current harmonics and reduce the total harmonic distortion (THD) of supply currents.

Keywords: high-voltage and high-power motor; soft starting; reactive power compensation; harmonic filtering

1. Introduction The high-voltage and high-power motor with its voltage over 6 kV and power over 10 thousand kilowatts has been used in a wide variety of fields. The major starting methods for a high-voltage and high-power motor are voltage reduction starting, voltage regulation soft starting, and variable frequency soft starting, which can be further divided into discrete variable frequency starting and continuous variable frequency starting. The high-voltage converter-based soft starting method performs better, yet its use is limited by its complicated technology, high price and heavy dependence on importing. The voltage reduction starting method consists of motor stator winding series reactor starting, autotransformer starting, and star-triangle switching starting. The voltage regulation soft starting method can be classified into motor stator winding series liquid resistance soft starting [1–3], thyristor voltage regulation soft starting [4–6], and magnetic saturation reactor voltage-reduction soft starting [7–9]. The above mentioned soft starting approaches can solve most starting problems faced by high-voltage and low-power motors, yet they fail to do the same for high-voltage and high-power motors. Therefore, the current-limiting of the high-voltage and high-power motor under no-load, light

Energies 2019, 12, 3068; doi:10.3390/en12163068 www.mdpi.com/journal/energies Energies 2019, 12, 3068 2 of 21 load, and heavy load multitasking conditions and the reactive power compensation of the motor while it starts remain to be explored [10–13]. To realize the soft starting of a high-voltage and high-power motor, the following problems should be addressed [14–16]: (1) The starting current of the high-voltage and high-power motor during soft starting should be restricted to avoid large impact on the power grid and damage to the motor and its driving equipment. (2) The power factor of the high-voltage and high-power motor during soft starting should be increased to save energy and ensure the voltage stability of power grid. At present, researchers have carried out lots of detailed studies on the motor voltage-regulating device. In Reference [17], as liquid resistance dropped gradually from the maximum value to zero, the motor could accelerate at a constant speed until it reached the nominal speed, thereby realizing soft starting. A middle-high power motor controller was proposed in [18], which used Programmable Logic Controller (PLC) as a magnetic control soft starter. A novel induction motor soft starter based on a magnetically controlled reactor was introduced in Reference [19]. In Reference [20], the researcher analyzed the torque oscillation of discrete variable frequency soft starting method during the starting process. Finally, in Reference [21], the performance of an on-off convertor-based soft starter of a middle-high voltage and high-power motor was investigated. According to the review, the existing high-voltage motor starters are suitable for low-power motors, but they are unable to meet the requirements on current-limiting and reactive power compensation during the starting process of high-voltage and high-power motors. The magnetic control soft starting method [22–27] controls high voltage with low voltage, so the starters are not restricted by the voltage withstand ability of thyristor and the magnitude of current, thereby widening the application range of soft starters. This soft starting method is able to solve the starting problems faced by the high-voltage and high-power motor. However, the high-cost magnetron voltage regulation part was removed after the starting process, resulting in a waste of resources. This paper, aiming at such starting problems of a high-voltage and high-power motor as large starting current, low power factor, a waste of resources, and a lack of harmonic control, develops a current-limiting soft starting method for a high-voltage and high-power motor. First, the principle of the proposed current-limiting soft starting method is discussed. The current-limiting starting characteristic curve of a high-voltage and high-power motor is obtained, the topological structure of integrated device is established, and a functional logic switching strategy is put forward. Second, the mechanisms of current-limiting starting, reactive compensation and dynamic harmonic filtering are analyzed. The direct starting and the current-limiting starting based on autotransformer voltage reduction–current limiting starting (AVRCLS), magnetron voltage regulation–current limiting starting (MVRCLS), and reactive power compensation (RPC) are performed by developing a simulation system. With a 10 kV/19,000 kW fan-loaded motor of a steel plant as the subject of study, a current-limiting soft starter with RPC used for the high-voltage and high-power motor based on the theoretical findings is designed, and the fan-loaded starting experiment is conducted with the soft starter. The experimental results demonstrate that the motor’s starting current decreases from 6.5 to 2 times as large as its rated current, that the power factor is raised to over 0.9 after RPC, and that the harmonic filter effectively eliminates current harmonics and reduces the total harmonic distortion (THD) of supply currents. The rest of the paper is organized as follows. Section2 illuminates the principle of the proposed current-limiting soft starting method. Section3 details and analyzes the proposed current-limiting soft starting method. Section4 reports the experimental results. Section5 concludes the paper and points out future work to be done.

2. Method Principle Aiming at cracking problems like large starting current, low power factor, a waste of resources, and a lack of harmonic control in a conventional soft starter for a high-voltage and high-power motor, a current-limiting soft starting method for a high-voltage and high-power motor was proposed. The Energies 2019, 12, 3068 3 of 21

method integrated functions like AVRCLS, MVRCLS, and RPC during starting. When the starting Energies 2019, 07, x FOR PEER REVIEW 3 of 22 process terminated, a power filtering subsystem was turned on to filter out harmonics in the power 96 system.system. This This section mainlymainly includes include current-limitings current-limiting starting starting characteristic characteristic curve, topological curve, topological structure, 97 structure,and function and functio logicaln switching logical switching strategy. strategy. 2.1. Current-Limiting Starting Characteristic Curve 98 2.1. Current-Limiting Starting Characteristic Curve As shown in Figure1, the high-voltage and high-power motor was expected to run by following 99 As shown in Figure 1, the high-voltage and high-power motor was expected to run by following the characteristic curve in stage I to III and to meet the requirements of a high-voltage and high-power 100 the characteristic curve in stage I to III and to meet the requirements of a high-voltage and high- motor on starting. 101 power motor on starting.

Us I II III IV Un UsM UsA δ·Un

t 0 t1 t2 t3 (a)

Is I II III IV k·In

In

IL

0 t t1 t2 t3 (b)

102 FigureFigure 1. 1.(a)( aVoltage) Voltage characteristic characteristic curve; curve; (b ()b Current) Current characteristic characteristic curve curve..

In Figure1, Un, In, IL, Us and Is stand for the nominal voltage, rated current, load current, starting 103 In Figure 1, Un, In, IL, Us and Is stand for the nominal voltage, rated current, load current, starting voltage, and starting current of the high-voltage and high-power motor, respectively. I, II, III, and IV 104 voltage, and starting current of the high-voltage and high-power motor, respectively. I, II, III, and IV denote the four stages for AVRCLS with RPC, MVRCLS with RPC, termination of starting, and power 105 denote the four stages for AVRCLS with RPC, MVRCLS with RPC, termination of starting, and power filtering. t1, t2, and t3 represent the switching time of the MVRCLS subsystem, full voltage run, and 106 filtering. t1, t2, and t3 represent the switching time of the MVRCLS subsystem, full voltage run, and power filtering subsystem respectively. UsA and UsM are output voltages of the AVRCLS subsystem 107 power filtering subsystem respectively. UsA and UsM are output voltages of the AVRCLS subsystem k 108 andand MVRCLS MVRCLS subsystem subsystem,, respectively. respectively. The The starting starting current current times k waswas thethe controlcontrol object,object, generallygenerally in the range of 2–4. 109 in the range of 2–4. Equation (1) could be satisfied by regulating the starting voltage Us during starting. 110 Equation (1) could be satisfied by regulating the starting voltage Us during starting.  δ U 0 t < t   Un n 0  t  t1 1   · ≤ UUs=  Uδ U  Un + (U t )sM t(t ) t t1 t t < t2 (1) s  n· sM 1 2≤ (1)   Un t = t2  Un t t2

111 where,where, δ δisis the the coefficient coefficient of of auto auto-voltage-voltage that that c couldould be be adjusted according toto thethe practicalpractical situation.situation. It 112 Itwas was aboutabout 0.4–0.7.0.4–0.7. The voltage characteristics characteristics of of the the motor motor shown shown in in Figure Figure 1a1a c couldould be be realized realized by by finding the optimal δ, t1, t2 and UsM(t). 113 finding the optimal δ, t1, t2 and UsM () t .

114 (1) AVRCLS with RPC (STAGE I, 0~t1) 115 First, the RPC subsystem was turned on, and then the AVRCLS subsystem was turned on. The 116 partial voltage of the motor obtained from the AVRCLS subsystem was δ·Un. The starting current Is of 117 the high-voltage and high-power motor was smaller than rated current In. 118 (2) MVRCLS with RPC (STAGE II: t1~t2) 119 When the starting current Is of the motor approached the rated current In, the MVRCLS 120 subsystem was switched on. In this stage, the magnetic control voltage regulation could be realized,

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(1) AVRCLS with RPC (STAGE I, 0~t1) First, the RPC subsystem was turned on, and then the AVRCLS subsystem was turned on. The partial voltage of the motor obtained from the AVRCLS subsystem was δ Un. The starting current Is of · the high-voltage and high-power motor was smaller than rated current In.

(2) MVRCLS with RPC (STAGE II: t1~t2)

When the starting current Is of the motor approached the rated current In, the MVRCLS subsystem was switched on. In this stage, the magnetic control voltage regulation could be realized, so the voltage of the motor increased from δ Un to Un progressively. The RPC subsystem provided the reactive power · requiredEnergies 2019 in the, 07, startingx FOR PEER process REVIEW of the motor, thus increasing the power factor. 4 of 22 The current-limiting of the high-voltage and high-power motor was realized in stages I and II. 121 so the voltage of the motor increased from δ·Un to Un progressively. The RPC subsystem provided

122 (3)the End reactive of starting power process required (STAGE in the III:startingt2~t3) process of the motor, thus increasing the power factor. 123 The current-limiting of the high-voltage and high-power motor was realized in stages I and II. The soft starting process of the motor terminated when the starting voltage Us reached Un and the 124 (3) End of starting process (STAGE III: t2~t3) current Is was equal to the load current IL. Afterwards, the motor worked normally under full voltage. 125 The soft starting process of the motor terminated when the starting voltage Us reached Un and the 126 current Is was equal to the load current IL. Afterwards, the motor worked normally under full voltage. (4) Power filtering subsystem (STAGE IV: AFTER t3~) 127 (4) Power filtering subsystem (STAGE IV: AFTER t3~) 128 WithWith the the motor motor working working normally normally under under full voltage, full voltage, the power the filteringpower subsystem filtering subsystem was switched was 129 toswitched filter out to harmonics filter out harmonic in powers system. in power system. 2.2. Topologial Structure 130 2.2. Topologial Structure Based on the research findings [28–32] and according to the above current-limiting starting 131 Based on the research findings [28–32] and according to the above current-limiting starting characteristic curve, a device was developed which integrated current-limiting starting, reactive 132 characteristic curve, a device was developed which integrated current-limiting starting, reactive compensation, and harmonic filtering (hereinafter referred to as integrated device). The topological 133 compensation, and harmonic filtering (hereinafter referred to as integrated device). The topological structure of the integrated device is shown in Figure2. 134 structure of the integrated device is shown in Figure 2.

Power Supply Voltage (U1)

QS

QA TA/TV 1KM1

1KM4 TS 1KM5 2KM1 2KM2 … 2KMn

SP Controller C C1 C2 Cn 1KM3

3KM ZS PTB D/A I/O SS TA1 Control System 1KM2 The main circuit PTB: Pulse trigger board SP: Signal processor M TS: Touch screen NL: Nonlinear load

135 FigureFigure 2. 2.Topological Topological structure structure of of the the integrated integrated device. device.

136 AsAs shown shown in in Figure Figure2, 2, QS QS is is a disconnector,a disconnector, QA QA is is a high-voltagea high-voltage circuit-breaker, circuit-breaker, TA TA/TV/TV is is a a 137 currentcurrent//voltagevoltage transformer, transformer, TA1 TA1 is an autotransformer,is an autotransformer, SS is a magnetic SS is a controlmagnetic reactance control transformer, reactance 138 transformer, ZS is an impedance transformer, C is a capacitor bank composed of several 139 compensation capacitors (C1, C2,…, Cn), and M is the high-voltage and high-power motor. 140 1KM1~1KM5, 2KM1-n, and 3KM are high-voltage contactors. 141 The integrated device consisted of a main circuit and a control system. The main circuit was mainly 142 composed of QS, QA, TA/TV, TA1, SS, ZS, 1KM1~1KM5, 2KM1-n, 3KM, and C. To be specific, the 143 AVRCLS subsystem was made up of QS, QA, TA1, 1KM2, and 1KM 3; the MVRCLS subsystem was 144 made up of QS, QA, SS, ZS, and 1KM3-5; the RPC subsystem was made up of C, 1KM1, and 2KM1-n; 145 and the power filtering subsystem was made up of QS, QA, SS, ZS, 1KM4, 3KM, 2KM1-n, and C. 146 The core components of the integrated device were TA1 and SS. Based on the principle of magnetic 147 amplifier, the working point of the core in SS is controlled to change its inductance value smoothly. The 148 primary winding of the SS was connected in series with the motor, the secondary winding was 149 connected with ZS, and ZS was a three-phase high impedance inverter composed of six thyristors V1~V6. 150 To go into details, V1, V3, and V5 were connected at the cathode, while V4, V6, and V2 were connected at

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ZS is an impedance transformer, C is a capacitor bank composed of several compensation capacitors (C1, C2, ... ,Cn), and M is the high-voltage and high-power motor. 1KM1~1KM5, 2KM1-n, and 3KM are high-voltage contactors. The integrated device consisted of a main circuit and a control system. The main circuit was mainly composed of QS, QA, TA/TV, TA1, SS, ZS, 1KM1~1KM5, 2KM1-n, 3KM, and C. To be specific, the AVRCLS subsystem was made up of QS, QA, TA1, 1KM2, and 1KM 3; the MVRCLS subsystem was made up of QS, QA, SS, ZS, and 1KM3-5; the RPC subsystem was made up of C, 1KM1, and 2KM1-n; and the power filtering subsystem was made up of QS, QA, SS, ZS, 1KM4, 3KM, 2KM1-n, and C. The core components of the integrated device were TA1 and SS. Based on the principle of magnetic amplifier, the working point of the core in SS is controlled to change its inductance value smoothly. The primary winding of the SS was connected in series with the motor, the secondary winding was connected with ZS, and ZS was a three-phase high impedance inverter composed of six thyristors V1~V6. To go into details, V1,V3, and V5 were connected at the cathode, while V4,V6, and V2 were connected at the anode. The thyristors were driven through phase control; according to the control principle of power electronic conversion technology, A-B, A-C, B-C, B-A, C-A, and C-B corresponded to V1,V3,V5,V4,V6, and V2 respectively. A pair of thyristors were triggered every 60 degrees to realize the impedance transformation of the three-phase high impedance inverter. The control system consisted of sensor (SP), controller, thyristor pulse trigger board (PTB), touch screen (TS), D/A converter (D/A), and digital input and output (I/O). The control system collected the information of starting current Is of the high-voltage and high-power motor via SP and made control decisions like data processing and starting algorithm via the controller. The results were obtained based on two approaches. The one was D/A, which controlled ZS through PTB, thereby realizing the transformation and continuous control of electromagnetic parameters of SS. The other one was I/O which controlled the dynamic switching of high-voltage contactors in the main circuit. TS communicated with the controller via RS485 (Kunluntongtai, Beijing, China), set the variable status, and displayed running parameters and device working status. The control system was used to coordinate and control the AVRCLS subsystem, MVRCLS subsystem, RPC subsystem, and power filtering subsystem so that these subsystems could perform the function properly. The high-voltage and high-power motor’s current-limiting soft starting with RPC was realized by the AVRCLS subsystem, MVRCLS subsystem, and RPC subsystem. The power filtering subsystem was turned on to filter out harmonics in power system as the starting process terminated.

2.3. Functional Logic Switching Strategy The control system switched the high-voltage contactors according to the topological structure of the integrated device, thereby realizing the current-limiting starting, reactive compensation of a high-voltage and high-power motor, and power harmonic filtering. The chronological order of high-voltage contactors is shown in Figure3. In Figure3, ‘ON’ meant the high-voltage contactor was closed, while ‘OFF’ meant it was open. In stage I (0~t1), the AVRCLS subsystem was turned on; in stage II (t1~t2), the MVRCLS subsystem was turned on; in stage III (t2~t3), the soft starting process of the high-voltage and high-power motor terminated; and in stage IV (t3~), the power filtering subsystem was turned on. The RPC subsystem provided the reactive power required in the starting process of the motor, thereby increasing the power factor in stage I and stage II. In this process, 2KM1-2KMn switched according to the compensation capacity. In stage IV, it switched according to the resonant frequency of harmonic current that needed to be filtered. Energies 2019, 07, x FOR PEER REVIEW 5 of 22

151 the anode. The thyristors were driven through phase control; according to the control principle of 152 power electronic conversion technology, A-B, A-C, B-C, B-A, C-A, and C-B corresponded to V1, V3, V5, 153 V4, V6, and V2 respectively. A pair of thyristors were triggered every 60 degrees to realize the impedance 154 transformation of the three-phase high impedance inverter. 155 The control system consisted of sensor (SP), controller, thyristor pulse trigger board (PTB), touch 156 screen (TS), D/A converter (D/A), and digital input and output (I/O). The control system collected the 157 information of starting current Is of the high-voltage and high-power motor via SP and made control 158 decisions like data processing and starting algorithm via the controller. The results were obtained based 159 on two approaches. The one was D/A, which controlled ZS through PTB, thereby realizing the 160 transformation and continuous control of electromagnetic parameters of SS. The other one was I/O 161 which controlled the dynamic switching of high-voltage contactors in the main circuit. TS 162 communicated with the controller via RS485 (Kunluntongtai, Beijing, China), set the variable status, 163 and displayed running parameters and device working status. The control system was used to 164 coordinate and control the AVRCLS subsystem, MVRCLS subsystem, RPC subsystem, and power 165 filtering subsystem so that these subsystems could perform the function properly. The high-voltage and 166 high-power motor’s current-limiting soft starting with RPC was realized by the AVRCLS subsystem, 167 MVRCLS subsystem, and RPC subsystem. The power filtering subsystem was turned on to filter out 168 harmonics in power system as the starting process terminated.

169 2.3. Functional Logic Switching Strategy 170 The control system switched the high-voltage contactors according to the topological structure of 171 the integrated device, thereby realizing the current-limiting starting, reactive compensation of a high- 172 Energiesvoltage2019 and, 12 high, 3068-power motor, and power harmonic filtering. The chronological order of high-voltage6 of 21 173 contactors is shown in Figure 3.

Stage I II III IV ON

OFF

(a) ON

OFF

(b) ON

OFF

(c) ON Energies 2019, 07, x FOR PEER REVIEW 6 of 22 OFF

176 In Figure 3, ‘ON’ meant the high-voltage contactor was closed, while ‘OFF’ meant it was open. (d) 177 In stage I (0~t1), the AVRCLSON subsystem was turned on; in stage II (t1~t2), the MVRCLS subsystem 178 was turned on; in stage III (t2~t3), the soft starting process of the high-voltage and high-power motor 179 terminated; and in stage IVOFF (t 3~), the power filtering subsystem was turned on. 180 The RPC subsystem provided the reactive power required in the startingt process of the motor, 0 t1 t2 t3 181 thereby increasing the power factor in stage I (ae)nd stage II. In this process, 2KM1-2KMn switched 182 according to the compensation capacity. In stage IV, it switched according to the resonant frequency 183174 of harmonicFigure 3.3.current (a) Chronological that needed orders to be of filtered. 1KM1,3; (b) ChronologicalChronological ordersorders ofof 1KM2;1KM2; ((cc)) ChronologicalChronological 175 orders of 1KM4; (d) Chronological ordersorders ofof 1KM5.1KM5. 184 3.3. Analysis Analysis of of Mechanisms Mechanisms 185 InIn this this section, section, the the mechanisms mechanisms of of current current-limiting-limiting starting, starting, reactive reactive compensation compensation and and dynamic dynamic 186 harmonicharmonic filtering filtering were were analyzed analyzed in in detail detail..

187 3.1.3.1. Mechanism Mechanism of of Direct Direct Starting Starting 188 FigureFigure 44 showedshowed the the Г-Γshaped-shaped circuit circuit of of the the motor. motor.

’ r1 r2

rm

189 FigureFigure 4. 4. ГΓ circuitcircuit of of the the motor. motor.

190 InIn Figure 4, 4,r 1rand1 andx1 σxare1σ are the statorthe stator resistance resistance and stator and leakage stator leakagereactance reactance of the motor, of respectively.the motor, 0 191 respectively.rm and xm are rm and the excitationxm are the excitation resistance resistance and excitation and excitation reactance reactance of the motor, of the respectively.motor, respectively.r2 and x'0 are the' converted values of rotor resistance and leakage resistance of the motor respectively. U1 192 r22σ and x2 are the converted values of rotor resistance and leakage resistance of the motor 1 s 0 is the root mean square (RMS) of power supply voltage. s is the slip ratio. −s r is the mechanical 2 1  s ' 193 respectively. U1 is the root mean square (RMS) of power supply voltage. s is the slip ratio. r is analog resistance. s 2 194 the mechanical analog resistance. 195 When the motor started, the speed was 0, s = 1, and the mechanical analog resistance reached 0. So 

196 the starting current I st was only limited by the short-circuit impedance, and its magnitude Ist was as 197 follows:

U1 Ist = ' 2 ' 2 (2) (r+r)+(x1 2 1σ +x 2σ )

198 It could be known from Equation (2) that as the leakage impedance was small, the starting 199 current was large. When the high-voltage and high-power motor started directly, the voltage at the 200 motor end was U1, and the starting current was Ist.

201 3.2. Mechanism of the AVRCLS Subsystem 202 The topological structure and equivalent circuit model of the AVRCLS subsystem are shown in 203 Figure 5 and Figure 6, respectively [24].

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When the motor started, the speed was 0, s = 1, and the mechanical analog resistance reached 0. · So the starting current Ist was only limited by the short-circuit impedance, and its magnitude Ist was as follows: U1 Ist = q (2) 2 2 ( + 0 ) + ( + ) r1 r2 x1σ x0 2σ It could be known from Equation (2) that as the leakage impedance was small, the starting current was large. When the high-voltage and high-power motor started directly, the voltage at the motor end was U1, and the starting current was Ist.

Energies3.2. Mechanism 2019, 07, x FOR ofthe PEER AVRCLS REVIEWSubsystem 7 of 22

The topological structure and equivalent circuitPower Supply model of the AVRCLS subsystem are shown in FiguresEnergies 20195 and, 076, ,x respectivelyFOR PEER REVIEW [24]. 7 of 22

QS Power Supply QA QS TA/TV

QA

1KM3 TA/TV

1KM3 TA1 M 1KM2

204 TA1 M 1KM2 205 Figure 5. Topological structure of the autotransformer voltage reduction–current limiting starting 206204 (AVRCLS) subsystem. Figure 5. Topological structure of the autotransformer voltage reduction–current limiting starting 205 (AVRCLS)Figure 5. Topological subsystem. structure of the autotransformer voltage reduction–current limiting starting 206 (AVRCLS) subsystem. . Is1

. . Is1 IsA

. TA1 . IsA U1 . UsA M . TA1 U1 . UsA M 207 Figure 6. Equivalent circuit model of the AVRCLS subsystem. 208 Figure 6. Equivalent circuit model of the AVRCLS subsystem. 207 When the AVRCLS subsystem ran, the transformation ratio of TA1 was set as KA (KA > 1). Let Is1, 209 IsA,WhenU1, and theU AVRCLSsA be the subsystem input current, ran, output the transformation current, input ratio voltage of TA1 and wa outputs set as voltage KA (KA of> 1 TA1). Let in Is the1, 208 Figure 6. Equivalent circuit model of the AVRCLS subsystem. 210 IsAAVRCLS, U1, andsubsystem, UsA be the input respectively, current, and output then current, we got: input voltage and output voltage of TA1 in the 211 AVRCLS subsystem, respectively, and then we got: 209 When the AVRCLS subsystem ran, the transformationU1 ratio of TA1 was set as KA (KA > 1). Let Is1, UsA = (3) 210 IsA, U1, and UsA be the input current, output current,U1 kinputA voltage and output voltage of TA1 in the UsA = (3) 211 AVRCLS subsystem, respectively, and then we gotk:A U 212 where, UsA was equal to δ·Un in Figure 1a. From Figure 6,1 when the high-voltage and high-power motor UsA = (3) 213 started, the starting current Is after being adjusted by autotransformerkA TA1 was:

212 where, UsA was equal to δ·Un in Figure 1a. From FigureU sA 6, when the high-voltage and high-power motor IsA = ' 2 ' 2 (4) 213 started, the starting current Is after being adjusted(r+r)+(x1 2 by autotransformer 1σ +x 2σ ) TA1 was: U 214 sA From Equations (2)–(4), we got: IsA = ' 2 ' 2 (4) (r+r)+(x1 2 1σ +x 2σ ) Ist IsA = (5) 214 From Equations (2)–(4), we got: kA

215 According to Equations (3) and (5), the AVRCLSI stsubsystem could reduce the voltage and limit IsA = (5) 216 the current of the motor during starting. kA

217215 3.3. MechanismAccording of theto EquationsMVRCLS Subsystem(3) and (5), the AVRCLS subsystem could reduce the voltage and limit 216 the current of the motor during starting.

217 3.3. Mechanism of the MVRCLS Subsystem

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where, U was equal to δ Un in Figure1a. From Figure6, when the high-voltage and high-power sA · motor started, the starting current Is after being adjusted by autotransformer TA1 was:

UsA IsA = q (4) 2 2 ( + 0 ) + ( + ) r1 r2 x1σ x0 2σ

From Equations (2)–(4), we got: Ist IsA = (5) kA Energies 2019, 07, x FOR PEER REVIEW 8 of 22 According to Equations (3) and (5), the AVRCLS subsystem could reduce the voltage and limit the 218 currentThe of topological the motor during structure starting. and equivalent circuit model of the MVRCLS subsystem are shown in 219 EnergiesFigure 2019 7 ,and 07, x FigureFOR PEER 8, respectivelyREVIEW [15]. 8 of 22 3.3. Mechanism of the MVRCLS Subsystem 218 TheThe topological topological structure structure and and equivalentPower equivalent Supply circuit circuitVoltage model model(U1) of of the the MVRCLS MVRCLS subsystem subsystem are are shown shown in in 219 FigureFigures 77 and and Figure8, respectively 8, respectively [15]. [15]. QS Power Supply Voltage (U1) QA TA/TV QS

QA 1KM4 1KM5TA/TV

1KM4 1KM5 1KM3 ZS M SS 1KM3 220 ZS 221 Figure 7. Topological structure of the magnetronM voltageSS regulation–current limiting starting 222 (MVRCLS) subsystem. 220 Figure 7. Topological structure of the magnetron voltage regulation–current limiting starting 221223 Figure(MVRCLS) 7. Topological subsystem. structure of the magnetron voltage regulation–current limiting starting 222 (MVRCLS) subsystem. . . Uss1 IsM 223 SS . . Uss1 . . . IsM Iss2 U1 Uss2 . SS UsM M . . . ZS Iss2 U1 Uss2 . UsM M ZS

224 FigureFigure 8. 8.Equivalent Equivalent circuit circuit model model of of the the MVRCLS MVRCLS subsystem. subsystem.

225 AsAs shown shown in in Figure Figure8, 8, the the high-voltage high-voltage winding winding of of SS SS was wa thes the primary primary side, side, with withN Nss1ss1as as the the 224226 numbernumber of of turns, turns,U Ussss1Figure1as as thethe 8. voltage voltageEquivalent and and circuitI ssIss11 asas model thethe current.current. of the MVRCLS TheThe high-voltagehigh subsystem.-voltage winding winding and and the the motor motor 227 (voltage:(voltage:U USS)) werewere connectedconnected inin series.series. Similarly,Similarly, thethe low-voltagelow-voltage windingwinding ofof SSSS was was the the secondary secondary 225228 side,As with shown Nss 2i nas Figure the number 8, the highof turns-voltage, Uss 2 windingas the voltage of SS waands theIss2 asprimary the current. side, with The Nlowss1 -asvoltage the 226229 numberwinding of andturns, ZS U wess1 reas alsothe voltageconnected and in I ssseries.1 as the current. The high-voltage winding and the motor 227230 (voltage:The U voltageS) were equationconnected could in series. be obtained Similarly, from the Figure low- 8voltage: winding of SS was the secondary 228 side, with Nss2 as the number of turns, Uss2 as the voltage and Iss2 as the current. The low-voltage UUUsM1  ss 1 229 winding and ZS were also connected in series. 230 The voltage equation could be obtained fromUIss1 Figure=Z ss 1 sM 8: (6)  UIss2=Z ss 2 ss 2 UUUsM1  ss 1  231 The ratio of the number (n) of turns of UIhighss1=Z-voltage ss 1 sM winding of SS to the number of turns of(6) low - 232 voltage winding was:  UIss2=Z ss 2 ss 2

NUIss1 ss 1 ss 2 231 The ratio of the number (n) of turns of nhigh = -voltage= winding of SS to the number of turns of low(7)- NUI 232 voltage winding was: ss2 ss 2 sM

NUIss1 ss 1 ss 2 n = =  (7) NUIss2 ss 2 sM

Energies 2019, 12, 3068 9 of 21

side, with Nss2 as the number of turns, Uss2 as the voltage and Iss2 as the current. The low-voltage winding and ZS were also connected in series. The voltage equation could be obtained from Figure8:   UsM = U1 Uss1  −  Uss1= Zss1IsM (6)   Uss2= Zss2Iss2

The ratio of the number (n) of turns of high-voltage winding of SS to the number of turns of low-voltage winding was: N U I n = ss1 = ss1 = ss2 (7) Nss2 Uss2 IsM Then the impedance correlation between the high-voltage winding and low-voltage winding of SS could be formulated as: U ss1 2 Zss1 = = n Zss2 (8) Iss1 In the thyristors in ZS work state, the positive and negative half-waves of current waveform of secondary winding were symmetrical, free from direct component and even-order harmonics. According to the Fourier formula, the current of secondary winding could be expressed as [21]:

X∞ iss2(ωt) = (an cos nωt + bn sin nωt) (9) n=1,3,5

where, R π = 2 ( ) ( ) a1 π a iss2 ωt cos ωtd ωt R π √ · = 2 2Uss2 cos ωtd(ωt) (10) π a Zssa √ · = 2Uss2 (cos 2a 1) 2πZssa − R π = 2 ( ) ( ) b1 π a iss2 ωt sin ωtd ωt √ · (11) = 2Uss2 [sin 2a + 2(π a)] 2πZss2 −

√2Uss2 1 1 1 a3 = [ cos 4a cos 2a + ] (12) πZss2 4 − 2 2

√2Uss2 1 1 b3 = [ sin 4a sin 2a] (13) 2πZss2 4 − 2 where, a is the triggering angle of thyristors. If a1 >> a3, a3 could be ignored; if b1 >> b3, b3 could be ignored. Similarly, a , a , ..., which was far less than a , and b , b , ..., which was far less than b , 5 7 ∞ 1 5 7 ∞ 1 were ignored. Then Equation (9) was simplified into:

iss2(ωt) = a1 cos ωt + b1 sin ωt (14)

The effective value of iss2 was: p I = 1 a 2 + b 2 ss2 √ 1 1 2 q (15) = Uss2 sin2 a + (π a) sin 2a + (π a)2 πZss2 − − Energies 2019, 12, 3068 10 of 21

From Equation (15), we got Zss1:

Z = Uss2 ss1 Iss2 πZss2 Energies 2019, 07, x FOR PEER REVIEW = q 10 of(16) 22 sin2 a+(π a) sin 2a+(π a)2 − − = fmZss2 244 where, f m was the coefficient of Zss1 with respect to Z ss 2 . The curve of the coefficient f m changing 245 with the triggering angle (a) of the thyristor is shown in Figure 9. where, fm was the coefficient of Zss1 with respect to Zss2. The curve of the coefficient fm changing with 246 the triggering angle (a) of the thyristor is shown in Figure9.

fm

a(。)

247 248 (a)

fm

a(。)

249 250 (b)

251 FigureFigure 9. 9. (a()a Coefficient) Coefficient f mfm changingchanging with with triggering triggering angle angle ranged ranged within within 0– 0–180;180; (b ()b Coefficient) Coefficient f mfm 252 changingchanging with with triggering triggering angle angle ranged ranged within within 140 140–180.–180.

253 TheThe following following conclusions conclusions could could be be drawn drawn from from Equations Equations (6) (6) and and (16) (16) and and Figure Figure 9: 254 (1) The equivalent impedance Zss1 would increase along with the triggering angle of thyristors. (1) The equivalent impedance Zss1 would increase along with the triggering angle of thyristors. (2) When the triggering angle ranged within 0–140, impedance Zss1 changed slowly; when the 255 (2) When the triggering angle ranged within 0–140, impedance Z changed slowly; when the triggering angle ranged within 140–170 from Figure9b, the impedance changess1 became obvious; and 256 triggering angle ranged within 140–170 from Figure 9b, the impedance change became obvious; and when the triggering angle ranged within 170–180, a sharp increase was observed in impedance Zss1. 257 when the triggering angle ranged within 170–180, a sharp increase was observed in impedance Z . (3) When the triggering angle of thyristors changed, the equivalent impedance of high-voltagess1 258 winding(3) When of SS the could triggering be seen angle as variable of thyristors impedance, changed, which the met equivalentZ impedanceZ Z of .high-voltage ss1min ≤ ss1 ≤ ss1max 259 winding(4) Asof SS the could triggering be seen angle as variable of thyristors impedance, decreased which from met 180 ◦ZZZtoss10 m in◦, the ss voltage 1  ss 1 m axU.sM and the current 260 IsM across(4) As the the triggering motor could angle be graduallyof thyristors increased decreased by from the MVRCLS 180° to 0°, subsystem. the voltage UsM and the current 261 IsM across the motor could be gradually increased by the MVRCLS subsystem. 3.4. Mechanism of the RPC Subsystem 262 3.4. MechanismThe high-voltage of the RPC and Subsystem high-power motor was a kind of inductive load which could reduce the 263 voltageThe ofhigh power-voltage grid and by establishinghigh-power amotor magnetic was a field kind and of consuminginductive load considerable which could reactive reduce power. the 264 voltageDuring of the power starting grid process by establishing of the motor, a magnetic the power field factor and graduallyconsuming increased considerable along reactive with the power. speed. 265 DuringThe compensation the starting process capacitor of Cthe was motor, connected the power in parallel factor withgradually the starting increased circuit along to providewith the reactivespeed. 266 Thepower, compensation which greatly capacitor reduced C was the totalconnected current in absorbed parallel fromwith the powerstarting grid circuit when to theprovide motor rea started.ctive 267 power, which greatly reduced the total current absorbed from the power grid when the motor started. 268 The topological structure and equivalent circuit model of the RPC subsystem are shown in Figure 10 269 and Figure 11, respectively [27]. 270 Let IR and IA be the active current and reactive current of Is respectively when the motor started, 271 then:

Energies 2019, 07, x FOR PEER REVIEW 11 of 22

Energies 2019, 07, x FOR PEER REVIEW 11 of 22 IIIs RA 0     90  (17)

272 When the RPC subsystem was turnedIII on, the  0starting    current  90  could be given by: s RA I sQ (17)

272 When the RPC subsystem was turned on, the starting current could be given by: IIIsQ s  C 90  I sQ (18) Energies 2019, 12, 3068 IIIRC 0  （ A   90    90 ） 11 of 21 IIIsQ s  C 90  (18) 273 where, IC was the reactive current providedIIIRC 0 by （ theA RPC  90 subsystem.    90 ） 274 The topological structure and equivalent circuit model of the RPC subsystem are shown in Figures 10 273 where, IC was the reactive current provided by the RPC subsystem. and 11, respectively [27]. Power Supply Voltage (U ) 274 1

Power Supply Voltage (U ) QS 1

QAQS

1KM1QA TA/TV Starting Circuit 1KM1 TA/TV 1KM4 Starting Circuit 2KM1 2KM2 … 2KMn 1KM4

2KM1 2KM2 … 2KMn C C1 C2 Cn 1KM3 C1 C2 Cn ZS C 1KM3 SS TA1 ZS SS RPC Subsystem 1KM2 TA1 RPC Subsystem AVRCLS Subsystem or M 1KM2 MVRCLS Subsystem AVRCLS Subsystem or M 275 MVRCLS Subsystem 276275 Figure 10. Topological structure of the reactive power compensation (RPC) subsystem. Figure 10. Topological structure of the reactive power compensation (RPC) subsystem. 276 Figure 10. Topological structure of. the reactive power compensation (RPC) subsystem. IsQ . IsQ . Is . . . IC Is U1 . . Starting UsA . CIC Circuit U1 . Starting UsA C Circuit 277 Figure 11. Equivalent circuit model of the RPC subsystem. 278277 Figure 11. Equivalent circuit model of the RPC subsystem. Let IR and IA be the active current and reactive current of Is respectively when the motor 279278 started,According then: to the principleFigure of11. reactive Equivalent power circuit compensation, model of the RPC the subsystem.relationship between the motor 280 current and voltage without and with theI sRPC= I Rsubsystem∠0◦ + IA∠ is90 shown◦ in Figure 12. (17) 279 According to the principle of reactive power compensation,− the relationship between the motor 280 currentWhen and thevoltage RPC without subsystem and was with turned the RPC on, thesubsystem starting is current shownI sQin couldFigure be 12 given. by:

I = I + I ∠90 sQ s C ◦ (18) = IR∠0 + (I ∠ 90 + I ∠90 ) ◦ A − ◦ C ◦

where, IC was the reactive current provided by the RPC subsystem. According to the principle of reactive power compensation, the relationship between the motor current and voltage without and with the RPC subsystem is shown in Figure 12. EnergiesEnergies 20192019, 07, 12, x, FOR 3068 PEER REVIEW 12 12of of22 21

281

. IC . IR . U

. . IsQ IA . Is . IC

282 FigureFigure 12. 12.Relationship Relationship between between the the motor motor current current and and voltage voltage without without and and with with the RPCthe RPC subsystem. 283 subsystem. The following conclusions could be drawn from Figure 12:

284 The(1) following RPC subsystem conclusions during could the starting be drawn of thefrom motor Figure could 12: reduce current, namely, IsQ < Is; (2) RPC subsystem during the starting of the motor could increase power factorII of the motor, 285 (1) RPC subsystem during the starting of the motor could reduce current, namely, sQ s ; as cos φ > cos φ . 286 (2) RPC2 subsystem1 during the starting of the motor could increase power factor of the motor, as Based on the empirical approach, this paper assumed that the compensation capacity was 287 cos2 cos  1 . correlated with the rated power PN of the motor. Then there existed: 288 Based on the empirical approach, this paper assumed that the compensation capacity was

289 correlated with the rated power PN of the motor.Q = Thenξ P Nthere existed: (19) · Q = ξ  P where, ξ denoted the coefficient of relationship whichN was generally around 0.3 and could be adjusted(19) 290 where,according  denote to the actuald the situation. coefficient of relationship which was generally around 0.3 and could be The optimal switching control strategy for the RPC subsystem was related with the time of 291 adjusted according to the actual situation. switching and grouping of capacitors. According to Equation (19), RPC effect was optimized 292 The optimal switching control strategy for the RPC subsystem was related with the time of by grouping switch. In general, the reactive power compensation of the motor required 293 switching and grouping of capacitors. According to Equation (19), RPC effect was optimized by undercompensating, and the power factor was about 0.90–0.95 after compensation. 294 grouping switch. In general, the reactive power compensation of the motor required 295 undercompensating,3.5. Mechanism of the and Power the Filteringpower factor Subsystem was about 0.90–0.95 after compensation.

296 3.5. MechanThe topologicalism of the Power structure Filtering and Subsystem equivalent circuit model of the power filtering subsystem are shown in Figures 13 and 14, respectively [29]. 297 The topological structure and equivalent circuit model of the power filtering subsystem are As shown in Figure 14, the power filtering subsystem was an LC series filter circuit. The 298 shown in Figure 13 and Figure 14, respectively [29]. impedances on the high-voltage winding and low-voltage winding of SS as well as the impedance of magnetic coupling were small, so they could all be neglected. Thus, SS showed inductance, then: ( Z X = ωL ss1 ≈ ss1 ss1 (20) Zss Xss = ωLss 2 ≈ 2 2 After substituting Equation (20) into Equation (16), the correlation between the equivalent inductance Lss1 on the high-voltage winding and the triggering angle α of thyristors could be obtained:

πLss2 Lss1 = q (21) sin2α + (π α)sin2α + (π α)2 − −

Therefore, by controlling the triggering angle of thyristors, Lss1 could be continuously adjusted. In the power system, the part of harmonic current generated by nonlinear load entered the power filtering subsystem, and the other part entered the power grid. When the power filtering subsystem worked normally, the power filtering subsystem resonated at harmonic frequency, forming a low impedance

Energies 2019, 12, 3068 13 of 21

Energiesbypass 2019 for, 07 the, x FOR harmonic PEER REVIEW current. In this case, most of the harmonic current was absorbed by the13 powerof 22 filteringEnergies 2019 subsystem., 07, x FOR PEER REVIEW 13 of 22

Power Supply Voltage (U1)

Power Supply Voltage (U1) QS QS QA QA TA/TV TA/TV

1KM4 … 1KM4 2KM1 2KM2 2KMn Nonlinear Load … 2KM1 2KM2 2KMn Nonlinear Load

C C1 C2 Cn 3KM C C1 C2 Cn 3KM ZS

SS ZS SS 299 Figure 13. Topological structure of the power filtering subsystem. 300 299 FigureFigure 13.13. Topological structurestructure ofof thethe powerpower filteringfiltering subsystem.subsystem. 300 . . Uss1 IF . . Uss1 IF SS C . .. Iss2 Uss2 SS C . .. . Iss2 Uss2 UC ZS . UC . ZS IL . IL Nonlinear . Nonlinear U1 . U1 301 FigureFigure 14 14.. EquivalentEquivalent circuit circuit model model of ofthe the power power filtering filtering subsystem. subsystem. 301 Figure 14. Equivalent circuit model of the power filtering subsystem. 302 AsLet shown the current in Figure offset 14 of, thethen- powerth harmonic filtering∆I n subsystembe: was an LC series filter circuit. The 303302 impedancesAs shown on the in high Figure-voltage 14, windingthe power and filtering low-voltage subsystem winding wa ofs SS an asLC well series as the filter impedance circuit. ofThe ∆In = InL InF (22) 304303 magneticimpedances coupling on the we highre small,-voltage so theywinding could and all lowbe| neglected.-voltage− | winding Thus, SS of show SS ased well inductance, as the impedance then: of 304 where,magneticI couplingwas the currentwere small, of the son- theyth harmonic couldZXL all of be the neglected. power filtering Thus, SS subsystem, showed inductance,I was the then: current nF ss1 ss 1 ss 1 nL n-  I (20) of the th harmonic on load side, and ∆ZXLnssZXL2wasss1 ssused 2 ss  1  to ss describe2 ss 1 the detuning degree of the power  (20) filtering subsystem. When the power filtering subsystem worked properly, ∆In was close to 0. In this ZXLss2 ss 2   ss 2 305 case,After the filter substituting did not detune Equation harmonics. (20) into Nevertheless, Equation (16), when the the correlation power filtering between subsystem the equivalent detuned 306 inductance L on the high-voltage winding and the triggering angle α of thyristors could be 305 harmonics,After substitutingthess1 ∆In of harmonic Equation current (20) intoincreased. Equation (16), the correlation between the equivalent

307306 obtained:inductanceFigure 15L ss1 shows on the the high schematic-voltage diagram winding of and tuning the filter trigger method.ing angle Through α of thyristors the synchronous could be 307 samplingobtained: of the currents IF and IL, the n-th harmonicπL currents InF and InL could be obtained via the L = ss2 ss1 (21) Discrete Fourier Transform. The controlsin objective2α +(π - wasα)sin2πL toα make+ (π - α current) 2 ∆In reach the minimum. L = ss2 ss1 (21) sin 2α +(π - α)sin2α + (π - α) 2 308 Therefore, by controlling the triggering angle of thyristors, Lss1 could be continuously adjusted. In

309308 the powerTherefore, system, by the controlling part of harmonic the trigger currenting anglegenerated of thyristors by nonlinear, Lss1 c ouldload enterbe continuouslyed the power adjusted. filtering In 310309 subsystem,the power and system, the otherthe part part of harmonicentered the current power generated grid. When by nonlinearthe power load filtering enter edsubsystem the power work filteringed 311310 normally,subsystem, the andpower the filteri otherng part subsystem entered resonatethe powerd at grid. harmonic When frequency,the power formingfiltering asubsystem low impedance worked 312311 bypassnormally, for the the harmonic power filteri current.ng subsystem In this case, resonate most ofd the at harmonicharmonic frequency,current wa sforming absorbed a lowby the impedance power 313312 filteringbypass subsystem. for the harmonic current. In this case, most of the harmonic current was absorbed by the power 314313 filteringLet the subsystem. current offset of the n-th harmonic ΔIn be: 314 Let the current offset of the n-th harmonic ΔIn be:

Energies 2019, 07, x FOR PEER REVIEW 14 of 22

In = I nL - I nF (22)

315 where, InF was the current of the n-th harmonic of the power filtering subsystem, InL was the current of 316 the n-th harmonic on load side, and ΔIn was used to describe the detuning degree of the power filtering 317 subsystem. When the power filtering subsystem worked properly, ΔIn was close to 0. In this case, the 318 filter did not detune harmonics. Nevertheless, when the power filtering subsystem detuned harmonics, 319 the ΔIn of harmonic current increased. 320 Figure 15 shows the schematic diagram of tuning filter method. Through the synchronous 321 samplingEnergies 2019 of, 12 the, 3068 currents IF and IL, the n-th harmonic currents InF and InL could be obtained via14 ofthe 21 322 Discrete Fourier Transform. The control objective was to make current ΔIn reach the minimum.

IL Sampling InL + CCS analysis ZS - InF Sampling IF analysis

Figure 15. Schematic model of tuning filter method. 323 Figure 15. Schematic model of tuning filter method. 4. Simulation and Experimental Evaluation 324 4. Simulation and Experimental Evaluation The direct starting simulation was performed, and then the current-limiting soft starting for 325 a high-voltageThe direct andstarting high-power simulation motor was wasperformed, verified and by simulationthen the current on AVRCLS-limiting and soft MVRCLS starting for with a 326 highRPC-voltage during starting.and high- Allpower the motor simulations was verified were performedby simulation using on AVRCLS MATLAB and/Simulink MVRCLS (R2018a, with RPC The 327 duringMathWorks, starting. Inc, Natick, All the MA, simulations USA). Then, were a model performed machine using was made MATLAB/Simulink to verify the performance (R2018a,of The the 328 MathWorks,current-limiting Inc, softNatick, starting MA, method. USA). T Thehen, harmonica model machine filter developed was made based to verify on the the harmonic performance filtering of 329 thesubsystem current was-limiting applied soft to starting eliminating method. harmonics The harmonic of the low-voltage filter developed distribution based system. on the harmonic 330 filtering subsystem was applied to eliminating harmonics of the low-voltage distribution system. 4.1. Simulation System on Current-Limiting Soft Starting 331 4.1. Simulation System on Current-Limiting Soft Starting The simulation systems were designed according to the topological structure of the integrated 332 deviceThe (Figure simulation2). AVRCLS system ands were MVRCLS designed proposed according in this to paper the topological were used forstructure current-limiting of the integrated starting 333 devicethrough (Figure a 10 kV 2)./19,000 AVRCLS kW motor. and MVRCLS Direct Starting proposed and Current-limitingin this paper were Soft used Starting for currentwere performed-limiting 334 Energiesstartingwith the2019 through developed, 07, x FOR a PEER 10 simulation kV/19 REVIEW,000 system, kW motor. as shown Direct in Starting Figure 16 and. Current-limiting Soft Starting15 ofw 22ere 335 performed with the developed simulation system, as shown in Figure 16.

336 Figure 16. Simulation system on current-limiting soft starting.

337 In Figure 16, theFigure AVRCLS 16. Simulation subsystem system was made on current up of-limiting CON1, SS,soft andstarting. ZS; the MVRCLS subsystem was made up of CON2, SS, and ZS; the RPC subsystem was made up of CON3 and C. CON1–CON4 338 In Figure 16, the AVRCLS subsystem was made up of CON1, SS, and ZS; the MVRCLS correspond to the high voltage contactor in Section 2.2 and the switching strategy in Section 2.3. The 339 subsystem was made up of CON2, SS, and ZS; the RPC subsystem was made up of CON3 and C. parameters of motor were saved and scoped by SASC module. 340 CON1–CON4 correspond to the high voltage contactor in Section 2.2 and the switching strategy in 341 Section 2.3. The parameters of motor were saved and scoped by SASC module 342 Three-Phase Source block with 10 kV/50 Hz in Simulink was selected as the power supply 343 voltage (U1). Asynchronous Machine block was selected as the motor whose parameters are listed in 344 Table 1.

Energies 2019, 12, 3068 15 of 21

Energies 2019, 07, x FOR PEER REVIEW 16 of 22 Three-Phase Source block with 10 kV/50 Hz in Simulink was selected as the power supply voltage 345 Table 1. Simulation Parameters of the Motor. (U1). Asynchronous Machine block was selected as the motor whose parameters are listed in Table1. Power (kVA) 19,000 Electrical Table 1. SimulationVoltage Parameters (kV of) the Motor. 10 Parameters Frequency (Hz) 50 Power (kVA) 19,000 Electrical Parameters Stator VoltageResistance (kV) (Ω) 0.124 10 Equivalent Stator LeakageFrequency inductance (Hz) (mH) 1.125 50 Circuit RotorStator Resistance Resistance (Ω (Ω) )0.123 0.124 Values RotorStator Leakage Leakage inductance inductance ( (mH)mH) 1.125 1.125 Equivalent Circuit Values MutualRotor inductance Resistance (mH (Ω)) 0.716 0.123 Rotor Leakage inductance (mH) 1.125 Inertia (kg·m2) 703.87 Mechanical Mutual inductance (mH) 0.716 Friction 0.00 Parameters Inertia (kg m2) 703.87 · Mechanical Parameters PoleFriction Pairs 2 0.00 Pole Pairs 2 346 The simulation, which mainly focuses on current limiting soft starting, is conducted to compare 347 current limiting soft starting and direct starting, and verify the effect of the soft starting method The simulation, which mainly focuses on current limiting soft starting, is conducted to compare 348 proposed in this paper. current limiting soft starting and direct starting, and verify the effect of the soft starting method proposed in this paper. 349 4.1. Simulation Results on Direct Starting 350 4.2. SimulationThe direct Results starting on simulation Direct Starting was finished with the developed simulation system when only 351 CON4The is directswitched starting on. simulation was finished with the developed simulation system when only 352 CON4According is switched to on.the simulation parameters of the motor in Table 1, speed reference was set to 1500 353 rpn, According load torque to thewas simulation set to 0 N parameters·M, and U1 of thewas motor applied in Table directly1, speed to the reference motor was for set direct to 1500 starting rpn, 354 loadsimulation. torque wasThe setsimulated to 0 N M, results and U1of the was current, applied current directly RMS to the, and motor speed for curve direct are starting shown simulation. in Figure · 355 The17. simulated results of the current, current RMS, and speed curve are shown in Figure 17. 356

Is (A)

t (sec)

357 358 (a)

IsRMS (A)

t (sec)

359 (b)

Figure 17. Cont.

Energies 2019, 12, 3068 16 of 21 Energies 2019, 07, x FOR PEER REVIEW 17 of 22

n (r/min)

t (sec)

360 (c)

361 Figure 17.17. (a) Current curve of motor direct starting; (b) CurrentCurrent rootroot meanmean squaresquare (RMS)(RMS) of motor 362 withwith directdirect starting;starting; ((cc)) SpeedSpeed curvecurve ofof motormotor withwith directdirect starting.starting.

363 ItIt could be known from F Figureigures 17 17a,ba,b thatthat thethe maximummaximum startingstarting currentcurrent reachedreached 56275627 A,A, whichwhich 364 waswas aboutabout 4.64.6 timestimes thatthat of thethe ratedrated currentcurrent (1253(1253 A).A). Therefore,Therefore, thethe powerpower gridgrid ofof thethe motormotor mightmight 365 be damaged.

366 4.3.4.2. Simulation Results on Current-limitingCurrent-limiting Soft Starting 367 AVRCLS andand MVRCLSMVRCLS proposedproposed in in this this paper paper were were used used for for current-limiting current-limiting starting starting through through a 368 10a 10 kV kV//19,00019,000 kW kW motor. motor. 369 Through assigningassigning didifferentfferent values values to toδ δ, ,t 1t1and andt t22,, thethe maximummaximum startingstarting current currentI smaxIsmax of the motor 370 waswas obtainedobtained byby simulation.simulation. From them, a set of parametersparameters suitable for practicalpractical projects couldcould bebe 371 obtained,obtained, and the motor'smotor's startingstarting current’scurrent’s relationshiprelationship withwith δδ,, tt11 andand tt22 are found. 372 According to functional functional logic logic switching switching strategy, strategy, δ δwaswas set set from from 1 to 1 to5 and 5 and increments increments of 0.1 of 0.1by 373 byCON1 CON1;; t1 wast1 was set set from from 0.1 0.1 to to 1 and1 and increments increments of of 0.1 0.1 by by CON2; CON2; and and t2t 2waswas set set from from 4 4 to 7 and and 374 incrementsincrements ofof 0.50.5 byby CON3.CON3. 375 The maximum startingstarting currentcurrent IIsmaxsmax ofof the the motor motor was was obtained obtained by by assigning assigning different different values of δ, 376 tt11 and t2.. Some Some of of the the simulation simulation results results are are li listedsted in in Table Table 22..

Table 2. Maximum starting current Isamax as t , t and δ varies. 377 Table 2. Maximum starting current Isamax as t1, t22 and δ varies. t (s) t (s) I (A) t1 (s) 1 t2 (s) δ δ samax Isamax (A) 0.1 0.1 4.5 0.70.7 4729 4729 0.1 0.1 4 0.70.7 4849 4849 0.2 4.5 0.4 5113 0.2 4.5 0.4 5113 0.2 4.5 0.5 4908 0.2 0.5 4.5 0.70.5 4731 4908 0.5 1 4.5 0.70.7 4733 4731 1 14.5 6 0.60.7 2987 4733 1 6 0.6 2987 The conclusions drawn from Table2 were as follows: 378 The conclusions drawn from Table 2 were as follows: (1) The starting current of the motor would be affected by δ, t1 and t2, showing a nonlinear 379 (1) The starting current of the motor would be affected by δ, t1 and t2, showing a nonlinear relationship. Among them, t2 made the greatest contribution to the starting current. 380 relationship.(2) Through Among simulation them, t2 analysismade the based greatest on AVRCLScontribution and to MVRCLS the starting with current. RPC, a set of parameters 381 (2) Through simulation analysis based on AVRCLS and MVRCLS with RPC, a set of parameters suitable for practical projects could be obtained. For instance, when t1 = 1, t2 = 6 and δ = 0.6, the 382 maximumsuitable for starting practical current projects of 10 could kV/ 119,000 be obtained kW motor. For instance, dropped when from 5627t1 = 1, A t to2 = 2987 6 and A, δ which = 0.6, was the 383 46.9%maximum lower starting than it current would of be 10 if thekV/119 motor,000 started kW motor directly dropped and wasfrom about 5627 2.4A to times 2987as A, large which as was the 384 rated46.9% current lower than (1253 it A). would The motor’sbe if the current motor curve, started current directly RMS and and was speed about curve 2.4 oftimes rotor as are large shown as the in 385 Figurerated current 18. (1253 A). The motor’s current curve, current RMS and speed curve of rotor are shown 386 in Figure 18.

Energies 2019, 12, 3068 17 of 21 Energies 2019, 07, x FOR PEER REVIEW 18 of 22

Is (A)

t (sec)

387 (a)

IsRMS (A)

t (sec)

388 (b)

n (r/min)

t (sec)

389 (c)

390 FigureFigure 18. 18. (a(a) )Current Current curve curve of of motor motor with with current current-limiting-limiting soft soft starting; starting; (b (b) )Current Current RMS RMS of of motor motor 391 withwith current current-limiting-limiting soft soft starting; starting; ( (cc)) Speed Speed curve curve of of motor motor with with current current-limiting-limiting soft soft starting. starting.

392 AVRCLSAVRCLS and and MVRCLS MVRCLS with with RPC RPC subsystem subsystem we werere used used for for current current-limiting-limiting starting starting through through a 10 a 393 kV/1910 kV,000/19,000 kW kWYKK1009 YKK1009-4-4 motor. motor. The The RPC RPC subsys subsystemtem wa wass turned turned on, on, provid providinging the the reactive reactive power power 394 requiredrequired in in the the starting starting process process of of the the motor. motor. The The simulation simulation results results show showeded that that the the maximum maximum starting starting 395 currentcurrent wa wass 2432 2432 A, A, 56.8% 56.8% lower lower than than that that of of direct direct starting starting and and 1.9 1.9 times times as as large large as as the the rated rated current current 396 (1253(1253 A) A).. Besides, Besides, the the power power factor factor increased increased to to 0.93. 0.93.

397 4.3.4.4. Experimental Experimental Evaluatiom Evaluatiom on on Soft Soft Starting Starting 398 WithWith a a1010 kV/19 kV/,19,000000 kW kWfan-loaded fan-loaded motor motor of a steel of aplant steel as plantthe subject as the of subjectthe study, of a the current study,- 399 limitinga current-limiting soft starter soft with starter RPC with wasRPC designed was designed based on based the theoretical on the theoretical findings findings.. The soft The starter soft starter was 400 manufacturedwas manufactured by Dayu by DayuElectric Electric Technology Technology Co., Ltd. Co., T Ltd.he fan The-loaded fan-loaded motor motor is shown is shown in Figure in Figure 19. 19.

Energies 2019, 12, 3068 18 of 21 Energies 2019, 07, x FOR PEER REVIEW 19 of 22

Soft starter Fan-loaded motor

401 Figure 19. Soft starter and load of 10 kV/19,000 kW motor (picture). 402 Figure 19. Soft starter and load of 10 kV/19,000 kW motor (picture). The fan-loaded starting experiment was performed with the soft starter. The starting current of 403 the motorThe fan was-loaded displayed starting by touchexperiment screen was in the performed control system. with the The soft results starter. of The motor starting soft starting current areof 404 thegiven motor in Table was 3displayed. by touch screen in the control system. The results of motor soft starting are 405 given in Table 3. Table 3. Results of soft starting of 19,000 kW/10 kV fan-loaded asynchronous motor. 406 Table 3. Results of soft starting of 19,000 kW/10 kV fan-loaded asynchronous motor. Time for Starting (s) Starting Motor Current (A) State Time for Starting Motor StateAutotransformer voltage 34Starting (t1) (s) Current (A) 2550 reduction-current limiting starting Autotransformer voltage(STAGE reduction I, 0~t-1) 34 (t1) 2550 Magnetron voltage regulation-current 92 (t2) 2400 current limiting starting (STAGE I, 0~t1) limiting (STAGE II: t1~t2) Magnetron voltage regulationEnd of starting-current process 14592 (t3 )(t2) 2400 2100 limiting (STAGE(STAGE II: t1~t2) III: t2~t3) 145 (t3) 2100 End of starting process (STAGE III: t2~t3) During the starting process, the maximum current of motor was 2550 A, about 2 times as large as 407 the ratedDuring current the starting of motor process, (1253 A), the and maximum the power current factor of increased motor was from 2550 0.32 A, to about 0.9. The2 times results as large agree 408 aswell the with rated the current simulation of motor results. (1253 A), and the power factor increased from 0.32 to 0.9. The results 409 agree well with the simulation results. 4.5. Experimental Evaluation of the Power Filtering Subsystem 410 4.4. Experimental Evaluation of the Power Filtering Subsystem The harmonic filter was developed based on the harmonic filtering subsystem. The harmonic 411 filterThe was harmonic applied tofilter eliminating was developed harmonics based of on the the low-voltage harmonic filtering distribution subsystem. system The in China harmonic resources filter 412 wascement applied (Chang to eliminat Jiang)ing Co., harmonic Ltd, withs of DC the transmission low-voltage distribution device. The system 5th harmonic in China current resources (I5) cement and its 413 (Changtotal harmonic Jiang) C distortiono., Ltd, with (THD) DC are transmission shown in Figure device. 20 . The 5th harmonic current (I5) and its total 414 harmonicThe filteringdistortion performances (THD) are shown of the in harmonic Figure 20.filter, as shown in Figure 16 (taking phase A as an example), were as follows: (1) Before filtering (Figure 20a), the 5th harmonic current and its THD of nonlinear load were 75.4 A and 30.0%, respectively. (2) With the harmonic filter developed in this paper (Figure 20b), the 5th harmonic current and its THD were reduced to 11.6 A and 5.7%, respectively. The harmonic filter absorbed 63.8 A of the 5th harmonic current, reaching an absorption rate of 84.6%. THD was decreased by 81%. It was found that the harmonic filter could effectively eliminate current harmonics, and reduce the THD of supply currents.

415 (a)

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Soft starter Fan-loaded motor

401

402 Figure 19. Soft starter and load of 10 kV/19,000 kW motor (picture).

403 The fan-loaded starting experiment was performed with the soft starter. The starting current of 404 the motor was displayed by touch screen in the control system. The results of motor soft starting are 405 given in Table 3.

406 Table 3. Results of soft starting of 19,000 kW/10 kV fan-loaded asynchronous motor.

Time for Starting Motor State Starting (s) Current (A) Autotransformer voltage reduction- 34 (t1) 2550 current limiting starting (STAGE I, 0~t1) Magnetron voltage regulation-current 92 (t2) 2400 limiting (STAGE II: t1~t2) 145 (t3) 2100 End of starting process (STAGE III: t2~t3)

407 During the starting process, the maximum current of motor was 2550 A, about 2 times as large 408 as the rated current of motor (1253 A), and the power factor increased from 0.32 to 0.9. The results 409 agree well with the simulation results.

410 4.4. Experimental Evaluation of the Power Filtering Subsystem 411 The harmonic filter was developed based on the harmonic filtering subsystem. The harmonic filter 412 was applied to eliminating harmonics of the low-voltage distribution system in China resources cement Energies 2019 12 413 (Chang Jiang), C, 3068o., Ltd, with DC transmission device. The 5th harmonic current (I5) and its total19 of 21 414 harmonic distortion (THD) are shown in Figure 20.

Energies 2019, 07, x FOR PEER REVIEW 20 of 22 415 (a)

416 (b)

417 FigureFigure 20. 20. (a()a I)5 Iand5 and total total harmonic harmonic distortion distortion (THD (THD)) before before filtering filtering;; (b ()b I)5 Iand5 and THD THD after after filtering filtering..

5. Conclusions 418 The filtering performances of the harmonic filter, as shown in Figure 16 (taking phase A as an 419 example)This, we paperre as has follows: proposed a current-limiting soft starting method which integrates autotransformer 420 voltage(1) Before reduction–current filtering (Figure limiting 20a), starting, the 5th magnetron harmonic voltagecurrent regulation–currentand its THD of nonlinear limiting load starting we andre 421 75.4reactive A and power 30.0%, compensation respectively. during starting for a high-voltage and high-power motor. Specifically 422 speaking,(2) With the the power harmonic filtering filter subsystem developed is in turned this paper on to (Figure filter out 20 harmonicsb), the 5th harmonic in power systemcurrent asand the 423 itsstarting THD we processre reduced terminates. to 11.6 A Based and 5.7%, on the respectively. current-limiting The harmonic starting characteristic filter absorbed curve, 63.8 thisA of paper the 424 5thhas harmonic established current, the topologicalreaching an structureabsorption of rate the integratedof 84.6%. THD device, was analyzed decreased the by switching81%. order of 425 high-voltageIt was found contactors, that the andharmonic studied filter the mechanismscould effectively of AVRCLS, eliminate MVRCLS, current RPC,harmonics, and power and reduce filtering. 426 theThe THD AVRCLS of supply subsystem currents. and MVRCLS subsystem are able to restrict the starting current of the motor and reduce the current absorbed form power grid. When the motor starts, the RPC subsystem is turned 427 5.on Conclusions to provide reactive power, and it increase power factor. As the power filtering subsystem is an LC 428 seriesThis filtering paper circuit, has a proposed variable equivalent a current inductance-limiting has soft been starting proposed method to realize which automatic integrates tuning 429 autotransformerand filtering of voltage harmonics. reduction–current limiting starting, magnetron voltage regulation–current 430 limitingThe starting simulation and reactive and experimental power compensation evaluation haveduring been starting completed. for a high In particular,-voltage and the directhigh-power starting 431 motor.and current-limiting Specifically speaking, starting the have power been filtering performed subsystem by developing is turned a simulationon to filter system. out harmonics In addition, in 432 powera 10 kVsystem/19,000 as the kW starting fan-loaded process motor terminates. of a steel Based plant on has the been current selected-limiting as thestarting object chara to verifycteristic the 433 curve,performance this paper of the has current-limiting established the soft topological starting method. structure As of indicated the integrated by the experimental device, analyzed results, the the 434 switchingmotor’s startingorder of currenthigh-voltage is about contactors, 2 times as and large studied as its ratedthe mechanisms current, the of power AVRCLS, factor MVRCLS, is raised toRPC, over 435 and0.9 power after RPC, filtering. and theThe harmonic AVRCLS filtersubsystem can eff ectivelyand MVRCLS eliminate subsystem current are harmonics able to restrict and reduce the starting the THD 436 currentof supply of the currents. motor and reduce the current absorbed form power grid. When the motor starts, the 437 RPC subsystemThe authors is ofturned the present on to provide paper believe reactive that power, the proposed and it increase method willpower be promisingfactor. As ifthe the power related 438 filteringtechnology subsyst is fullyem is developed. an LC series The details filtering for circuit, realizing a variable the power equivalent filtering subsystem inductance in has this been study 439 proposedcan be found to realize in the automatic research tuning results and of the filtering authors of [harmonics.27], and as a result are excluded from this paper. 440 Moreover,The simulation it is necessary and experimentalto add a power evaluation filtering subsystem have been and completed. to develop Inan particular,integrated device the direct in the 441 starting and current-limiting starting have been performed by developing a simulation system. In 442 addition, a 10 kV/19,000 kW fan-loaded motor of a steel plant has been selected as the object to verify 443 the performance of the current-limiting soft starting method. As indicated by the experimental 444 results, the motor’s starting current is about 2 times as large as its rated current, the power factor is 445 raised to over 0.9 after RPC, and the harmonic filter can effectively eliminate current harmonics and 446 reduce the THD of supply currents. 447 The authors of the present paper believe that the proposed method will be promising if the 448 related technology is fully developed. The details for realizing the power filtering subsystem in this 449 study can be found in the research results of the authors [27], and as a result are excluded from this 450 paper. Moreover, it is necessary to add a power filtering subsystem and to develop an integrated 451 device in the future research. Hopefully, the research findings in this paper are of great theoretical 452 and practical significance in developing soft starters with independent intellectual property rights 453 and in promoting the development of soft-start technology and industry.

Energies 2019, 12, 3068 20 of 21 future research. Hopefully, the research findings in this paper are of great theoretical and practical significance in developing soft starters with independent intellectual property rights and in promoting the development of soft-start technology and industry.

Author Contributions: Y.W. provided the conceptualization, methodology of this manuscript; K.Y. and J.C. provided the experimental evaluation and revised the manuscript in English; Y.Y. support the simulation evaluation and reviewed the manuscript. Funding: This work was supported by the National Support Program of China 358 (2015BAG20B05), the Zhejiang Province Natural Science Foundation of China (LY14E070003), and the Independent Innovation Fund Project of Wuhan University of Technology (2018-JL-004). Acknowledgments: The authors thank for the kind help of Dayu Electric Technology Co., Ltd. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Abbreviations

PLC Programmable Logic Controller AVRCLS Autotransformer voltage reduction–current limiting starting MVRCLS Magnetron voltage regulation–current limiting starting RPC Reactive power compensation TS Touch screen D/A Digital analog converter I/O Digital input and output RMS Root mean square

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