Comparative Study of Passive and Active Islanding Detection Methods for PV Grid-Connected Systems

sustainability

Article Comparative Study of Passive and Active Islanding Detection Methods for PV Grid-Connected Systems

Ahmed G. Abokhalil 1,2,*, Ahmed Bilal Awan 1 ID and Abdel-Rahman Al-Qawasmi 1 ID

1 Department of Electrical Engineering, College of Engineering, Majmaah University, Al Majmaah 11952, Saudi Arabia; [email protected] (A.B.A); [email protected] (A.-R.A.-Q.) 2 Electrical Engineering Department, Assiut University, Assiut 71515, Egypt * Correspondence: [email protected]; Tel.: +966-53-082-3972

Received: 17 April 2018; Accepted: 26 May 2018; Published: 30 May 2018

Abstract: Photovoltaic (PV) power grid-connected systems have the advantages of being prompt and reliable supplies of electrical power. Nevertheless, the installation and operation requirements from the grid side have to be fulfilled in order to guarantee the security of the PV system technicians and the efficiency of the power system. Particularly, the potential for “islanding” is one of the dreads that are brought about by PV grid-connected systems. To be able to tackle these concerns, this paper investigates recent islanding detection techniques and topologies for PV systems. Active islanding detection techniques apply regular disturbances to the inverter system and then analyze the output voltage or frequency to investigate the islanding and stability of the grid. If the injected disturbances influence the load voltage or frequency, the controller forces the intermediate inverter to stop sending power to the connected load. In addition, several islanding detection techniques that inject a periodical signal to the reference current that causes a change in the magnitude of inverter output voltage when islanding happens in a three-phase photovoltaic grid-connected system are discussed. The validity of the proposed technique is tested and verified through PSIM software.

Keywords: photovoltaic; islanding detection; active; passive

1. Introduction Recently, the distributed generation (DG) system has witnessed increasing contributions in the field of power generation all over the world. Nowadays, various DG systems have penetrated power generation systems for the smart grid, which introduces the bi-directional flow of electricity and information, protects the electric machines and equipment from a complete failure caused by natural disasters and wrong operations, and enhances the system reliability, security, and efficiency of generation, transmission, and distribution [1–3]. On the other hand, the increasing penetration of DGs impose challenges on the existing distribution infrastructure such as power coordination, anti-islanding, voltage harmonics, and voltage regulation problems. In a micro-grid with several DGs, the optimum operation of the power and voltage controllers can be obtained by controlling the islanding operations. According to IEEE standards, the grid-connected inverter should use an islanding detection technique. The grid parameters threshold values of the STD 929 and STD 1547 standards are given in References [4–6]. The voltage limits are divided into ranges and the trip time for each range is defined in each standard. The normal operating range for the frequency is the same in both standards as are the current harmonic distortion limits. Different algorithms have been used in the last decade or so. All the existing methods can be broadly divided into 3 categories as shown in Figure1 and summarized as follows:

Sustainability 2018, 10, 1798; doi:10.3390/su10061798 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 1798 2 of 15 Sustainability 2018, 10, x FOR PEER REVIEW 2 of 15

- - passivepassive methods; methods; - - activeactive methods; methods; - - communications-basedcommunications-based methods. methods. ThisThis paper paper is dividedis divided into twointo parts. two parts The ﬁrst. The part first surveys part surveysthe existed the islanding existed detection islanding methods detection andmethods presents and the presents theory of the operation theory of of operation several passive of several and active passive islanding and active detection islanding methods detection in grid-connectedmethods in grid PV-connected systems. The PV secondsystems. part The introduces second part an inverter introduces current an inverter reference current perturbation reference algorithmperturbation for a algorithm three-phase for grid-connected a three-phase PV grid system-connected for islanding PV system detection. for islanding The injected detection. signal The periodicallyinjected signal perturbs periodically the reference perturbs current the with reference the same current amount with ofpositive the same and amount negative of components. positive and Thisnegative keeps components. the mean PV This output keeps power the mean unchanged PV output during power the perturbation unchanged during time. When the perturbation the grid connectiontime. When is lost, the the grid magnitude connection of is the lost, Point the of magnitude Common Coupling of the Point (PCC) of Commonvoltage deviates Coupling because (PCC) ofvoltage the current deviates magnitude because perturbation.of the current magnitude To implement perturbation. the proposed To implement method, a the grid-connected proposed method, PV systema grid is-connected designed andPV system used. The is designed validation and of used. the proposed The validation control algorithmsof the proposed is applied control through algorithms the PSIMis applied software. through the PSIM software.

Anti-islanding Method

Remote Local

- Power Line Carrier Communication - Supervisory Control and Data Passive Active Acquisition (SCADA) system - Under and Over Voltage - Under and Over Frequency - Frequency Shift - Phase Change - Phase Shift -Harmonic Detection - Current Magnitude Variation

Figure 1. The anti-islanding methods classiﬁcation. Figure 1. The anti-islanding methods classification.

2.2. Islanding Islanding Detection Detection Methods Methods 2.1. Passive Islanding Methods 2.1. Passive Islanding Methods The passive techniques look for some parameter deviations like voltage magnitude [7], rate of The passive techniques look for some parameter deviations like voltage magnitude [7], rate of change of frequency [8], harmonics [9], or Phase angle displacement [10]. In the islanding mode, change of frequency [8], harmonics [9], or Phase angle displacement [10]. In the islanding mode, these these parameters vary largely at the PCC. The difference between the grid-connected and islanding parameters vary largely at the PCC. The difference between the grid-connected and islanding mode mode depends on the setting of the threshold values. A lower setting for the threshold for the depends on the setting of the threshold values. A lower setting for the threshold for the permissible permissible disturbances in these quantities may cause nuisance tripping. On the other hand, if the disturbances in these quantities may cause nuisance tripping. On the other hand, if the setting is too setting is too high, then the protective devices will not respond to the islanding condition. The merits of high, then the protective devices will not respond to the islanding condition. The merits of these these methods are the fast response and the absence of system disturbances. However, these methods methods are the fast response and the absence of system disturbances. However, these methods have have the disadvantage of having a large non-detectable zone (NDZ) and these protective methodologies the disadvantage of having a large non-detectable zone (NDZ) and these protective methodologies might have variable or unpredictable reaction times. More speciﬁc passive anti-islanding methods can might have variable or unpredictable reaction times. More specific passive anti-islanding methods be discussed as follows: can be discussed as follows: 2.1.1. Under/over Voltage and under/over Frequency 2.1.1. Under/over Voltage and under/over Frequency Under/over frequency protection techniques (UOF) and under/over voltage protection Under/over frequency protection techniques (UOF) and under/over voltage protection techniques (UOV) are required for all grid-connected PV inverters. These UOF/UOV protective techniques (UOV) are required for all grid-connected PV inverters. These UOF/UOV protective devices are used as anti-islanding detection techniques and also used to protect the equipment of devices are used as anti-islanding detection techniques and also used to protect the equipment of customers. In Figure2, the system conﬁguration of a PV inverter that is connected to a variable load is customers. In Figure 2, the system configuration of a PV inverter that is connected to a variable load shown. When the PV-inverter is connected to the grid, the active and reactive powers (Pt + jQt) are is shown. When the PV-inverter is connected to the grid, the active and reactive powers (Pt + jQt) are delivered by the PV. If the PV power rating is lower than the load power, then the power differences delivered by the PV. If the PV power rating is lower than the load power, then the power differences (ΔP and ΔQ) will be supplied from the utility grid. In the case of islanding, a mismatch of some parameters as voltage and frequency will occur. This behavior of this method may vary depending Sustainability 2018, 10, 1798 3 of 15 Sustainability 2018, 10, x FOR PEER REVIEW 3 of 15

Sustainability 2018, 10, x FOR PEER REVIEW 3 of 15 on(∆P ΔandP and∆ QΔ)Q will. For be more supplied details, from if Δ theP is utility not equal grid. to Inzero the then case the of islanding,voltage amplitude a mismatch at PCC of some will deviate and the islanding will be detected by the UOV protective relay. Similarly, the phase voltage parameterson ΔP and Δ asQ voltage. For more and details, frequency if Δ willP is occur. not equal This behaviorto zero then of this the methodvoltage mayamplitude vary depending at PCC will on at PCC changes when ΔQ is not equal to zero, causing a frequency deviation. Additionally, the UOF deviate∆P and ∆andQ. Forthe moreislanding details, will if be∆P detectedis not equal by the to zeroUOV then protective the voltage relay. amplitude Similarly, at the PCC phase will voltage deviate protection relays will then detect these variations and, consequently, islanding will commence. If the atand PCC the changes islanding when will Δ beQ detectedis not equal by theto zero UOV, causing protective a frequency relay. Similarly, deviation the. Additionally, phase voltage the at UOF PCC power differences are too large, both voltage and frequency exceed the adjusted limits of the protectionchanges when relays∆Q willis not then equal detect to zero, these causing variations a frequency and, consequently, deviation. Additionally, islanding will the commence UOF protection. If the UOF/UOV protective devices, which ultimately trips the circuit breaker. In cases of small differences powerrelays will differences then detect are these too large, variations both and, voltage consequently, and frequency islanding exceed will the commence. adjusted limits If the power of the between the load power and the PV output power, this method may not be able to detect islanding UOF/UOVdifferences protective are too large, devices both, which voltage ultimately and frequency trips the exceed circuit the breaker. adjusted In case limitss of of small the UOF/UOVdifferences due to the large NDZ. An example of NDZ is given in Figure 3; the mismatch regions of the power protectivebetween the devices, load power which and ultimately the PV tripsoutput the power, circuit breaker.this method In cases may of not small be able differences to detect between islanding the components are represented by the shaded area where this method cannot detect the islanding events dueload to power the large and theNDZ. PV An output example power, of thisNDZ method is given may in notFigure be able 3; the to detectmismatch islanding regions due of tothe the power large [9,11–14]. componentsNDZ. An example are represented of NDZ is by given the shaded in Figure area3; thewhere mismatch this method regions cannot of the detect power the componentsislanding events are represented by the shaded area where this method cannot detect the islanding events [9,11–14]. [9,11–14]. Power Meter #1 Power Meter #2

Power Meter #1 Power Meter #2 P + jQ ΔP + jΔQ PV I I PV array ● ● ● Utility InverterV P + jQ ΔP + jΔQ PV I I P + jQ PV array ● ● L ●L Utility InverterV

PL + jQL R L C Local PR QL QC R L C loads Local PR QL QC loads Figure 2. The test circuit for the islanding detection function in a photovoltaic (PV) power inverter. Figure 2. The test circuit for the islanding detection function in a photovoltaic (PV) power inverter. Figure 2. The test circuit for the islanding detection function in a photovoltaic (PV) power inverter.

FigureFigure 3. AnAn example example of a a non non-detectable-detectable zone ( (NDZ).NDZ).

2.1.2. Harmonics Harmonics Detection Figure Method Method 3. An example of a non-detectable zone (NDZ).

2.1.2.The Harmonics Harmonics Detection Detection Method method method concept is based based on on monitoring monitoring the the Total Total Harmonic Harmonic Distortion (THD) of of the the PCC PCC voltage voltage to determine to determin whethere wh islandingether islanding occurs. Inoccurs. normal In operation, normal operation, the impedance the The Harmonics Detection method concept is based on monitoring the Total Harmonic Distortion impedanceof the power of gridthe power is small; grid the is small; inverter the current inverter harmonic current harmonic component component mainly ﬂows mainly into flows the grid.into (THD) of the PCC voltage to determine whether islanding occurs. In normal operation, the theThe grid. PCC The voltage PCC is voltage decided is bydecided the utility-grid by the utility voltage-grid and, voltage usually, and the, usually voltage, the harmonic voltage harmonic content is impedance of the power grid is small; the inverter current harmonic component mainly flows into contentsmall. When is small. the powerWhen supplythe power is disconnected supply is disconnected due to the non-linear due to the characteristics, non-linear characteristics the non-linear, load the the grid. The PCC voltage is decided by the utility-grid voltage and, usually, the voltage harmonic nonharmonic-linear currentload harmonic ﬂowing current into the flowing inverter into will the also inverter have large will harmonics also have becauselarge harmonic the non-linearitys because content is small. When the power supply is disconnected due to the non-linear characteristics, the theof thenon transformer-linearity of the hysteresis transformer output hysteresis current output produces current a distortion produces voltage a distortion in the voltage transformer. in the non-linear load harmonic current flowing into the inverter will also have large harmonics because transformer.The islanding The conditions islanding can conditions be decided can by be detecting decided the by harmonic detecting distortion the harmonic of the distortion output voltage. of the the non-linearity of the transformer hysteresis output current produces a distortion voltage in the output voltage. transformer. The islanding conditions can be decided by detecting the harmonic distortion of the output voltage. Sustainability 2018,, 1010,, 1798x FOR PEER REVIEW 4 of 15

This method has several advantages: it is simple, viable, and has a high detection accuracy. It does Thisnot methodinfluence has the several power advantages: quality and it isthe simple, detection viable, range and is has wide; a high the detection change accuracy.of the detection It does noteffect influence is not yet the investigated power quality under and the detectionparallel operation range is wide;condition the change of the ofinverters the detection; the islanding effect is notdetection yet investigated during the underpower thematch parallel with operationa load is still condition very high. of the inverters; the islanding detection duringThe the di powersadvantages match of with this a method load isstill are veryas follows high.: a large amount of network voltage harmonics due toThe the disadvantages non-linear load of thiscauses method significant are as limitations follows: a largeand difficulties amount of in network practice. voltage This method harmonics has duean NDZ to the [15]. non-linear load causes significant limitations and difficulties in practice. This method has an NDZ [15]. 2.1.3. Phase Monitoring Method 2.1.3. Phase Monitoring Method The phase monitoring method is based on detecting any abrupt “jumps” in the phase shift betweenThe phasethe terminal monitoring voltage method of the is inverter based on and detecting its output any abruptcurrent “jumps”, as shown in the in phaseFigure shift 4. However, between thewhen terminal a fast Phase voltage Locked of the inverterLoop (PLL) and is its implemented, output current, this as phase shown jump in Figure would4. However, not be detected when a since fast Phasethe inverter Locked terminal Loop (PLL) voltage is implemented,and current are this always phase in jump phase. would Therefore, not be this detected method since is presented the inverter to terminalenhance the voltage phase and jump current technique. are always Under in phase.the normal Therefore, condition, this methodthe inverter is presented produces to zero enhance reactive the phasepowerjump and the technique. phase angle Under between the normal the condition, terminal vo theltage inverter and theproduces output zero current reactive at PV power inverter and theterminals phase angleis zero. between During thethe terminalislanding voltage condition, and a the displacement output current in the at PVphase inverter takes terminalsplace because is zero. of Duringthe shift the of the islanding voltage condition, vector. After a displacement a few cycles, inthe the measured phase takes angle place is compared because ofwith the the shift detected of the voltageangle, h vector.ence, After a few cycles, the measured angle is compared with the detected angle, hence,

∆θ=θt−t θtt−1

Figure 4. The d diagramiagram showing the phase jump due to islanding.

The calculated load phase angle is given by the following equation: The calculated load phase angle is given by the following equation:

1   1    tan RC  1  (1) θ = tan−1 R ωC − L  (1)   ωL  When the grid power is disconnected and the load frequency is as the same as the frequency of When the grid power is disconnected and the load frequency is as the same as the frequency of the  grid,the grid,θ remains remains at zero at without zero without any change any change in the in phase the phase angle angle of the of output the output current. current. In contrast, In contrast, if the loadif the and load grid and frequencies grid frequencies are not are same, not thesame, phase the differencesphase differences and θ differs and  from differs zero. from The zero. resulting The phaseresulting change phase is thenchange used is tothen detect used the to islanding detect the condition. islanding When condition. that phase When change that phase exceeds change the threshold,exceeds the the threshold, controller the will controller turn off will the inverter.turn off the inverter. The accuracy of this methodmethod is highly affected by the reactive elements of the power circuit circuit.. However, the output output power power quality quality of of the the inverter inverter and and the thesystem system dynamics dynamics are not are affected. not affected. The effectiveness of phase monitoring is not affected when the parallel inverters are connected to the islanded system [9]. Sustainability 2018, 10, 1798 5 of 15

The effectiveness of phase monitoring is not affected when the parallel inverters are connected to the islanded system [9].

2.1.4. Change Detection of Key Power When islanding occurs, due to the instability of the system, frequency, power oscillations, and their rate of change, this method continuously monitors the power, frequency rate, harmonic distortion, and the partial derivative of the frequency with respect to power. The second step is to compare these variables with the limit values and then detect the islanding condition [16–18].

2.1.5. Change of Frequency with Respect to Power df /dp In this method, the islanding detection implemented by monitoring the rate of frequency change over power df /dp because the small power generation capacity system df /dp is greater than that in the larger power generation capacity. In the condition in which the local load and the capacity of DG are equal, df /dp will be more accurate than df /dt to detect islanding [19,20]. All the above-stated methods belong to the passive detection methods and their characteristics are almost the same. These methods commonly monitor other electrical quantities to enhance the detection performance. However, the need to measure these quantities leads to an increase in the cost of measurement [21,22]. Practically, to achieve the purposes of anti-islanding protection, one or several islanding detection methods can be used [23]. Islanding detection methods are classiﬁed together, even though they have different principles.

2.2. Active Islanding Detection Methods Active islanding methods (AIMs) send and receive a periodical perturbation signal between the PV inverter and grid to make sure that the inverter is still connected to the power grid. That is to say, with this method, even a small disturbance signal will become clear and noticeable when inferring the islanding mode of operation so that the inverter will deal with the power change [24]. On the other hand, these methods, which depend on using disturbance signals, degrade the power quality because they can cause a variation in the magnitude of the inverter output current or output frequency. The PV inverter output current is given by the following equation [25]:

Iinv = Im sin(2π f t + θ) (2) where f is the frequency, Im is the inverter current amplitude, and θ is the initial phase angle. All the perturbation signals can be set or modiﬁed based on these three parameters. More speciﬁc AIMs can be discussed as follow:

2.2.1. Active Frequency Drift (AFD) In this method, a small disturbance current signal is added to the inverter reference output current to disturb its output voltage. In the grid-connected mode, this distortion does not affect any of the inverter output voltages and frequencies, but in the grid-disconnected mode, this perturbation affects the inverter output frequency and, hence, this change in frequency may force the UOF to disconnect the DG. Therefore, the AFD method adds a zero-conduction time into each half cycle of the current waveform, as shown in Figure5. If T is the utility voltage period and tz is the zero time, the ratio of tz to T/2 is deﬁned as the “chopping fraction” Cf, which is used to characterize the intensity of the disturbance: 2t C = z (3) f T In the grid-disconnected mode, the inverter output power factor is normally unity and will have the same grid frequency. During the islanding condition, the introduced distortion to the current Sustainability 2018, 10, 1798 6 of 15 referenceSustainability will 2018 introduce, 10, x FOR PEER a constant REVIEW drift in the inverter frequency, which will ﬁnally reach the frequency6 of 15 boundary limits set for the islanding detection. This method can easily be implemented with the help of a microprocessormicroprocessor-based-based controller. controller. However, thisthis methodmethod degradesdegrades thethepower power quality quality because because of of the the disturbance disturbance signals, signals, which which causes cause as variationa variation in in the the frequency frequency or or magnitude magnitude of of the the output output current current [26 [26–39–3].9].

Figure 5. The operational waveform of active frequency drift (AFD)(AFD) method.

2.2.2. Slip-ModeSlip-Mode Frequency Shift (SMS) SMS introduces a positive feedback to destabilize the inverter output for islanding detection. The phase of the inverterinverter output current changes relative to the grid voltage that makes the voltage frequency at thethe PCCPCC deviate from thethe nominalnominal valuevalue once thethe gridgrid hashas disconnected.disconnected. ComparedCompared to other active techniques, SMS has a small NDZ and is very effective in the prevention of islanding. However, this method has some issues such as a transienttransient response and power quality at very high penetration levels and feedbackfeedback loop gains. This This is is a similar issue in all the methods that utilize positive feedback. SMS also introduces introduces a aphase phase shift shift perturbation perturbation which which can can lead lead to noise, to noise, measurement measurement error, error, and andquantization quantization error. error. This This limitation limitation can can be beanswered answered by by introducing introducing an an extra extra phase phase shift shift called called the improved-SMSimproved-SMS (IM-SMS).(IM-SMS). The IM-SMSIM-SMS was verified veriﬁed through digital simulation and experimentation, experimentation, which resultedresulted inin aa highhigh reliability,reliability, easyeasy implementation,implementation, andand simplicitysimplicity [9[9].].

2.2.3.2.2.3. Impedance Measurement The impedance impedance measurement measurement method method is somehow is somehow similar similar to a passive to a technique,passive technique, which measures which themeasures system the impedance system impedance variations caused variations by islanding.caused by In islanding. AIMs, however, In AIMs, a however, parallel inductor a parallel is temporarilyinductor is temporarily connected acrossconnected the utilityacross grid.the utility Theinductor grid. The current inductor and current supply and voltage supply reduction voltage areredu usedction to are calculate used to the calculate power source the power impedance source [40 impedance]. The impedance [40]. The measurement impedance methodsmeasurement have recentlymethods been have used recently because been ofused the because belief that of the they belief have that no NDZ,they have especially no NDZ in, theespecially single-inverter in the single case.- inverterTherefore, case. the experimental studies verify the robustness of the impedance measurement test basedTherefore, on the islanding the experimental detection studies in a single-inverter verify the robustness case [41 of]. the The impedance experiment measurement shows that test the impedancebased on the measurement islanding method detection based in a on single islanding-inverter detection case does [41]. have The an experiment NDZ in the shows single-inverter that the systems.impedance However, measurement this method method has based the main on disadvantage;islanding detection in the does case of have parallel an NDZ multiple in the inverters, single- eachinverter one systems. would be However, forcing a this slightly method different has the signal main into disadvantage the line. ; in the case of parallel multiple inverters,Ensuring each thatone thewould inverter be forcing maximum a slightly power different point MPPTsignal into is enabled the line. with impedance detection is importantEnsuring because that the the inverter impedance maximum measurement power point islanding MPPT detection is enabled method with impedance is reliable detection for large oris smallimportant duty because ratios. The the accuracyimpedance of thismeasurement method in islanding the single-inverter detection casemethod is highly is reliable improved for large by the or additionsmall duty of aratio variables. The length accuracy phase of shift.this method This enhancement in the single produces-inverter a case small is number highly ofimproved sub-harmonics by the addition of a variable length phase shift. This enhancement produces a small number of sub- harmonics in the PV inverter output. Therefore, other active techniques can be used to overcome the limitation of impedance measurement methods. Sustainability 2018, 10, 1798 7 of 15 in the PV inverter output. Therefore, other active techniques can be used to overcome the limitation of impedance measurement methods.

2.2.4. Sandia Frequency Shift (SFS) SFS is basically a modiﬁed form of the AFD method. SFS uses positive feedback by introducing a little phase shift at the inverter output current through adding dead-conduction times to the inverter reference current. Therefore, the frequency of the inverter current will deviate from the power system frequency. The chopping frequency shown in Equation (4) is intended to be proportional to the difference between the grid and the measured frequency.

Cf = Cf 0 + K( f0 − f ) (4) where K is an accelerating gain, Cf0 is the chopping frequency, f is the grid frequency, and f 0 is the measured frequency of the inverter output voltage. The chopping frequency becomes low when the frequency error is zero because the utility grid stabilizes the PCC voltage by giving a concrete reference for frequency and phase. The phase error between the inverter output voltage and current waveforms arise when the utility grid is lost. This causes an increase in the inverter output current in order to overcome this phase shift. Due to the continuously increasing phase error, the chopping frequency also increases at the same time and the inverter still detects a phase error and keeps on increasing it far from the frequency. As a result, the value of the chopping frequency increases, the frequency drifts so far from f 0 that the under and over frequency protection systems detect this and stop the inverter operation [42–48].

2.2.5. Sandia Voltage Shift (SVS) Islanding is prevented by the Sandia Voltage Shift (SVS) with a positive feedback method, which is based on the voltage amplitude at the PCC. There will be little or no effect on the system power when the utility grid link is maintained. However, once the link of the utility grid is broken, there is a reduction in the PCC voltage. As per the load impedance’s relationship, this reduction continues, which results in the reduction of the current and output power. Therefore, the under voltage protection system can detect this reduction in the amplitude of the PCC voltage. It is possible to either decrease or increase the inverter output power, which leads to a tripping of the corresponding under and over voltage protection systems and the cease of the operation of the inverter [49–52].

2.2.6. Detection of Impedance at the Specific Frequency In this technique, a low frequency current harmonic is intentionally injected into the PCC. During the connection of the grid, if the impedance at the harmonic frequency is much lower than the load impedance, then the harmonic current flows into the grid and the voltage abnormality is observed. When the utility is disconnected, then the harmonic current has only one way to flow, which is the terminal load. This method’s name is derived from the relationship between the amplitude of the produced harmonic voltage and load impedance at the harmonic current frequency.

3. Proposed Islanding Method The system considered in the proposed islanding detection periodically perturbs the inverter reference q-axis current to be 10% more than the current reference value at one line cycle and, in the next line cycle, the current reference value is decreased by the same value, as shown in Figure6. This method maintains the inverter average output power to be constant so as to not deteriorate the MPPT process. For several cycles, the reference current command is set to be nominal without perturbation. This process is implemented regularly with the same intervals as shown in Figure6. In a string of consecutive line cycles, if the measured RMS output voltage is different from the nominal Sustainability 2018, 10, 1798 8 of 15 value, the islanding factor is incremented steadily to the maximum value in which the inverter is tripped. The islanding factor is set to zero during the grid connection time when the rigid grid voltage forces the inverter terminal voltage to be equal to the grid voltage. On the other hand, withoutSustainability perturbing 2018, 10, x FOR the PEER current, REVIEW the inverter voltage remains unchanged during the islanding mode.8 of 15 Therefore, the increases and decreases in the reference current make the RMS output voltage deviate from the the nominal nominal value value when when the the grid grid is lost. is lost. As a As result, a result, the islanding the islanding factor increases factor increases and the andinverter the isinverter deactivated is deactivated when the when factor the exceeds factor theexceeds threshold the threshold value. value.

dV(RMS) V(RMS)

Islanding Factor I f

Islanding confirmation Time

Figure 6.6. The operational waveforms of reference q-axisq-axis current, voltage value,value, islanding factor, and islanding conﬁrmation.confirmation.

The overall system configuration of the proposed islanding detection method comprises of the The overall system configuration of the proposed islanding detection method comprises of the PV array, voltage source inverter (VSI), a low-pass filter, and a step-up  –Y transformer, as shown PV array, voltage source inverter (VSI), a low-pass filter, and a step-up ∆–Y transformer, as shown in Figure 7. The inverter circuit synchronizes the grid voltage with the inverter output current and in Figure7. The inverter circuit synchronizes the grid voltage with the inverter output current and its control unit is developed to implement the MPPT algorithm for extraction of the PV maximum its control unit is developed to implement the MPPT algorithm for extraction of the PV maximum power [53]. To reduce the high-frequency switching harmonics of the perturbation signals on the grid power [53]. To reduce the high-frequency switching harmonics of the perturbation signals on the grid voltage, line filters are included. voltage, line filters are included. To achieve the MPPT and a unity output power factor, the abc voltages and currents are To achieve the MPPT and a unity output power factor, the abc voltages and currents are transformed into a rotating dq reference frame synchronized with the grid voltages. This reference transformed into a rotating dq reference frame synchronized with the grid voltages. This reference frame is then used to represent the grid voltage and currents. frame is then used to represent the grid voltage and currents. The main point is adjusting the q-axis current to trace the MPP of the PV system, while the other The main point is adjusting the q-axis current to trace the MPP of the PV system, while the current component is set to be zero to maintain the unity output power factor of the inverter output other current component is set to be zero to maintain the unity output power factor of the inverter [53]. Figure 8 shows the general control structure in the synchronous reference frame. output [53]. Figure8 shows the general control structure in the synchronous reference frame.

Sustainability 2018, 10, x FOR PEER REVIEW 8 of 15 from the nominal value when the grid is lost. As a result, the islanding factor increases and the inverter is deactivated when the factor exceeds the threshold value.

dV(RMS) V(RMS)

Islanding Factor I f

Islanding confirmation Time Figure 6. The operational waveforms of reference q-axis current, voltage value, islanding factor, and islanding confirmation.

The overall system configuration of the proposed islanding detection method comprises of the PV array, voltage source inverter (VSI), a low-pass filter, and a step-up  –Y transformer, as shown in Figure 7. The inverter circuit synchronizes the grid voltage with the inverter output current and its control unit is developed to implement the MPPT algorithm for extraction of the PV maximum power [53]. To reduce the high-frequency switching harmonics of the perturbation signals on the grid voltage, line filters are included. To achieve the MPPT and a unity output power factor, the abc voltages and currents are transformed into a rotating dq reference frame synchronized with the grid voltages. This reference frame is then used to represent the grid voltage and currents. The main point is adjusting the q-axis current to trace the MPP of the PV system, while the other currentSustainability component2018, 10, 1798is set to be zero to maintain the unity output power factor of the inverter output9 of 15 [53]. Figure 8 shows the general control structure in the synchronous reference frame.

Sustainability 2018, 10, x FOR PEER REVIEW 9 of 15

FigureFigure 7 7.. TheThe power power circuit circuit for for a a PV PV system. system.

abc/dq e

e eab bc

+ C V dc - dc

ia ib

abc/dq * * *  va vb vc e

SVPWM iqe ide * * v qe v de

q-axis d-axis current L L current * controller controller i qe * ide MPPT

Pout 1.5(Vqe  I qe Vde  Ide ) i iqe de

FigureFigure 8. TheThe control control circuit circuit for the PV system.

4. Results 4. Results To implement and verify the proposed islanding detection algorithm, a simulation of a three- To implement and verify the proposed islanding detection algorithm, a simulation of a three-phase phase PV system is simulated in the PSIM software. The electrical characteristics of a 50 Wp PV array PV system is simulated in the PSIM software. The electrical characteristics of a 50 Wp PV array to be to be used as the reference module for simulation are as follows: NS = 50, NP = 200, 17.4 (V) and 2.87 used as the reference module for simulation are as follows: NS = 50, NP = 200, 17.4 (V) and 2.87 (A) are (A) are the voltage and current at maximum power. At the irradiation level of 1000 W/m2, the 60 the voltage and current at maximum power. At the irradiation level of 1000 W/m2, the 60 strings of 50 strings of 50 modules PV generates 500 kWp. modules PV generates 500 kWp. To continuously investigate the islanding condition, the inverter reference q-axis current is To continuously investigate the islanding condition, the inverter reference q-axis current is periodically disturbed. Figure 9 shows the perturbed q-axis current and its effect in varying the periodically disturbed. Figure9 shows the perturbed q-axis current and its effect in varying the inverter phase current and output phase voltage. The value of the injected signal is ±10% to the rated q-axis current. The figure shows the disturbed inverter output current when the reference current is perturbed when the reference current is disturbed. The inverter current follows the reference current when its value is controlled by the nominal value. During the grid connection mode, Figure 10 shows the grid voltage, which remains constant, and the islanding factor that also remains zero. Figure 10 shows that the PV average output power remains constant even though it oscillates due to the variations in the q-axis current. Sustainability 2018, 10, 1798 10 of 15 inverter phase current and output phase voltage. The value of the injected signal is ±10% to the rated q-axis current. The ﬁgure shows the disturbed inverter output current when the reference current is perturbed when the reference current is disturbed. The inverter current follows the reference current when its value is controlled by the nominal value. During the grid connection mode, Figure 10 shows the grid voltage, which remains constant, and the islanding factor that also remains zero. Figure 10 shows that the PV average output power remains constant even though it oscillates due to Sustainability 2018, 10, x FOR PEER REVIEW 10 of 15 theSustainability variations 2018 in, 10 the, x FOR q-axis PEER current. REVIEW 10 of 15

Figure 9. The q-axis current, phase current, and phase voltage without islanding. FigureFigure 9. 9.The The q-axisq-axis current, current, phase phase current, current, and and phase phase voltage voltage without without islanding. islanding.

Figure 10. The fault timer and PV power without islanding. Figure 10. The fault timer and PV power without islanding. Figure 10. The fault timer and PV power without islanding. During the islanding mode, the perturbation of the q-axis current in Figure 11 deviates the During the islanding mode, the perturbation of the q-axis current in Figure 11 deviates the outputDuring current the and islanding terminal mode, voltage the permanently perturbation from of the the q nominal-axis current values, in as Figure shown 11 in deviates Figure 12 the. output current and terminal voltage permanently from the nominal values, as shown in Figure 12. Theoutput islanding current detection and terminal algorithm voltage then permanently starts to observe from the the nominal voltage values deviation, as shown incrementing in Figure the 12. The islanding detection algorithm then starts to observe the voltage deviation incrementing the islandingThe islanding factor detection to the limit algorithm value as shown then starts in Figure to observe 13. When the the voltag islandinge deviation factor reaches incrementing the limit, the islanding factor to the limit value as shown in Figure 13. When the islanding factor reaches the limit, theislanding inverter factor controller to the shuts limit itvalue down as and shown the in PV Figure output 13. power When falls the toislanding zero as shownfactor reaches in Figure the 14 lim. it, the inverter controller shuts it down and the PV output power falls to zero as shown in Figure 14. the inverter controller shuts it down and the PV output power falls to zero as shown in Figure 14.

Sustainability 2018, 10, x FOR PEER REVIEW 10 of 15

Figure 9. The q-axis current, phase current, and phase voltage without islanding.

Figure 10. The fault timer and PV power without islanding.

During the islanding mode, the perturbation of the q-axis current in Figure 11 deviates the output current and terminal voltage permanently from the nominal values, as shown in Figure 12. The islanding detection algorithm then starts to observe the voltage deviation incrementing the islanding factor to the limit value as shown in Figure 13. When the islanding factor reaches the limit, Sustainabilitythe inverter2018 controller, 10, 1798 shuts it down and the PV output power falls to zero as shown in Figure11 14. of 15

Sustainability 2018, 10, x FOR PEER REVIEW 11 of 15 Sustainability 2018, 10, x FOR PEER REVIEW 11 of 15 Figure 11. The perturbed q-axis reference and actual current. Figure 11. The perturbed q-axis reference and actual current. Figure 11. The perturbed q-axis reference and actual current.

Figure 12. The instantaneous waveforms of grid voltages and currents. Figure 12. The instantaneous waveforms of grid voltages and currents. Figure 12. The instantaneous waveforms of grid voltages and currents.

Figure 13. The islanding factor and islanding conﬁrmation. Figure 13. The islanding factor and islanding confirmation. Figure 13. The islanding factor and islanding confirmation.

Figure 14. The PV power and current before and after islanding. Figure 14. The PV power and current before and after islanding. Sustainability 2018, 10, x FOR PEER REVIEW 11 of 15

Figure 11. The perturbed q-axis reference and actual current.

Figure 12. The instantaneous waveforms of grid voltages and currents.

Sustainability 2018, 10, 1798 Figure 13. The islanding factor and islanding confirmation. 12 of 15

Figure 14. The PV power and current before and after islanding. Figure 14. The PV power and current before and after islanding. To compare the performance proposed method with the other methods, it is found that the proposed method is faster than most of the active islanding detection methods such as the active impedance method, SFS, SMS, and AFD. However, this method has a larger NDZ than the other active methods. Due to the positive and negative perturbation sequence, this method has a better power quality. In addition, this method is more difﬁcult to implement due to its complexity.

5. Conclusions This paper introduced and analyzed several recent islanding detection methods for grid-connected PV systems. According to the discussion, the research trend of anti-islanding is mainly divided into passive methods, which are based on the measurement of system parameters such as voltage and frequency. Additionally, the active methods which are based on making disturbances on the inverter output voltage or current. As a conclusion of the comparison, the active methods provide faster response, high reliability, and a smaller power degradation. However, the active methods have a large NDZ and are difﬁcult to implement as compared with the passive methods. The latter category is simple to implement and has no effect on the output power quality. However, the passive methods are not trusted for all load conditions as well as it is difﬁcult to set the NDZ thresholds due to its large size. In addition, this paper proposes a new perturbation current signal based on the islanding detection of a three-phase PV. The injected signal results in a terminal voltage deviation when islanding occurs. However, the rigid grid voltage prevents the changes of the inverter output voltage magnitude during the connection mode. The proposed islanding detection algorithm was simulated under various load conditions and the simulation results show that the islanding detection time was about 0.1 s, which is very much faster than the required detection time of 2 s.

Author Contributions: A.G.A. and A.-R.A.-Q. suggested the research idea, collected the relevant papers, analyzed the data, performed simulations, and contributed in writing the manuscript. A.B.A. collected and analyzed the data and contributed in writing the manuscript. Acknowledgments: The authors would like to thank the Deanship of Scientific Research, Majmaah University and his office for providing an opportunity and support to take up this research project. The Deanship of Research, Majmaah University (Contract No. 37/68). Conflicts of Interest: The authors declare no conflicts of interest.

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