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Low Voltage Ride Through Control Strategy of Directly Driven with System

Kun Zhang, Yuping Duan, Jiandong Wu, Jun Qiu, Jiming Lu, Shu Fan, Senior Member, IEEE, Hui Huang, and Chengxiong Mao, Senior Member, IEEE

impact on the stability of the power grid. The grid-fault Abstract—The increasing penetration poses conditions may cause the wind power generators to trip offline significant technical problems for the systems. The for self protection until the grid recovers. This may make the intermittent and fluctuant output power of wind generators has a grid recover more difficult and deteriorate the grid condition. great impact on the power quality and power system stability. On Therefore, the new grid operation codes require that the wind the other hand, the grid-side faults influence the transient generators remain connected during grid fault conditions, processes of wind generators. In this paper, an integrated control strategy is proposed based on the characteristics of directly helping the grid to resume its normal state. Many driven wind turbine with permanent magnet alternator (D- investigations have been conducted to enhance the low PMA). The D-PMA is incorporated with energy storage system, voltage ride-through (LVRT) capability of D-PMA [9-14]. which smoothes the output power and enhances the low voltage Researchers in [9] adopted a new control strategy to improve ride-through (LVRT) capability of the wind generator. The the LVRT capability. Crowbar circuits to fulfill the LVRT effectiveness of the control strategy has been verified in the requirements of the D-PMA were proposed in [10-13]. numerical simulations. Researchers in [14] investigated series dynamic braking Keywords—Directly driven wind turbine, Energy storage resistor to absorb the remaining power to fulfill LVRT system, Smoothing of output power, Low voltage ride-through requirements. However, the majority of the research focuses on either smoothing wind power fluctuations or improving I. INTRODUCTION LVRT capability of D-PMA. It is therefore necessary and ORE and more directly driven wind turbines with important to consider both aspects, and propose a feasible Mpermanent magnet alternator (D-PMA) have been used methodology to fulfill all the requirements of the two aspects. in wind farms, as they have the advantages such as low In this work, a combined control strategy is proposed to mechanical consumption, high reliability, high power smooth the power fluctuations and fulfill the LVRT generating efficiency and easy maintenance, compared to the requirements of D-PMA with energy storage system. Using doubly-fed induction generators. the proposed control strategy, the output power of wind The increasing wind power penetration poses significant generators is smoothed by energy storage system, and the technical problems for the electric power systems. The wind turbines can ride severe grid disturbances during grid- fluctuations in the output power of wind farms have a great side fault conditions. impact on the power quality and stability of the host power system [1-3]. Some researchers were seeking to smooth wind II. THE PRINCIPLE OF THE SYSTEM power fluctuations in [4-8]. Researchers in [4] made use of the The topology of the system considered in this work is pitch control and variable speed control of the wind turbine to showed in Fig.1. The D-PMA is connected to a host AC grid smooth the wind power fluctuations. Researchers in [5-8] took network via a controlled full-scale power converter system advantage of the energy storage system to smooth the wind (PCS) and a step-up . The PCS comprises of a power fluctuations and enhance the stability of the host power three-phase uncontrolled rectifier bridge, a filter capacitor, a system. boost converter at the generator side, and a pulse width High levels of wind power penetration have a significant modulated (PWM) three-phrase voltage source converter (VSC) at the grid side. The VSC controls the reactive power This work was supported in part by the National Basic Research Program transmitted to the AC grid network and keeps the voltage of of China ( 2009CB219702) and the National Basic Research Program of China (2010CB227206 ) and the Key Project of National Natural Science the DC bus constant, using decoupled pq current control Foundation of China (50837003). methodology. The grid side and the generator side convertors Kun Zhang, Jiandong Wu, Jiming Lu, Hui Huang, and Chengxiong Mao, are interconnected by a common DC bus. The energy storage are with the Department of Electrical and Electronic Engineering , HuaZhong system comprised of a super capacitor stack is connected to University of Science and Technology ,WuHan,430074, China. Shu Fan is with Business and Economic Forecasting Unit Monash University, the common DC bus via a bi-directional DC/DC converter, Australia( email: [email protected] ; [email protected] ) which controls the active power transmitted to the AC grid Yuping Duan and Jun Qiu are with Wuhan Iron and Steel (Group) Corp. 978-1-4577-1002-5/11/$26.00 ©2011 IEEE 2 network. taking advantage of the ultra capacitor, the fluctuation within The control block diagram of the system is presented in milliseconds can be smoothed rapidly and dynamically. Fig. 2. The power transferred to the AC grid network is During grid-side fault conditions, the wind turbine works * determined by P W. In normal operating conditions, if the normally to maintain optimized energy capture, the boost output power of the D-PMA PG is bigger than the reference converter at the generator side will delivery as much energy as * power transferred to the grid P W, the remaining energy is the wind generator generates, but the power transferred to the absorbed by the super capacitor stack, if the output power of grid is much smaller than normal conditions, the redundant the D-PMA PG is smaller than the reference power transferred power will be absorbed by the ultra capacitor stack to keep the * to the grid P W, the super capacitor stack will release energy to voltage of the comment bus constant, to insure that the system the common DC bus to keep the DC voltage constant. The work normally. The ride through time scale of the system is power transferred to the grid can be smoothed by the energy mainly determined by the capacity of the ultra capacitors and storage system.. the state of charge (SOC) of the ultra capacitors when the ride Ultra capacitor, which attracts more and more attention, has through happens. The larger the capacity of the ultra the features of high power density, long cycle life, high capacitors is and the lower SOC the ultra capacitors have, a efficiency and easy to maintain [15]. In this paper, ultra longer time the system will work normally for during grid-side capacitor is introduced into the wind turbine system. By fault condition [16].

PG PT

PF

Fig1.Direct-driven wind generation system based on the ultra capacitors From (1) (2) (3), the PWM duty cycle of the boost III. CONTROL STRATEGY OF THE SYSTEM controller is calculated as follows:

⎛⎞Ki * A. Control strategy of the generator side converter dK0P=−⎜⎟ + iiuuu 000 − +()DCDC − (4) S () The control block diagram of the generator side boost ⎝⎠ converter is depicted in Fig. 2. The boost converter is B. Control strategy of the grid side converter * controlled to deliver the output power of the wind turbine P G During grid-side fault conditions, the apparent power * to the DC bus. P G is calculated by a certain maximum power- transferred to the grid calculated with positive and negative tracing algorithm to capture maximum power from the wind sequence components is： [17]. The reference current of the boost controller is produced PPNN by a PI controller from the deviation between power command ⎡⎤PT0 ⎡⎤uuuudqdq * ⎢⎥NNPPP P G and the power boost convertor transferred, as shown in ⎢⎥ PTs2 uuuuiqdqdd−− ⎡ ⎤ (1). ⎢⎥⎢⎥ ⎢⎥NNPPP⎢ ⎥ ⎢⎥PTc2 uuuuidqdqq **⎛⎞K ⎢ ⎥ i ⎢⎥= ⎢⎥PPNNN (5) iK0P=− + PP GG − (1) ⎢ ⎥ ⎜⎟() QuuuuiT0 ⎢⎥qdqdd−− ⎝⎠S ⎢⎥ ⎢ ⎥ ⎢⎥Quuuui⎢⎥−−NNPPN The PWM duty cycle of the boost controller is produced by Ts2 ⎢⎥dqdqq⎣⎢ ⎦⎥ a PI controller from the boost current error, as shown in (2). ⎢⎥ NNPP ⎣⎦⎢⎥QuuuuTc2 ⎢⎥qdqd−− ∧ ⎣⎦ ⎛⎞Ki * (2) Where PT0 and QT0 are the mean active and reactive power, dK0 =−⎜⎟P00 +() ii − ⎝⎠S PTs2 and QTs2 are the second harmonic sine components of the To restrain the effect of the voltage fluctuations of the active and reactive power, PTc2 and QTc2 are the second comment DC bus, a feed forward control of the DC voltage harmonic cosine components of the active and reactive power. P P N N is introduced, as shown in (3). id , iq , id , iq are the positive and negative dq components of the grid currents. Considering the adverse effects of the duuu0DC0DC=−() (3) 3 negative sequence current, the references of the negative N* N* ⎧ P*⎛⎞Ki P* P P P components of the grid currents, id , iq are set to zeros. The ⎪VKddddq=−⎜⎟P +() iiuLi − + +ω mean reactive power Q* is set to zero to keep the grid side ⎪⎝S ⎠ T0 ⎨ （9） converter working at unity . The reference DC ⎛⎞K ⎪VKP*=− +i iiuLi P* − P + P −ω P current of the VSC is derived by a PI controller from the DC ⎪ qqqqd⎜⎟P () voltage error, as showed in (6), and the reference mean active ⎩ ⎝⎠S * power P T0 can be calculated from the reference DC voltage ⎧ N*⎛⎞Ki N* N N N and reference DC current, as shown in (7). ⎪VKddddq=−⎜⎟P +() iiuLi − + −ω ⎪⎝S ⎠ **⎛⎞Ki ⎨ (10) iKDC=−⎜⎟ P + vv dc − dc (6) K S () ⎪ N*⎛⎞i N* N N N ⎝⎠ VKqqqqd=−⎜⎟P +() iiuLi − + +ω ⎩⎪ ⎝⎠S ⎡⎤⎛⎞K ***i (7) The current of the grid can be controlled to be symmetrical PKT0=−⎢⎥⎜⎟ P +() vvv dc − dc dc ⎣⎦⎝⎠S with the control strategy depicted above, which can restrain N* N* * As id , iq , Q T0, are set to zeroes, from equations (5) and the negative current when the voltage of the grid is P* P* (7), the reference current id , iq can be calculated as asymmetrical. PT0 can be calculated as follows： follows： PP PP PuiuiT0 =+dd qq (11) 2 ⎧ uP The VSC is controlled in a synchronous rotating dq axis ⎪ P*()d * iPd = 22T0 frame, with the d-axis oriented along the positive component ⎪ uuPP P ⎪ ()dq+ () of the grid voltage vector such that Ud =0, PT0 is： (8) PP ⎨ 2 P PuiT0 = qq (12) ⎪ uq P*() * i P=0, as the VSC works at unity factor mode, and the ⎪iPq = 22T0 d ⎪ uuPP current transferred to the grid is calculated as follows： ⎩ ()dq+ () PP2P2P PI, as well as forward feedback control strategy is iiii=+=()dqq () (13) introduced to control the decoupled positive and negative p sequences of the VSC’s currents, which are controlled by the Suppose that i_MAX is the maximum current the VSC can decoupled positive and negative dq components of the VSC’s transferred to the grid, and the maximum power it can voltage. As shown in (9) and (10). transferred to the grid is calculated as follows: PP PP PuiuiT0 _ MAX==qqq _ MAX _ MAX (14) * As showed in Fig. 2 , PPW2= T0 _ MAX .

u0 i0 uDC uDC

uabc iabc

θθ udq udq idq idq * d0 uDC

∧ d u u u u i i i i d 0 0 u d q d q d q d q SC i1 θ θ

d1 P* P* N* N* * V V V V 1/u i d q d q 1 d ∧ DC ω i ∗ u 1 d1 0 0 ω ω uSC i * * PG 1 P 1/uDC T0 QT0 ω ∗ ud u i1 P ∗ DC PI PI G PI PI uq id P* * P* P* i id iq q _Max PW2 i * q P* uq PW i * d id PG 1 PW1 P* iq iq 1+ ST1 PT0 N* N* id iq Fig2.Block diagram of converters in Direct-driven wind generation system based on the ultra-capacitors 4

normally without withdrawing from the grid. C. The control strategy of the energy storage system The energy storage system is presented in Fig. 2. The IV. SIMULATION RESULTS reference power transmitted to the grid is: A simulation model is established in Matlab/Simulink in * 1 PPW1= G (15) order to verify the proposed control strategy, and the 1+ ST1 simulation parameters are presented bellow. For the D-PMSG, where PG is the pre-filtering wind power. The final the rated capacity is 1.5MW. Regarding to the SVC, the DC reference power transmitted to the grid is the minimum of capacitance is 60000uF, the reference DC voltage is 2400V * * P W1 and P w2, as shown in (16) and the grid voltage (ph-ph, rms) is 690V, the connecting **** inductance is 1.3mH. About energy storage system the rated PPPPPWW1W2W1T0_MAX= Min( , )=Min( , ) (16) capacity is 600kVA/6MJ, the rated voltage is 600V, the The reference current of the bi-directional DC/DC connecting inductance is 0.04mH. The rated voltage of the converter is produced by a PI controller from the deviation * transformer is 0.69kV/20kV. between P W and PT0, as shown in (17) A. Simulation results at normal conditions **⎛⎞Ki iK1P=−⎜⎟ + PP WT0 − (17) S () Fig. 3 presents the output power of the wind turbine, which ⎝⎠ is much fluctuant. Fig. 4 presents the output power of the The PWM duty cycle of the bi-directional DC/DC wind turbine and the power transferred to the grid, which is converter is produced by a PI controller from the current much smoother. The output power of the energy storage error, as shown in (18) system is shown in Fig. 5. ∧ ⎛⎞Ki * 1 (18) dK=−⎜⎟P11 +() ii − ⎝⎠S To restrain the effect of the voltage fluctuations of the comment DC bus, a feed forward control of the DC voltage is introduced, as shown in (19)

duuu1DCSCDC=−() (19) From (17) (18) (19), the PWM duty cycle of the bi- directional DC/DC converter is calculated as follows. ⎛⎞K Fig. 3 The output power of the wind turbine i * (20) dK1=−⎜⎟ P +() iiuuu 1 − 1 +() DC − SC DC ⎝⎠S In normal conditions, the VSC works at unity factor such p p p that u q=1, i d=0, and suppose that i_MAX =1.5, then *PP * PuiW2==qq _ MAX 1.5. PPW1=+≤ G(1 ST 1 ) 1 , as PG is smaller than 1. The reference power transferred to the grid is * ** * PW = Min( PW1,)P W2 = PW1 , which means that the power transferred to the grid is smoothed by the energy storage system. Fig. 4 The output power of wind turbine and VSC During grid-side fault conditions, the wind turbine and the boost converter at the generator side work normally, the energy storage system works to keep the voltage of the comment bus constant, helping the wind turbine to be ** connected to the grid. If PPW1≤ W2 , then **** PP PWW1W2W1= Min(PP , )= P, PPT0≤ T0_MAX , ii≤ _MAX, the power transmitted to the grid is smaller than the maximum ** power the VSC allowed. If PPW1≥ W2 , then * ** * PW = Min (,PPW1 W2 )= P W2 , the power transferred to the grid is Fig.5 The output power of the energy storage system the maximum power the VSC allowed, the remaining power is B. Simulation results during fault conditions absorbed by the energy storage system to keep the voltage of When a certain fault occurs, the wind turbine and the boost the comment DC bus constant, the hole system can work 5 converter at the generator side work normally, but the output current and output power of the VSC changes a lot, as shown below. Fig. 6(a), (b), (c) present the grid voltage, grid current and active power transferred to the grid when a single phase voltage dip, 85%-2s, which occurs at 0.1s and is cleared at 0.3s, is applied to the wind turbine. The current rises to 1 pu, and the active power oscillates as there are harmonic components in the current. Fig. 7(a), (b), (c) present the grid voltage, grid current and active power transferred to the grid (d) . DC link voltage when a two phase voltage dip, 85%-2s, which occurs at 0.1s and is cleared at 0.3s, is applied to the wind turbine. The current rises to 1.4 pu, and the active power drops from 0.78 pu to 0.61 pu. Fig. 8(a), (b), (c) present the grid voltage, grid current and active power transferred to the grid when a symmetrical voltage dip, 60%-2s, which occurs at 0.1s and is cleared at 0.3s, is applied to the wind turbine. The current rises to nearly 1.4 pu, and the active power drops from 0.78 pu to 0.56 pu. (e). Output power of the ESS When a certain fault occurs, the energy storage system absorbs the redundant power in the DC capacitor to keep the Fig.6 Simulation results of one phase voltage drop

DC voltage constant, as showed in Fig. 6(d), 7(d), 8(d). The b. Simulation results of two phase voltage drop energy absorbed by the energy storage system is shown in Fig.

6(e), 7(e), 8(e). a. Simulation results of one phase voltage drop

(a). Grid voltages

(a) . Grid voltages

(b). Grid currents

(b). Grid currents

(c). Mean active power

(c). Mean active power 6

(d) .DC link voltage (d) . DC link voltage

(e). Output power of the ESS (e). Output power of the ESS

Fig.7 Simulation results of two phase voltage drop Fig.8 Simulation results of three phase symmetrical voltage drop c. Simulation results of three phase symmetrical voltage drop V. CONCLUSION

In this paper, a combined control strategy is proposed to smooth the power fluctuations and fulfill the LVRT requirements of D-PMA with energy storage system. The wind power quality is improved using the proposed control strategy and the energy storage system, and the wind turbine can ride-through severe grid disturbances. The simulation results have verified the correctness and effectiveness of the control strategy.

(a).Grid voltages VI. REFERENCES

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