Low Voltage Ride Through Control Strategy of Directly Driven Wind Turbine with Energy Storage System

1 Low Voltage Ride Through Control Strategy of Directly Driven Wind Turbine with Energy Storage 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 wind power penetration poses conditions may cause the wind power generators to trip offline significant technical problems for the electric power 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 transformer. 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 power factor. 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.

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