1710 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 4, APRIL 2014 A Low-Voltage Ride-Through Method With Transformer Flux Compensation Capability of Renewable Power Grid-Side Converters Shih-Feng Chou, Chia-Tse Lee, Student Member, IEEE, Hsin-Cheng Ko, and Po-Tai Cheng, Senior Member, IEEE Abstract—With the growing penetration of renewable energy resources, the grid operators place higher emphasis on their grid integration requirements. The low-voltage ride-through (LVRT) capability is one of the most important issues for maintaining the grid stability. This paper proposes a LVRT technique for effective utilization of the ampere capacity of the grid-connected converter to meet the LVRT requirement, and to reduce the potential post- sag inrush current of the transformer within the grid-connected converter system. This technique precisely manages the peak value of converter’s phase current while performing the transformer ﬂux compensation and the current injection for full utilization of the ampere capacity without the risk of triggering the overcurrent protection. The operation principles of the proposed method is explained, and laboratory test results are presented for validation. Index Terms—Distributed energy resources (DERs), ﬂux com- pensation, grid-connected converter, inrush current, low-voltage ride-through (LVRT). I. INTRODUCTION R ENEWABLE energy resources have become more and more important these days and have been taken as the main solution for reducing the green house gases. As the increasing use of renewables are installed in the utility grid, their impact on the power system’s stability becomes a signiﬁcant issue. As a result, the grid operators start outlining speciﬁc requirements for the integration of renewables into the utility grid. Low-voltage ride-through (LVRT) capability is one of the most important issues among grid codes. In wind power sys- tem, the grid codes demand the power converters to remain connected to the grid in the event of voltage drop for a certain period of time [1]–[9]. The grid codes also require reactive cur- rent injection (RCI) during the fault to support the grid stability. Manuscript received November 18, 2012; revised February 1, 2013 and April 11, 2013; accepted May 23, 2013. Date of current version October 15, 2013. This work was supported by the National Science Council of Taiwan under Grant NSC-98-2221-E-007-121-MY3. Recommended for publication by Asso- ciate Editor F. W. Fuchs. S.-F. Chou is with the Delta Electronics, Inc., Taipei 11491, Taiwan (e-mail: miracle6923@gmail.com). C.-T Lee and P.-T. Cheng are with the Center for Adavanced Power Tech- nologies, Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan (e-mail: atonis5@gmail.com; ptcheng@ieee.org). H.-C. Ko is with the Taiwan Army, Taipei Songshan Airport, Taipei 10548, Taiwan (e-mail: jacobko0805@gmail.com). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TPEL.2013.2266511 In photovoltaic power system, the grid-connected converters are also required to be capable of fault ride-through [10]–[15]. The German grid code further demands the mandatory reactive power generation to provide the voltage support for the photo- voltaic power system [10]. Various control methods have been proposed [16]–[18] to meet the RCI speciﬁcations of the LVRT requirement. These methods produce the converter current command based on the active and reactive power ﬂow, and the grid-connected converter may experience excessive high current stress due to the unbal- anced and decreased grid voltages as shown in [16]. Thus, the commanded current needs to be scaled down; otherwise, the grid-connected converter is exposed to the risk of triggering overcurrent protection and fail the LVRT operation. In [19], an LVRT technique has been proposed to precisely manage the peak value of converter’s phase current so the full ampere ca- pacity can be utilized without the risk of overcurrent during the LVRT operation. In the LVRT operation, the step-up transformer, an essential part of the grid-connected converter system, is also exposed to the faulted line voltages. Such exposure develops ﬂux offset in the transformer’s core. This ﬂux offset decays very slowly in the low-loss core, and easily leads to core saturation when the line voltage recovers, and results in inrush current [20], [21]. In this paper, a ﬂux compensation method is proposed to mitigate the sag-induced ﬂux offset, and hence reduce the po- tential post-sag inrush current. Combined with the aforemen- tioned RCI control method, the proposed technique commands the converter current to meet the RCI speciﬁcations of LVRT requirement, and to compensate the transformer ﬂux offset. Re- garding the converter’s peak current issue, the proposed ﬂux compensation method extends the concept stated in [19]; thus, the aforementioned functionalities are performed within prede- termined current constraints of the converter, and the converter current capacity can be fully utilized without the risk of trigger- ing the overcurrent protection or damaging the converter itself. II. SYSTEM CONFIGURATION The system conﬁguration of the three-level neutral-point- clamped converter (three-level NPC converter) being connected to the grid through the transformer is shown in Fig. 1. During the LVRT operation, the power produced by the renewable en- ergy resources needs to be reduced or redirected, so that the grid-connected converter can maintain its dc bus voltage and 0885-8993 © 2013 IEEE