Scenario-based analysis and probability assessment of sub-synchronous oscillation caused by wind farms with direct-driven wind generators

Journal of Modern Power Systems and Clean Energy, Jul 2018

Recently, explanations of the sub-synchronous oscillation (SSO) caused by wind farms based on direct-driven wind generators (DDWGs) have been published in the literatures, in which the controller parameters of DDWGs and the system equivalent parameters play an important role. However, more than one set of parameters can cause weakly damped sub-synchronous modes. The most vulnerable and highly possible scenario is still unknown. To find scenarios that have potential oscillation risks, this paper proposes a small disturbance model of wind farms with DDWGs connected to the grid using a state-space modeling technique. Taguchi’s orthogonal array testing is introduced to generate different scenarios. Multiple scenarios with different parameter settings that may lead to SSOs are found. A probabilistic analysis method based on the Gaussian mixture model is employed to evaluate the consistency of these scenarios with the actual accidents. Electromagnetic transient simulations are performed to verify the findings.

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Scenario-based analysis and probability assessment of sub-synchronous oscillation caused by wind farms with direct-driven wind generators

Scenario-based analysis and probability assessment of sub-synchronous oscillation caused by wind farms with direct-driven wind generators Zhi AN 0 1 2 3 4 Chen SHEN 0 1 2 3 4 Zetian ZHENG 0 1 2 3 4 Feng LIU 0 1 2 3 4 Xiaoqing CHANG 0 1 2 3 4 Wei WEI 0 1 2 3 4 Zetian ZHENG 0 1 2 3 4 0 China State Key Laboratory of Power System and Generation Equipment , Beijing , China 1 Department of Electrical Engineering, Tsinghua University , Beijing , China 2 Electric Power Research Institute of Sichuan Electric Power Company , Chengdu , China 3 The Author(s) 2018 & Chen SHEN 4 Xiaoqing CHANG Recently, explanations of the sub-synchronous oscillation (SSO) caused by wind farms based on directdriven wind generators (DDWGs) have been published in the literatures, in which the controller parameters of DDWGs and the system equivalent parameters play an important role. However, more than one set of parameters can cause weakly damped sub-synchronous modes. The most vulnerable and highly possible scenario is still unknown. To find scenarios that have potential oscillation risks, this paper proposes a small disturbance model of wind farms with DDWGs connected to the grid using a state-space modeling technique. Taguchi's orthogonal array testing is introduced to generate different scenarios. Multiple scenarios with different parameter settings that may lead to SSOs are found. A probabilistic analysis method based on the Gaussian mixture model is employed to evaluate the consistency of these scenarios with the actual accidents. Electromagnetic transient simulations are performed to verify the findings. Direct-driven wind generator; Subsynchronous oscillation; Probabilistic assessment; Randomness 1 Introduction 1.1 Motivation With the vigorous exploitation of wind power generation, stability problems associated with wind farms have increased [ 1?5 ], among which sub-synchronous oscillation (SSO) is a prominent example. SSO was initially defined as an electric power system condition where the electric network exchanges significant energy with a turbine generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system [ 6 ]. With deepening research, the concept of SSO has expanded to systems with wind generators. In October 2009, a series of offline wind generators and cracked crowbar circuits were found in the Electric Reliability Council of Texas event [ 3 ]. In December 2012, a wind farm in North China reported that a part of their generators were shut down and that a sub-synchronous current was sent out to the main grid during the accident [ 4, 5 ]. In July 2015, several thermal generators were tripped off by shaft torsional vibration relay in a thermal power plant in Hami, Xinjiang Uygur Autonomous Region of China. This event led to the emergent power reduction of a nearby highvoltage direct-current (HVDC) transmission line and caused sharp fluctuation in the frequency of the grid. Analysis of the records of the phasor measurement units after the accident revealed that the sub-synchronous current that aroused the torsional vibration came from wind farms north of the region [7]. This accident exhibited unique characteristics. First, most wind generators in Hami wind farms are direct-driven wind generators (DDWGs), i.e., Type-4 wind generators. Second, there is no series compensator in the region. Finally, the thermal generators and HVDC line showed no indication of SSO at the very beginning that the SSO current arouse. According to [ 7 ], this accident was the first reported SSO event caused by wind farms based on DDWGs. Thus far, researchers have not reached a consensus on the mechanism of this SSO. This paper focuses on the new type of SSO and provides a methodology for finding a system scenario consistent with the actual accident, considering the stochastic variation in the operating conditions of the wind farms. 1.2 Literature review Recently, a series of researches presented various explanations about this event. Reference [ 8 ] indicated that this event should be categorized as a new type of subsynchronous interaction. Reference [ 9 ] constructed a small signal model for grid-connected wind farms regarding the wind farms as a single equivalent generator and then reproduced the SSO under specific operating conditions. The authors inferred that there might be an unstable mode with sub-synchronous frequency that was strongly correlated with the controller of the converter in DDWGs. In this system, the equivalent impedance of the wind farms behaved as a capacitive impedance with a small negative resistance. Therefore, oscillation, whose frequency was determined by the equivalent capacity and system reactance, might arise. Reference [ 10 ] proposed an impedance model, based on which the authors stated that interactions between wind farms and the weak grid might produce negative damping for the SSO. The interaction is associated with the control (...truncated)


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Zhi AN, Chen SHEN, Zetian ZHENG, Feng LIU, Xiaoqing CHANG, Wei WEI. Scenario-based analysis and probability assessment of sub-synchronous oscillation caused by wind farms with direct-driven wind generators, Journal of Modern Power Systems and Clean Energy, 2018, pp. 1-11, DOI: 10.1007/s40565-018-0416-2