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)