考虑储能变流器容量限制的电网低频振荡抑制策略

Suppression Strategy for Low Frequency Oscillations of Power Grid Considering ESS Converter Capacity Limit

下垂控制与惯量控制是储能系统(energystorage system,ESS)抑制电网低频振荡的主要控制模式,但由于储能变流器的容量有限,当控制作用增强到一定程度时,ESS的输出功率将被限制在变流器的最大功率,即最大功率调制模式,从而失去预期的输出功率调节作用。为此,首先从评价转子转速动态特性的指标出发,对比了ESS不同有功控制模式对转速变化率的抑制作用。研究表明:当变流器工作在最大功率调制模式时,ESS抑制电网低频振荡的能力明显优于下垂控制与惯量控制;但ESS长时间运行在最大功率调制模式会频繁地以大电流进行充放电,导致其寿命骤减。鉴于此,提出了融合最大功率调制与下垂控制模式的电网低频振荡抑制策略:最大功率调制模式使储能变流器以功率限值为基准交替发出或吸收有功,快速补偿系统中的缺额功率或吸收系统中的过剩功率,从源头上抑制产生低频振荡的内在驱动力,即不平衡功率;当转子的转速变化率、偏移量等动态评价指标满足阈值要求后,再利用下垂控制使ESS平滑地退出最大功率调制模式,并进一步减小转子转速偏差,提高系统频率质量。仿真结果验证了所提策略的有效性与先进性。

Droop control and inertia control are the typical control modes for energy storage system(ESS)suppressing low frequency oscillations.However,when the control effect is enhanced to some certain extent,ESS will limit the output power to the device maximum power(i.e.,the maximum power modulation mode)and be out of the expected power control because of the limited converter capacity.This paper focuses on the evaluation indexes of the rotor speed dynamic characteristics and compares the suppression abilities of the rate of changes of the rotor speed with different ESS control modes.The study shows that the low frequency oscillations can be better suppressed with the ESS control modes than with the droop control and inertia control modes?;however,the long-term operation under the maximum power modulation mode will cause the ESS frequent charge and discharge with large currents,resulting in the sharp decrease in its service life.Therefore,a control strategy based on the maximum power modulation and the droop control is proposed to damp low frequency oscillation.The former can alternately output or absorb active power with the converter capacity to quickly make up for the system power shortage,suppressing the driving force of low frequency oscillations,i.e.,the unbalanced power.When the evaluation index meets the limited requirements,under the effect of the droop control,the ESS drops out of the maximum power mode smoothly and further suppresses the rotor speed deviation to improve the frequency quality.Simulation results verify the effectiveness and advancement of the proposed control mode.