网侧故障下光伏直流并网系统不平衡功率快速平抑方法

A Fast Method for Suppressing Unbalanced Power in Photovoltaic DC Grid-Connected System under Grid-Side Faults

网侧故障下,光伏直流并网系统的并网变流器将根据电网电压变化实时控制输出功率,而直流侧光伏单元需接收系统功率指令后控制有功输出,两者间无法实现有功功率的快速平衡,导致直流电压持续偏移。传统方法通过投入耗能装置消耗盈余有功功率,以抑制直流电压抬升,但投资成本高。为此,该文提出一种网侧故障下系统不平衡功率快速平抑方法,在分析故障期间电网电压幅值变化趋势基础上,构建并网变流器内部计算电压变化轨迹算式。该方法利用故障初期内部计算电压数据,计算带权邻近度以求解电压变化轨迹算式关键项系数,进而求得所需系统功率指令。所提方法的优点在于有效地缩短了盈余功率持续时间,减少了对耗能装置的依赖,并且易于实现。仿真结果表明,所提方法可维持直流电压在安全范围内,为系统低电压穿越提供可靠保障。

In the case of grid-side faults,the grid-connected converter(GCC)of the photovoltaic DC grid-connected system(PV-DCGS)should reduce the output power in real time based on the variations of AC grid voltage.At the same time,the PVs are required to reduce active power according to the power command issued by the central controller.During the low voltage ride through(LVRT)period,due to the difference in control response speed between GCC and PVs,and the delay in system command processing,the unbalanced power within PV-DCGS will occur,resulting in DC bus voltage deviation.Traditionally,energy-dissipating devices are used to absorb the excess power to suppress the rising magnitude of the DC bus voltage.However,the investment cost and maintenance complexity will increase accordingly.To address these issues,this paper proposes a fast method for reducing the unbalanced power of PV-DCGS under grid-side faults,which can limit the deviation of DC bus voltage to an acceptable range.First,the structure of PV-DCGS is introduced and the influence of the variation of the system power command on the DC bus voltage is analyzed.Second,the variation characteristic of the internal calculation voltage of GCC during LVRT is revealed.Then,the voltage variation trajectory formula is constructed and the variation characteristics of the key coefficients of the constructed formula are analyzed.Third,the weighted proximity is introduced to calculate the key coefficients of the constructed formula.The exact voltage variation trajectory formula can be obtained by the calculated key coefficients.Finally,the expected power command can be obtained by putting appropriate length of voltage data into the constructed formula,and effective DC bus voltage suppression can be achieved.The simulation model of the PV-DCGS is built on the PSCAD/EMTDC electromagnetic simulation platform,which verifies the feasibility and effectiveness of the proposed method.Simulations on different types of short-circuit faults are performed.The comparison of the voltage variation trend between the simulation system and the actual power system shows that the variation trend of the internal calculation voltage of GCC in the simulation system is similar to that of the actual power system. In order to quickly calculate the key coefficients of the constructed voltage formula, the data window length should be selected appropriately. The simulation results show that when the data window length is selected between 5~20 ms, the calculation error of key coefficients is less than 5%. Considering the reliability of calculation results, the data window length of this paper is selected at 20ms. Under the selected data window length, different transition resistances of AC grid faults are tested. The results show that the key coefficients of the constructed voltage formula can be calculated reliably, and the calculation error of the dominant term is smaller than 10%. To further illustrate the effectiveness of the proposed calculation method for key coefficients, the simulation waveforms of three-phase symmetrical fault are carried out. The results show that the numerical results of the constructed voltage formula are similar to those of the simulation waveform, the calculation error is within 5%. Compared with the traditional centralized control method, the proposed method can effectively suppress the peak value of the DC bus voltage to the acceptable range (≤1.05(pu)) during LVRT period. In conclusion, the proposed method can quickly and reliably calculate the system power command required for LVRT by using the voltage data of the external AC system at the initial fault stage, which shortens the excess power and its duration, resulting in the reduction of DC voltage deviation. With the proposed method, the GCC and PVs are free from operating at unexpected DC overvoltage range (>1.05(pu)). In addition, the proposed method weakens the dependence on additional energy-dissipating equipment and reduces the investment cost.