Abstract
Based on the equilibrium point equations of a classic three-node system integrated with a large-scale photovoltaic cell (PV) power plant, the impact of PV output fluctuation on the saddle-node bifurcation (SNB) was derived and analyzed. When PV runs in a unity power factor and the PV output active power Ppv is not too large (several hundred MW and below), the PV output fluctuation has little effect on the SNB point position and load margin index, so that the load margin index can be calculated online using the SNB point at Ppv = 0 pu. On the other hand, the local reactive power compensation in the load center can effectively raise the load bus voltage and make the voltage stability problem become more concealed; the traditional under-voltage load-shedding (UVLS) strategy only carries out load shedding when the bus voltage amplitude is below the specified value and cannot effectively maintain the system static voltage stability in some occasions. In this paper, a fuzzy load-shedding strategy considering the impact of PV output fluctuations for the large-scale PV grid-connected system was designed, taking the load bus voltage amplitude and load margin index as fuzzy input variables, and the load-shedding command as a fuzzy output variable. Nine fuzzy IF-THEN rules were extracted for the fuzzy controller and the corresponding practical calculation method of load-shedding quantity was put forward. The simulation results of the classic three-node system and IEEE 14-bus system, both with a 100 MW PV power plant, verified the effectiveness of the fuzzy load-shedding controller whose input variable load margin index was calculated using the SNB point when the PV active power output was 0. The designed fuzzy load-shedding strategy can compensate for the defect—that the traditional UVLS strategy cannot effectively guarantee the system static voltage stability—and it can be widely used in power grids integrated with PV power plants whose scales are at a level of several hundred MW and below.
Highlights
With the rapid development of grid-connected new-energy power generation including wind power and solar photovoltaic cell (PV) power, energy structures are undergoing profound changes all over the world, and the safe and stable operation of power grids is facing enormous challenges [1,2,3].China has become the country with the largest installed capacity of grid-connected PV power generation since the end of 2015
The impact of the PV output fluctuation on the position of the saddle-node bifurcation (SNB) point is derived based on the equilibrium point equations of a classic three-node system with a large-scale PV power plant, firstly
We found that when the PV plant is running with a power factor of 1 or close to 1 and the installed capacity of the PV plant is not too large, the PV output fluctuation had little effect on the SNB position of the system and the impact on the load margin index was smaller
Summary
With the rapid development of grid-connected new-energy power generation including wind power and solar photovoltaic cell (PV) power, energy structures are undergoing profound changes all over the world, and the safe and stable operation of power grids is facing enormous challenges [1,2,3]. The emergency control methods for voltage instability, when the voltage stability margin of the large-scale PV grid-connected system is smaller and the power grid is close to the critical point of voltage collapse, lack further study. With the change of energy structures, the output fluctuation of grid-connected new-energy power generation has to be taken into account in the load-shedding method. In PV grid-connected systems, the PV output fluctuation has, to a certain extent, an inevitable impact on the calculation of the conventional voltage stability index, so an adaptive voltage stability index has to be studied and it should be used as another reference for load shedding when the system is under an emergency state close to voltage instability. This paper mainly studies the load-shedding problem taking the voltage stability of large-scale PV grid-connected systems into consideration using bifurcation theory and fuzzy control theory. Based on the analyses of the impact of the PV output fluctuation on the saddle-node bifurcation (SNB) point and on the voltage stability load margin index, a fuzzy load-shedding strategy, with the load margin index and voltage amplitude as reference variables, is designed for maintaining the system’s static voltage stability, the purpose being to shed enough load in time when the system is under a heavy load and to ensure that the static voltage stability and load bus voltage amplitude are both qualified
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