Research On Strategy Avoiding Out-of-step Protection Act-error As Oscillation Center Invades Generators

In recent years,the characteristics of large-scale parallel operation of synchronous generator units have become increasingly prominent in large-scale power transmission bases.As the units develop towards large capacity and high voltage,the impedance of the sending system becomes larger increasingly,while the equivalent impedance of the external system is reduced due to the distributed load access.When out-of-step oscillation occurs between the sending system and the receiving system with this feature,the tendency of the oscillation center to migrate from the transmission corridor to the inside of the sending system becomes more significant.For power plants with large-scale units in parallel,there is a serious risk that the fleet will be switched off due to out-of-step protection and low-voltage protection.Therefore,it is urgent to analyze the characteristics of the oscillating center's intrusion into the sender group,and study effective extrapolation strategies.More in-depth research was carried out and certain results were obtained around the internal impacts,causes and countermeasures of the invasion of the oscillation center.Here are the main research contents and results.(1)For large-scale electric power delivery bases,the conditions of the oscillation center invading the generator group and the impact on the out-of-step protection of generators and the low-voltage protection of auxiliary machines are analyzed.First,the characteristics of large-scale power transmission bases are summarized.Based on the calculation of the system's equivalent impedance,the conditions and reasons for the intrusion of the oscillation center into the generator are analyzed.Taking three-impedance out-of-step protection and low-voltage protection as examples,the characteristics of the protection action during the intrusion of the oscillation center into the machine group are analyzed.(2)Starting from changing the impedance parameters of the transmission corridor,a flexible extrapolation strategy of the oscillation center for adjusting the TCSC parameters was proposed.First of all,when the system is oscillating,the reactive parameter values that may be adjusted for the TCSC group outside the oscillation center are predicted,and the system non-parallel resonance constraints are formulated.Further system stability during TCSC parameter adjustment is judged,and the TCSC trigger angle under constraints is calculated.(3)Starting from the adjustment of system energy to change the impedance of the system,a flexible extrapolation strategy of oscillation center based on energy rebalance was proposed.When the TCSC does not achieve the desired effect,firstly the standby energy at the transmitting,the transferred energy at the transmission corridor,and the cutting-off energy at the receiving are determined.Based on the law of energy rebalancing,the generator group is pushed out of the center of oscillation by maximizing the input and removal of generators and lines.Further,the transferred energy of the DC system is predicted,taking into account the change in the equivalent impedance of the system caused by the change of the DC power,and establishing the energy transfer restriction constraints.Finally,under the premise of satisfying the system stability,the energy adjustment schemes of the transmitting end,the transmission corridor,and the receiving end are specified.(4)Starting from the load shedding at the receiving,a rigid extrapolation strategy of the oscillation center based on the receiving end source load adjustment was proposed.When the flexible strategy fails to achieve the desired results,firstly based on the necessary conditions for launching the cluster outside the oscillation center,the number of end-system units and load shedding is predicted,and migration moderation constraints are formulated.Further system stability was judged,and the specific scheme of receiving source and load adjustment was determined.PSCAD is used to build a large-scale power delivery base model,and the internal scene of the oscillation center invading the terminal group is simulated and verified.Further,three different flexible or rigid extrapolation strategies were simulated,verifying the feasibility of the three strategies.The simulation results show that the effect of the flexible extrapolation strategy is small,the adjustment capacity is limited,but the load is not lost.The rigid extrapolation strategy is better,but there is a load loss.