| China's power grid has become the world's largest interconnected power grid with the highest voltage level.The dynamic characteristics and operation mode of the power grid are more complex and changeable,and the challenges to the secure and stable operation of the system are more severe.In order to prevent the occurrence of blackout accidents,China's power grid has constructed three lines of defense for safety and stability,in which the relay protection device is the first line of defense,which is responsible for quickly removing the fault components and preventing the fault from expanding;the stability control measures such as generator tripping and load shedding are the second line of defense,which is to prevent the system from losing stability.However,according to the current operation status of the power grid,the information received by the second line of defense from the relay protection device after the fault occurs only contains limited information such as fault components and circuit breaker opening or closing.By comparing the limited information with the offline generated strategy table to judge the current stable state of the system,the first and second lines of defense are still in the split state.With the development of relay protection technology,the existing relay protection devices are fully capable of informing the second line of defense of important fault information such as fault time,fault location,fault type,grounding resistance and overload degree.If the rich fault information of relay protection devices can be transmitted to the control center in time and used effectively and reasonably,and the relationship between the first and second lines of defense can be changed from the traditional"situation driven" to "information driven",it will help to reduce the calculation error,achieve more accurate stability analysis and more efficient control strategy,so as to further improve the secure and stable operation level of power system.Based on the fault information provided by relay protection device,this thesis makes theoretical analysis and research on three aspects:transient stability analysis method,transient stability emergency control strategy and power flow transferring overload emergency control strategy.The following research results are obtained:(1)A power system transient stability analysis method based on fault panoramic information is proposed.First,the factors affecting the system transient stability are analyzed,and the fault panoramic information is constructed,which includes fault time,fault location,fault type and grounding resistance.Then,the fault panoramic information is fused into the final admittance matrix by shrinking the system admittance matrix twice.On this basis,the extended equal area criterion is used to form three-dimensional surface of the system transient stability margin in different fault scenarios.The transient stability boundary of the system is obtained by extracting the intersection line between the surface and the zero margin surface.Finally,IEEE 3-machine system and New England 10-machine system are used as test systems for simulation,and the results exhibit that this method can improve the accuracy of transient stability assessment considering the fault panoramic information under different faults.(2)An analytical evaluation method for transient stability margin of multi machine power system based on fault current distribution coefficient is proposed.Firstly,the mathematical expression of generator electromagnetic power on fault current distribution coefficient under different fault scenarios is derived from the fault mechanism.Based on this,the analytical expression of single machine energy function varying with fault current distribution coefficient and time is obtained.From the expression,the energy function of single machine can be calculated directly,thus avoiding the time complexity of power angle and angular velocity of generator in traditional methods Finally,combined with the energy function of single unit,the single unit stability margin of key units is obtained and used to judge the transient stability of the system.The simulation results show that the proposed method can effectively determine the key units in the system under different fault scenarios,and the obtained single machine stability margin and fault limit clearing time have high accuracy.(3)A generator tripping strategy considering fault information is proposed.First,the part of the transient stability margin related to the controlling unstable equilibrium point is extracted,then the modified equation of the generator tripping amount and the controlling unstable equilibrium point is established,and the change of the controlling unstable equilibrium point is determined by the Newton-Raphson method.On this basis,the sensitivity of the transient stability margin of the system to different generators is calculated.Then,the transient stability margin of the system is calculated based on the fault information provided by relay protection device,such as fault time,fault location and fault type,and finally the generator tripping strategy is made according to the sensitivity and transient stability margin.The simulation results show that the sensitivity of this strategy is more accurate than that of the cutting strategy without considering the change of the controlling unstable equilibrium point,which can effectively reduce the total generator cutting amount.At the same time,the solution steps of the strategy are direct and clear,which avoids the tedious calculation process and saves the calculation time.(4)A cross voltage level emergency control strategy for power flow transferring overload is proposed.Firstly,considering the characteristics of the grid structure under different voltage levels,the hierarchical emergency control model is established.On the one hand,for the system of 500kV and above voltage level,the overload area is divided according to the overload information of relay protection device.Taking the minimum control cost as the objective function and considering the voltage and frequency stability constraints,the optimal generator/load shedding scheme is established by using the improved particle swarm optimization algorithm.On the other hand,for the system under 500kV voltage level,based on the load information transmitted between substations,the emergency control scheme with the optimal comprehensive cost is developed by AHP fuzzy comprehensive evaluation method,and the emergency control of the cross voltage level is realized.The example shows that compared with other strategies,the strategy can effectively eliminate the overload of the line and reduce the calculation time greatly,which is conducive to the realization of online application of engineering. |
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