Research On DC Power Modulation Strategy Of Improveing AC-DC Interconnected System Trasient Stability

Large-scale AC/DC hybrid system has become the development trend of China’s power grid, which shows the characteristics of high-capacity and transient stability issues have become prominent. Through flexible using the UHVDC-contact-line’s capability of short time overload and fast adjustable power, it can achieve the purpose of improving the system transient stability. For the actual grid transient stability problems, this paper uses DC active power modulation technique to improve the system transient stability, researches the modulation strategy of DC active power great mode modulation and DC active power upgrade/back down modulation.Firstly, it is introduced the basic principles of DC active power modulation, started at the generator swing equation, explained the DC active power modulation mechanism of improving the system transient stability. On this basis, two modulation models in BPA are introduced and verified effectively in2DC system sample.Secondly, the paper introduces a practical grid, analyses AC and DC outgoing channel failures influence to the system indexes such as generator angles, outgoing channel power and bus voltage. Through a large number of simulation analysis, a failures set which has a strong impact on system transient stability is obtained. On the basis of the failures set, the paper researches parameter selection of power great mode and power upgrade/back down modulations respectively, and proposes two applicable parameter estimation algorithms.For power great mode modulation, parameter selection algorithm is proposed based on Fourier spectrum analysis. Through calculating the trend of the power flow transfer situation under various fault in the failures set, it is selected the outgoing channel power which has the largest proportion of power flow transfer as a modulation input signal. The parameter selection algorithm compares and analyses input AC power signals with additional output DC power signals, uses hierarchical search algorithm, and obtains suitable modulation parameters for serious problems. Simulation results show that the modulated signal selection and parameter estimation method are effective and feasible with generally applicable, the modulation parameter is more reasonable than engineering empirical one, it also proves that the capability of resisting serious failures is improved by DC active power high manner modulation.For power upgrade/back down modulation, combined Particle Swarm Optimization, a modulation parameters optimization algorithm based on the transient stability indicator is proposed. According to the Equal Area Criterion, this paper defines the transient stability composite indicator using generator accelerated area and decelerated area. It is combined of this indicator with traditional PSO algorithm, carried out the optimal modulation parameter search, hybrid Simulated in Matlab and BPA, obtained the optimal modulation results. Simulation results show that the proposed algorithm is effective for modulation parameter search, the index considered the angle swings, and modulation effect is satisfactory.