On Underlying Principles And Countermeasures Of Subsynchronous Oscillation In Power Systems

Series-compensated AC transmission and HVDC transmission are the two major ways for long-distance bulk power transmission, which are of great importance to solve the unbalance problem of energy resource distribution in China. However, both series-compensated AC transmission line and HVDC system my cause subsynchronous oscillation (SSO) of power systems, which is harmful to generator shafts. If there are multiple generators operating in parallel, their SSO characteristic will be more complicated because of interactions among them. At the same time, as the operating conditions of power systems alter frequently, SSO countermeasures should meet a higher requirement of adaptation. Under such background, this thesis deals with some problems about underlying principles and countermeasures of SSO. The main works are organized as follows.(1) The SSO characteristic of multiple generators in the vicinity of rectifier station of HVDC system is studied. Also studied is the designing method of SSDC to mitigate SSO of multiple generators. Modal analysis of non-identical generators in parallel under their common torsional natural frequency is carried out, based on which the phase relationship of their speed deviation signals is investigated. Then the method to eliminate anti-mode component in the input signal of SSDC is put forward, so that mode separation is achieved. Theoretical analysis and time-domain simulation verify that after mode separation, the SSDC is effective in mitigating SSO of multiple generators simultaneously.(2) The designing method of SSDC that takes electromagnetic power signal as its input signal, instead of speed deviation signal, is studied. The phase relationship of speed deviation, electromagnetic torque and electromagnetic power when SSO occurs is analyzed, based on which the SSDC designing procedure is investigated, so that the SSDC can provide the generator with positive additional damping under each torsional natural frequency. With a typical case study, the effectiveness of the SSDC taking electromagnetic power as its input signal in mitigating SSO is verified by complex torque coefficient method and time-domain simulation.(3) The SSO damping characteristic of induction machine damping unit (IMDU), which is installed on the generator shaft to mitigate SSO, is studied. The reason that analyzing IMDU dynamic performance based on its torque-slip characteristic curve is not adequate is explained. Based on its dynamic equations, the method of directly calculating the additional electrical damping of IMDU under various oscillation frequencies is deduced. The calculating results of this direct calculation method are compared with that of the test signal method thus its accuracy is verified. This direct calculation method is a useful tool for finding ways to increase the electrical damping of IMDU. In addition, the damping characteristic of an IMDU with typical parameters is investigated, showing that the IMDU may even present negative damping if no special measure is taken to increase its electrical damping, so that it may even aggravate the SSO problem.(4) Three ways to increase the electrical damping of IMDU in the subsynchronous frequency range, namely, parameter optimization, frequency raise and applying active damping scheme, are studied. IMDU parameter optimization is carried out by genetic algorithms, so that the electrical damping of IMDU is increased and negative damping is avoided. By raising its rated frequency and also raising the number of poles of its stator winding, the electrical damping of IMDU in the subsynchronous frequency range can be largely increased. In addition, the voltage-source inverter used to drive the IMDU has the extra advantage that it is capable of keeping the magnitude of the terminal voltage of IMDU during disturbance, so that the effectiveness of IMDU can be further guaranteed. By applying the active damping scheme, namely, adding a feedback path from the speed deviation of IMDU to the output voltage waveform of the voltage-source inverter, the electrical damping of IMDU can be further increased. By eigenvalue analysis, damping torque analysis and time-domain simulation with IEEE SSR first benchmark model, and time-domain simulation with IEEE SSR second benchmark model, the effectiveness of the three method of increasing electrical damping of IMDU is verified. With increased electrical damping, the required capacity of IMDU to stabilize the system can be reduced and the SSO will damp out in a faster rate.