Study On Regulating Mode And Control Strategy Of Controllable Reactor Of Transformer Type

Controllable reactor of transformer type(CRT) is a reactive power compensator applied to EHV transmission line, it plays an important role in stabilizing the bus voltage. The previous research shows that the performance of CRT varies considerably in different modes. In this thesis, the concept of multi-winding regulating mode of CRT is proposed, it contains the only 3 single-winding regulating modes discussed at present. Based on the multi-winding regulating mode, the following theoretical problems are discussed, such as the equivalent circuit of CRT, the method of directly generating each winding’s current waveform, the harmonic analysis in multi-winding regulating mode, the matching method of the currentlimiting inductance, the harmonic optimization of work winding current, the decoupling of control windings’ currents, the improvement of voltage control system. All the works are to provide theoretical guidance for improving the performance of CRT.(1) Based on the circuit equations expressed by self and mutual inductances of multiwindings transformer, the corresponding nodal voltage equations are derived, and then the equivalent circuit of CRT is established, which lays a foundation for the later simulation and verification.(2) Overcoming the modeling difficulty caused by the switching sequence in multiwinding regulating mode by arranging the windings’ serial numbers according to the switching sequence and based on the circuit equations expressed by self and mutual inductances, the instantaneous expression of each winding current at arbitrary period is obtained by means of subsection linearization, and then a simple method that can directly generate each winding’s current waveform is presented.(3) Using Fourier series, the function relationship in matrix form between Fourier coefficients of all the winding’s harmonic currents and the trigger angle vector is deduced, and then the process for calculating the RMS of each winding’s harmonic currents and the total current based on all the triggle angles are acquired.(4) Based on the definitions of regulating process and capacitance range, the simplified method for analysing the single-winding regulating mode of CRT is provided, and then a novel method for matching the current-limiting inductance of CRT at the fixed-single-branch mode is proposed, which is based on the principle of ensuring continuous output power and the minimum overlap among all the capacitance ranges.(5) The current harmonic injected into the power system by CRT is optimized in both the single-winding and multi-winding regulating mode. Specifically, in single-winding regulating mode, the optimization can be obtained by the method of enumeration, and in multi-winding regulating mode, the corresponding nonlinear optimal model is built firstly, and then the optimization can be obtained after the model is solved by calling the algorithm named fmincon which can find the minimum of constrained nonlinear multivariable function in MATLAB. The results show that multi-winding harmonic optimization regulating mode has a much better harmonic characteristic than the single-winding harmonic optimization regulation mode, the primary cause of which is that the harmonic current can be offset by each other in multi-winding regulating mode, hence, which has a more considerable effect than the traditional method based on harmonic dilution.(6) Based on the basic concept of multi-winding regulating mode, the decoupling regulating mode of CRT is achieved by coordinating all the triggered angles, thus, each winding’s current curve of CRT in which there is a close electromagnetic coupling among its control windings can imitate the current curve of split CRT which has little electromagnetic coupling among its control windings. This will decrease each control winding’s rated capacity of CRT considerably and increase its efficiency.(7) A common look-up table is designed for the linearization model of the CRT voltage control system discussed in the relevant references, and the novel regulating modes can be applied in the voltage control system. The simulation result shows that the novel regulating modes can not only stabilize the bus voltage, but also improve the performance of CRT.