Research On Controlled Switching Of No-load Transformers

With the rapid development of China's economic power, the power system network increases, so that the number of transformers in use gradually increased, and their operations become more and more frequent also. The no-load closing as one of the most common operations to transformer, its transient processes will bring more and more serious influences to power system with the number of operations increases.Transformers are electrical equipment that based on the principle of electromagnetic induction, therefore necessary to take some of the exciting current to create a magnetic field in the transformer windings, and thus go into electromagnetic energy conversion. During normal operation, the transformer is running at the linear section of magnetization curve, so the magnetic flux is not saturated, and excitation current is small. But when no-load closing using circuit breaker, as the influence of the core residual flux and the uncertainty of closing phase angle, it is easy to take the magnetic flux into the saturated zone, results in inrush current.Because the amplitude of inrush current is large and the harmonic content is rich, it may lead to protective relay malfunction, and cause a huge impact to the power system, causing over-voltage and damaging the equipment insulation. In addition, the inrush current waveforms have dead angle and are rich in high-order harmonics (mainly the second and third harmonic) and the DC component, it is not only pollute the electricity network, but also reduces the power quality and equipment insulation.Improvement of living standards promote people growing emphasis on high quality,stable and reliable power supply. For inrush current caused various hazards to the power system and for many traditional methods all have their own application defects, scholars have been seeking the best solution. Controlled switching technology has been raised at the 70s of last century; it is a control strategy that considering transformer remanence affects. Its essence is measured transformer core remanence firstly, and then with the moment that the prospective flux equals to remanence as a special phase, and then control the high-voltage circuit breaker contacts closing at this particular phase of the reference signal (voltage signal),to reduce the transient process when no-load transformer put into power system,and eliminate inrush current.This paper studied the transient process of transformer no-load closing and the mechanism of inrush current, explained the principle of controlled switching strategy when considering transformer core remanence.And used ATP/EMTP to establish the system model of transformer no-load closing,realized the dynamic co-simulation and verification with Matlab.Finally constructed a hardware system based on DSP theory,be used to achieve the remanence measurements and the calculation of the best closing time.The main work of this paper were showed as follows:1,Detailed studied of the inrush current generation mechanism,size and hazards, changes in the core flux in the transient process of transformer no-load closing, the principle and application of controlled switching strategy.2,Established the transformer no-load closing model in ATP/EMTP which used the datas that got by transformer no-load test and short-circuit test,mainly studied the convertion of magnetization curve and simulation results.3,After completed the ATP/EMTP system model,achieved the dynamic co-simulation with Matlab.And used of Matlab M-function implemented parameter changes to the ATP file,achieved on flexibility,diversity control the ATP/EMTP program.4,Based on the transient process of breaking no-load transformer with circuit breaker, studied the principle of remanence measurement and the influence of Transient Recovery Voltage(TRV) to remanence measurement.Studied the impact of the remanence measurement error to inrush current and controlled switching technology,the impact of circuit breaker operating time dispersion to inrush current.5,Based on TMS320F2812 constructed a minimum DSP system,included the design of external circuit. Finally, realized the remanence measurements and the calculation of the best closing time.