The Key Technologies Of Distribution Transformer Integration STATCOM (DT-STATCOM)

With the rapid development of the power system and the modern power electronic technique, the STATCOM has been the hot spot of and power quality control. Recently there were two major trends of STATCOM technology:one is the high voltage and large power STATCOM technology and another is local compensation D-STATCOM technology. However, the high voltage and large power STATCOM must adopted multiple converters or chain structure to meet the high voltage level. That made the STATCOM structure complicated, expensive and low safty. Because of the low connect voltage, the D-STATCOM's capacity was limited and its compensation current became large. In order to fulfill the local compensation, the D-STATCOM was very dispersing. It led to the low utilization rate, lack co-operation, resource waste and harmonic pollution.Toward the characteristic and problems of STATCOM technology, this paper consider the intersection point of high voltage and low voltage is the most ideal, flexible, and effective location for reactive power compensation and power quality control. Many distribution transformers were set in this point and connect two different voltage power grids. And it's easy for hierarchically and partitioning controlling. Using the windings characteristic of transformer, an integration design of transformer and STATCOM could be put forward. And this integration technology can provide a new solution for reactive power compensation and power quality improvement of power system. So this paper proposed a new technology:it integrated the distribution transformer and the static var compensator (Distribution Transformer STATCOM, DT-STATCOM). This technology take full advantage of the surplus capacity of distribution transformer to compensate the reactive power; by setting the connect voltage flexible, it extended the switching devices selection, which benefit the economic and technical indicators; the integration structure reduce the volume and cost of compensation device. Revolve around the DT-STATCOM, this paper discuss about the basic theory and key technology.The DT-STATCOM use the surplus capacity of transformer to compensate reactive power and control the power quality. So it will change the current distribution of transformer winds. It is important to avoid the transformer overload. Toward that, this paper researched the mathematical relation of load ratio and DT-STATCOM capacity and realized the compensation under the normal operation. The DT-STATCOM need the connect taps to realize the integration structure. But the transformer had many winding structure, which made the integration design difficult. This paper researched the single group and multi-group connection taps integration structure based on different link group of transformer. On the same time, in order to suitable for the different DT-STATCOM structure and the tap voltage selection range, the paper analized and research the topology of compensation units. The result showed that the NPC structure was the most economical and practical topology. Besides, the DT-STATCOM connected with the transformer, it can reduce the volume of equipment and could take use of the leakage reactance as the parts of the output filter. So the paper analized the mechanism of LCL filter first, and then proposed the multi-restrictions design method. The simulations demonstrated the effectiveness. The paper also compared the highvoltage STATCOM, D-STATCOM and DT-STATCOM. The result showed that the DT-STATCOM had better technology and economy.The compensation instruction curren detection was the basis of reaction power compensation and power quality control. But because of the special structure of DT-STATCOM, the compensation current detection point and the compensation current injection point was located at different ports of the transformer. The two ports septum transformer's windings, so the voltage and current of the tow ports has deviation on amplitude and phasor. That led the traditional p-q and ip-iq detection method was not suitable. Toward that, the paper proposed three crossing-ports command current detection method. Based on the thdory derivation and the simulation, the paper analyzed the detection accuracy of different method. According to the analysis and the simulation, we defined the application of three crossing-ports detection methods. Use the detection method, the paper designed the control method and follow-up control of DT-STATCOM.The DT-STATCOM is a complicated nonlinear system, the AC output and the DC voltage has strong coupling relationship.The paper combined the characteristic of DT-STATCOM operation, adopted the feedforward decoupling strategy. It could realize the decoupling of inner current ring, which could simplified the design of the control system. The PWM pulses trigger strategy influence the compensation effect directly. The paper compared triangle carrier compare strategy, hysteresis comparison and deadbeat strategy. The result showed that the hysteresis comparison is the most suitable trigger strategy. In order to reduce the waveform of switching frequency, the paper optimized the hysteresis comparison method and realized the variable hysteresis quasi constant frequency comparison strategy. It improved the adverse effect of DT-STATCOM filter.The DT-STATCOM was acomplicated hybrid DC-ACA discrete system. And because of the special integrated structure of transformer and compensation units, the dynamic compensation and control strategy was different from the traditional equipment.Inorder to examine the DT-STATCOM proposal mentioned above, the paper built up a DT-STATCOM simulating model with simulink workbox in MATLAB. The main contents of the simulation model are as follows:establishion the DT-STATCOM simulation system based on multi-tap primary winding access mode and the three-level NPC based inverter circuit. Take advantage of mixed programme technology of simulation model and S function, the simulation model realized the real time sampling calculation and pulse trigger control. The simulation results showed that, the DT-STATCOM could compensation the reactive power quickly and control the power quality effectively.On the foundation of simulating research, a dynamic-simulating experiment system was built with multi-tap winding distribution transformer and DT-STATCOM module. With the experiment system, the paper carried out no-load loss test, reaction power compensation test, harmonic restrict test and three-phase umbalance compensation test. The experiment results verify the correction and validity of DT-STATCOM.