| With the rapid development of Chinese national economy, there are more and more industrial and civil electrical devices in distribution power net. These loads, which have the electric characteristics of low power factor, nonlinear, asymmetry or impact, bring so much power quality problems as low power factor, voltage fluctuation and flicker, three phase voltage unbalance, high-order harmonic pollution and etc. AC EAFs and rolling mills are the most typical ones of the loads.In all kinds of electrical devices that can solve well these power quality problems synthetically, static var compensator (SVC) of the type TCR+FC is one of them, whose ratio of cost and performance has very high correspondingly. It can track the loads' running change automatically; adjust the TCR's capacity output by controlling the thyristors' passing angle continuously. It can have good effects in the terms of improving the loads' power factor, stabilizing and balancing voltage, decreasing the high-order harmonic currents which flow to the net. In west countries, SVC has already been one kind of products of ripe technology. In China, there are few providers. But their technology has still had biggish difference comparing with the west countries'.The author undertakes the research and development work on SVC of the type TCR+FC for industrial loads in SUNTEN Co., Ltd. That is introduced in details here such SVC's engineering design, including the calculation of SVC's dynamic compensation power and passive filters' parameters design, the thyristor valves' parameters and structure design, triggering and cooling measure, the research on the control strategies based on instantaneous reactive power theory, the realization of the control hardware and the results of the tests and simulation based on PSCAD.AC EAF and rolling mills' running will bring very big reactive power impact and voltage flicker to the power net. SVC can resolve this problem. Its dynamic compensation power depends mainly on the maximum reactive power fluctuation and SVC's responding speed. SVC will have no effects on restraining the voltage flicker if its compensation ratio is 100% and its response time is 20 ms. And when the SVC's response time is over 20 ms, higher the compensation ratio is, worse voltage flicker is. This paper analyses the two loads' electric characteristics and provides the calculation methods of SVC's power separately. And there are two actual examples.Three usual types of the passive filters in SVC are introduced in details in this paper, which are single tuning type, 2-rank highpass type and C-type highpass type. The single tuning filter has a narrow passing frequency band. Its filtering effect is good, its waste is low and its tuning is easy. Its use is very wide. 2-rank highpass filter has a broad passing frequency band, but its waste is higher than the single tuning filter. It is used to filter higher order harmonic currents. C-type highpass filter is new in terms of the structure; it is only used in the SVC imported in Chinese. Its filtering effect is not as good as the 2-rank highpass filter, but its waste is lower under the basic frequency. It is used to filter 2nd and 3rd order harmonic currents when there is a continuous spectrum. It is needed to see the different order filters as a whole to design. And the filter capacitors should pass the three kinds of safety performance checking: overload checking, overcurrent checking and overvoltage checking.Thyristor valves are very pivotal and special primary devices in SVC. Several thyristors are linked in series to achieve the rated voltage that is demanded in TCR because the rated voltage of the TCR used in distribution network is higher than the single thyristor. Averaging voltage is needed to ensure every thyristor having the same running condition, that is to say, the voltage across every thyristor is same. The steady averaging voltage measure is to link the same resistor in parallel with every thyristor. The dynamic averaging voltage measure is to link the same resistor-capacitor in parallel with every thyristor. Reliable trigger is necessary for SVC's intelligent running. The driving circuit of an electric-triggered thyristor valve can realize every thyristor's trigger and state signals feedback. It is composed of two parts: trigger unit and every thyristor's gatetrode driving unit. The trigger unit receives the optical trigger signals and changes them to the electric signals. Gatetrode driving unit achieve the trigger electric signal through the pulse transformer's electromagnetic coupling and trigger every thyristor. At the same time, LBDs in the unit show every thyristor's state. When the thyristor valve is under an overvoltage or some thyristor does not-pass, the BODs will force it to trigger. Comparing Forced air cooling and hot pipe cooling, forced water cooling is better to ensure the thyristor valves' effective cooling and its long time running safety and reliability. The closed cycle water cooling system's structure and working principle are introduced in details here.It is needed to adopt the open loop control way and the closed loop control way in SVC. The former can bring the SVC a faster response speed and the latter can guarantee a steadier operation to the SVC. The theory base of the unbalance compensation is C.P. Steinmetz balance principle: The SVC is composed of positive and negative sequence parts. The former has balanceable three phase conductance, it produces positive sequence currents under the positive sequence voltage, which is used to counteract theloads' positive sequence reactive current. The latter has unbalanced three phase conductance, it produces negative sequence currents under the positive sequence voltage, which is used to counteract the loads' negative sequence current. Fast and exact detecting the positive and the negative parts of the loads' current or power is the key to increase the SVC open control accuracy and speediness. This paper introduces several open loop control algorithm based on instantaneous reactive power theory. And the simulation results show that they are practical. The signals detected have harmonic components because the loads' current has harmonic components. So it is very important that digital filter is fast and good. Fourier algorithm of full wave and half wave slip window and digital low pass filter are researched here. The same open loop control algorithm can be adopted in balance compensation. Another kind of instantaneous reactive power can be adopted too, which is based on the general definition of three phase instantaneous quantity. The closed loop control way is the PI adjustment, which is conventional but effective. From the simulation results, we can see that the control system's output will oscillate if the PI parameters are not adopted properly.Referring to the practice of the famous SVC's supplier, we adopt SMADYN-D digital controller and digital single phase trigger pulse generators as the SVC's control hardware. Their reliability is very high even in bad industrial environment.In order to check the operation effects of the SVC's control system, we have simulated in PSCAD and test the 380 V simulated circuit. The results of the simulation and the tests are nearly same: firstly, the accuracy of the SVC's closed loop steady control is high; secondly, the response speeds are fast under the open and close loop control; thirdly, the unbalance compensation accuracy is not so good in the influence of the low quality factor of the TCR reactor.The control system is the most characteristic part in the SVCs which are made by different providers. According to the results of our simulation and tests, it is showed that: Comparing with the control hardware based on singlechip or simple DSP technology and the control algorithm based on the electric parameters' conventional definition adopted by some domestic providers, the control strategies based on the instantaneous reactive power theory and the control hardware based on SMADYN-D controller adopted by us are the advanced technology. It is believed that the SVC based on this technology will be very good market worth.
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