| Unified Power Quality Conditioner (UPQC) is a kind of combined device for adjusting power quality. Based on power electronic technology, it can not only compensate distorted waveform ,but also restore dynamic voltage and implement flexible power supply. And it is one of the most important branches of customer power technology(Cuspow).It changes the traditional ways that compensating voltage waveform and current waveform respectively, and has capacity of simultaneously compensating both of them, which satisfying the compensating requires of current and voltage and realizing a great variety of function, so it must have a prospective future and characteristics attracting the eye ball of the researchers. Aiming at three-phase four-wire UPQC system, this paper works in the five areas as listed below.First of all,topology circuit of three-phase four-wire UPQC was carried on and its work principle ,function as well as compensating purpose was introduced under the condition that unbalance or distortion is included in source voltage. The power characteristic was then analyzed as well as the expressions of active power and reactive power were derived, based on which the power flow diagram gotten. On the purpose of obtaining the linearization model of UPQC,the Low Frequency model in the three phase stationary and d-q-0 coordinates as well as small-signal model in d-q-0 coordinates were deduced, and the linearization expresses of voltage and current of UPQC were deveried. Based on that, the linear relationship between source active current and voltage on DC side is discussed, the related expression is deduced at the same time, which supplying the theory gist for the dc voltage control strategy discussed subsequently.Secondly, the method of extracting of compensation component when supply voltage or load current is unbalance and distort was discussed. Correct extraction of compensation component is the prior condition for UPQC to realize its function. Two methods adopting in this thesis was studied in detail. One was based on synchronous coordinate transform- ation, namely the measure method in d-q-0 coordinate, the conventional measure methods on which can not accurately get the value of compensation signals when supply voltage was unbalance and/or distort .After analyzing the reasons of nonpreciseness, a new sort of method having no use for conventional PLL to get the phase angle needed in the transform was put forward. Based on that, an improved and practical method of extraction of compen- sation signals in d-q-0 coordinate was then presented, which worked well even when supply voltage was unbalance and/or distort. The other method was based on p-q-r theory, a new coordinate transformation theory it was presented recent years. Although it had many advantages, its transformation is so sophisticate that it was only used to the measuring compensation component according the available reference, the control strategy on p-q-r axle can not achieve. By promoting and improving this theory, complexity of the transform- ation decreased, so the improved p-q-r theory can be used for actual control strategy, which brings independent control at p-q-r axis into effect. The simulation result shows that the two improved actual method can abstract the compensation component precisely.And then,two aspect of coordinated-control of series-parallel compensator were discussed systematically. The cooperation of function of series-parallel compensator was introduced, with the two usual control strategy as example,meanwhile,the reasons of the interaction between series and parallel compensator on account of external electric connection were analyzed in detail, based on that, basic undertakings to cancel the interaction were presented. Simulation result showed that, this method could increase control accuracy and decrease steady-state error. The voltage on the dc side might fluctuate when source voltage and load current are unbalanced and distort, which in turn effect on the inverters of the series and parallel compensator respectivly.It was concluded that, under this condition, voltage fluctuation of the two capacitance on the dc side is inevitable. Two ways were brought into effect to assure the equivalence of the output wave of the two invert when dc voltage fluctuating and stabilization. One was that the modulation wave in SPWM was multiplied by a regulation factor, the other is adjusting the switch time in particularity switching cycle according to the impulse equal theory, so that the effect of voltage fluctuation on dc side can be eliminated.Next, the indirect control strategy of three-phase wire UPQC in d-q-0 coordinates was discussed in detail. The theory of indirect control strategy was firstly introduced and a new method to design the voltage conditioner on dc side was then presented. After that, the limitations of the conventional voltage control scheme for series compensator as well as current control scheme for parallel compensator were discussed. In order to eliminate the interaction existing in series and parallel side, a two-closed-loop with voltage feedforward control scheme was presented for series compensator and a load voltage feedforward with state discoupling control strategy for shunt compensator. When source voltage or load current are unbalance or distort, ordinary DC proportional-intergral controller on the d-q-0 axis can not eliminate steady-state error, the reason of that was analized.For the purpose to solve this question, enhance the control accuracy and achieve zero steady-state error control, an AC equivalent proportional-intergral controller in d-q-0 axis, referring to the zero steady-state error regulator of AC signal in stationary coordinates, was proposed. Its principle of eliminating steady-state error to zero was analyzed and a new control strategy by using of it was presented under the condition that system voltage and/or load current are unbalance and/or distort. Then the simulation and experiment waveforms of different voltage and current under different electric network and loading condition were gave out. The simulation and experiment result shows that UPQC efficiently carry out its compensation function while adopting the proposed control strategy. Finally, the direct control strategy of three-phase four-wire UPQC in p-q-r coordinates was discussed in detail. The principle of direct control strategy was firstly introduced. The control equation of current and voltage of series and shunt compensator were deduced on the p-q-r branches as well as the process to calculate the regulator parameter was discussed. Considering the unbalance or distortion of source voltage and load current, for the sake of eliminating voltage fluctuation on dc side, a direct current voltage regulation coefficient was added into the adopting control strategy, meanwhile, in order to eliminate the effect of shunt compensator on series compensator, a series compensation voltage feedforward control was joined into the control strategy on series side. In order to cancel the effect of series compensator on parallel compensator, load current feedforward control was led into the control strategy on parallel side. In addition, the equivalent proportional-intergral controller was added into current and voltage loop of control strategy of shunt compensator for the sake of decreasing steady-state error, the schematic diagram of control system for shunt and series compensator were gave out as well. Dynamic response of source current and load voltage were discussed on the condition that source voltage and load both or one of them changed. Finally, simulation and experiment waveform of voltage and current under different electric network and loading condition were given out. The simulation and experiment result manifested that, adopting forementioned control strategy; UPQC can ensure the THD of its output voltage and input current less than 5%, input power factor in proximity to 1 and with good dynamic response.
|
PDF Full Text Request