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Abstract
There are two compensation methods during distribution system currently: high voltage centralized compensation and low voltage user end distributed compensation. Towards the problems of the two compensation method, this paper proposes a technology of integrating distribution transformer and static var compensation(DT-STATCOM). The technology takes the distribution transformer of box substation as the control node of reaction power and power quality. It uses the surplus capacity of transformer to compensate the reactive power. And by setting the connect voltage reasonably, it decreases the withstand voltage level and current capacity of switching devices, which benefit the economic and technical indicators. In addition, the integration structure reduces the volume and cost of compensation device. Meanwhile, towards the command current detection problem caused by the special structure of DT-STATCOM, this paper proposes a novel reactive current detection method across the transformer's ports. Then the paper designs the control system based on the multi-lateral information. At last, the simulation and prototype test show that, the DT-STATCOM system had significant effect on dynamic reactive power compensation, transformer voltage support and imbalance inhibition.
New types of energy-saving transformers have been applied in many Chinese corporations, under this background, it’s essential to study energy conservation and reconstruction methods and techniques of distribution transformers for individual line enterprises. A brief review was made on domestic and abroad energy conservation and reconstruction methods and techniques, including transformer material optimization, life-cycle management, structure modification, condition-based maintenance, reasonable capacity selection, economic operation and reactive power compensation devices application, etc. Moreover, the economic switch method of transformer operation modes is given by a case study, and compared with conventional means. Corresponding operation losses are made in contrast, which shows large energy conservation potential exists in conventional operation modes. Another actual case is studied on transformer energy efficiency diagnosis via reactive power compensation. Finally, weak links in current transformer energy conservation and reconstruction methods are discussed. It’s concluded that new-type energy-saving transformers should be promoted vigorously in Chinese enterprises. Hence, it can fully dig energy conservation potentials and improve productive benefits for transformers, as well as give certain reference and guidance for the management departments in aspect of energy conservation and reconstruction.
This paper proposes a kind of method of three-phase voltage phase locked loop under three-phase unbalanced voltage. When the positive dq synchronous reference frame transforms, the dq axis voltage component will contain AC component caused by negative sequence voltage component which leads to the error of the three-phase voltage phase locked loop (PLL) based on the dq synchronous reference frame transformation. After the dq axis DC voltage component extracted from negative sequence dq synchronous reference frame transformation compensates positive sequence dq transformation synchronous reference frame q axis voltage component, the positive sequence voltage phase can be locked accurately and quickly. By the way of the negative sequence dq synchronous reference frame d axis and q axis DC voltage component detecting negative sequence voltage initial phase angle, the negative sequence voltage phase can be locked. The simulation results show that, this method can achieve the purpose that three-phase power supply frequency, positive sequence voltage phase and the negative sequence voltage phase can be fast and accurately locked in case of unbalanced three-phase voltage.
In order to improve the power factor, reduce line loss, stabilize user terminal voltage in low voltage power distribution system, this paper proposes a new method to realize VAR adjustment, in which a fast dynamic multi DSTATCOM static VAR compensation and TSC VAR compensation are combined, and DSTATCOM adjusts VAR by changing voltage and frequency between the fixed capacitor capacity. The method can realize stepless speed dynamic compensation, TSC can realize step VAR compensation, and the combination of the two achieves the stepless adjustment of high-capacity VAR compensation. The control principle and control strategy of the system are analyzed. Two-stage VAR compensation system has the characteristics of small volume, low cost, fast dynamic response, etc. Experiments show that the method is reliable and practical.
Control delay significantly affects the performance of distribution static synchronous compensator (D-STATCOM) in harmonic suppression and reactive power compensation. To overcome the drawback of the conventional control method, which has long compensation delay and low compensation accuracy, a novel control strategy based on software optimization is proposed. Software flow is adjusted based on the analysis of the reference compensation currents. It shortens the calculation delay of harmonics and reactive power compensation reference current, so compensation delay occurs less and compensation accuracy of harmonic suppression and reactive power compensation is improved. Results obtained by D-STATCOM simulations system built in Matlab/Simulink and prototype experiment of a 10 kVA three-phase four-wire D-STATCOM show the validity and feasibility of the proposed strategy.
The request of power quality is more and more strict in our country these years, and the reactive power compensation and harmonic control problems are the key factors to improve the power quality. STATCOM is the most advanced technology in middle and low voltage reactive power compensation and harmonic control fields now, and has been widely used. Now due to the dependence of the stability in power system, multiple transformers were using in most mining substation with parallel operation mode. But because of the capability of single STATCOM was very limited, Multiple STATCOM devices were used on the secondary side of parallel transformers. Affected by the load's unbalance, there were some energy flowing between the transformers. The flowing energy will cause the current's circulation and oscillation among the devices, even cause the devices outputting current radiating and make the system worse, and it's a new challenge for the reactive power compensation and harmonic control in the parallel transformers' operation. A new control strategy of improved distributed interconnection based on new sampling method in hardware was proposed, avoids the problems of the current's circulation among devices and the current's divergence effectively, it has a important practical significance for the reactive power compensation and harmonic control in the parallel operation of multiple transformers.
As nonlinear loads largely increases and much more power electronic devices being used, harmonics and active component increases a lot in the grounding fault current. After compensating the reactive current, residual current is enough to maintain the arc, which may cause overvoltage and fault continue to develop. This paper proposes an electromagnetic hybrid petersen coil (EHPC) based on magnetically controllable reactor (MCR) and active power compensator (APC), expounds it's two operating statuses and principle, analyses the factors of influence of frequency scanning to ground capacitor and proposes an improving method. We also discuss the relationship of neutral displacement voltage, fault phase voltage, zero sequence current and grounding fault current, put forward a decoupling compensating and flexible control strategy pointing at the active, reactive and harmonic component of grounding fault current. Simulations and experiments verify that grounding fault current can be fully compensated fast and effectively, meeting both the requirement of current and voltage, suggesting the correctness and feasibility of the principle and the control strategy.
The paper deals with the economic operation of the power transformer, which is of a great significance for analyzing the technical economic area of the network considering the selection of the transformer and the line. Taking into account of the cost of the transformer, the charge of the loss and the maintenance, the transformer's technical economic loading rate is deduced, which is higher than the traditional rate obtained from that the transformer's copper loss equals to the iron loss. The mathematic representation of the loading rate is given, but the load fluctuates in a certain rang, finding out the economic area of the transformer's loading rate has more useful meaning to verity the transformer's operation, then the technical economic area is presented. The technical economic area of every line is obtained according to the economic current density of the line and the Kirchhoff current law. According to the former, the optimal economic area and line loss rate area is calculated, which can be used to estimate whether the line and the transformer are matching each other. It is useful for the loss reduction and the network evaluation. Finally, a line with 10 wires and 6 transformers is given, which is evaluated using the forming theory.
The purpose of optimal reactive power planning is to determine the optimal sizing and location of VAR sources, so that the security and the economic operation of power system at different load levels and operation conditions can be maintained. Considering right, normal, heavy load operation conditions and fault conditions, the mathematical model is established in which the sum of yearly active power loss fee and minimized investment in new addition of VAR equipment divided into each year is used as the objective function. A kind of optimal method for reactive power compensation planning in radial distribution system considering fault condition giving attention to both security and static voltage stability constraints is presented in this paper. Various operating conditions are considered so that the result is precise and the algorithm is simple and can be easily realized. The calculation of reactive power compensation and regulating transformer rate are done with the backward and forward sweep adapting to the radial distribution system. The result of example systems proves that the method is feasible in technique and has obvious economic benefits.
The 50 MVA STATCOM to be installed at Shanghai Xijiao substation employs chain-circuit inverter based on IGCTs. For strict type tests, it's necessary to do equivalent rated-state operational tests and overcurrent protection tests for each link. The so called back-to-back test method is employed for the equivalent rated-state operational tests, two Links are connected back-to-back via an inductor, one is controlled to operate in leading mode and another in lagging mode, power losses are provided by a DC power supply connected to one of the dc capacitors. The equivalent overcurrent protection test is carried out through a inductive load. The parameters of the links, the principle and the results of equivalent tests are given. Tests results confirmed that all performance requirements were met.
The phase lock loop detection error and the time delay often occur in the digital control circuit for harmonics detection. To mitigate the influences of the two problems, the performance of the harmonics detection method without phase lock loop was discussed, and on the basis of the universal instantaneous power theory, the effect of the time delay on the reference compensation current signal was investigated. Then an improved synchronous reference frame method for harmonics and reactive currents detection was proposed, in which there is no phase lock loop and took the time delay into account. The phase lock loop detection error was avoided by set the rotation speed of the synchronous reference frame in advance and the time delay was compensated by importing the compensatory angle in the inverse transform. This method is easy to be implemented for digital control application and is not affected by the distorted source voltage. The simulation results have also shown its accuracy and feasibility.
The region of attraction of one-machine infinite-bus power system with STATCOM is studied. The mathematic model of the system is given and the energy function of the system is derived with first integral based on the model. The conservation of the model to reflect damping effect is discussed. To tide over the conservation, a damping-reflected energy function is derived. Based on the energy function, the region of attraction of the system is estimated and simulation result is given. The simulation results show the changing tendency of the stable region of the system as the equivalent var current of STATCOM increases, which indicates STATCOM is beneficial for stable operation of the system and efficiently expand the asymptotic stable region (region of attraction) of the system.
The detection of compensator current has important influence on the compensation performance of DSTATCOM when it is used for load compensation. A detection method based on synchronous reference frame is proposed. A single-phase voltage is derived and used to form virtual symmetrical three-phase system. The d axis of the synchronous reference frame is always synchronous with the voltage vector in virtual symmetrical three-phase system. The load current is projected on voltage vector. The component of d axis of the load current, which synchronous with the voltage vector is feed into low pass filter and the compensation current is derived. Compared with traditionally detection method based on synchronous transformation the detection method proposed in this paper need simple calculation and can detect compensation current accurately under unbalance voltage conditions. The principle of the detection method is interpreted by power balance theory. The feasibility and effectiveness of the detection method is verified by theoretic analysis and simulation results.
A control scheme of the advanced static var generator (ASVG) is determined. First, the instantaneous reactive-power theory is analyzed, Then, the closed-loop indirect current control strategy is introduced. Based on the instantaneous reactive-power theory, the reactive current is detected in time, and this method increases the speed of systematic response. On configuration of system, the converter's power module based on IGBT is adopted in main circuit; and a TMS320LF2407 is adopted as the main controller to deal with sampling, calculating and controlling on-line, its performance satisfies the request of the speedy on-line control and heightens the precision of PWM-pulse. Finally, a simulation is done through the software of MATLAB. The ASVG is tested as the most promising technique for the current control.
Under unbalanced voltage of power system, the influence on voltage output performance of distribution network static synchronous compensator (D-STATCOM) is analyzed. Two times frequency wave occurs in voltage of capacitor at DC side of D-STATCOM under unbalanced voltage, which result in negative sequence and 3 times harmonics in AC voltage and converter may be overcurrent caused by negative sequence current and 3 times harmonic current. According the problem above, improved switching function modulation is proposed to improve voltage output performance of D-STATCOM and suppress 3 times harmonic current. An unbalanced control strategy of feed-forward control for negative sequence voltage is also carried out to suppress converter overcurrent caused by negative voltage in order to make D-STATCOM safe. Simulation results show the good performance.
A mixed integer dynamic reactive power optimization model, which adopts the entire system energy loss in a day as the objective function and considers the constraint of the permissible number of equipment switching, is presented. A plant growth simulation algorithm (PGSA) is used to optimize the objective function from both time and space synchronously, which avoids separating the optimization problem into two sub-problems of switching time optimization and capacity optimization. The constraints are smartly handled for improving the efficient of calculation. The proposed method is applied to IEEE 69 bus sample system and the optimal switching scheme of capacitors in a day is gained. The effect of different maximum switching number constraints on dynamic reactive power optimization results is also analyzed. Furthermore, the result based on the proposed method is compared with that based on the static optimization method. The obtained results indicate the correctness and effectiveness of the proposed method.
Reactive power control through static var generator (SVG) has been gaining wide attentions due to its outstanding performance. But for a SVG with traditional control strategies, unbalanced utility voltages have greatly affect the performance of the SVG and in severe case may even cause shut-down of the SVG for overcurrent protection. This paper proposes a voltage controlled voltage source (VCVS) strategy to assure normal operation of SVG with series-connected IGBTs when severe system voltage unbalance happens, and furthermore to mitigate voltage unbalance at the point of common coupling (PCC) whether it is caused by utility or load. The control law, operation principle, compensation characteristics and parameter optimization of the proposed control strategy are analyzed in detail. Based on the VCVS control, a modified VCVS strategy is proposed and the difference between them is presented. Finally, the simulation and experimental investigations were carried out respectively for resistive and rectifier load conditions. The results verified that the current passes through the SVG is under control and the voltage at PCC is much more balanced than that before compensation. The compensation performance of the SVG is thus proved satisfactory
This paper provides an adaptive filter for synchronous detection and extraction of harmonics. The filter can be used as integral part of the control system of a power electronic apparatus (e.g., STATCOM, APF, and UPFC) to generate the desired control signals. Stability and convergence analyses of the adaptive filter are presented based on the dynamical systems theory. Performance of the filter is verified as a means for reference signal generation in a shunt active power filter.