An optimized compensation strategy of DVR for micro-grid voltage sag
© The Author(s) 2016
Received: 12 May 2016
Accepted: 16 May 2016
Published: 2 July 2016
This paper uses a dynamic voltage restorer (DVR) to improve the voltage quality from voltage sags. It is difficult to satisfy various of compensation quality and time of the voltage sag by using single compensation method. Furthermore, high-power consumption of the phase jump compensation increases the size and cost of a dynamic voltage restorer (DVR).
Methods & Results
In order to improve the compensating efficiency of DVR, an optimized compensation strategy is proposed for voltage sag of micro-grid caused by interconnection and sensitive loads. The proposed compensation strategy increases the supporting time for long voltage sags.
Firstly, the power flow and the maximum compensation time of DVR are analyzed using three basic compensation strategies. Then, the phase jump is corrected by pre-sag compensation. And a quadratic transition curve, which involves the injected voltage phases of pre-sag strategy and minimum energy strategy, is used to transform pre-sag compensation to minimum energy compensation of DVR.
The transition utilizes the storage system to reduce the rate of discharge. As a result, the proposed strategy increases the supporting time for long voltage sags. The analytical study shows that the presented method significantly increases compensation time of DVR. The simulation results performed by MATLAB/SIMULINK also confirm the effectiveness of the proposed method.
Since micro grid can integrate various kinds of renewable energy resources, it has been widely investigated in recent years . However, there also exists some power quality problems that cannot be ignored, such as voltage sag, swell, and harmonic . Furthermore, micro grid contains a variety of powers, sensitive loads and non-linear power electronic devices. As a result, the frequent start-up and shutdown operation of renewable power generators will lead to voltage fluctuations, especially sag. The voltage sag can damage the power quality, may even cause huge economic losses . To improve and manage the power quality of micro grid, the flexible alternating current transmission technology (FACT) has been introduced, such as SVC, D - STATCOM, DVR and UPQC [4–6]. Among these devices, DVR is one of the most powerful apparatus that can protect voltage-sensitive equipment from voltage sag, swell and flicker. In some cases, it can also suppress the voltage harmonics and unbalanced three-phase voltage effectively .
DVR needs some power exchange when it is used in compensation. In order to reduce power exchange during the effective compensation of voltage drop, the compensation strategy of DVR should be optimized . The present research and application of DVR compensation strategies for voltage sag are pre-sag compensation, in-phase compensation and the minimum energy compensation [9–12]. In-phase compensation method can compensate the sagged voltage with minimum amplitude . The minimum energy method adjusts the injected active and reactive power of DVR to reduce consumption of the dc stored energy . However, both can’t correct the phase jump, even lead to trip of sensitive loads. The pre-sag compensation can fully restore the amplitude and phase angle of the sagged load voltage . Though pre-sag compensation can guarantee the continuity of grid voltage, the compensation of phase jump needs a large amount of active power in an energy storage system.
Taking advantage of three basic compensation strategies, some comprehensive compensation strategies [15, 16] were proposed. The comprehensive strategy of pre-sag and in-phase compensation  reduced the injected voltage while extending compensation time, but it sustainably used the energy of dc unit. The control that combined pre-sag method and minimal energy method  were lacked of the time control though it made good use of dc energy. To avoid these drawbacks, an optimized strategy based on the strategy of paper  is discussed in this paper. Firstly, the DVR compensating voltage and power flow for three basic strategies is analyzed and the maximum compensation time is derived from the analysis. Then, pre-sag strategy is used to correct phase jump at the beginning of compensating. According to a quadratic transition curve, the injected phase angle varied from pre-sag to the minimum energy compensation is rotated in multi-step. The rest process is completed by the minimum energy strategy. Owing to the introduction of the transition process, the compensation time of the comprehensive strategy is significantly increased. The performance of the proposed method is validated by using MATLAB/Simulink simulation.
Three basic compensation strategies
Minimum energy compensation
In an ideal system, the DVR dc power in Eq. (11) is equal to the ac power in Eq. (17), so the capacity of capacitor C can be obtained from the relation. However, the dc voltage will decrease with the flow of power, so as the output compensating voltage. Accordingly, the DVR output active power is also limited by the capacity of dc capacitor and the minimal dc voltage, which is sufficient for a proper restoration of the load voltage. In addition, the dc voltage gradient dU dc /dt is proportional to the DVR injected active power P DVR directly. The less P DVR is, the smaller the slope of dc voltage is. Hence, the compensating time is extended. It can be known that improving the rate of dc voltage drop can prolong the compensation time.
The maximum compensation time t max directly reflects the utilization level of stored energy in dc capacitor.
The optimized compensation strategy
The dc voltage gradient is controlled by adjusting the magnitude of DVR injected active power to extend the DVR compensating time. Firstly, pre-sag compensation is used to restore the amplitude and phase angle of sagged voltage. Then a transition process is introduced to shift pre-sag strategy to the minimum energy strategy gradually. The transition avoids direct jump of phase angle at running point. This process is usually implemented in one to two cycles, and the sagged voltage can be restored to stabilization in a short time. The compensating control process is described in detail as follows.
The pre-sag compensation
In order to reduce the voltage distortions at the load side, the sagged voltage should be restored to the normal value by the pre-sag compensation. After a certain time, the modulation index will increase as a result of discharging of the dc capacitor. If there is no sufficient energy to support the compensation, DVR will not be able to compensate properly. Therefore, as soon as a certain modulation index is reached, a smooth transition from pre-sag compensation to the minimum energy compensation is initiated to reduce consumption of energy.
The transition process
After the sagged voltage recovers to normal, pre-sag compensation will be smoothly transformed to the minimum energy compensation in one to two cycles.
The operation process
As mentioned above, the maximum compensation time plays an important role in reflecting the performance of DVR. It is directly connected to the amount of energy needed for the compensation and the needed capacitance. Based on the mentioned analytic equations, a comparative study is presented to compare the DVR maximum compensating time for different sags and jump phases by using the mentioned strategies (i.e. pre-sag strategy, in-phase strategy, and the proposed strategy.).
The sag depth is varied from 10 % to 80 % of the rated voltage. As shown in Fig. 4(a), the proposed method has the longest supporting time among three strategies at the same sag depth. Taking 50 % sag depth for example, the compensation time of the in-phase compensation is longer than that of the pre-sag compensation. The proposed method further increases the compensation time to 20 cycles based on in-phase compensation. When the sag depth is lower than 30 %, the proposed strategy works in the self-supporting mode. The minimize utilization of dc active power makes the slowest discharge of capacitor rate among three compensation methods, so the compensation time can be infinitely prolonged. Figure 4(b) depicts that the phase jump is varied from -60° to 60° for a 50 % sag depth when other conditions don’t change. The compensating time of in-phase compensation and the proposed compensation have no obvious fluctuation with the change of phase jump, while pre-sag compensation is opposite. The proposed strategy has the longest compensation time among three strategies.
According to Eq. (20), the voltage sag in grid is monitored by logical unit. Control system includes two sub-modules, one is the calculation of the injected initial angle and final angle of DVR, and another is the transition to the minimum energy compensation. The line current, which is acted as a reference, can be gotten from PLL to realize the decoupling control of active and reactive power. For the phase angle detection module, PLL is used to calculate the DVR initial angle of pre-sag compensation and the final angle of the minimum energy compensation. After the transition, the reference voltage is given by the minimum energy module, i.e. U L-abc * = U opt . The reference voltage U DVR * is generated by DVR. U DVR * is compared with the actual voltage U DVR in the stationary reference frame. The difference of U DVR and U DVR * is tracked by the PI controller accurately. A feed forward control with gain is added to compensate the loss of DVR system.
As shown in Fig. 6, the sag depth was 40 %, and the start time of sag was 0.1 s. The DVR injected voltage and power were shown in the last two graphs of Fig. 6. In the power graph, it was noticed that DVR injected a large number of active power to compensate the phase jump in first cycle of initial stage. After two cycles, pre-sag compensation was gradually shifted to the minimum energy compensation mode. Accordingly, the injected voltage made a smooth transition, which can be seen from the third graph. In the last process, the injected active power was gradually reduced and became steady. After the recovery by DVR, the amplitude and phase of load voltage returned to normal.
To manage the power quality in a micro grid, an optimized compensation strategy of DVR has been proposed based on three basic strategies in the paper. The proposed strategy protects sensitive loads against the grid voltage sags accompanied with the phase jump. At the beginning of voltage sag, pre-sag compensation is adopted to detect and compensate phase jump. The full compensation of voltage amplitude and phase is achieved. After two cycles, pre-sag compensation is shifted to the minimum energy compensation to reduce the consumption of DVR active power. To mitigate the influence of phase jump on sensitive loads, the voltage phase is converted smoothly according to a quadratic transition curve. Although the presented method has good compensation effects to three-phase symmetrical voltage sags with different characteristics, a more accurate mathematical model needs to be established for unbalanced three-phase voltage drop. Furthermore, the compensation time of DVR under pre-sag compensation can be significantly increased when applying the proposed optimized strategy. In practical application, the proposed method can meet the efficient and accurate requirements of DVR.
This work is supported by National Nature Science Foundation under Grant 51477070, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
ZML proposed the optimized compensation strategy of DVR for voltage sag, carried out a quadratic transition which involves the injected voltage phases of pre-sag strategy and minimum energy strategy, finished the related calculation of voltage amplitude and phase. WWL carried out the DVR compensating strategy studies, derived the maximum compensation time of DVR by control requirements, participated in the comparison of the maximum compensation time for different methods and drafted the manuscript. THP established the model and carried out the simulation study of the proposed compensation of DVR, gave the voltage and power change during the compensation, made the summary. All authors read and approved the final manuscript.
About the Authors
W.W. Li, received her B.S degree from Jiangsu University in 2013. Now she has been a postgraduate student in School of Electrical and Information Engineering, Jiangsu University, Zhenjiang, China. Her current research interests include micro-grid, power electronic devices.
Z. M. Li, received his B.S. degree from zhenjiang agricultural machinery institute and received his M.S degree from Xi’an Jiao Tong University in 1987. Now he has been a professor in School of Electrical and Information Engineering, Jiangsu University, Zhenjiang, China. His current research interests include industrial computer network, process control, power system remote monitoring etc. He published over 100 papers, more than 20 articles were included by SCI and EI. He obtained second prize for technological progress of the original ministry of machine and the second prize of scientific and technological progress of Jiangsu province, and outstanding teaching achievement prizes of Jiangsu province.
T.H. Pan, received his B.S. degree from Anhui Agriculture University and M.S. degree from Gansu University of Technology in 1997 and 2000 respectively. And he received his Ph.D. degree in control theory and control engineering from Shanghai Jiao Tong University in 2007. Now he has been a professor in School of Electrical & Information Engineering, Jiangsu University, Zhenjiang, China. His current research interests include multiple model approach and its application, machine learning, virtual metrology, predictive control and Run-to-Run control theory and practice, etc.
The authors declare that they have no competing interests.
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