贾变——英文文献

兰州交通大学毕业设计英文翻译（英文）毕业设计英文翻译（原文） 专 业 电气工程及其自动化 姓 名 贾 变 学 号 200909025 指导教师 赵 峰 李 红 Analysis and Comparison on the Control Strategies of MultipleVoltage Source Converters in Autonomous MicrogridYang Wang, Zongxiang Lu, Yong MinDepartment of electric engineering, Tsinghua University, Beijing 100084, China AbstractDifferent from traditional utility grid, a microgrid contains a lot of VSC (voltage sourceconverter)interfaced distributed generators which has more controlled variables than the common used synchronous generator. VSC can be operated in active power - reactive power (PQ) mode, active power - voltage (PV) mode, voltage - frequency mode (Vf), etc. The control strategy selection has large impact on the microgrid system performance. This paper analyzes and compares two typical control schemes (PQ and Vf) of the VSCs. A single bus microgrid contains four VSCs was taken as the example. The dynamic performance and load sharing methods of the two control scheme were discussed from three scenarios: (1) all the VSCs operated in Vf mode, (2) one VSC operated in Vf mode and the other ones in PQ mode, (3) a group of VSCs operated in Vf mode and another group in PQ mode. The performances of the system are simulated in PSCAD software.The difference between the control strategies can be obtained from the simulation results and the effect of the control modes on frequency variation were summarized. Keywords: microgrid, autonomous operation, voltage source converter, control strategies.1 Introduction Distributed generation (DG) is emerging as a complementary infrastructure to the traditional central power plants. DG encompasses a wide range of prime-mover technologies, such as internal combustion engines, gas turbines, wind power, micro turbines, photovoltaics and fuel cells 1. DG interconnected to distribution systems can form a new type of power system, the so-called microgrid. A microgrid is defined as a portion of a power system, usually a distribution subsystem, which includes one or more DG units capable of supplying the subsystem, independent of the main utility grid 4. The microgrid paradigm can provide considerable control flexibility to realize reliable and economically operation: it can be connected with the utility grid, or it can be isolated from the utility grid in case of disturbances or faults. Many DGs adopt VSC as the interface to the microgrid. Different form the traditional synchronous generator, the control strategies of the VSC are more flexible such as active reactive (PQ) control, active voltage (PV) control and voltage frequency (Vf) control 2. In autonomous operation, a key issue is how to coordinate the control of the parallel VSCs so as to achieve high performance of power and voltage regulations in the microgrid. Previous research has been given to power sharing management in parallel VSC operations 357. Similar to traditional power system, the concept of droop characteristics are used in autonomous microgrid to share the load so as to ensure stable operation. For an autonomous microgrid contains synchronous generator which decides the voltage and frequency of the system, the VSCs can operate in active - reactive power (PQ) control to share the load by droop lines 3. For an autonomous microgrid only contains VSCs, voltage - frequency (Vf) control should be used to regulate system voltage and frequency 5. For a VSC-based microgrid, due to planned or accidental islanding, the microgrid will transit from grid-connected mode to autonomous mode. In this situation, one or more VSCs should be switched to Vf mode. The other VSCs can still be operated in PQ mode. This paper focuses on the impact of different control strategies on the system frequency dynamics and load sharing from grid-connection to islanding. Two typical control strategies (PQ and Vf) were considered and three scenarios were discussed: (1) All the VSCs operated in Vf mode, (2) One VSC operated in Vf mode and the others in PQ mode, (3) A group of VSCs operated in Vf mode and another group of VSCs in PQ mode. The system dynamic performances were simulated by PSCAD software. This paper is divided into five major sections as follows. Section II introduces the Vf and PQ control strategy of a single VSC in details. Section III briefly describes the microgrid study system. Section IV addresses three scenarios of VSCs assigned with different control strategies to ensure stable operation of the autonomous microgrid. Then the simulation results in PSCAD are given out. The conclusions of the results are obtained in section V. 2 Control strategies for single VSC Different from traditional power system, a microgrid contains a lot of VSC (voltage source converter) interfaced distributed generators. The typical structure of VSC consists of a DC link and power electronics converter 6. The VSC connected to the power grid via the equivalent resistance and inductance is shown in Fig.1. Fig. 1. Typical diagram of a voltage source converterActive - reactive power (PQ) control and voltage - frequency (Vf) control are the two typical control strategies for VSCs in microgrid. When the microgrid is operated in grid-connected mode, the VSCs are usually operated in PQ mode. The references of active and reactive power of each VSC can be commanded by microgrid central controller or by MPPT control strategy. When the microgrid transits to autonomous operation, the system will become unstable if all the VSCs continue to operate in PQ control mode. In order to establish system frequency/voltage and share the load among parallel VSCs, one or more VSC should be switched to Vf mode and other VSCs can still be in PQ mode.2.1 PQ control strategy The VSC utilizes independent active - reactive power control strategies to determine the output power requirements of the unit. The block diagram of a VSC adopted PQ control strategy is shown in Fig. 2. Controls are implemented in a dq0reference frame that determines d- and q-axis components of the ac-side currents. The controller contains two loops: the power control loop and the current control loop. The current set-points are determined by the power control loop. By setting the d-axis aligned with the grid voltage the dq axis currents are proportional to the active and reactive power respectively 3. Thus, the set-point of d-axis current component is determined by active power control loop and the q-axis current component is determined by reactive power control loop. Fig. 2. Diagram of the PQ control strategyIn an autonomous microgrid, if the load need to be shared between the VSCs operated in PQ control mode, the active and reactive power references can not derived from the MPPT control strategy. A widely used method is using the droop line control to share the load which is shown in Fig.3. The RMS voltage and frequency were calculated from voltage waveforms. The active and reactive power reference can be calculated by the droop setting. The slope of the droop is often set by a central controller. The droop relations can be written as:Whereandare the rated active and reactive power,andare the rated value of the system voltage and frequency. is the slope ofdroop line, is the slope of droop line.Fig. 3. Droop line for the PQ control strategy2.2 VF control strategy In an autonomous microgrid, at least one VSC should be operated in Vf control mode to regulate the system voltage and frequency within the allowed limits. Some Vf control strategies are proposed in the previous publication 5,6. In an autonomous microgrid, if the VSC operated in Vf control mode, the voltage and frequency references are obtained directly from the droop line and the load will be shared according to the droop slope. The principle is that when there is an increase in the load, the frequency reference will be decreased. Similarly, reactive power is shared by introducing the droop characteristic in the voltage magnitude.Fig. 4. Droop line for the VF control strategyThe block diagram of droop line for the Vf control is shown in Fig.4. The active and reactive power of the VSC were calculated from the voltage and current. The voltage and frequency reference can be then calculated by the droop setting. The slope of the droop is often set by a central controller. The droop relations can be written as:Whereandare the rated active and reactive power,andare the rated value of the system voltage and frequency.is the slope ofdroop line,is the slope of droop line.3 Microgrid study systemSince the prime mover does not have instantaneous tracking capability of the power command, usually an energy storage unit is connected to the DC bus. With storage, the value of the voltage at the DC bus is quite stiff and it can be regulated by he prime mover. Because there is a need for some sort of available storage, then it is possible to eliminate the prime mover altogether from the model, without loss of generality. This simplification allows to obtain conclusions on the behavior of inverter based sources, without having the need to use an actual prime mover. Fig. 5. Diagram of a VSC-based single bus microgridTo investigate the control strategies for VSC in a microgrid a single bus grid was constructed and the diagram is shown in Fig. 5. It is a 380V microgrid system consists of a radial distribution system. The microgrid can be connected to the power grid through the isolation switch. The microgrid system includes four electronically interfaced DG units. Here the DC bus voltage of VSC is considered to be constant and the prime mover is omitted. Four VSCs, i.e., VSC1VSC4 are connected to feeders 14 respectively. It is assumed that all VSCs each has adequate capacity to supply independently controlled active and reactive power to the system. For simplicity, the load is considered to be resistive loads. L1L4 represent the line impedances. 4 Operation scenarios and simulation results While the microgrid transits to autonomous operation, VSCs are assigned with different control strategies to ensure stable operation of parallel VSCs. Three scenarios are tested in this autonomous microgrid: (1) All the VSCs are all operated in Vf mode and share the load by the droop lines; (2) One VSC is operated in Vf mode and the others are operated in PQ mode, the droop method was used to share the load between the VSCs in PQ mode; (3) One group of VSCs is operated in Vf mode and another group is operated in PQ mode. The droop method was used to share the load between the VSCs with same control strategy. Every scenario has at least one VSC operated in Vf mode so as to establish frequency and voltage for the autonomous microgrid. The performances of the system are simulated in PSCAD software In the simulation, the rated frequency is 50Hz, all the active power references of droop lines are 30kW and the reactive power references of droop lines are 0kVar. 4.1 All VSCs in Vf control strategy In this scenario, the system was in grid-connected mode at first and then transited to islanding mode. The active power reference for each VSC is 30kW. The isolation switch opened at 0.5s which made the microgrid islanding. The reference frequency is determined by droop line as shown in (2). With different droop slope Kpf the loading sharing was different between these VSCs in autonomous operation. When the load was 140kW and 100kW, the simulation results were shown in Fig.6 (a) and (b) respectively. Fig. 6. Load sharing between the VSCs in Vf control strategyIt can be seen from Fig.6 that when the active power provided by the VSCs is larger than the load, the VSCs will increase the active power output to make the system stable as shown in Fig.6 (a). When the active power provided by the VSCs is smaller than the load, the VSC will reduce the active power output as shown in Fig.6 (b). The larger the droop slopeis, the smaller power the VSC will be shared. This can be explained by (2), the frequency reference will become smaller when the droop slopebecomes larger, and this will make the VSC share smaller active power. The frequency variation of the microgrid under different combination of droop slopewas shown in Fig.7. The load was 140kW. The three curves in Fig.7 represent three combinations of droop slope: All the droop slopeare 20; the droop slopeare 20, 40, 60, 80 respectively; all the droop slopeare 80. Fig. 7. Microgrid frequency variation under different active power indexIt can be seen from Fig.7 that when all the droop slopeare 20, the steady state frequency variation is the smallest. When all the droop slopeare 80, the steady state frequency variation is the largest. The microgrid steady state frequency is determined by all the VSCs droop slopeThe frequency variation becomes smaller when the slope becomes smaller. 4.2 One VSC in Vf and the others in PQ control strategy In this scenario, the system was in grid-connected mode at first and then transited to islanding mode. The active power reference for each VSC is 30kW. The isolation switch opened at 0.5s which made the microgrid islanding. The system frequency and voltage are determined by the VSC operated in Vf control mode. The reference frequency and voltage were set according to droop lines. The other three VSCs were operated in PQ control strategy and the loads were shared by the droop line. The active power of each VSC is shown in Fig.8, the droop slopeof the VSCs are 1, 5 and 10 respectively.Fig. 8. Load sharing between the VSCs in PQ control strategyIt can be seen from Fig.8 that the active power output of each VSC increased after islanding to make the system stable. The larger the the droop slopeis, the more active power will be shared. This can be explained by (1), larger the droop slopewill make the active power larger and the load sharing will be larger.The frequency variation of the microgrid under different combination of droop slopeof the PQ mode VSCs was shown in Fig.9. The three curves in Fig.9 represent three combinations of the droop slope: All the droop slope are 10; the droop slopeare 1, 5, 10 respectively; all the droop slopeare 1. Fig. 9. Microgrid frequency variation under different frequency indexIt can be seen from Fig.9 that the variation of the microgrid frequency has relationship with the droop slopeof the VSC in PQ control strategy. The frequency variation will become smaller when the the droop slopebecome larger. Because larger droop slope can increase the active power output which reduced the frequency variation. 4.3 One group of VSCs in Vf and anther group in PQ control strategy In this scenario, the system was in grid-connected mode at first and then transited to islanding mode. The active power reference for each VSC is 30kW. The isolation switch opened at 0.5s which made the microgrid islanding. After islanding one group of VSC were operated in Vf control strategy and the reference frequency and voltage were set according to droop lines. Another group were operated in PQ control strategy and the active and reactive power were set according to droop lines. The droop slopefor the Vf droop controller are 20 and 80. The droop slopefor the PQ droop controller are 1 and 10. The simulation results were shown in Fig. 10.It can be seen from Fig.10 that the unbalanced active power can be shared between the VSCs in different control strategies. The Vf control group of VSC satisfied the conclusion in section 4.1 and the PQ control group satisfied the conclusion in section 4.2. By selecting different droop slopeand droop slope, the load power can be shared between these two groups.Fig. 10. Load sharing between the VSCs in Vf and PQ control strategyThe microgrid frequency is decided by the group in PQ control and the group in Vf control together. The frequency variation of the microgrid under different control strategy was shown in Fig.11. The frequency variation became smaller when theincreased and the decreased.Fig. 11. Microgrid frequency variation under Vf and PQ control5 ConclusionsThe VSC control strategy has large impact on the microgrid performance. This paper analyzed and compared two typical control strategies: PQ and Vf. The results which verified by the simulation of a study system can be listed as follows: (1) The VSC operated in Vf control strategy can share the load active power by the droop slope.The VSC shares more active power when the droop slopedecreases. The microgrid frequency variation becomes smaller