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Control Method of Islanded Microgrid to Improve Reactive Power Sharing and Power Quality* Huang Jing1, Liu Xiaohua1, Li Hongping2 1Foshan Power Supply Bureau,Guangdong Power Grid Co., Ltd. Foshan, 528000; 2Sichuan University, Chengdu; *Fund Project: Scientific and Technological Project of China Souther Grid Corp(GD2014-2-0230) Abstract: Due to the influence of feeder impedance, reactive power supplied by distributed generation (DG) based on traditional droop control could not be shared according to their droop gains. This paper proposes a control strategy which is based on virtual impedance, frequency/voltage and reactive power regulation for islanded microgrid. Virtual impedance is added to existing control loops to improve the characteristics of output impedance, weaken the power coupled caused by resistive component. Secondary regulation on the voltage and frequency deviations from droop control, improves the quality of voltage and frequency. Secondary reactive power regulation directly controls the distribution of reactive power. Reactive power sharing can be obtained independently from inverter output voltage, and it can limit the reactive power sharing errors. A microgrid simulation model is built in Matlab/Simulink simulation platform, and simulation analyses verify the validity and feasibility of the proposed strategy. 1 Microgrid and Traditional Droop Control In recent years, in order to solve the problem of energy crisis and environmental pollution, a variety of renewable energy and distributed generation has been widely concerned. Microgrid can be defined as a cluster of elements such as energy storage devices, energy conversion devices, protection devices, loads and distributed generations. Microgrid allows the DG units work in stand-alone mode and grid-connected mode. Due to the difference of feeder impedance, reactive power supplied by distributed generation based on traditional droop control could not be shared according to their droop gains and cause circulation current. When loads change large, traditional droop control could make the grid voltage and frequency deviate from its ratings, affecting the power quality of microgrid. To design a control strategy, which make the reactive power distribution is not affected by line impedance mismatch, is very necessary. 2. 2.1 Proposed Control Strategy Virtual Impedance Traditional droop control is proposed when inverter output impedance and line impedance present inductance. DG units always work in low voltage microgrid, and the line impedance present resistance, so traditional droop control method will no longer apply. By adding virtual impedance can make the inverter output impedance present inductance, reduce inverter output active and reactive power coupling, make droop control method continue to be used in low voltage microgrid. 2.2 Secondary Control Strategy In order to guarantee the quality of voltage and high precision of reactive power distribution, voltage/frequency and reactive power secondary control is proposed. It can make the frequency and voltage maintain rated value, and reactive power is shared according to their load capacity. When the load reactive power increase (or decrease) lead to voltage amplitude decrease (or increase), through the secondary voltage regulation control, hanging up (or down) droop control curve to restore the voltage to the rating voltage; When load active power increase (or decrease) lead to frequency decrease (or increase), through the secondary frequency regulation, hanging up (or down) frequency control curve to restore frequency to the rating. Secondary reactive power control adjust the allocation of reactive power directly, reactive power distribution is not affected by the voltage of DG unit. It can eliminate reactive power share error caused by mismatch of line impedance, to achieve high precision distribution of reactive power. the secondary voltage control first compare inverter output voltage with its reference and then adjust the difference value using PI controller; next the result add with the output voltage U of the traditional droop control; eventually it can receive compensated voltage U which is the input to the voltage and current double-loop control, and preserves the rated output voltage. By the same token, second frequency and reactive power control can control frequency and reactive power, to preserve the rated output frequency and reactive power sharing of high precision. 3. Simulation results To test and verify the validity and feasibility of the proposed control method, simulation model of two parallel DG units is built in Matlab/Simulink platform. 3.1 Calculation Case 1 With the traditional droop control, DG can share same active power even if load changes. But due to the mismatch of line impedance, DG cannot share same reactive power. DG1 share more reactive power because of smaller line impedance than DG2. DG output frequency and voltage have large deviation from its reference when load changes. When DG active and reactive power increases, the frequency and voltage are smaller than their ratings. DG with proposed control method can share same power of load. What is more, when load changes, compared with the conventional droop control method, it has smooth transition process. Even if line impendence is different between each DG, DG still can realize accurate allocation of reactive power that is DG1 and DG2 evenly distributed load reactive power. When load changes large, it can keep DG output frequency maintain the reference value 50Hz and output voltage maintain the reference value 310 V. 3.2 Calculation Case 2 Because DG units join voltage/frequency and reactive power secondary control based on the traditional droop control, it can turn into a stable operation state in a short time, continuing to supply power for load, when a DG exit of microgrid. When a DG malfunction or exit the runtime, the network can continue to run normally. 4. Simulation results Considering the reactive power allocation accuracy of inverters parallel system using traditional droop control and the voltage and frequency deviations caused by droop control in microgrid, a balance control strategy of reactive power and voltage/frequency for inverters parallel operation is proposed based on virtual impedance. Through the secondary control regulation, it can maintain the voltage and frequency in the rating, and reactive power distribution also can achieve higher accuracy. The proposed control strategy works without central controller, avoiding the whole system cannot run normally when a DG unit fails. It turned out that the simulation results are fairly well matched, thereby confirming the validity and feasibility of the proposed approach. Keywords: Islanded microgrid, voltage/frequency regulation, power sharing, droop gains. Author’s brief introduction and contact information: 1) Author’s Name: Huang Jing 2) Author’s Affiliation: Foshan Power Supply Bureau, Guangdong Power Grid Co., Ltd. 3) Author’s Tel: 13438039532 4) Author’s E-mail: [email protected] 5) Author’s Address: Foshan, Guangdong Province, China. 6) Fund Project: Scientific and Technological Project of China Souther Grid Corp(GD2014-2-0230)