An Adaptive Virtual Impedance Based Droop Control Scheme for Parallel Inverter Operation in Low Voltage Microgrid
Authors: Hai Nguyen Tien , Kim Kyeong-Hwa 
Published: February 2017

<span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">This paper presents an adaptive virtual impedance based droop control scheme for parallel inverter operation in low voltage microgrid. Because it is essential to achieve power sharing between inverters in microgrid</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">, various </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">droop control scheme</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">s</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> ha</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">ve</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> been </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">proposed</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">. In practice, </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">the </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">line impedance between inverters and the point of common coupling (PCC) </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">in</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> microgrid are not always equal. This </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">im</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">balance</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> in</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> line impedance often results in </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">a </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">reactive power mismatch among inverters. This problem has been solved by introducing a virtual impedance loop in the conventional droop control scheme. However, the reactive power sharing performance of this method </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">is still </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">deteriorate</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">d</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> when the line impedances change during operation. To overcome such </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">a </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">problem</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">, a new control</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> scheme </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">that is based on </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">a virtual impedance loop and an impedance estimation scheme </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">is proposed. To monitor </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">the changes </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">in </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">line impedances</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">, t</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">he impedance estimator </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">is implemented by using </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">the output voltages and currents of inverters as well as </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">the </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">voltages at the PCC. To compensate </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">for the </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">reactive power mismatch </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">due to </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">the line impedance changes</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">, t</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">h</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">e</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> estimated line impedance is fed to the virtual impedance loop in which it adjusts the virtual impedance</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> value</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">. </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">Comparative s</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">imulation results </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">with the conventional ones </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">verify the effectiveness of the proposed </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">adaptive </span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">virtual impedance based droop control</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: KO; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US"> scheme</span><span style="font-family: 'Times New Roman','serif'; font-size: 9pt; mso-fareast-font-family: '맑은 고딕'; mso-fareast-theme-font: minor-fareast; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; mso-bidi-font-style: italic;" lang="EN-US">.</span>