Chapter 2 - Distributed generating system integration: Operation and control

Abstract

Distributed energy resource (DER)-based voltage source inverters (VSIs) have become an inalienable part of the modern day power system. Traditionally, the power generation sources constituted synchronous generators (SGs). However, the present day generation sources are composed of VSI-based DERs, such as photovoltaic, battery, wind, and fuel cell. To maintain the harmony of generation between grid and the DER, the control of VSI is an important area which needs to be properly devised. Such integration of DERs along with the grid is categorized under distributed generating system. The DER-led VSIs are operated in three modes: (i) grid following (GFL) mode in which the VSI follows voltage and frequency of the grid at the local point of common coupling (PCC), (ii) grid supporting mode in which the VSI supports the grid, and (iii) grid forming (GFM) mode in which VSI operates in the islanded mode. In the GFL mode of operation, the VSI has a synchronizing unit known as phase locked loop, which latches the VSI to the network. Post synchronization, with the grid, the export of power takes place from the DER to local load and the grid. Active (P*) and reactive (Q*) power references serve as the control input, and voltage at PCC is used to provide input to the VSI. Power regulatory authority of different countries impose grid codes for the operation of generating units comprising of DER in the distribution system. DER operator adheres to the grid codes to maintain operation of the VSI in GFL mode. During the absence of grid, the DERs in the distributed generating system will form an islanded micro-grid which will operate in GFM mode. The DERs in the micro-grid are operated in conventional droop to handle active and reactive power flows to maintain demand and generation balance of the system. The widely used conventional droop is P-f and Q-V droop (depends on the R/X of the network) which is used in power sharing among the DERs. The droop equation for low R/X network is modified for proper sharing of power among the DERs. The choice of droop coefficient of the DERs is a critical task, as it is highly motivated by the economics of the plant as well as the network parameters. Apart from the GFM and GFL control, in this chapter, grid supporting (GS) mode of VSI control is discussed. Some of the popular GS functionalities are frequency, reactive, inertial, and voltage support.