Control schemes for distributed generators connected to distribution networks

Since 1970, there has been a growing interest in use of distributed generators connected to distribution networks. In a stand along operation, a distributed generator´s Governor and AVR (Automatic Voltage Regulator) are used to ensure the output frequency and voltage are maintained to the set points. When operating in parallel with the grid supply, the distributed generator´s output frequency and voltage are determined by the network´s frequency and voltage controllers and the generator´s control systems are responsible for delivering the set power level. The paper reviews the various methods used to control the different types of distributed generators and their influence on the operation of both the generator and the network. These include several types of asynchronous generators as well as synchronous generators. It also explores the techniques used for synchronous machines where the generator offers the opportunity of providing auxiliary services to the network operator in ensuring that the supply is maintained within regulatory operating constraints and maintaining supply during network disruptions.