Guide for AC Generator Protection
|Publication Date:||16 November 2006|
This application guide for the relay protection of synchronous generators presents a review of the generally accepted forms of protection for the synchronous generator and its excitation system. It summarizes the use of relays and devices and serves as a guide for the selection of equipment to obtain adequate protection. The guide is primarily concerned with protection against faults and abnormal operating conditions for large hydraulic, steam, and combustion turbine generators.
Basing generator protection on machine size is difficult because the desired protection may be determined more by the importance of the generator to the power system than by the size of the generator. The recommendations made pertain to typical synchronous generator installations. However, sufficient background information relating to protection requirements, applications, and setting philosophy is given to enable the reader to evaluate the need, to select, and to apply suitable protection for most situations.
The protective functions discussed in this guide may be implemented with a multifunction microprocessor based protection system (digital system). The protection philosophy, practices, and limits are essentially identical to those of the implementation using discrete component relays. The algorithms used to perform some of the protection functions may be different, but should produce equal or better protection. However, the performance and capability may be superior using the digital systems such as improved frequency response (bandwidth) and thresholds (pickup settings). Other additional features may be available from these digital systems that enhance the functionality.
This guide does not purport to detail the protective requirements of all generators in every situation. For example, standby and emergency-use generators are specifically excluded.
Description of the guide
Clause 3 presents a brief description of typical generator design and connections, generator grounding practices, excitation systems design, and generating station arrangements. The intent of this clause is to present information that affects the protection arrangement and selection of protective relays.
A discussion of auxiliary system transfer and the possible negative impacts of misoperation and faults on these systems are beyond the scope of this guide.
The methods employed for grounding and fusing the secondary circuits of voltage transformers (VTs) and the methods for grounding current transformer (CT) secondary circuits are not generally the same for all installations. CT and VT secondary circuits should be grounded in accordance with IEEE Std C57.13.3 ™ .1
Clause 4 briefly describes the damaging effects of faults and abnormal operating conditions and the type of devices and their settings commonly used to detect these conditions. A clear understanding of the effects of abnormalities on generators will assist the reader in evaluating the need for and the means of obtaining adequate generator protection in any specific situation.
Clause 5 presents a discussion of other forms of protection and factors that may be considered in the generator zone.
Clause 6 describes multifunction generator protection systems (MGPS) and presents an application on a typical generating unit and includes information on testing of MGPS.
Clause 7 presents detailed diagrams that are classified according to the method by which the generator is connected to the system. These diagrams show the combination of relays (and their control function) often applied for generator and excitation system protection in accordance with good engineering practices. These diagrams also consider the protective devices on other equipment in or adjacent to the generating station that are connected to trip or shut down the generator.
Annex A describes sample calculations for settings of generator protection functions.
1 Information on references can be found in Clause 2.