Protection
1. Overview
1.1 Scope
This application guide describes the generally accepted forms of protection for synchronous generators and their excitation systems. It summarizes the use and selection of relays and other protective devices that provide generator protection. The guide is primarily concerned with protection against faults and abnormal
operating conditions for large hydraulic, steam, and combustion turbine generators (CTGs).
The guidance pertains 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 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 usually similar to
those of the implementation using discrete component relays. The algorithms used to perform some of the protection may be different, but typically provide equivalent or better protection. The protection
performance and capability may be superior using 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 such as monitoring and reporting.
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.
1.2 Purpose
This guide summarizes the use of relays and other protective devices such that the reader may select the necessary equipment to provide adequate protection for ac generators from short circuits and abnormal
operating conditions.
1.3 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. This clause intends to present information that affects the protection arrangement and selection of protective relays.
A discussion of auxiliary systems 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 may be grounded in accordance with IEEE Std C57.13.3™.
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 many situations.
Clause 5 presents a discussion of other forms of protection and factors that may be considered in the generator zone.
Clause 6 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 functions) 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.
Table 2 provides a summary of the protective elements and their subclause references.
Table 3 provides guidance on equipment to be tripped or alarm for the different protective elements and their subclause references.
Annex A describes sample calculations for settings of some common generator protection functions.
Annex B provides derivations for the steady-state stability limit (SSSL) equations.
Annex C provides the bibliography.
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