Power System Protection And Switchgear By Badri Ram Dn Vishwakarma Pdf 44
the switchgear consists of the following items:
- the switchgear enclosure which is a housing for the distribution components and switches.
- the switches, contactors, buses and other distribution components.
- the switchgear oil.
the breaker is an electromechanical contactor which can be connected in series or parallel with the line. the breaker provides the lowest cost of fault protection in the power system. the standard breaker is a mechanical version of the vacuum relay, having the movable element to be a flat plate. a mechanical breaker uses contact arms which are usually bimetallic and spring loaded.
most digital protection relays are analog in nature, and although there are also digital relays, the analog nature provides a more flexible design structure. the performance of the relay is measured by a pre-determined curve which is set into the relay by the factory.  the user cannot modify the curves and the curves are limited to the protection provided by the manufacturer. digital protection relays are commonly used in power systems and switchgear to measure the power level, to provide frequency response, to provide the time delay or the magnitude response or to provide special functions such as the ability to measure the phase angle and to perform harmonic distortion analysis  :ch 7, pp 52. a protection relay is a device that measures and compares the current flowing in a protected circuit. it compares the measured current value with the value that is known to be safe and provides the protection signal when it senses a fault condition or a change in the power level.  :ch 7, pp 52. a protection relay is typically used to protect equipment by detecting overloads, short circuits, phase-to-phase shorts, and other disturbances that damage equipment. protection relays are also used to protect human life and equipment that are vital to the operation of the power system, such as generators, transformers, cables and switchgears.  :ch 7, p 51. protection relays are divided into four main categories, either of which may be used in the protection of electrical equipment:
the relay’s primary winding is supplied from the power systems current transformer via a plug bridge,  which is called the plug setting multiplier (psm). usually seven equally spaced tappings or operating bands determine the relays sensitivity. the primary winding is located on the upper electromagnet. the secondary winding has connections on the upper electromagnet that are energised from the primary winding and connected to the lower electromagnet. once the upper and lower electromagnets are energised they produce eddy currents that are induced onto the metal disc and flow through the flux paths. this relationship of eddy currents and fluxes creates torque proportional to the input current of the primary winding, due to the two flux paths being out of phase by 90. a self-powered protector is a device that derives its operating power from a low-voltage bus. it may be a simple relay or an active device. a self-powered protector may also be called an autonomous protector, or an isolated protector. a protector can be used as a reactive load to isolate other equipment from a low-voltage bus. the analysis of electrical equipment is based on the conventional time-domain approach. the primary focus of the author is on the understanding of the basic principles, applications of the analysis and the simulation software, and the design of all types of protection equipment. this book provides an introduction to the principles of the operation of a power system, which is essential to understand the operations of a power system. the primary focus of the author is on the understanding of the basic principles, applications of the analysis and the simulation software, and the design of all types of protection equipment. the concepts in this book are related to distribution and sub-station design. 5ec8ef588b