Power System Protection

By

Dr. Neeraj Gupta, (Ph.D IIT Roorkee), SMIEEE

Assistant Professor

NIT Srinagar, J&K

drngupta.com

Introduction

Secondary systems in a Power system

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• Protection

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• Auto control for voltage, frequency, reactive power compensation, power flow, network configuration and stability

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• Metering for billing, operational control and statistical data

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• Local manual control (plant status, voltage level reactive power support, network configuration)

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Secondary systems in a Power system

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• Remote manual control via communications links (SCADA)

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• Plant condition monitoring and alarming (temperature, malfunction, maintenance need, operating duty)

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• Communications infrastructure

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• Instrument transformers - current and voltage transformers

Substation fire

Substation fire Aftermath

Transformer fires due to fault

Generator Faults

Expensive consequences of Fault

Protection scheme

• Power System Protection is a fascinating subject.

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• A protection scheme in a power system is designed to continuously monitor the power system to ensure maximum continuity of electrical supply with minimum damage to life, equipment, and property.

Designing the protective schemes

• While designing the protective schemes, one has to understand the fault characteristics of the individual power system elements.

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• One should also be knowledgeable about the tripping characteristics of various protective relays.

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• The job of the protection engineer is to devise such schemes where closest possible match between the fault characteristics and the tripping characteristics is obtained

Designing the protective schemes

• The design has to ensure that relays will detect undesirable conditions and then trip to disconnect the area affected, but remain restrained at all other times.

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Why Study Power System protection

• However, there is statistical evidence that a large number of relay trippings are due to improper or inadequate settings than due to genuine faults. It is therefore necessary that students should be equipped with sound concepts of power system protection to enable them to handle unforeseen circumstances in real life.

Very Interesting subject

• Whenever a tripping takes place it has all the elements of intrigue, drama, and suspense. A lot of detective work is usually undertaken to understand the reason behind the tripping.

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• It needs to be established why the relay has tripped.

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• Whether it should have tripped at all.

• What and where was the fault?

Power system is complex

• Power system is a highly complex and dynamic entity.

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• It is always in a state of flux.

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• Generators may be in or out of service.

• New loads are added all the time.

Power system is complex and surgical accuracy required.

• A single malfunction at a seemingly unimportant location has the potential to trigger a system-wide disturbance.

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• In view of such possible consequences, a protective system with surgical accuracy is the only insurance against potentially large losses due to electrical faults.

Elements of power system protection

Elements of power system protection

Elements of power system protection

Elements of power system protection

Elements of power system protection

Introduction: CTs and PTs

• CTs and PTs are the ‘eyes’ and the ‘ears’ of the protection system while the protective relays and the circuit breakers are the ‘brain’ and the ‘brawn’ of the system. Each of these components is important in its own right. All of them have to work in tandem with each other to mitigate the effects of faults.

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• Failure of any one of these components is treated as failure of the protective system.

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Work of protection Engineer

• It is ironic that every additional protective relay or a protective feature that we add to a relay’s characteristics also increases the possibility of maloperation of the relay. This is possibly an area where protection tends to become an art. The protection engineer has to strike a balance between the threat perception and the security offered by the protective scheme.

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Dependence of Modern Society on Electric Supply

• The modern society has come to depend heavily upon continuous and reliable availability of electricity—and a high quality of electricity too.

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• Computer and telecommunication networks, railway networks, banking and post office networks, continuous process industries and life support systems are just a few applications that just cannot function without a highly reliable source of electric power.

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• And add to this, the mind-boggling number of domestic users of electricity whose life is thrown out of gear, in case the electric supply is disrupted. Thus, the importance of maintaining continuous supply of electricity round the clock cannot be overemphasized.

Faults and Abnormal Operating Conditions

• No power system can be designed in such a way that it would never fail. So, one has to live with the failures.

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• In the language of protection engineers, these failures are called faults.

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• What is more important is, how to prevent the faults and how to mitigate the consequences of the faults.

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• The ill effects of faults are minimized by quickly isolating the faulty element from the rest of the healthy system; thus limiting the disturbance footprint to as small an area in time and space as possible.

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Fundamentals

• Overview

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• Natural Mode of synchronous operation (close knitting of system).

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• Why do we need protection?

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• Types of protection?

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• Analogy with functioning of Human being?

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• Evolution of Relays

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