CHAPTER 1INTRODUCTION

ET 601 – POWER SYSTEM ET 601 – POWER SYSTEM PROTECTIONPROTECTION

CHAPTER OUTLINES

After completed this chapter, students should be able to:

1. Understand the constitution of power system

2. Know the need of protection system

3. Understand basic requirement of protection evaluation 4. Understand the type of fault and their effect

5. Know the basic components of protection

6. Understand basic concept of protection system.

POWER SYSTEM PROCESS

EHV TransmissionSystem (275 kV)

Generator TransformerYd1

Syncronous Generator -275 kV

(Base Load Generator)

Super GridTransformer

275/132 kV Yy0d1

Gas Turbine(Pick-up Generation)

TransmissionTransf.

132/11 kV Yy0d1

TransmissionTransf.

132/33 kV Yd1

Distribution Transf.33/11 kV Dy1

33 kV11 kV

132 kV

11 kV

132 kV

275 kV

Hydro Power Plant(Spinning Reserve )

FLOW OF POWER SYSTEM

SINGLE LINE DIAGRAM

PROTECTION SYMBOL

two-winding transformer

current transformer

two-winding transformer

generator

bus

voltage transformer

capacitor

circuit breaker

transmission line

delta connection

wye connection

circuit breaker

fuse

surge arrestor

static load disconnect

Safeguard the entire system to maintain continuity of supply

Minimize damage and repair costs where it senses fault

Ensure safety of personnel.

Why need protection system?

1. Normal operation means that there are no failures of equipment and/or no mistakes committed by personnel. This also means that there is no effect of any natural calamity on the power system.

2. Prevention of electric failure means the provision of features in the design that are aimed at preventing failures. These features include adequate insulation, insulation co-ordination in accordance with the capabilities of lighting arresters, providing overhead ground wires and low tower footing resistance, and adoption of proper operation and maintenance procedures.

3. Mitigation of the effects of electric failure when it occurs implies that certain ‘protection provisions should be incorporated in the system. These provision include limiting the magnitude of short circuit current, provision for promptly disconnecting the faulted section, and features that investigate and send out an alert in the case of an incipient fault.

Philosophies

Objective to achieve philosophies

These requirements are necessary, firstly for early detection and localization of faults, and

Secondly for prompt removal of faulty equipment from service.

In order to carry out the above duties, protection must have the following characteristics: Reliability Selectivity Sensitivity Speed simplicity

Basic requirement/characteristics

• minimum fault duration and consequent equipment damage and system instability.

• assurance that the protection will perform correctly.

• maximum continuity of service with minimum system disconnection.

• To detect even the smallest fault, current or system abnormalities and operate correctly at its setting

A power system fault may be defined as any condition or abnormality of the system which involves the electrical failure of primary equipment such as generators, transformers, busbars, overhead lines and cables and all other items of plant which operate at power system voltage.

Electrical failure generally implies one or the other (or both) of two types of failure, namely insulation failure resulting in a short-circuit condition or conducting path failure resulting in an open-circuit condition, the former being by far the more common type of failure.

FAULT IN POWER SYSTEM

Symmetrical faultFaults giving rise to equal currents in lines displaced by

equal phase angles i.e 120o in three phase systems.Example: short circuit of all three phase conductors of a cable at a single location

Unsymmetrical faultFaults in which not all the line currents are equal and not all have the same phase.Example (any one): single phase line to ground fault (L-G), two phase to ground (LL-G) fault and phase to phase (L-L) fault.

FAULT IN POWER SYSTEM

SYMMETRICAL FAULT

THREE- PHASE FAULTTHREE- PHASE FAULT

THREE PHASE - EARTH THREE PHASE - EARTH FAULTFAULT

UNSYMMETRICAL FAULT

PHASE – PHASE FAULTPHASE – PHASE FAULT

TWO PHASE – EARTH TWO PHASE – EARTH FAULTFAULT

SINGLE PHASE - EARTH SINGLE PHASE - EARTH FAULTFAULT

OPEN CIRCUIT FAULT

SINGLE- PHASE OPEN SINGLE- PHASE OPEN CIRCUITCIRCUIT

TWO- PHASE OPEN TWO- PHASE OPEN CIRCUITCIRCUIT

THREE- PHASE OPEN THREE- PHASE OPEN CIRCUITCIRCUIT

BASIC COMPONENT

Figure shows basic connections of circuit breaker control for the opening operation. The protected circuit X is shown by dashed line. When a fault occurs in the protected circuit the relay (2) connected to CT and PT actuates and closes its contacts (6).

Current flows from battery (5) in the trip circuit (4). As the trip coil of circuit breaker (3) is energized, the circuit breaker operating mechanism is actuated and it operates for the opening operation.

Thus the fault is sensed and the trip circuit is actuated by the relay and the faulty part is isolated.

BASIC COMPONENT

The power system is divided into protection zones defined by the equipment and the available circuit breakers. Six categories of protection zones are possible in each power system:

1. Generators and generator-transformer units

2. Transformers3. Bus bars4. Lines (transmission and distribution)5. Utilization equipment (motors, static

loads, or other)6. Capacitor or reactor banks (when

separately protected)

PROTECTIVE ZONE

PROTECTIVE ZONE DIAGRAM

Although the fundamentals of protection are quite similar, each of these six categories has protective relays, specifically designed for primary protection, that are based on the characteristics of the equipment being protected. The protection of each zone normally includes relays that can provide backup for the relays protecting the adjacent equipment.

The protection in each zone should overlap that in the adjacent zone; otherwise, a primary protection void would occur between the protection zones. This overlap is accomplished by the location of the CTs the key sources of power system information for the relays.

Main or primary protectionFirst in line of defense is main protection which ensures quick action and selective clearing of faults within the boundary of the circuit section or the element it protects. Main protection is essentially provided as a rule.

Back-up protectionIt is the second line of defense in case in failure of primary protection. It is designed to operate with sufficient time delays so that primary relaying will be given enough time to function if it is able to. Back up protection gives back up to the main protection, when the main protection fails to operate or is cut out for repairs etc.

PRIMARY & BACK UP PROTECTION

PRIMARY & BACK UP PROTECTION

WHICH ONE PRIMARY AND BACK UP???

UNIT PROTECTION

- Able to detect and respond to fault occurring only within its own zone protection.

- Have absolute discrimination. Its zone of protection is well defined.

- It does not respond to the fault occurring beyond its own zone protection.

- Also referred to as “main protection”- Fast operating time- Example: differential protection of

alternators, transformer or bus bars, frame leakage protection, pilot wire and carrier current protection.

NON UNIT PROTECTION

- Does not have absolute discrimination(selectivity).

- No defined area of coverage- Also known as “back-up protection”- Slower operating time - Example: distance protection and time

graded, current graded or both time and current graded.