Protection Of Electrical Equipment And Systems Engineering Essay

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This document outlines the mandatory requirement for protection of electrical equipment and power systems, as well as the information required for conducting the design and implementation.

However, this document will not cover areas where protection requirements are special, that is, oil well pump protection and control. Special engineering shall be taken into consideration when work is involved with these particular areas.

Introduction To Typical Power Systems

It is mandatory that each individual involved with protection of power systems be familiar with it to ensure that the overall protection and coordination is maintained.

System Reference Drawings

Listed in the following are single Line diagrams and related drawing that provide most of the information required for system protection as well as system operation.

System Grounding

There are many different grounding methods, table 1 below shows the grounding methods that are typically used at the specified voltage levels.

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Table 1 - Grounding Methods

Voltage Level

Grounding Method

138 kV

Solid Ground

34.5 kV

Solid Ground

33 kV

Low-Resistance Grounding

13.8 kV

Low-Resistance Grounding

4.16 kV

Low-Resistance Grounding

480 V

High-Resistance Grounding

Supplemental System Information

Usually, generation power systems are designed with emphasis on high reliability and continuity. In addition, the power systems networks are designed with the capability to fully utilize the generated power and transfer of power to other locations via transmission lines.

Codes & Standards

The design and implementation of protection systems shall conform to the latest edition of standards and codes to which a project is built to.

Listed below are some of the codes and standards are typically considered when dealing with electrical power systems.

Canadian Electrical Code Part I CSA-C22.1 CEC

Canadian Standards Association CSA

National Electrical Code NEC

Institute of Electrical & Electronic Engineers IEEE

Protection Requirements

Protection requirements are grouped into the following typical zones:

Zone of Protection

Zone Description

Generator Zone

This Includes a generator and its interrupting devices

Motor Zone

This includes a motor and its starter

Transformer Zone

This Includes a transformer and feeder between its primary and secondary interrupting devices

Grounding Transformer

This includes a grounding (neutral derivative) transformer and interrupting devices

Feeder and Bus Zone

This includes a feeder and a main bus connected down stream

Generators

For most generators the following protections shall be equipped:

27 Undervoltage

32 Reverse Power

40 Loss of field

46 Phase balance current

49 Thermal protection

51V Overcurrent and voltage restraints

51GN Ground Overcurrent

64 Field Ground

87G Differential

87GN Restricted Ground Fault

Motors

Synchronous Motors

All synchronous motors shall be equipped with the following protection functions

27 Undervoltage

32 Reverse Power

40 Loss of field

46 Phase balance current

49 Thermal protection

50G Ground overcurrent instantaneous

50 overcurrent instantaneous

51 overcurrent inverse time

55 power factor

64 field ground

78 out of step

81 frequency

87 differential

99* Locked Rotor

Note: * when synchronous motor accelerating time is greater than its allowable stall time, speed sensor shall be installed to prevent false tripping

Refer to appendix A for protective device configurations

Induction Motors

For motors in this class, the following protection functions shall be equipped:

27* Undervoltage

46 Phase balance current

49 Thermal protection

50* overcurrent instantaneous

50GS Ground overcurrent instantaneous

51 overcurrent inverse time

87** differential

99 Locked Rotor

Note: * for motors protected by starter, device 50 and 27 are not required if starters are equipped with fuses and will dropout automatically due to undervoltage

** Required for motors 3000 hp and higher

Transformers

For Station Service Transformers the following protection functions shall be equipped

50 overcurrent instantaneous (primary)

50N residual neutral overcurrent instantaneous (primary)

51 overcurrent inverse time (primary)

51N overcurrent inverse time (primary)

51TN transformer neutral overcurrent inverse time (solidly grounded neutral)

63 Gas pressure (slow and fast)

71Q Low oil (alarm)

Grounding Transformer

Grounding or neutral derivative transformer shall be protected by the following:

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50 overcurrent instantaneous

51 overcurrent inverse time

Feeder and Bus Protection

4.5.1 Feeder and bus with high resistance ground shall be protected by the following:

27 undervoltage

50 overcurrent instantaneous

50N residual neutral overcurrent instantaneous

51 overcurrent inverse time

60 phase voltage unbalance or fuse failure (applicable to feeders with fuse protection upstream)

Feeder and bus with low resistance ground shall be protected by the following:

27 undervoltage

50 overcurrent instantaneous

50N residual neutral overcurrent instantaneous

51 overcurrent inverse time

60 phase voltage unbalance or fuse failure (applicable to feeders with fuse protection upstream)

64 equipment ground, insulation failure

Selection of Protective Devices

Lightning Arrestors

Lightning/Surge arrestors shall be provided:

At all junction points between underground and overhead systems

At all stations connected to overhead lines

At incoming circuits entering every plant and at intermediate switching stations upstream of each point

For equipment protection, as indicated in the equipment specification/datasheet/single line diagram/purchase orders. The responsible design engineer shall demonstrate his decision making based on criticality of application, requirements of process continuity, reliability and economics of protection.

Lightning arrestors shall be zinc oxide (ZnO) type with appropriate classes, voltage levels and mounting enclosures for the applications. Location of lightning arrestors shall be not more than 50 feet from the equipment protected. Any deviation shall be supported by calculations and prior approval from the company.

In General, lightning arrestors selected shall have:

a voltage level no more than 140 percent of the system or equipment operation voltage to ground

a current rating which can adequately handle the calculated maximum discharge current based on ANSI C62.2 suggested formula:

ia = (2 x e0 - ea)/Z

Where ia = surge current in amperes

e0 = 1.2 x line insulation level in volts (1.2 x 50 micro-sec critical flashover)

Z = Line surge impedance in ohms

ea = arrestor discharge voltage in volts

Power Fuses

Power fuses shall have current limiting, non-deteriorating characteristics that conform to the latest edition of ANSI standards C37.46. for indoor applications, power fuses shall be non-vented type, complete with blown fuse indicator.

Power fuses shall be capable of:

Successfully interrupt the maximum short circuit current

Carry the maximum allowable load current

Withstand the maximum normal transient overcurrent such as magnetizing inrush current of a transformer or starting current of motors.

Isolate designated fault conditions

In addition to the above, it is desirable that the power fuse can provide:

Proper coordination between the source and the loads

Backup overload protection

Potential Transformers (P.T.'s)

Potential transformers used for protective relaying shall have accuracy rating of 0.3 WXYZ conforming to the latest edition of CSA standard C13 and ANSI/IEEE c57.13. Each potential transformer shall be loaded not more than 200VA. Transformer ratios shall be selected such that such that the secondary line to line voltage (of a set of transformers) is 200 VAC

Current Transformers (C.T.'s)

Current transformers for protective relaying shall be conforming to the latest edition of CSA standard C13 and ANSI/IEEE C57.13 and the following accuracy ratings:

C.T. Ratio Accuracy Class

Below 300A on primary 2.5L20

300A and above 2.5L200

Current transformer loading shall be limited to:

0.2 ohm for C.T.'s having a ratio of less than 300:5

2.0 ohm for C.T.'s having a ration of 300:5 and above

Current transformer ratio shall be selected such that:

Secondary current does not exceed 100 amperes when the primary carries maximum fault current

Secondary current is approximately 5 amperes when the primary caries ultimate fill load current

If both conditions (i) and (ii) cannot be met, then (i) shall be precedence and it shall be submitted to the engineering department for prior approval.

For special applications, for example, neutral current transformer with relays of high burden or current sensor for special relays primary wound type C.T.'s with high burden capability or manufacturer's recommended sensors shall be used. All special applications shall be submitted for prior approval.

Protective Relays

Electromechanical Relays

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Selection of electromechanical protective relays should be made from those meet the following requirements:

It shall be accessible for testing and inspection without deenergizing other associate devices

It shall have manually reset mechanical trip indicators integrally or non-integrally mounted, adjustable trip settings and, if applicable, adjustable time settings

It shall have good proven history in its applications at least 2 years

It shall maintain rated accuracy throughout its life expectatancy

It shall be immune against surges, radio wave interference and ambient temperature changes

Static or Microprocessor Driven Type

Selection of electromechanical protective relays should be made from those meet the following requirements:

It shall be accessible for testing and inspection without deenegizing other associate devices

It shall have manually reset mechanical trip indicators integrally or non-integrally mounted

It shall have non-voltile memory trip indicator and easily readable indicator

It shall have adjustable trip settings via easy access dip switches or user interface keypad programming. If applicable, adjustable time settings via dip switches or user interface keypad programming

It shall have good proven history in its application for at least 2 years.

It shall maintain rated accuracy throughout life expectancy.

It shall be immune against surges, odd order harmonics interface such as 180,300,420 Hz, high frequency radio wave interface and ambient temperature changes

It shall have built-in continuous self-supervision of both hardware and software

It shall relay failed indicator LED and will remains light up and initiate a alarm relay to pick up if relay failed or supply voltage is interrupted

Relay Settings and Co-ordination

Protective relay settings shall be chosen with high sensitivity to prevent damage and also with positive selectivity to minimize system disruptions. Hence, the relay settings must be set at the lowest point yet to be able to:

Accommodate harmless system fluctuations and temporary transients.

Coordinate with all related protective zones within the system

Further to that above, conditions outlines in the following paragraphs shall also be observed while choosing relay settings.

Inverse Time Current Relays

When setting these relays, a minimum margin of 0.4 seconds shall be maintained between cascaded protective zones.

MV Power Plant Systems

General

The system is at 13.8kV level, the decay in generating current and voltage shall be taken into account when choosing relay settings at this voltage level. Calculations for decreasing values as follows:

IEEE Transaction of Industry and General Applications

Power Plant and Distribution Systems

The protective relay performance shall be such that multi-phase and phase to ground faults which could result in major system disturbance based on the following conditions:

13.8 kV faults occurring within the bonds of a generating/switching station shall be cleared instantaneously with no intentional delay

13.8 kV faults occurring on a circuit or a transformer located adjacent to a major generating/switching station shall be cleared instantaneously with no intentional delay

13.8 kV fault occurring on a line circuit adjacent to generating/switching station shall be cleared in not more than 0.5 second, breaker operating time.

Load interrupting Switches

For load interrupting switches equipped with automatic ground fault interrupting capability, an instantaneous overcurrent relay (device number 50) shall be employed and be set at no more than 90 percent of the interrupting current rating of the switch to block the switch from tripping excessive fault current.

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