Transmission Network Protection

Theory and Practice

Yeshwant G. Paithankar, YeshwantG. Paithankar

• Publisher: Routledge
• Year: 2017
• Print ISBN: 9780824799113
• eBook ISBN: 9781351407083
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

From the basic fundamentals and principles of protective relaying to current research areas in protective systems and future developments in the field, this work covers all aspects of power system protection. It includes the implementation of relays using electromechanical devices, static devices and microprocessors; distance protection of high voltage and extra high voltage lines, including distance relay errors; and adaptive, dynamic, travelling wave and noise-based relays.

1

Basic Philosophy of Relaying

1.1. CAUSES OF FAULTS

Every power system element is subjected to a fault or a short circuit. Power system elements are generators, step-up transformers, busbars, transmission lines, step-down transformers and then distribution network feeding to loads.

The causes of faults are any of the following:

1. Healthy insulation in the equipment subjected to either transient overvoltages of small time duration due to switching and lightning strokes, direct or indirect. This causes the failure of insulation, resulting in fault current or short-circuit current. The fault current magnitude may be any-where from 10 to 30 times the full-load or rated current of the equipment. For example, if a turboalternator has 1.0 p.u. internal voltage and the d-axis reactance is 0.1 p.u., the short-circuit current, for a three-phase fault, would be E/Xd = 1.0/0.1, or 10 times the rated current. Fault currents for other faults, such as L-----L, L-----L-----G and L-----G, must be calculated from symmetrical component theory. Note that the fault current magnitudes normally go down from three-phase faults to L-----L-----G faults to L-----L faults, and finally, to L-----G faults.

2. Another cause of faults is insulation aging, which may cause breakdown even at normal power frequency voltage.

3. The third cause of faults is an external object, such as a tree branch, bird, kite string, rodent, etc., spanning either two power conductors or a power conductor and ground.

1.2. TYPES OF FAULTS

The type and nature of faults in a three-phase system are normally classified as (a) phase and ground, (b) permanent, (c) transient, and (d) semitransient.

1.2.1. Phase Faults and Ground Faults

Faults involving more than one phase with or without ground are designated as Phase faults. Faults involving any phase with ground are called ground faults. Thus, a system is subject to a total of 10 types of faults. Note that an L-----L-----G fault is classified as a phase fault rather than as a ground fault.

Phase fault	No. types	Ground fault	No. types
Three-phase	1	L-----G	3
L-----G	3
L-----L	3

1.2.2. Permanent Faults

Permanent faults are created by puncturing or breaking insulators, breaking conductors, and objects falling on the ground conductor or other phase conductors. These faults are detected by relays and trip the circuit breaker, which remains locked out.

1.2.3. Transient Faults

Transient faults are of short duration and are created by transient overvoltages. These types of faults will be dealt with in Chapter 2. Basically, this fault is caused by a flashover across the insulation due to abnormal transient overvoltages, which of course are bypassed, but results in subsequent power follow current of power frequency. The relaying system sees this fault and clears it by tripping the circuit breaker (CB). After a time the fault path is deionized, and the CB can be closed automatically to restore the supply to the equipment.

1.2.4. Semitransient Faults

Semitransient faults are created by an external object such as a tree branch or rodent. In medium-voltage lines a multishot automatic CB reclosure can burn out the object, causing a fault, restoring the equipment and improving supply reliability. Such multishot reclosures are employed only in medium-voltage lines, since the fault levels are low. They cannot be applied to high-voltage lines due to abnormal fault currents and the subsequent damage.

1.3. EFFECTS OF FAULTS

The three-phase fault is the most dangerous since it causes maximum abnormal short-circuit current. In general, if faults are not cleared rapidly, the following statements are true.

1. Generators, transformers, busbars, and other equipment are likely to be damaged due to overheating and the sudden mechanical forces developed.

2. Arcing faults invariably are a fire hazard and permanently damage the equipment. The fire can also spread in the substation unless the fault current is eliminated by suitable relaying equipment and circuit breakers. Note that the relay and CB must work together for short-circuit protection.

3. Faults can reduce the voltage profile on the entire electrical system, thereby affecting the loads. A frequency drop may lead to instability among interconnected, synchronously running generators, which, unless halted by suitable means, result in cascade tripping of generators. Hence, the purpose of interconnecting power stations (power transfer over tie lines) is lost.

4. Unsymmetrical faults result in voltage imbalance and negative sequence currents, which lead to overheating.

1.4. FAULT STATISTICS

As pointed out, a three-phase system is subjected to transient or permanent faults. The majority of L-----G faults are transient or arcing faults. One can overcome these faults by single-shot and high-speed autoreclosing to ensure supply reliability. On permanent faults the automatic CB reclosure will be unsuccessful and the CB will remain in the open position. A considerable amount of dislocation to the load takes place.

Therefore, the choice of autoreclosing depends on the statistical nature of the faults. If most faults are of a transient nature, autoreclosing is a must and will be successful. No relaying scheme can, by itself, detect whether a fault is transient or permanent. Statistically, about 80% of faults are transient and 20% are permanent. Thus, CB reclosures will always be applied, irrespective of the fault. It will be successful on transient faults, ensuring reliability, whereas it will be unsuccessful on permanent faults, leading to partial loss of supply.

1.5. PURPOSE AND REQUIREMENTS OF PROTECTIVE RELAYS

The type of failure that causes the greatest concern is the short circuit or fault. The fault may lead to various abnormal conditions on the system, such as changes in current, voltage, frequency, phase angle, rate of change of these quantities, direction of power flow (active as well as reactive), and perhaps many others, still unnoticed and unutilized to synthesize new relays. A new development is a high-impedance ground-fault relay based on fault-generated harmonics.

Most existing relays are energized by voltage and/or current supplied by current and voltage transformers. The purpose of current transformers (CTs) and potential transformers (PTs) is to reduce voltages and currents to levels manageable by the relays and to physically isolate relays from high voltage.

The basic or primary function of the relay can now be defined. A relay detects the faulty element in the integrated power system and removes it, with the help of the circuit breaker, from the remaining healthy system as quickly as possible to avoid damage and ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Introduction
• Preface
• 1. Basic Philosophy of Relaying
• 2. Difference in Protection Requirements for Increasing Line Voltage
• 3. Line Protection: Overcurrent and Directional Relays
• 4. Distance Protection: HV and EHV Line Protection
• 5. Carrier Schemes for HV and EHV Lines
• 6. Current and Potential Transformers
• 7. Basics of Differential Relays
• 8. Generator Protection
• 9. Transformer Protection
• 10.  Bus Bar Protection
• 11.  Test Procedures, Benches, and Maintenance Schedules
• 12.  Recent Advances and Futuristic View
• 13.  Central Computer Control and Protection
• References
• Index