Training _ Power System Protection _AREVA

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TRAINING ON POWER

SYSTEM PROTECTION

APPS COMBINED 'COURSE

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INTRODUCTION

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POWER SYSTEM PROTECTON

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Protection Generators Setting Criteria

Distance Protection Notes Distance Protectiorr Schemes Busbar Protection

Motor Protection A C Motor Protection Motor Setting Criteria Notes 1 C T S

Notes Additional Analysis Notes Unbalanced Faults Tutorial Balanced Faults Tutorial Grading Examples Tutorials Generator Protection Tutorial C T Selection

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Overview

Of

Protection

Fundamentals

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APPS- Corn bined course

Overview Of Protection Fundamentals Page 2 of 0

OVERVIEW OF PROTECTION FUNDAMENTALS

1.0 INTRODUCTION

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Relays are compact devices that are connected throughout the power system to detect intolerable or unwanted conditions within an assigned area. They are in effect, a form of active insurance designed to maintain a high degree of service continuity and limit equipment damage. They are "Silent Sentinels". While protective relays will be the main emphasis of this chapter, other types of relays, applied on a more limited basis or used as part df a total protective relays system will also be covered.

2.0 CLASSIFICATION OF RELAYS

I Relays can be divided into five functional categories:

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I a. P r o t e c t i v e R e l a y s , which detect defective lines, defective apparatus, or

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other dangerous or intolerable conditions. These relays can either i initiate or permit switching or simply provide an alarm.

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b. M o n i t o r i n g R e l a y s , which verify conditions on the power system or in the

protection system. These relays include fault detectors, alarm units, channel-monitoring relays, synchronism verification, and network phasing. Power system conditions that do not involve opening circuit

, _{breakers during faults can be monitored by these relays.}

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I _{c. P r o g r a m m i n g R e l a y s , which establish or detect electrical sequences.}

Programming relays are used for reclosing and synct-~ronising.

I d. R e g u l a t i n g R e l a y s , which are activated when an operating parameter

deviates from predetermined limits. Regulating relays function through supplementary equipment to restore the quantity to the prescribed limits.

e. A u x i l i a r y R e l a y s , which operate in response to the opening or closing of

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the operating circuit to supplement another relay or device. These include timers, contact-multiplier relays, sealing units, receiver relays, lock-out relays, closing relays and trip relays.

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Overview Of Protection Fundamentals Page 3 of O-

In addition to these functional categories, relays may be classified by input, operating principle or structure and performance characteristic:

Input Current voltage Power Pressure Frequency - Temperature Flow Vibration

(ii) Operating Principle of Structure Percentage

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Multi-restraint

>

Product

>

Solid state

>

Electromechanical

>

Thermal.

The above c~assifi~ation and definitions are based on the ANSI Standard 37.90 (IEEE 31 3).

3.0 PROTECTIVE RELAYING SYSTEMS AND THEIR DESIGN

Technically, most relays are small systems within themselves. Throughout this chapter, however, the term systems will be used to indicate a combination of relays of the same or different types. Properly speaking, the protective relaying system includes circuit breakers as well as relays. Relays and circuit breakers must function together; there i s little or no value in applying one without the other.

Protective relays or systenls are not required to function during normal power system operation, but must be immediately availa,ble to handle intolerable system conditions and avoid serious outages and damage. Thus,. the true operating life of these relays can be on the order of a few seconds, even though they are connected in a system for many years. In practice, the relays operate far more during t.esting and maintenance than in response to'adverse service conditions.

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In theory, a relay system should be able to respond to the infinity of abnormalities that can possibly occur within the power system. In practice, the relay engineer must arrive at a compromise based on the four factors that influence a n y re!oy rrpp!icatisn:

a. Economics - Initial, operating and maintenance.

b. Available measure of fault or trouble - Fault magnitudes and location of

current transformers and voltage transformers.

c. Operating practices - Conformity to standard and accepted practices; ensuring efficient system operation.

d. Previous experience - History and anticipation perhaps better expressed of trouble likely to be encountered within-the system-.

The third and fourth considerations are perhaps better expressed as the "personality of the system and the relay engineer".

Since it is simply not feasible to design a protective relaying system capable of handling any potential problem, compromises must be made. In general, only those problems, which according to past experience are likely to occur, receive primary consideration. Naturally, this makes relaying somewhat of an art. Different relay engineers will, using sound logic, design significantly different proteclive systems for essentially the same power system. As a result there is little standardisation in protective relaying. Not only may the type of relaying system vary, but also will the extent of the protective coverage. Too much protection i s almost as bad as little.

Nonetheless, protective relaying i s a highly specialised technology requiring an in-depth understanding of the power system as a whole. The relay engineer must know, not only the technology of the abnormal, but have a basic understanding of all the system components and their operation in the system. Relaying, then, i s a "Vertical" specialty requiring a "horizontal" viewpoint. This horizontal, or total system, concept of relaying includes fault protection and the performance of the protection system during abnormal system operation such as severe overloads, generation deficiency, out-of-step conditions, and so forth. Although these areas are vitally important to the relay engineer, his concern has not always been fully appreciated or shared by his colleagues. For this reason, close and continued communication between the planning, relay design, and operation systems should be mandatory, since power systems grow and operating conditions change.

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APPS- Combined course

Overview Of Protection Fundamentals Page 5 of 0

A complex relaying system may result from poor system design or the economic need to use fewer circuit breakers. Considerable savings can be realized by using fewer circuit breakers and a more complex relay system. Such systems usually involve design compromises requiring careful evaluation, if acceptable protection is to be maintained.

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4.0 DESIGN CRITERIA

The application logic of protective relays divides the power system into --

several zones, each requiring its own group of relays. In all cases, the five design criteria listed below are common to any well-designed and efficient protective system or system segment:

a. Reliability - the ability of the relay p r relay system to perform correctly

when needed (dependability) and to avoid unnecessary operalion (security).

b. Speed

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minimum fault time and equipment damage:

c . Selectivity

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maximum service continuity with minimum system disconnection.

d. Economics

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maximum protection at minimum cost. e. Simplicity - minimum equipment and circuitry.

Since it is impractical to fully satisfy all these design criteria simultaneously the necessary compromhes must be evaluated on the basis of comparative risks.

4.1 Reliability

System reliability consists of two elements - dependability and security. Dependability is the certainty of correct operation in response to system trouble, while security i s the ability of the system to avoid mis-operation between faults. Unfortunately, these aspects of reliability tend to counter one another: increasing security tends to decrease dependability and vice versa. In general, however, modern relaying systems are highly reliable and provide practical compromise between security and dependability.

Protective relay system must perform correctly under adverse sysfem and environmental conditions. Regardless of whether other systems are momentarily blinded during this period, the relays must perform accurately and dependably. They must either operate in response to trouble in their assigned area or block correctly i f the trouble is outside their designated area.

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Dependability c a n be checked relatively easily in the laboratory or during installation by simulated tests or staged faults. Security on the other hand is much more difficult to check. A true test of system security would have to measure response to an almost infinite variety of potential transients and counterfeit trouble indicalions in the power system and i t s environment. A secure system is usually the result of a good background in design combined with extensive miniature power system testing and can only be confirmed in the power system itself and its environment.

4.2 Speed

Relays that could anticipate a fa~llt w o ! ~ l d be utopian. But, even if 'available, they would doubtlessly raise the question of whether or not the fault gr trouble really required a trip-out. The development of faster relays must always be measured against the increased probability of more unwanted or unexplained operations. Time, no matter how short, i s still the best method of distinguishing between real and counterfeit trouble.

Applied to a relay, high speed indicates that the operating time usually does not exceed 50 ms (3 cycles on a 60-hertz base). The term instantaneous indicates that no delay i s purposely introduced in the operation. In practice, the terms high speed and instantaneous are frequently used interchangeably.

4.3 Selectivity versus Economics

High speed relays provide greater service continuity by reducing fault damage and hazards to personnel. These relays generally have a higher initial cost, which, however, cannot always be justified. Consequently, both low and high-speed relays are used to protect power systems. Both types have high reliability records. Records on protective relay operations consistently show 99.5% and better relay performance.

4.4 Simplicity

As in any other engineering discipline, simplicity in a protective relay system is always the hallmark of a good design. The simplest relay system, however, is not always the most economical. As previously indicated, major economies are possible with a complex relay system that uses a minimum number of circuit breakers. Other factors being equal, simplicity of design improves system reliability - if only because there are fewer elements that can malfunction.

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APPS- Combined course

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i 5.0 FACTORS INFLUENCING RELAY PERFORMANCE

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Relay performance i s generally classed as: ( 1 ) Correct

(2) No conclusion

( 3 ) lncorrect

lncorrect operation may be either failure to trip or false tripping. The cause of incorrect operation may be, a) Wrong application, b) lncorrect settings, c ) A personnel error or 4) Equipment mal-function. Equipment that can cause an incorrect operation includes current transformers, voltage transformers, circuit breakers, cable and wiring, relays, channels or station batteries.

lncorrect tripping of circuit breakers not associated with the trouble area is often as disastrous as

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failure to trip. Hence, special care must be taken in both appiication and installation to ensure against the possibility of incorrect tripping. -

" No conclusion" is the last resort when no evidence is -available for a correct or incorrect operation. Quite often this is a personnel involvement. 6.0 Zones of Protection

The general philosophy of relay application is to divide the power system into protective zones that can be protected adequately with the mininwm amount of the system disconnected. The power system is divided into protective zones for:

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Generators ii) Transformers iii) Buses

iv) Transmission and distribution circuits

v ) Motors

. A typical power system and its zones of protection are shown in Figl. The purpose of the protective system is to provide the first line of protection, within the guide-lines outlined above. Since failures .do occur, however some form of backup protection is provided to trip out the adjace13f breakers or zones surrounding the trouble area. Protection in each zone is overlapped to avoid the possibility of unprotected areas

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The device switching equipment are referred to by numbers, with appropriate suffix letters when necessary, according to the functions they

- perform.

These numbers are based on a system adopted as standard for automatic switchgear by IEEE and incorporated in American Standard C37.2 - 1970. 'This system is used in connection diagrams, in instruction books and in specifications.

8.1 Device Numb'erina Device Number 1 Definition Master Elemenl Function

It is an initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly or through such permissive devices as protective and time

delay relays. t o

place an equipment in or out of

Starting or Closing Relay 2

a desired amount of time delay before or after any point of operation in a switching sequence or protective relaying system, except as specifically provided by device function 48, 62 and 79

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described later.

Checking or Interlocking Relay

Time Delay

It is a device which operates in response to the position of a number of other devices (or to a number of predetermined conditions), in an equipment, to allow an operating sequence to proceed, to stop, or to provide a check of the position of these devices or of these conditions

i for any purpose.

operation.

It i s a device which functions to give

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Overview Of Protection Fundamentals

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Device Number Definition

Master Contactor

Page 1 1 of 0

Function

It is a device, generally controlled by the device No.1 or equivalent, and the required perrr~issive and protective devices, that serve to make and break the necessary control circuits to place an equipment into operation under the desired conditions and to take it out

to shut down an equipment and hold it out of operation This device

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may be manually or Electrically

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actuated, but excludes the function of electrical lockout (see device

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5 - Anode Circuit Breaker Stopping Device Control Power Disconnecting Device

of operation under other or abnormal conditions.

It is a control device used primarily

function 86) on abnormal conditions. 1

It is a device whose principal) function is to connect a machine.to its source of staitina voltaae.

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It is one used in-the anode circuits of a power rectifier for the primary purpose of interrupting the rectifier 6

circuit if an arc back should occur. It is a disconnecting device - such as a knife switch, circuit breaker or pullout fuse block, used for the purpose of connecting and disconnecting the source of control power to and from the control bus or equipment.

Note: Control power is considered to

include auxiliary power, which supplies such apparatus as sn~all Starting Circuit

Breaker I

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any other reversina functions.

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Reversing

Device

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Unit Sequence

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It is used to change the sequence in

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motors and heaters.

It is used for the purpose of reversing a machine field or for performing

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Switch

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which units may be placed in and

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The device switching equipment are referred to by numbers, with appropriate suffix letters when necessary, according to the functions they

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perform.

These numbers are based on a system adopted as standard for automatic switchgear by IEEE and. incorporated in American Standard

C37.2 - 1970. This system is used in connection diagrams, in instruction books and in specifications.

8.1 Device Numb'erina Device Number 1 -- Definition Master Element Function

It is an initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly or through such permissive devices as protective and time

delay relays. to

Checking

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P

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except as specifically provided b y device function 48, 62 and 79 described later.

It is a device which operates in Time Delay

Stariing or Closing Relay

predetermined conditions), in an equipment, to allow an operating sequence to proceed, to stop, or to provide a check of the position of these devices or of these conditions place an equipment in or out of operation.

It is a device which functions to give a desired amount of time delay before or after any point of operation in a switching sequence or protective relaying system,

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Interlocking

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Relay

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for any purpose.

response to the position of a number of other devices (or to a number of

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Overview Or Protection ~undamental~ page 1 1 of 0

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Function

It is a device, generally controlled b y the device No.1 or equivalent.

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the required permissive and

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protective devices, that sr=rL/rs to make and break the rlecessarY control circuits to place a r t , equipment into operation vrde! 'the I desired ~onditiof?s end to toke i i out of operation ~ n d m TJ~I-!F-: Or

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abnormal conditic:;~. ____----

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It is a coritiol d,-\:jCe usee ;,:irr~arily I to shut down

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equipn.5~1: a r i d )

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hold it out of

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may be mancc:ii. or I:5c.::i~~lly , actuated, but exc,,zss ;r,5 f;,..ctior~

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Definition

Master Contactor

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Stopping Device

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purpose

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circuit if ar: arc F - : A 7 : T ; ~ , . J ~ .

It i s a discanne;- -; ,I. 'p L - , - ;:,C~I

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as a knife switc: L-.-=_ - L-3r;/er

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pullout fuse b!.zqzq - - -

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purpose o f I:---

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disconnecting ii-5 5

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include zct..rilic-. 1 . . :- ,,.,< ~f-r supplies sctci, r -

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Anode Circuit

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Breaker 1

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an other re\,ersir,z -- -.y .T; : -- . .

Unit Sequence It is used :p , - ~ ? C ~ . - . Z - - --.-

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Disconnecting Device

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Overview Of Protection Fundamentals Page 14ofO Device Number

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the armature winding of a machine, or that of a load limiting or load shifting resistor or of a liquid or other

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medium exceeds a p.redefermined value ; or if the temperature of the

protected appa~.atus, such as a

Definition

Synchronising or Synchronism-

power rectifier, or of any medium decreases below a predetermined

Function

It is a device that operates when two a c circuits are within the desired Check Device

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Apparatus Thermal Device

limits of frequency, phase angle and voltage, to permit' or to cause the paralleling of these two circuits. It is a device, which functions when the temperature of the shunt field or

presence of the pilot or main flame In such apparatus as a gas turbine

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value.

It is a device, which functions on a given value 'of undervoltage.

It is a device that monitors the

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Isolating

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Direc:'anal

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It is a device which functions on a

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Under Voltage

Relay

Flanie detector

or a steam boiler.

It is a device used for disconnecting

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one circuit from another for the purposes of emergency operation, maintenance, or test.

It is a non-automatically reset device that gives a number of separate visual indications upon the functioning of protective devices and which may also be arranged to

perform a lockout function.

It connects a circuit such as the shunt field of a synchronous converter, to a source of separate excitation during the starting sequence ; or one which energises the excitation and ignition circuits of a nower rectifier.

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Contcctor 30 3 1 Annunciator relay - Sepcrzl te Excitciion Device

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. Overview Of Protection Fundamentals

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Power Relay

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desired value of power fiow in

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Definition

given direction, or upon reverse power, like resulting from arc back in

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the anode or cathode circuits of a

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Function

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main device or piece of apparatus,

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Sequence Device

Position Switch

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operated multi-conjact switch, or the equiv'alent, or a programming device, such 0 s a computer, that establishes or determines the power rectifier.

It makes or breaks contact when the

operating sequence of the major devices in an equipment during starting and stopping or during other Master

which has no device function number, reaches a given position. It is a device such as a motor-

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Operating, or

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shifting, the brushes of a machine, or

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Voltage Device Brush-

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predetermined polarity only verifies the presence of a polarising sequential operations.

It is used for raising, lowering, or Slip-ring-short- circuiting Device Polarity or Polarising

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Relav

for short circuiting i t s slip rings, or for engaging or disengaging the contacts of a mechanical rectifier. It operates or permits the operation of another device on a ,

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37 Bearing Protective Device Mechanical Condition Monitor Undercurrent or Under power predetermined value.

It functions on excessive bearing temperature, or on other abnormal mechanical conditions, such as undue wear, which may eventually result in excessive bearing voltage in an equipment.

It functions when the current or power flow decreases below a

occurrence abnormal

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Overview Of P r ~ ? x t ! o n Fundamentals Page 1 6 of 0

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Device Number

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Definition Function

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- Field Relay

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Field Circuit

associated with bearings as covered under device function 38), such as excessive vibration, eccentricity, expansion, shock, tilting, or seal failure.

It functions on a given or abnormally low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an a c machine indicating abnormally low field excitation.

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It is a device, which functions to

Breaker

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function i s to connect a machine to its source of running or operating voltage. This function may also b e used for a device, such as a contactor, that is used in series with Breaker

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Device transfers the switching equipment or of some

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of the devices.

apply, or to remove the field excitation of a machine. 42

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- Manual Transfer Selector Unit Sequence

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Starting Relay I

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a circuit breaker or other fault protecting means, primarily for frequent opening and closing of the circuit.

It transfers the control circuits so as to modify the plan of operation of

It is a device, whichfunctions to start the next available unit in a multiple- unit equipment on the failure or on.

I Atmospheric

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_Condition

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Monitor

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I I preceding unit.

It is a device that functions upon the occurrence of an abnormal atmospheric condition, such as damaging fumes, ex'plosive mixture, smoke or fire.

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the non-availability of the normally

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Reverse-Phase. It is a relav which functions when the

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Overview Of Protection Fundamentals Page 1 7 of 0

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Device Number

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Phase-Balance, Current Relay

poly-phase currents are of reverse phase sequence, or when the poly- phase cu:cer?,?s fire vnbolonced zl; contain negative phase-sequence components above a given

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Sequence -

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value of poly phase voltage in the

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4 7 Voltage Relay Incomplete Phase - Sequence Relay amount.

It functions, upon a predetermined desired phase sequence.

It is a relay that generally returns the equipment to the normal, or off, position and locks it out if the normal starting, operating or stopping sequence is not properly completed within a predetermined time. If the

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device is used for alarm purpose only, it should preferably be

Transformer, Thermal Relay

,

l+achine, or

temperature - of a machine armature, or other -load carrying winding or element of a machine, or the temperature of a power rectifier designated as 48A (alarm).

It is a relay that functions w.hen the

or power transformer (including a power rectifier transformer) exceeds an medetermined value.

Ilnstantaneous [ I t is a relay that functions1 overcurrent, or Rate of rise Relay - AC Time Overcurrent Relay I 52

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AC Circuit instantaneously on an excessive value of current, or on an excessive current rise, thus indicating a fault in the apparatus or circuit being protected.

It is a relay with either a definile or an inverse time characteristic lhat functions when the current in an ac circuit exceeds a predetermined value.

It i s a device that is used to close

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Breaker and interrupt an ac power circuit

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. . Overview Of Pr~?w-t!on Fundamentals

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Device Number

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Field Relay

associated with bearings as covered under device function 38), such as

excessive vibration, eccentricity, expansion, shock, tilting, or seal failure.

It functions on a given or abnormally low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an ac machine indicating abnormally low field

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Running Circuit It is a device whose principal

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Breaker

' excitation.

It i s :a device, which functions to apply, or to remove the field

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function i s to connect a machine to its source of running or operating voltage. This function may also b e

4 1 Field Circuit Breaker

protecting means, primarily

frequent opening and closing of the

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used for a device, such as- a contactor, that i s used in series with a circuit breaker or other fault

.

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the non-availability of the normally

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Monitor

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atmospheric condition, such damaging fumes, explosive mixture, Manual Transfer

or Selector Device transfers

circuit.

It transfers the control circuits so as to modify the plan of operation of the switching equipment or of some

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OALSTOM Limited, Energy Automation & Information of the devices.

It is a device, which functions to start the next available unit in a multiple- unit equipment on the failure or on

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~e/erie-phase,

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smoke or fire.

l t i s a reiay which functions when the Unit Sequence

Starting Relay

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Overview Of Protection Fundamentals Page 1 7 of 0

4 7 It functions upon a predetermined

value of poly phase voltage in the

Device Number

Volta e Rela Incomplete

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Sequence Relay

desired phase sequence.

It is a relay that generally returns the equipment to the normal, .or off, position and locks it out i f the normal starting, operating or stopping sequence is not properly completed within a predetermined time. If the device is used for alarm purpose

Definition

Phase-Balance, Current Relay

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only, it should preferably be

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Function

poly-phase currents are of reverse phase sequence, or when the poly- phclse currents

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unbalaficzd o i contain negative phase-sequence cornponents above a given amount. Transformer, Thermal Relay I 49

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Machine, or Instantaneous overcurrent, o i Rate of rise , Relay -

designated as 48A {alarm).

It i s a relay that functions when the temperature of a machine armature, or other load carrying winding or element of a machine, or the temperature of a power rectifier or power transformer (including a power rectifier transformer) exceeds an predetermined value.

It is a relay that functions instantaneously on an excessive value of current, or on an excessive current rise, thus indicating a fault in the apparatus or circuit being ~rotected.

AC Time

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It i s a relay with either a definile or Overcurrent

Relay

an inverse time characterisiic l l m t functions when the current in an ac

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circuit exceeds a predetermined

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value.

AC Circuit It is a device that is used to c:lose Breaker

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and interrupt an ac power circuit

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under normal condilions or to

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Overview Of Protec?bn Fundamentals Page 20 of 0

Overcurrent

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desired value of ac overcurrent

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Relay

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flowing in a predetermined

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Device Number

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for blocking of tripping on external faults in a transmission line or in other apparatus under predetermined

Definition

AC directional

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Blocking Relay

conditions, or co-ordinates with other devices to block tripping or to

Function

mechanical positioning.

It is a relay that functions on a

direction.

It is a relay that initiates a pilot signal

block re-closing on an out-of-step

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condition or on powerswing:.

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ermissive It is - generally a two position,

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Control ~ e v i c e ' manudlly operated switch that in

I one position permits the closing of a

circuit breaker, or the placing of an equipment into operation, and in the other posilion prevents the circuit breaker or the equipment

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Level switch

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Rheostat

used in an electric circuit, which is electrically operated or has other electrical accessories, such as auxiliary position or limit switches.

It is a switch which operates on from being operated.

It is a variable resistance device

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given values, or on a given rate of

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7 2

I

DC circuit

i

OALSTOM Limited, inergy Automation & Informalion change, of level.

It is used to close and interrupt a d c

1

Breaker -

1

power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

73 i Load - Resistor

' Contactor

1

_I

I

_I

It is used to shunt or insert a step of load limiting, shifting, or indicating resistance in a power circuit, or to switch a space heater in circuit, or to switch a light, or regenerative load resistor of a power reclifier or

(22)

Overview Of Protection Fundamentals Page 21 of 0

1

Device Number Definition

1

Function

other machine in and out of circuit. Alarm ~ e b y It i s a device other than the

annunciator, as covered under device No.30, which is used to operate in connection with a visual that isused for Changing

Mechanism

moving a muin device from one position to another i n a n equipment ;as . for example, shifting a removable circuit breaker unit to

1

and from the connected, DC Overcurrent

1

77 /

Pulse Transmitter

1

It i s used to generate and transmit

I

disconnected, and test positions. It is a relay that functions when the Relay

I

pulses over a telemetering or pilot-

/

wire circuit to the remote indicating current in a dc circuit exceeds a aiven value.

(

out-of-ste-) Protective -

i

78 Angle

/

',","::ring, or

between two voltages or between two currents or between voltage or receiving device.

It is a relay that funclions at a predetermined phase angle

that controls Relay automatic reclosing and locking out

8

0

Flow Switch It is a switch, which operates on

1

I

1

given values, or on a given rate of

I

Relay

1

8

1 I

~iequency

predetermined value of frequency, either underlover on normal system frequency or rate of , change of change, of flow.

1 I

Relay

-

1

automatic closing and reclosing of a

(

1-

82

dc circuit interrupter, generally in resoonse to load circuit conditions.

- -

OALSTOM Limited, Energy Automation & Information DC Re-closing

frequency. 1

(23)

Overview Of Protec?i@r! Fundamentals Page 20 of 0

Device Number

1

I

1

67

1

AC directional

apparatus under predetermined conditions, or co-ordinates with mechanical positioning.

/

Overcurrent

(

Relay I 68

_I

Blocking Relay

-

I

/

desired value of ac overcurrent flowing in a predetermined direction.

It is a relay that initiates a pilot signal for blocking of tripping on external faults in a transmission line or in other

-

circuit breaker, or the placing of an equipment into operation, and in

!

! -

69

1

Permissive i Control Device

other devices to block tripping or to block re-closing on an out-of-step condition or on po\der swings:

It is generally a two position, manually operated switch that in one position permits the closing of a

1

electrically operated or has other I

I

70

I

Rheostat

the other position prevents the circuit breaker or the equipment from being operated.

It i s a variable resistance device used in an electric circuit, which i s

I -

-

7 1 i Level Switch

electrical accessories, such a: auxiliary position or limit switches. It is a switch which operates or

72

I

DC circuit

1

Breaker - I Contactor I I

given values, or on a given rate of change, of level.

It is used to close

power circuit under normal conditions or to interrupt this circuit under fault or emergency

7 3 : Load - Resistor

load limiting, shifting, or indicating resistance in a power circuit, or to switch a space heater in circuit, or conditions.

It is used to shunt or insert a step of

OALSTOM Limited, inergy Automation & Information

1

i

to switch a light, or regenerative load resistor of a power rectifier or

(24)

i

OALSTOM Limited, Energy Automation & information

APPS- Combined course \

Overview Of Protection Fundamentals

Page 21 of 0

Function

other machine in and out of circuit. It is a device other than the annunciator, as covered under device No.30, which i s used i o operate in connection with a visual or audible alarm.

It is a mechanism that i s used for moving a main device from one position to another in an equipment ;as for example, shifting a removable circuit breaker unit to and from the connected, disconnected, and test posi lions. It is a relay that functions when the 1

1

i i ? I Device Number 74 75

I

76 Definition Alarm ~ e l a y Position Changing Mechanism DC 0ve:current

current in a dc circuit exceeds a given value.

It is used to generate and transmit pulses over a telemetering or pilot- wire circuit to the remote indicating or receiving device.

It is a relay that functions at a predetermined phase angle between two voltages or between two currents or between voltage and current.

It is a relay that controls the automatic reclosing and locking out of an ac circuit interrupter.

It is a switch, which operates on given values, or on a given rate of change, of flow.

It is a relay that functions or-) a predetermined value of frequency, e~ther underJover on normal system frequency or rate of change of frequency.

It is a relay that controls automatic closing and reclosing of a dc circuit inter~upter, generally in response to load circuit conditions.

I

i

l

1

77 Pulse Transmitter I I I 78

I

Phase Angle

I

Measuring, or I out-of-step

I

I Protective

'

79 Relay I AC Re-closing , -

80

8 1 -

82

Relay Flow Switch ~ t e q u e n c ~ Relay DC Re-closing Relay -

(25)

Overview Of Protection Fundamentals Page 24 oi 6

1

Device Number

I

Definition

1

Function

(

to 94 i s suitable.

I

8.2 Devices Performing

ore

Than One Function

If one device performs two relatively important functions in an equipment so that it is desirable to identify both of these functions, this may be done by using a double function number and name such as:

50/51 - Instantaneous and Time Overcurrent Relay.

8.3 SuffixNumbers

If h.10 or more devices with the same function number and suffix letter (if

used) are present in the same equipment, they rlay be distinguished by numbered suffixes as for example, 52X-1, 52X-2 and 52X-3, when necessary.

8.4 Suffix Letters

Suffix letters are used with device function numbers for va~~ious purposes. In order to prevent possible conflict each suffix letter should have only one meaning in an individual equipment. All other words should use the abbreviations as contained in ANSI Y 1.1 latest revision, or should use some other distinctive abbreviation, or be written out in full each time they are used. The meaning of each single suffix letter, or combination of letters, should b e clearly designated in the legend on the drawings or publications applying to the equipment.

Lower case (small) suffix letters are used in practically all instances on electrical diagrams for the auxiliary, position, and limit switches. Capital letters are generally used for all other suffix letters. Th,e letters should generally form part of the device function designation, are usually written directly after the device function number, as for example, 52CS. 71 W, or 49D. When it is necessary to use two types of suffix letters in connection with one function number, it is often desirable for clarity to separate them by a slanted line or'dash, as for example, 20D/CS or 2OD-CS. .

The suffix letters which denote parts of the main device, and those which cannot or need not form part of the device function designation, are generally written directly below the device function number on drawings, as for example, 52/CC or 43/A.

-

8.9 Standard reference positions of some typical devices

(26)

Overview Of Protection Fundamentals Page 250f 0 ,

Device

Power Circuit Breaker Disconnecting Switch Load-break swiich

-

Valve

Gate Clutch

Standard Reference Position

Main Contacts Open Main Contacts Open Main Contacts Open

Closed Posilion

)

Closed Posilion Disengaged Position

Adjusting Means Position

Relay (2)

1

Pressure Switch 131

I

Lowest Pressure

1

Disengaged Position Maximum Gap Position Rheostat - Maximum resistance Position

Contactor (21 Contactor (latched-in-type) Temperature Relay (3) Level Detector (3) Flow Detector (3) Speed Switch (3) Vibration Detector

(3)

7

)

Vacuum Switch (3)

1

Lowest Pressure i.e., Highest Vacuum

Note : If several similar auxiliary switches are present on the same device, they should be De-energised Position

Mairi Contacts Open Lowest Temperature

Lowest Level I

Lowest Flow Lowest Speed Minimum Vibration

designated numerically 1.2.3 etc, when necessary.

( 1 ) 'These may be speed, voltage, current, load, or similar adjusting devices comprising rheostats, springs. levers, or other components for the purpose.

( 2 ) These electrically operated devices are of the non-latched-in type, whose contact p,osition is dependent only upon the degree of energisation of the operating or restraining or holding coil or coils which may or may not be suitable for continuous energisation. The de-energised position of the device i s that with all coils de- energised.

(3) The energising influences for these devices are considered to be, respectively, rising temperature, rising level, increasing flow, rising speed, increasing vibration, and increasing pressure.

The simple designation "a" or "b" is used in all cases where there is no need to adjust the contacts to change position at any particular point in the travel of the main device or where the part of the travel, where the contacts change position is of no significance in the control or operating scheme. Hence fhe "a" or "b" designations usually are sufficient for circuit breaker auxiliary switches.

(27)

I

Overview Of Protection Fundamentals - .

Page 24 oi O

1

Device Number

1

Definition

I

Function

to 94 is suitable.

8.2 Devices Performing ~ 6 r e Than One Funciion

If one device performs two relatively important functions in an equipment so that it i s desirable to identify both of these functions, this may be done by using a double function number and name such as:

I

50151 - Instantaneous and Time Overcurrent Relay.

8.3 Suffix Numbers

If two or more devices with the same-function number and suffix letter (if used) are present in the same equipment, they may be distinguished by numbered suffixes as for example, 52X-1, 52X-2 and 52X-3, when necessary.

8.4 Suffix Letters

Suffix letters are used with device function numbers for various purposes. In order to prevent possible conflict each suffix letter should have only one meaning in an individual equipment. All other words should use the abbreviations as contained in ANSI Y 1 . l latest revision, or should use some other distinctive abbreviation, or be written out in full each time they are used. The meaning of each single suffix letter, or combination of letters, should be clearly designated in the legend on the drawings or publicalions applying to the equipment.

Lower case (small) suffix letters are used in practically all instances on electrical diagrams for the auxiliary, position, and limit switches. Capital letters are generally used for all other suffix letters. The letters should generally form part of the device function designation, are usually written directly after the device function number, as for example, 52CS, 71 W, or

490. When it is necessary to use two types of suffix letters in connection

with one function number, it is often desirable for clarity to separate them by a slanted line or'das!?, as for example, 20DJCS or 20D-CS.

The suffix letters which denote parts of the main device, and those which cannot or need not form part of the device function designation, are generally written direcily below the device function number on drawings, as for example, 52lCC or 43lA.

-

8.9 Standard reference positions of some typical devices

i

(28)

. . .- Overview Of Protection Fundamentals

Page 25 of 0

Device

I

Clutch

I

Disengaged Position

Standard Reference Position

Valve Gate

7

Main Contacts Open Main Contacts Open Main Contacts Open Closed Position

Closed Position

I

Level Detector 131

!

Lowest Level

1

-

1

Vacuum Switch (3)

1

Lowest Pressure i.e., Highest Vacuum

Note : If several similar auxiliary switches are present on the same device, they should be Turning Gear

Power Electrodes

Flow Detector (3)

1

Lowest Flow

designated numerically 1,2,3 etc, when necessary.

Disengaged 'Position

Maximum

Posi lion

Rheostat - Adjusting Means ( 1 ) Relay (2) Contactor (2) Contactor (latched-in-type) Temperature Relay ( 3 ) Speed Switch (3)

( 1 ) These may be speed, voltage, current, load. or similar adjusting devices conlprising rheostats, springs, levers, or other components for the purpose.

Maximum resistance Posilion Low or Down Position

De-energised position De-energised Position . Main Contacts Open

Lowest Temperature I

Lowest Speed

(2) These electrically operated devices are of the non-latched-in type, whose contact position is dependent only upon the degree of energisation of the operating or restraining or holding coil or coils which may or may not be suitable for continuous energisation. The de-energised position of the device is that with all coils de- energised.

(3) The energising influences for these devices are considered to be, respectively, rising temperature, rising level, increasing flow, rising speed, increasing vibration, and increasing pressure.

I

The simple designation "a" or "b" is used in all cases where there is no need to adjust the contacts to change position at any particular point in the travel of the main device or where the part of the travel, where the contacts change position is of no significance in the control or operating scheme. Hence fhe "a" or "b" designations usually are sufficient for circuit breaker auxiliary switches.

Vibration Detector (3) Pressure Switch (3)

OALSTOM Limited, Energy .4utomation & Informalion

-

I Minimum Vibration

(29)

Overview Of Pr~:ec:Icn Fundamentals Page 28 of 0 The following Chart gives a birds-eye view of the relay classifications based on technology.

I

Relays I

1

Electromechanical

1

Analogue .--

-+

I

Numerical

I

(30)

(31)

(32)

TYPES OF PROTECTION FUSES

The simplest form of overcurrent protection is the fuse. The fuse is capable of operating in less than 10ms for very large values of current, thus considerably limiting fault energy. However, it does have a number of disadvantages, namely;

Can be difficult to co-ordinate Its characteristic is fixed

Needs replacing ioiiowing iauit ciearance

Has limited sensitivity to earthfaults since it is rated above the full load current of the feeder

Operation of single fuse results in a condition refereed to as single phasing. Single phasing .can be disastrous for rotating plant such as motors.

The fuse characteristic is split into two sections, the 'Pre-arcing Time' and the 'Arcing Time'. The addition of these times is referred to as the 'Total Operating Time'.

Fault

PRINCIPLE OF OVERCURRENT PROTEClION

The purpose of overcurrent protection, as with other forms of protection, is to detect faults on a power system and as a result, initiate the opening of switchgear in order to isolate the faulty part of the system. The protection must thus be discriminative, that is to say it shall, as far as possible, select and isolate only the faulty part of the system leaving all other parts in normal operation.

Discrimination can be achieved by overcurrent, or by time, or by a combination of overcurrent and time.

DISCRIMINATION BY CURRENT

Discrimination by current relies upon the fact that the fault curren't varies with the position of the fault. This variation is due to the impedance of various items of plant, such as cables and transformers, between the source and the fault. Relays throughout the system are set to operate at suitable values such that only the relay nearest to the fault operates.

Relays which adopt this of operation are generally termed Instantaneous

overcurrent relays.

(Where the fault level does not vary greatly between two relay location then the use of

i n s t a n t a n e ~ s overcurrent relays is not possible).

DISCRIMINATION BY TIME

(33)

. -

:

If the fault level over a system is reasonably constant then discrimination by current will

not be possible. An alternatlile Is tc use time discrimination in which each overcurrent

relay is given a fixed ?irr?e delay with the relay farthest away from the source having the

shortest time delay. Operating time is thus substantially,independent of fault level but the main disadvantage is that the relay nearest the source will have the longest time delay and this is the point with the highest fault level.

Relays which adopt this principle of operation are generally termed definite (independent) time overcurrent relays.

NOTE : When applying definite time overcurrent relays care must be taken to ensure

that the thermal rating of the current measuring element is not exceeded.

(Relay Current Setting)

D1SCRIMlNATlON BY BOTH TlME AND CURRENT

Due to the limitat~ons imposed by the independent use of either t ~ m e or current, the

, inverse time overcurrent characteristic has been developed. With this character~st~c the

time of operation is inversely proportional to the current applied, i.e.; basically the higher

the current applied, the faster the relay operates. Thus, the actual characteristic IS a

function of both t ~ m e and current settings, thereby gaining the advantages of the

previous mentioned methods and eliminating some the disadvantages.

TlME

IS

Applied Current'

(34)

PRINCIPLES OF CO-ORDINATION

The principle of co-ordination refers to the procedure of setting overcurrent relays to ensure that the relay nearest the fault operates first and all other relays have adequate additional time to prevent them from operating. If the relay nearest fo the fault fails to clear the fault, and the co-ordination is correct, then the next up-stream relay should operate and so on towards the source, thus isolating the minimum amount of plant. The principle of co-ordination is often referred to as 'grading'.

When performing any co-ordination exercise the following need to be considered:

Relay Characteristics -

Relay Current Setting Grading Margin Time Multiplier Setting Relay Characteristics

There are numerous characteristics, however they all confirm to either BS142lIEC or ANSIIIEEE standards. The BS142lIEC standard incorporates the following

characteristics. Standard Inverse Very lnverse

Extremely Irlverse Long Time Inverse

The ANSIIIEEE standard incorporates the following characteristics: Moderately Inverse-

- Very lnverse

Extremely lnverse Short Time lnverse Inverse

The BS142lIEC standard curves are mainly adopted in the LIK and the most commonly

used ones are explained in more detail below:

Standard lnverse - This characteristic is commonly known as the 3110 characteristic,

i.e. at ten times setting current and TMS of 1 the relay will operate in 3 secs.

The characteristic curve can be defined by the mathematical expression :

where I - - applied current

-

15 - setting current

111, - - multiple of setting current

The standard inverse time characteristic is widely applied at all system voltages - a s back up protection on EHV systems and as the main protection on HV and MV distribution systems.

I

'

In general, the standard inverse characteristics are used when

There are no co-ordination requirements with other types of protective equipm2nt further out on the system, e.g. Fuses, thermal characteristics of transformers, motors etc. The fault levels at the near and far ends of the system do not vary significantly.

(35)

There is minimal inrush on cold load pick up. Cold load inrush is that c u i e n t which occurs when a feeder is energised after a prolonged outage. In general the relay cannot be set above this value but the current should decrease below the relay setting before the relay operates.

Very lnverse Time

-

This type of characteristic is normally used to obtain greater time

selectivity when the limiting overall time factor is very low, and the fault current at any point does not vary tno :vlde!y with system conditions. It is particularly suitable, if there is a substantial reduction of fault current as the distance from the power source increases. The steeper inverse curve gives tonger time grading intervals. Its operating time is

approximately doubled for a reduction in setting from figures 7 to 4 times the relay

current setting. This permits the same time multiplier setting for several relays in series. The characteristic curve can be defined by the mathematical expression :

13.5

t = -

-{i:

-

I]

Extremely lnverse Time

-

With this characteristic the operating time is approximately

inversely proportional to the square of the current. The long operating time of the relay at peak values of load current make the relay particularly suitable for grading with fuses and also for protection of feeders which are subject to peak currents on switching in, such as feeders supplying refrigerators, pumps, water heaters etc., which remain connected even after a prolonged interruption of supply.

For cases where the generation is practically constant and discrimination with low tripping times is difficult to obtain, because of the low impedance per line section, an extremely inverse relay can be very useful since only a small difference of current is necessary to obtain an adequate time difference.

-

Another application for this relay is with auto reclosers in low vo'ltage distribution circuits. As the majority of faults are of a transient nature, the relay is set to operate before the

normal operating time of the fuse: thus preventing perhaps unnecessary blowing of the fuse.

Upon reclosure, if the fault persists, the recloser locks itself in the closed posjtion and allows the fuse to blow to clear the fault.

This characteristic is also widely used for protecting plant against overheating since overheating is usually an I,t function.

(36)

This characteristic curve can be defined by the mathematical expression :

t = 80

{ti'

-

b o n g Time Inverse

-

This type of characteristic has a long time characteristic and may

be used for protection of neutral earthing resistors (which normally have a 30 sec rating).

The relay operating time at 5 times current setting is 30 secs at TMS of 1.

This can be defined by :

- Current Setting

The current setting of a relay is typically described aS either a percentage or multiple of

the current transformer primary or secondary rating.

If the CT primary rating is equal to the normal full load current of the circuit then the percentage setting will refer directly to the primary system. This is an important point as if, for example, the normal primary full load current was, say, 400 amp but the CT ratio was 50015 then a relay with setting range 50-200% of 5 amp set at 100% would not

represent a "full load" setting;-the actual setting would in fact be 125% of full load current.

The choice of current setting thus depends on the load current and the CT ratio and is

normally close to but above the maximum load current (typically'lO%) - assuming of

course the circuit is capable of carrying the maximum foreseeable load. It should be stressed at this point, that the relay is neither designed nor intended to be used as an overload relay but as a protective relay to protect the system under fault conditions. It is also important to consider the resetting of the relay. The relay will reset when the

current is reduced to 90%-95% of the setting (Depending on relay design) and if the

normal load current is above this value the relay will not reset after starting to operate under through fault conditions which are cleared by other switchgear.

The setting for a typical overcurrent relay with a reset ratio of 95% can be determined

using the following:

Where: Is = Setting

IF^ = Full Load Current

Grading Margin

As previously mentioned, to obtain correct discrimination it is necessary to have a time interval between the operation of two adjacent relays. This time interval or grading margin depends upon a number of factors :

a) The circuit breaker fault interrupting time

b) The overshoot time of the relay

C) Errors

d) Final margin on completion of operation (safety margin)

The discriminating relay can only be de-energised when the circuit breaker has

completely interrupted the fault current. It is now normal practice to use a value of 50 -

(37)

100 ms for circuit breaker overall interrupting time but obviously if it is known that the switchgear is slower than this time, this must be taken..injo account.-

Operating of the relay may continue for a short time after the relay is de-energised until a n i stored energy is dissipated. For example, an induction disc 'element will have stored kinetic energy (or inertia) and a numerical relay may have stored energy in capacitors. Although these factors are minimised by design, some allowance is usually necessary. It is common to use a figure of 50 ms.

NO-TE : The overshoot time is not the actual time during which some forward

operation takes plan but is the time that the relay would have taken to travel the same distance had the relay remained energised.

Travel

I

l o o O / o

Overshoot

Travel t l = relay de-energised

t3 - t l = actual overshoot time t2 - t l = overshoot time used in the

calculation of margin

t l t2 t3

All measuring dev~ces such as rejays and current transformers are subject to some degree of error The t ~ m e characteristic of either or both of the relays involved may have positive or negat~ve errors. Current transformer errors are mainly due t o the

rnagnetis~ng characteristic. It should be noted the CT errors do not affect definite time overcurrent relays.

A safety margin of 100 ms is normally added to the final calculated margin to ensure correct discrirn~nation. This additional time ensures a satisfactory contact gap (or equivalent) is maintained.

In the past, a fixed margin of 0-4 secs was considered adequate for correct

discrimination. With faster modern switchgear and lower overshoot times a figure of 0.35 secs is quite reasonable and under the best possible conditions 0-3 secs may be feasible.

However, rather than using a fixed margin it is better to adopt a fixed time for circuit breaker operation and relay overshoot and add to this a variable time value which takes into account relay and CT errors and the safety margin. This is particularly so when grading at low values multiples of setting current where the relay operating time is longer and a fixed total margin may be of the same order as the relay timing error. A fixed value 0-25 secs is chosen which is made up of 0.1 secs for circuit breaker operating time. 0.05 secs for relay overshoot time and 0.1 sec for safety margin.

In considering the variable time value, it is assumed that each IDMT relay complies with basic assigned error class 7.5 according to British practice in BS 142. The error for a class 7.5 relay IS 5 7.5%, but allowance should be made for the effects of temperature,

frequency and departure from the reference conditions as laid down in the BS. A more practical approximation is to assume a total effective error of 2 x 7-5 i.e. 15% and is to apbly to the relay nearest the fault which'is considered slow. To this total effective relay error a further 10% is added to allow for overall CT error.

Training _ Power System Protection _AREVA

TRAINING ON POWER

SYSTEM PROTECTION

APPS COMBINED 'COURSE

wwvv.areva-td.com/training

m

I

INTRODUCTION

TO

POWER SYSTEM PROTECTON

CONTENTS

-

Overview

Of

Protection

Fundamentals

OVERVIEW OF PROTECTION FUNDAMENTALS

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