PRINCIPLES AND PHILOSOPHY OF RELAYING PROTECTION

INTRODUCTION

Relaying protection is used to prevent or minimize damage to equipment and maintain continuous supply of Electricity at barest minimum cost. The need for relaying protection comes into play in providing the most efficient protection for power system equipment. This can be very expensive. To reduce such cost, a balance needs to be struck between the cost of the protection and the degree of safety to the equipment.

The main purposes of relaying protection are as stated below:

(i) To ensure uninterrupted power supply.

(ii) To reduce equipment damage.

(iii) To maintain quality of service.

(iv) To guarantee safety of life and property.

(v) To ensure operation of equipment at peak efficiency.

The earliest method of protection was the fuse. The fuse finds its use primarily in Distribution Circuits due to its cheapness and simplicity. Its use in system protection in the power industry is limited by the following disadvantages:

(a) The fuse is slow in operation

(b) Before power supply can be restored the fuse has to be replaced

(d) It cannot be used for very high voltage protection.

As a result of the above shortcomings, the use of fuses has generally been replaced with the protective relays.

THE RELAY

This is an electrical device that behaves in a prescribed way to an applied input so as to cause, by its contact operation, abrupt changes in associated control circuits.

In protective relaying, there are important parameters required for effective performance. They include:

(a) Sensitivity

A relay must be sensitive to the least fault conditions for which it has been configured.

(b) Reliability

It must be relied upon at all times to respond to any fault by relaying signals that will cause the faulty part to be isolated.

(c) Selectivity

The relay must be able to discriminate between faults and abnormal conditions.

(d) Simple

For a relay to be effectively used, its construction and operation has to be simple in nature.

(e) Speed of Operation

To be able to prevent damage to the associated equipment the relay is protecting, it must act fast before the damage is done.

(f) Cost

The relay should not be so expensive as to outweigh the benefit of using it to protect the associated equipment.

Fault Conditions

In power systems, faults occur as a result of breakdown in equipment insulation. These faults can be categorised as follows:

Single phase to ground fault
Double phase to ground fault
Three phase to ground fault
Phase to phase fault
Three phase fault.

The commonest, in occurrence, of the above fault conditions, is the single phase to ground fault which is about 70%.

Damage to equipment can be caused by other abnormal conditions in a power system. Such conditions are:

Over heating
Over voltage (surge)
Over load
Fire disaster
Unbalanced loading
Loss of synchronism

For the faults and abnormal conditions enumerated above protective relays are

designed to isolate and reduce damage to the system equipment.

RELAY TYPES AND CLASSIFICATIONS

Relays are classified according to the following:

· Input - voltage, current, frequency.

· Operating Principle – percentage or restraining.

· Function - Monitoring, Regulating, Auxiliary, Programming or Protection.

· Performance characteristics - Definite time, Inverse time or Distance.

· Structure - Static, Electromechanical or Thermal

Sometimes relays are also classified using a combination of the above terms, e.g. inverse time over current.

RELAY PERFORMANCE

Performance of relays can be classified as:

(i) Correct

(ii) Incorrect

(iii) Inconclusive.

Incorrect Operation

This can be due to the following factors:

(a) Poor Application

(b) Incorrect Relay Setting

(d) Equipment Malfunction

Incorrect tripping may be either failure to trip or false tripping.

Failure to trip can be caused by faulty associated instrument transformer, circuit breakers, control cables and wiring and station batteries.

Inconclusive Operation

This is the last resort when no evidence is available either for a correct or incorrect operation. Quite often, this is a personal involvement.

RELAY OPERATING TIME

Relays can be classified in terms of their operating times as follows:

· High Speed Relays - operate in less than three (3) cycles

· Slow speed relays - operate in three (3) cycles or more

· Time delay relays - have built in time delay facility to allow co-ordination with other relays within the power system.

· Instantaneous relays - have no deliberate time delay facility. They operate instantaneously.

ZONES OF PROTECTION

For effective protection of the system with minimum part disconnected during fault, protection zones are mapped out. These zones are created in such a way that each overlap around an isolating device such as a circuit breaker.

This method guarantees total protection of power system sub circuits. These zones follow common logical boundaries to cover such equipment as Lines, Transformers, Buses, Generators, Motors and any combinations of the above equipment.

For such boundaries to be genuine, there must be:

Measuring devices such as Current Transformer or Voltage Transformer.
Isolating devices such as breakers.

Generator Protection Zone

Typical faults occur within generators such as - Winding faults, Field Ground fault, A.C.

Over Voltage faults and Field Loss faults. The protective relay, on occurrence of any of these faults must act very fast to isolate the faulty part in order to save both the life of the equipment and the personnel around it.

Transformer Protection Zone

In this zone, the usual faults that can occur are as follows:

Winding faults, Phase to Ground faults, Phase-to-Phase faults and Inter turn faults.

For these faults, differential protection is the major type used for transformer protection. Oil/Winding temperature relays are also provided along with Bucholz gas Alarm/Bucholz surge trip protection.

These four methods are used to protect the transformer against faults within the windings. Over current/Earth fault protection are also provided.

Adequate protection to the transformer is cumbersome due to the following transformer constraints:

(i) There is phase shift in star connected transformers.

(ii) There are different voltage levels between the primary, the secondary and or the tertiary side of the transformer.

(iii) There is a high magnetizing inrush current especially during transformer energization. These currents show harmonic currents of the 2nd order and above.

In order to compensate for the above constraints, transformer differential schemes are designed taking them into account.

As an example:

(a) To prevent tripping of transformer on high magnetizing inrush currents, a harmonic restraint device is embedded in the differential relay, which prevents it from operating on inrush currents during transformer energization.

(b) Matching current transformers are used to correct the voltage level differences at both sides of the transformer and also to correct the differences in transformer characteristics of the current transformers on both sides of the transformer.

(c) Phase shift in the Star-Delta windings of transformers are taken care of by connecting star winding CT’s in Delta and Delta winding CT’s in Star.

In some cases the relay may be unstable as a result of spill current on the Delta side of the transformer due to zero sequence. These currents are filtered out by wiring part of the matching C.T. winding to cancel itself out of the delta side of the transformer.

The vector group of the transformer windings also plays a prominent role when the differential relay is wired. To take care of this, the delta wiring of the matching CT's must be wired according to the vector group of the winding as stipulated by the manufacturer.

Details of transformer differential are discussed fully in Chapter 9 on Power Transformer Protection.

Bus bar Protection Zone

The most common fault within this zone is the phase to ground fault generally caused

by flash over on insulators as a result of lightening. Other causes of this flash over are:

Ø Cracked insulators, birds and reptiles, dirty or broken insulators or animals that may walk close to the bus.

The bus has several lines/feeders tied to it. The current transformers on the bus get easily saturated due to these lines. The usual type of protection for the bus zone is the differential type. This method compares the current entering the bus zone with that leaving it.

Current transformers installed on each bus feed are used to make this comparison. Under a fault condition, the CT's on the faulted circuit get the sum of all the currents from the other circuits.

Lines Protection Zone

The transmission and distribution lines comprise the major means where by electric power is transported from generating source to the points where the energy is to be used. These lines run into thousands of kilometers.

Faults occurring on power transmission lines can be due to the following causes:

Lightning
Wind
Birds
Bush fire

There are four types of line faults namely: Line to Ground, Line to Line, Double Line to Ground faults.

Transmission Line protection can be classified as follows:

(a) Instantaneous/inverse time over current (non-directional)

(b) Instantaneous/inverse time over current (directional)

(d) Pilot wire using communication channels

(e) Current balance.

For effective line protection, the different protection schemes must be properly coordinated. Distribution lines are adequately protected using over current relays and fuses.

Distance protection is frequently used for voltages of 66KV and above. The scheme functions by comparing the system voltage and current and operates when the voltage-current ratio is less than a pre-set value.

Thus V = IZ, where V, I, Z, are system voltage, current and impedance respectively.

In normal operation, the system Z is fixed. This value reduces or increases depending upon an external or internal fault within the zone of protection. Impedance diagrams are usually used to show the characteristics of the distance relay and are usually known as Mho characteristics. Distance relays can be single-phase type or three-phase types.

In modern line distance protection, tripping in remote stations is facilitated through the use of communication channels. This method is called carrier-assisted distance protection scheme.

PRIMARY AND SECONDARY RELAYS

Primary Relays are the first line of defense in the system. They are generally high-speed relays. The primary relay scheme is designed to remove minimum equipment

from service.

Secondary Relays also called backup relays are intentionally delayed in their operation so as to give the primary relays a chance to operate first. The backup relays scheme is independent of the primary relay scheme and operates if the primary relay scheme fails to operate. The equipment removed from service by the backup protection is more of the equipment including the faulty ones. Backup protection comes in overlapping zones.

ELECTRICAL POWER SYSTEM DEVICE NUMBERS AND FUNCTIONS

The devices in 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.

This system is used in connection diagrams, in instruction books and in specifications.

Device Definitions and Functions

Number

1 Master Element is the 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 place equipment in or out of operation.

2 Time Delay Starting or Closing Relay 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 relay system except as specifically provided by Device Functions 48, 62 and 79 described later.

3 Checking or Interlocking Relay is a device that 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 for any purpose.

4 Master Contactor is a device, generally controlled by Device No. 1 or equivalent and the required permissive and protective devices that serve to make and break the necessary control circuits to place equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions.

5 Stopping device is a control device used primarily to shut down equipment and hold it out of operation (this device may be manually or electrically actuated but excludes the function of electrical lock out (See Device Function 86 on abnormal conditions).

6 Starting Circuit breaker is a device whose principal function is to connect a machine to its source of starting voltage.

7 Anode Circuit breaker is one used in the anode circuits of power rectifiers for the primary purpose of interrupting the rectifier circuit if an arc back should occur.

8 Control Power Disconnecting Device is a disconnection device such as a knife switch, circuit breaker or pull out 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 small motors and heaters.

9 Reversing Device is used for the purpose of reversing a machine field or for performing any other reversing functions.

10 Unit Sequence Switch is used to change the sequence in which units may be placed in and out of service in multiple unit equipment.

11 RESERVED FOR FUTURE APPLICATION

12 Over Speed device is usually a directly connected speed switch, which functions on machine over speed.

13 Synchronous Speed device such as a centrifugal speed switch, a slip frequency relay, a voltage relay, an under current relay or any type of device, that operates at an approximate synchronous speed of a machine.

14 Under Speed device functions when the speed of a machine falls below a predetermined value.

15 Speed or frequency matching device functions to match and hold the speed or the frequency of a machine or of a system equal to or approximately equal to that of another machine, source or system.

16 RESERVED FOR FUTURE APPLICATION

17 Shunting or Discharge switch serves to open or to close a shunting circuit around a piece of apparatus (except a resistor) such as a machine field, a machine armature, a capacitor or a reactor.

Note: This excludes devices which perform such shunting operations as may be necessary in the process of starting a machine by Devices 6 or 42 or their equivalent, and also excludes Device 73 which serves for the switching of resistors.

18 Accelerating or Decelerating device is used to close or to cause the closing of circuits, which are used to increase or to decrease the speed of a machine.

19 Starting to Running Transition Contactor is a device, which operates to initiate or cause the automatic transfer of a machine from the starting to the running power connection.

20 Electrically Operated Valve is electrically operated, controlled or monitored valve in a fluid line.

Note: The function of the valve may be indicated by the use of suffixes.

21 Distance Relay is a device which functions when the circuit admittance, impedance or reactance increases or decreases beyond predetermined limits.

22 Equaliser Circuit breaker is a breaker, which serves to control or to make and break the equaliser or the current balancing operations for a machine field, or for regulating equipment in a multiple unit installation.

23 Temperature Control device which functions to raise or lower the temperature of a machine or other apparatus or of any medium, when its temperature falls below, or rises above, a predetermined value.

Note: An example is a thermostat, which switches on a space heater in a switchgear assembly when the temperature falls to a desired value as distinguished from a device, which is used to provide automatic temperature regulation between close limits and would be designated as 90T.

24 RESERVED FOR FUTURE APPLICATION

25 Synchronising or Synchronism Check device operates when two A.C circuits are within the desired limits of frequency, phase angle or voltage to permit or to cause the paralleling of these two circuits.

26 Apparatus Thermal device functions when the temperature of the shunt field or the armotisseur windings of a machine or that of a load limiting or load shifting resistor or of a liquid of other medium exceeds a predetermined value; or if the temperature of the protected apparatus, such as a power rectifier, or of any medium decreases below a predetermined value.

27 Under Voltage relay is a device, which functions on a given value of under voltage.

28 Flame Detector is a device that monitors the presence of the pilot or main flame in such apparatus as a gas turbine or a steam boiler.

29 Isolating Contactor is used expressly for disconnecting one circuit from another for the purpose of emergency operation, maintenance or test.

30 Annunciator Relay 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 lock out function.

31 Separate Excitation device 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 energizes the excitation and ignition circuits of a power rectifier.

32 Directional Power relay is one which functions on a desired value of power flow in a given direction, or upon reverse power resulting from arc back in the anode-cathode circuits of a power rectifier.

33 Position Switch makes or breaks contact when the main device or piece of apparatus, which has no device function, reaches a given position.

34 Master Sequence device is a device such as a Motor operated multi-contact switch, or the equivalent, or a programming device such as a computer that establishes or determines the operating sequence of the major devices in an equipment during starting and stopping or both during other sequential switching operations.

35 Brush Operating or slip ring short circuiting device is used for raising, lowering or shifting the brushes of a machine, or for short-circuiting its slip rings, or for engaging or disengaging the contacts of a mechanical rectifier.

36 Polarity or Polarizing Voltage device operates or permits the operation of another device on a predetermined polarity only or verifies the presence of a polarizing voltage in an equipment.

37 Undercurrent or under power relay functions when the current or power flow decreases below a predetermined value.

38 Bearing Protective device functions on excessive bearing temperature or on other abnormal mechanical conditions, such as undue wear, which may eventually result in excessive bearing temperature.

39 Mechanical Condition monitor is a device that functions upon the occurrence of an abnormal mechanical condition (except that associated with bearings as covered under Device function 38) such as excessive vibration, eccentricity, expansion, shock, tilting or seal failure.

40 Field relay functions on a given or abnormally low value or failure of machine field current, or an excessive value of the reactive component of armature current in an A.C. machine indicating abnormally low field excitation.

41 Field Circuit breaker is a device, which functions to apply, or to remove, the field excitation of a machine.

42 Running Circuit breaker is a device whose principal function is to connect a machine to its source of running or operating voltage. This function may also be used for a device, such as a contactor, that is used in series with a circuit breaker or other fault protecting means, primarily for frequent opening or closing of the circuit.

43 Manual Transfer or Selector device transfers the control circuits so as to modify the plan of operation of the switching equipment or of some of the devices.

44 Unit Sequence starting relay is a device, which functions to start the next available unit in multiple unit equipment on the failure or on the non-availability of the normally preceding unit.

45 Atmospheric Condition monitor is a device that functions upon the occurrence of an abnormal atmospheric condition such as damaging fumes, explosive mixtures, smoke or fire.

46 Reverse phase or Phase balance current relay is a relay which functions when the polyphase currents are of reverse phase sequence, or when the polyphase currents are unbalanced or contain negative phase sequence

components above a given amount.

47 Phase sequence Voltage Relay functions upon a predetermined value of polyphase voltage in the desired phase sequence.

48 Incomplete Sequence Relay 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 device is used for alarm purposes only, it should preferably be designated as 48A (Alarm).

49 Machine or Transformer Thermal Relay is 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 rectifying transformer) exceeds a predetermined value.

50 Instantaneous over current or rate of rise relay is a relay that functions instantaneously on an excessive value of current, or an excessive rate of current rise, thus indicating a fault in the apparatus or circuit being protected.

51 A.C. Time Over current relay is a relay with either a definite or inverse time characteristic that functions when the current in an A.C. circuit exceeds a predetermined value.

52 A.C. Circuit breaker is a device that is used to close and interrupt an A.C. power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

53 Exciter or D.C. generator relay is a relay that forces the D.C. machine field excitation to build up during starting or which functions when the machine voltage has built up to a given value.

54 RESERVED FOR FUTURE APPLICATION

55 Power Factor Relay is a relay that operates when the power factor in an A.C. circuit rises above or drops below a predetermined value.

56 Field Application Relay is a relay that automatically controls the application of the field excitation to an A.C. motor at some predetermined point in the slip cycle.

57 Short circuit or grounding device is a primary circuit switching device that functions to short circuit or to ground a circuit in response to automatic or manual means.

58 Rectification Failure Relay is a device that functions if one or more anodes of a power rectifier fail to fire or to detect an arc back or failure of a diode to conduct or block properly.

59 Over voltage Relay is a relay that functions on a given value of over voltage.

60 Voltage or current balance Relay is a relay that operates on a given difference in voltage, or current input or output of two circuits.

61 RESERVED FOR FUTURE APPLICATION

62 Time Delay stopping or opening relay is a time delay relay that serves in conjunction with the device that initiates the shutdown, stopping or opening operation in an automatic sequence.

63 Pressure switch is a switch, which operates on given values or on a given rate of change of pressure.

64 Ground Protection Relay is a relay that functions on failure of the insulation of a machine, transformer or of other apparatus to ground, or on flash over of a D.C. machine to ground.

Note: This function is assigned only to a relay, which wired to operate the relay in front is always equal to or less than the primary current required to operate the relay behind it.

65 Governor is the assembly of fluid, electrical or mechanical control equipment used for regulating the flow of water, steam or other medium to the prime mover for such purposes as starting, holding speed or load or stopping.

66 Notching or Jogging device functions to allow only a specified number of operations of a given device or equipment or a specified number of successive operations within a given time frame. It also functions to energise a circuit periodically or for fractions of a specified time interval or that used to permit intermittent acceleration or jogging of a machine at low speeds for mechanical positioning.

67 A.C Direction or Over-current Relay is a relay that functions on a desired value of A.C over-current flowing in a predetermined direction.

68 Blocking relay is a relay that initiates a pilot signal for blocking of tripping on external faults in a transmission line or in other apparatus under predetermined conditions or co-operates with other devices to block tripping or to block

re-closing on an out of step condition or on power swings.

69 Permissive control device is generally a two position manually operated switch that in one position permits the closing of a circuit breaker or the placing of equipment into operation and in the other position prevents the circuit breaker or the equipment from being operated.

70 Rheostat is a variable resistance device used in an electric circuit, which is electrically operated or has other electrical accessories, such as auxiliary position or limit switches.

71 Level switch is a switch, which operates on given values, or on a given rate of change of level.

72 D.C Circuit breaker is used to close and interrupt a D.C power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

73 Load Resistor contactor 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 rectifier or other machine in and out of circuit.

74 Alarm relay is a device other than an Annunciator, as covered under Device No. 30, which is used to operate in connection with a visual or audible alarm.

75 Position Changing mechanism is a mechanism that is 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 positions.

76 D.C. Over-current relay is a relay that functions when the current in a D.C circuit exceeds a given value.

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

78 Phase angle measuring or out of step protective relay is a relay that functions at a predetermined phase angle between two voltages or between two currents or between voltage and current.

79 A.C. Reclosing relay is a relay that controls the automatic re-closing and locking out of an A.C. circuit interrupter.

80 Flow Switch is a switch, which operates on given values or on a given rate of change of flow.

81 Frequency Relay is a relay that functions on a predetermined value of frequency either under or over or on normal system frequency or rate of change of frequency.

82 D.C. Reclosing relay is a relay that controls the automatic closing and re-closing of a D.C. circuit interrupter, generally in response to load circuit conditions.

83 Automatic Selective control or Transfer relay is a relay that operates to select automatically between certain sources or conditions in an equipment or performs a transfer operation automatically.

84 Operating mechanism is the complete electrical mechanism or servo-mechanism, including the operating motor, solenoids, position switches, etc., for a tap changer, induction regulator or any similar piece of apparatus which has no device function number.

85 Carrier or Pilot Wire Receiver Relay is a relay that is operated or restrained by a signal used in connection with the carrier current or D.C pilot wire fault directional relaying.

86 Locking out relay is an electrically operated hand or electrically reset relay that functions to shut down and hold an equipment out of service on the occurrence of abnormal conditions.

87 Differential Protective Relay is a protective relay that functions on a percentage or phase angle or other quantitative difference of two currents or of some other electrical quantities.

88 Auxiliary motor or motor generator is one used for operating auxiliary equipment such as pumps, blowers, exciters, rotating magnetic amplifiers etc.

89 Line switch is used as disconnecting load interrupter or isolating switch in an A.C or D.C power circuit when this device is electrically operated or has electrical accessories such as an auxiliary switch, magnetic lock etc.

90 Regulating device functions to regulate a quantity or quantities such as voltage, current, power, speed, frequency, temperature and load at a certain value or between certain (generally close) limits for machines, tie lines or other apparatus.

91 Voltage directional relay is a relay that operates when the voltage across an open circuit breaker or contactor exceeds a given value in a given direction.

92 Voltage and power directional relay is a relay that permits or causes the connection of two circuits when the voltage difference between them exceeds a given value in a predetermined direction and causes these two circuits to be disconnected from each other when the power flowing between them exceeds a given value in the opposite direction.

93 Field changing contactor functions to increase or decrease in one-step the value of field excitation on a machine.

94 Tripping or trip free relay functions to trip a circuit breaker, contactor or equipment or to permit immediate tripping by other devices; or to prevent immediate re-closure of a circuit interrupter, in case it should open automatically even though its closing circuit is maintained closed.

DEVICES PERFORMING MORE 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 device function such as: 50/51 - An over-current relay with an instantaneous element and an inverse element.

SUFFIX NUMBERS

If two or more devices with the same function number and suffix letter (if used) are present in the same equipment then these are distinguished as follows 52x-1, 52x-2, 52x-3 etc

SUFFIX LETTERS

Suffix letters are used with device numbers for various purposes. The meaning of each suffix letter or combination of letters should be clearly indicated in the legend on the drawings or publications accompanying the equipment. This is to avoid possible confusion. These letters should be written directly after the device function number to indicate that they are a part of the device.

Commonly used letters are as follows:

R - Raising relay or for remote operation

L - Lowering relay or for local operation

O - Opening relay or contactor

C - Closing relay or contactor

CS - Control Switch

CC - Closing Coil

TC - Trip Coil

PB - Push Button

G - Generator

T - Transformer

L - Line

F - Feeder etc

Example: 52 TC – Tripping coil of the breaker.

REPRESENTATION OF DEVICE CONTACTS

There are almost in all electrical devices, particularly in circuit breakers and relays, a set of contacts which are normally open and another set of contacts which are normally closed. When the device operates, the contact position reverses. Those normally open become closed and vice versa. These are generally indicated as ‘a’ and ‘b’ contacts. When the device has not operated or de-energised or open contacts, they are designated thus:

Normally Open (NO) contacts – ‘a’ Normally Closed (NC) contacts – ‘b’

NAPTIN FIELD EXPERIENCE CLASS REVIEW

Friday, December 28, 2012

History of Proection For Power Engineers