General Protective-Relay Features, Summaries of Power Distribution and Utilization

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Current, Voltage, Directional, Balance,

and Differential Relays

A H Chowdhury, PhD

Professor, EEE, BUET

C. Russell Mason, The Art & Science of Protective Relaying, Chap. 3

General Protective-Relay Features

Contact Ratings

• Protective-relay contacts are rated on their ability to close and to open inductive or noninductive circuits at specified magnitudes of circuit current and a-c or d-c circuit voltage
• Protective relays that trip circuit breakers are not permitted to interrupt flow of trip-coil current → they require only a circuit-closing and momentary current-carrying rating
• If a breaker fails to trip, contacts of relay will almost certainly be damaged
• Circuit-opening rating is applicable only when a protective relay controls operation of another relay, such as a timing relay or an auxiliary relay → in such a case, the protective relay should not have a holding coil or else it may not be able to open its contacts once they have closed
• If a seal-in relay is used, current taken by controlled relay must be less than pickup of seal-in relay

Contact Ratings

• When a “over and-under” type relay with “a” and “b" contacts is used to control operation of some other service, the relay can be relieved of any circuit breaking duty [see Fig.]
• When protective relay picks up, it causes an auxiliary relay to pick up and seal itself in around the protective-relay contacts
• Other auxiliary-relay contacts may be used for control purposes, thereby relieving protective-relay contacts of this duty
• When protective relay resets, it shorts the auxiliary-relay coil, thereby causing the auxiliary relay to reset

Burdens

• Impedance of relay-actuating coils is called relay burden
• Burden must be known to determine if relay’s voltage- or current-transformer sources will have sufficient capacity and suitable accuracy to supply the relay load together with any other loads that may be imposed on the transformers
• Relay impedances are listed in relay publications [ This subject will be treated further when we examine characteristics of VTs and CTs ]

Overcurrent, Undercurrent,

Overvoltage, Undervoltage Relays

Adjustment – Pickup or Reset

• Overcurrent relays have a range of adjustment to make them adaptable to wide a range of application circumstances - Range of adjustment limited because of coil-space limitations and to simplify relay construction - Various relays are available, each having a different range of adjustment
• Plunger or attracted-armature relays – adjustment of initial air gap, adjustment of restraining- spring tension, adjustable weights, or coil taps
• Current-actuated induction relays – adjustment by coil taps
• Voltage-actuated relays - adjustment by taps on series resistors or by auxiliary autotransformer taps
• Voltage relays and undercurrent relays do not have wide range of adjustment < they are expected to operate within a limited range from normal magnitude of actuating quantity >

Adjustment – Time

• Operating time of inverse-time induction relays adjustable by choosing amount of travel of rotor from its reset position to its pickup position → by adjustment of position of reset stop Example: disc with spiral periphery
• As disc turns toward pickup position when reset stop is advanced, or whenever relay operates to pick up, increase in amount of disc area between poles of actuating structure causes an increase in electrical torque that just balances increase in control-spring torque
• Slight increase in restraining torque of control spring, as reset stop is advanced toward pickup position → compensated for by shape of disc

Time Characteristics

• A family of inverse-time curves of one widely used induction-type relay
• A curve is shown for each major division of adjustment scale
• Any intermediate curves can be obtained by interpolation since adjustment is continuous
• Even if relay closes its contacts, contact pressure may be so low that contamination of contact surface may prevent electrical contact
• For reliable relay operation actuating quantity must be at least 1.5 times pickup, but not too much more

Time Characteristics

• Time curves is used to estimate relay operating time, and also how far relay disc will travel toward contact- closed position within any time interval
• Consider multiple of pickup is 3, and no. 5 time-dial setting is used - 2.45 sec. to close contacts - 1.45 sec. to close contacts if No. 3 time-dial setting were used
• i.e., in 1.45 seconds, disc travels a distance corresponding to 3.0 time-dial divisions, or three-fifths of total distance to close contacts

Reset Time

• Reset time vary directly with the time-dial adjustment
• Estimating amount of disc travel during short time intervals + reset time → estimated operation of inverse-time relays
• Important for cases of successive application and removal of actuating quantity
• Example cases
  • when a motor is “plugged”
  • when a circuit is tripped and then automatically reclosed on a fault several times
  • during power surges accompanying loss of synchronism

DC Directional Relays

Voltage-Directional Relays

• Same as current-directional relays except for number of turns and resistance of the armature coil, and possibly except for the polarizing source
• Used in d-c power circuits to respond to a certain polarity of voltage across circuit
• Polarized by a permanent magnet if intended to respond to reversal of circuit-voltage polarity
• Otherwise, both permanent-magnet polarization or a field coil energized from circuit voltage is used
• “Differential” relay operation – when used for closing of circuit breaker only when voltage across open breaker has a certain polarity < “differential,” because it operates only in response to a predetermined difference between magnitudes of circuit voltages on either side of breaker >

Current-and-Voltage Directional Relays

• Such relay has two coils
• Controls closing and opening of a circuit breaker in a circuit between a d-c generator and a bus to which another source of voltage may be connected < to avoid motoring of generator >
• Voltage coil connected across breaker → picks up the relay to permit closing the breaker only if generator voltage is a certain amount greater than bus voltage
• Current coil connected in series with circuit, or across a shunt → resets relay to trip breaker whenever a predetermined amount of current starts to flow from bus into generator