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

Continuous and Short-Time Ratings

• Continuous rating specifies what a relay will withstand under continuous operation in an ambient temperature of 40° C
• Relays having current coils carry a 1-second current rating, such relays should not be subjected to currents in excess of the 1-second rating
• Overcurrents lower than the 1-second-rating value are permissible for longer than 1 second, so long as the I 2 t value of the 1-second rating is not exceeded ▪ Example – if a relay will withstand 100 A for 1 sec, it will withstand [100√(0.5)] A for 2 seconds ▪ Not always safe to assume that a relay will withstand any current that it can get from current transformers for as long as it takes a circuit breaker to interrupt a short circuit after the relay has operated to trip the circuit breaker ▪ Should a relay fail to succeed in tripping a circuit breaker, thermal damage should be expected unless back- up relays can stop flow of short-circuit current soon enough to prevent such damage

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

Holding-Coil or Seal-in-Relay and Target Ratings

• Two different current ratings are generally available in same relay ▪ Higher current rating is for use when relay trips a circuit breaker directly ▪ Lower current rating is for use when relay trips a circuit breaker indirectly through an auxiliary relay
• Ratings are low enough so that seal-in and target operation will be obtained for two or more relays in parallel < and dividing total available trip circuit current between parallel protective- relay-contact circuits >
• Depending on tripping speed of breaker, trip-circuit current may not have time to build up to its steady-state value
• Resistances of seal-in and target coils are given so that trip-circuit currents can be calculated

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

Overcurrent, Undercurrent, Overvoltage, Undervoltage Relays

• These relays are derived directly from basic single-quantity electromagnetic-attraction or induction types
• “Over” – relay picks up to close a set of “a” contacts when actuating quantity exceeds magnitude for which relay is adjusted to operate
• “Under” – relay resets to close a set of “b” contacts when actuating quantity decreases below reset magnitude for which relay is adjusted to operate
• Some relays have both “b” and “a” contacts → prefix before the actuating quantity in their name is “over-and- under”
• “Current” relay – actuating source is a current in a circuit supplied to relay either directly or from a current transformer
• “Voltage” relay – actuating source is a voltage of a circuit obtained either directly or from a voltage transformer

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 typical inverse time curve for a high-speed relay

• “High-speed” because 3-cycle operating time slightly above the pickup value - Curve plotted in terms of multiples of pickup value, so that same curves can be used for any value of pickup - This is possible with induction-type relays where pickup adjustment is by coil taps, because ampere-turns at pickup same for each tap → at a given multiple of pickup, coil ampereturns, and hence torque, same regardless of tap used - If air-gap or restraining pickup adjustment is used, shape of time curve varies with pickup

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

Overtravel

• “Overtravel” – owing to inertia, motion will continue even when actuating force is removed < Important only in time-delay relays, particularly for inverse-time overcurrent relays, where selectivity is obtained on a time-delay basis > Example – basis for specifying overtravel
• Suppose that, for a given adjustment and at a given multiple of pickup, a relay picks up and close its contacts in 2.0 seconds
• Now suppose that we make several tests by applying that same multiple of pickup for time intervals slightly less than 2.0 seconds, and we find that, if time interval is any longer than 1. seconds, relay will still close its contacts → overtravel is 0.1 second < Higher multiple of pickup, longer overtravel time >
• Overtravel time of ~0.1 second is generally assumed in application of inverse-time relays

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

Current-Directional Relays

• Used for protection in d-c power circuits
• Armature coil connected either directly in series with circuit or across a shunt in series with circuit
• Polarized either by a permanent magnet or by a field coil connected to be energized by voltage of the circuit
• Can be calibrated to have overcurrent (or overpower) or undercurrent (or underpower) characteristics, or both, in addition to being directional

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

Voltage-Balance Directional Relays

• Two voltage coils → used to protect a three-wire d-c circuit against unbalanced voltages
  • Two coils connected in such a way that their magnetomotive forces are in opposition
  • Double-throw contacts and two restraining springs provide calibration for movement of armature in either direction
• When one voltage exceeds the other by a predetermined amount, armature will move one way to close one set of contacts
• If the other voltage is higher, armature will close other set of contacts

Current-Balance Directional Relays

• Similar to voltage-balance type
• Two current coils → used for current-balance protection of a three-wire d-c circuit, or to compare loads of two different circuits

Polarizing Magnet vs. Field Coil

• Field coil is generally preferred < except where a permanent magnet is the only suitable polarizing source available > - Field coil is required if a relay is to respond to power (watts) - A coil provides more flexibility of adjustment → series resistors can be used to vary polarizing mmf - For some types of relays, it is necessary to remove polarization to permit to reset → requires a field coil

AC Directional Relays

Power Relays

• Relays that must respond to power, i.e. used for protection against conditions other than short circuits
• Connected to be polarized by a voltage of a circuit; current connections and relay characteristics are chosen so that maximum torque in relay occurs when unity-power-factor load is carried by circuit → relay will now picks up for power flowing in one direction through the circuit and resets for opposite direction of power flow Connections and vector diagram for a power relay where phase-to-neutral voltage is not available