PROTECTIVE RELAYS
STANDARDISATION
Phase overcurrent (50 / 51)
3.1. FUNCTIONS OF PROTECTION AND THEIRS APPLICATIONS
Relays or multifunction devices fulfil the functions of protection. At the origin, the relays ofprotection used only one function and were analogue type.Now, the technology is digital and then it is possible to conceive increasingly advanced functions and the same device generally carries out several functions.
This is why, we say rather multifunction devices.
The relays of protection are devices which verify permanently the electric network (current,voltage, frequency, power, impedances...) with predetermined thresholds and which make an automatic action (generally opening of a circuit breaker) or an alarm. The role of the protection relays is to detect any abnormal phenomenon being able to occur on an electrical network such as short-circuit, variation of voltage, dysfunction of a machine, etc.
The relay can be:
1) Without auxiliary power supply (autonomous) when the necessary energy for its operation is taken directly from the supervised circuit. The actuator must be
sensitive because the power taken on the circuit is weak;
2) With auxiliary power supply when the necessary energy for its operation is taken
from an auxiliary source of voltage (DC or AC) independent of the supervised circuit
All types of electrical faults have been “codified” with the ANSI code giving one reference number (+ letter), one symbol, for each.
Three phase connection and equipment protection against phase-to-phase faults. The
following types of time delay settings are available: definite, standard inverse, very
inverse and extremely inverse.
Tank earth leakage (50 / 51)
Quick, selective detection of earth leakage current in transformer primary and secondary
windings. This is an additional overcurrent protection function. For it to be used the
transformer tank must be insulated and a current sensor must be installed on the frame
earthing connection.
Neutral unbalance (50 / 51)
Detection of unbalance current between the neutral points of star-connected capacitor
banks; unbalance current indicates that components of one of the capacitors is
damaged.
Voltage restrained overcurrent (50 / 51V)
Three phases' protection against alternator phase faults. Its characteristics are suitable
for the weak current supplied by the alternator when short-circuit occurs.
Earth fault (50N / 51N)
Protection against earth faults. The following types of time delay settings are available:
definite, standard inverse, very inverse and extremely inverse.
Neutral (50N / 51N) or Sensitive earth fault (50G / 51G)
Overload protection of neutral earthing impedance and sensitive overall network
protection against earth faults. This is an additional earth fault protection function
Directional overcurrent (67)
Incomer protection, which provides quick, selective protection against upstream faults
when there are several parallel transformer incomer in the network.
Directional earth fault (67N)
This protection has several uses:
highly sensitive earth fault protection of transformer feeders supplied by long cables
characterised by high capacitive current
quick, selective detection of upstream earth faults when there are several parallel
transformers in the network
Thermal overload (49)
Thermal overload (49)
Protection of equipment against thermal damage caused by overloads. Thermal
overload is calculated according to a mathematical model, with 2 time constants, taking into account the effect of negative sequence current by means of an adjustable
weighting coefficient: an adjustable setting to define motor warm state an adjustable tripping setting
Negative sequence / unbalance (46)
Protection of equipment against overheating caused by an unbalanced power supply,
phase inversion or phase break, and against low levels of overcurrent between 2
phases.
Locked rotor / excessive starting time (48/51LR)
Protection of motors that are liable to start with overloads or insufficient supply voltage
and/or drive loads that are liable to become locked (e.g.: crusher). It is an overcurrent
protection that is only confirmed after a time delay, which corresponds to the normal
start time.
Starts per hour (66)
Protection against overheating caused by too frequent starts. Checking of:
number of starts per hour number of consecutive warm starts (detected by the thermal overload protection) number of consecutive cold starts The protection inhibits motor energising for a preset time period when the permissive limits are reached.
Undercurrent (37)
Pump protection against the consequences of priming loss. This protection detects
delayed undercurrent corresponding to motor no-load operation, which is typical of a
loss of pump priming. Other relays: 37P (under active power) and 37Q (under reactive
power)
Undervoltage (27)
Protection used either for automated functions (changeover, load shedding) or for the protection of several motors against undervoltage. This protection checks for
undervoltage in each of the system voltages measured.
Positive sequence undervoltage (27D)
Protection which prevents motor malfunctioning due to insufficient or unbalanced supply voltage
Remanent undervoltage (27R)
Monitoring of the clearing of voltage sustained by rotating machines after the opening of
the circuit. This protection is used with automatic changeover functions to prevent
transient electrical and mechanical phenomena that are caused by fast re-supply of power to motors.
Direction of rotation (47)
Protection which prevents the changing of direction of motor rotation following a power
supply modification
Overvoltage (59)
Protection against abnormally high voltage and checking that there is sufficient voltage
for power supply changeover
Neutral voltage displacement (59N)
Detection of insulation faults in ungrounded systems by measurement of neutral voltage
displacement. This protection is generally used for transformer incomer or busbars.
Under frequency (81)
Detection of variances with respect to the rated frequency, in order to maintain high
quality power supply. This protection can be used for overall tripping or for load
shedding.
Over frequency (81)
Protection against abnormally high frequency
Reverse true (active) power (32P) Protection of synchronous motors against operation as generators when driven by their load
Reactive overpower (32Q / 40R)
Protection of synchronous drives against field loss, which causes over consumption of
reactive power and leads to the loss of synchronism
Temperature monitoring by RTD (38 / 49T)
Protection, which detects abnormal overheating of motors (bearings and/or windings),equipped with PT100 type platinum resistive temperature probes.
Motor differential (87M)
Fast, sensitive motor protection against internal faults due to damaged insulation. The
protection is based on the principle of percentage differentials, i.e. starting current
restraint to sustain stability in spite of its high level of sensitivity
Synchronism check (25)
Authorises the closing of the breaking device, only if the two circuits have voltage,
frequency and phase gaps within the planned limits. The choice of an operating mode
with voltage absent allows the generator to be coupled with a de-energised installation.
Restricted earth fault (64REF)
A restricted earth fault protection is available on the secondary side of the protection. It
is used to detect phase-to-earth faults in the transformer winding connected to the
secondary side of the protection. It may be used when the secondary neutral point is
earthed inside the protected zone
