Electrical System Protection Tips and Guidelines

 

Protection Relay Panel | Source: Schnieder Electric

Electrical system protection should: 

  • detect faults
  • cut off of the faulty parts of the electrical system, keeping the fault-free parts in operation. Protection systems are chosen according to the electrical system configuration (parallel operation of AC generators or transformers, loop or radial system, grounding system…). 

Protection against each of the following types of faults are to be considered: 

  • phase-to-phase faults
  • earth faults (protections related to electrical system grounding). 


Related Article: Fundamental of Short Circuit Currents


This will be done by successively examining the following cases: 

  • a single incoming line
  • two incoming lines
  • a busbar

Electrical system with a single incoming line


Phase-to-phase faults 
The protection device at D detects faults at 1 on the outgoing lines and is tripped following a time delay tD. The protection device at A detects the faults at 2 on the busbars and is tripped following a time delay tA. It also acts as back-up in the event of a malfunction of protection D. 

  • Choose : IrA ≥ IrD and tA ≥ tD +∆t ∆t : discriminator interval (generally 0.3 s).

Phase to Phase Fault



Phase-to-earth faults 

Grounding resistance on the transformer (see figure below)
Outgoing lines, the incoming line and the grounding connection are equipped with earth fault protection devices. These devices are necessarily different from multiphase fault protections as the fault currents are in a different range. 

Outgoing line protections are set selectively in relation to the incoming line protection, which is itself set selectively in relation to the protection equipping the grounding connection (respecting discrimination intervals). The fault current is fed back by the capacitances of the fault-free outgoing lines and the grounding resistance. All the fault-free outgoing line sensors detect capacitive current. 


Grounding resistance on transformer


So as to prevent inadvertent tripping, the protection device on each outgoing line is set at a setting higher than the outgoing line's own capacitive current. 
  • fault at 1 : the D1 circuit breaker trips, actuated by the protection device linked to it,
  • fault at 2 : the A circuit breaker trips, actuated by the incoming line protection device, 
  • fault at 3 : the protection device located on the neutral grounding connection causes circuit breaker H to trip at the transformer



Grounding by resistance on the busbar (see figure below)
The outgoing and incoming line protections are selectively set in relation to the protection equipping the grounding impedance. As in the previous case, the protection on each outgoing line is set at a setting higher than the outgoing line's own capacitive current. In the event of a fault on outgoing line 1 only the D1 outgoing line circuit breaker trips. 

Grounding by resistance on the busbar



In the event of fault on the busbar 2, only the protection equipping the grounding connection detects the fault. It causes tripping by circuit breaker A. In the event of fault on the transformer secondary 3, the incoming line protection detects the fault. It causes tripping by circuit breaker H. Note: when circuit breaker A is open, the transformer secondary is ungrounded.


Ungrounded Neutral
A fault, regardless of its location, produces current which is fed back by the capacitance of the fault-free outgoing lines. in industrial system, this current is generally weak (a few amperes), allowing operations to carry on while the fault is being tracked. 

The fault is detected by a permanent insulation monitor (Vigilhom) or a neutral voltage displacement protection device. In the case of a system with a high total capacitive current (tens of amperes), added measures are required to quickly clear the fault. Directional earth protection can be used to selectively trip the faulty outgoing line.

Ungrounded Neutral



System with two incoming lines

Phase- to- phase fault (System with two transformer incomers or with two incoming lines)
The outgoing lines are equipped with phase overcurrent protection with a time delay of tD. The two incoming lines A1 and A2 are equipped with phase overcurrent protections selectively set with the outgoing lines, i.e. at a value of 

  • t A ≥ tD + ∆t

System with 2- incoming lines


They are also equipped with directional protection devices with time delays set at 
  • tR < tA - ∆t

Therefore, 

  • A fault at 1 is cleared by the opening of D2 with a time delay of tD. 
  • A fault at 2 is cleared by the opening of A1 and A2 with a time delay of tA (the directional protections do not detect the fault). 
  • A fault at 3 is detected by the A1 directional protection which opens at time tR, allowing continued operation of the fault-free part of the system. The fault at 3 however, is still fed by T1. At time tH ≥ tA + ∆t, H1 is actuated by the phase overcurrent protection with which it is equipped.



Phase-to-earth faults (System with two transformer incomers)
Grounding by resistance on the transformers. The outgoing lines are equipped with earth fault protection devices set at a setting higher than the corresponding capacitive current with a time delay of tD. 


System with 2 Transformer Incomers



The incomers (A1 and A2) are equipped with directional protections with a time delay of tR. The grounding connections are equipped with earth fault protections, the setting of which is higher than the settings of the incomer and outgoing line protections with a time delay of tN ≥ tD + ∆t. 

Therefore, 

  • A fault at 1 is cleared by the opening of D1.
  • A fault at 2 is cleared by the opening of A1, A2, H1 and H2, triggered by the protections located on the grounding connections of the 2 transformers. 
  • A fault at 3 is detected by the A1 directional earth fault protection which opens at time tR, allowing continued operation of the fault-free part of the system. 
  • However, fault 3 is still fed up to time tN, the moment at which the protection located on the corresponding transformer's grounding connection triggers the opening of the H1 circuit breaker.


Busbars
In addition to the protections described earlier, a busbar can be equipped with a specific protection device referred to as high impedance differential protection, the aim of which is to be sensitive, quick and selective. 

The differential protection below takes the vectorial sum per phase of currents entering and leaving the busbar; whenever this sum is not equal to zero, it trips the busbar power supply circuit breakers. 

differential protection


Logic discrimination below applied to overcurrent protections provides a simple, simple solution for busbar protection. A fault at 1 is detected by the D1 protection which transmits a blocking input to the A protection. The D1 protection is tripped 0.6 sec. later A fault at 2 is detected only by the A protection which is tripped 0.1 sec. later.

Logic discrimination



Open Loop and Closed Loop
In a distribution system comprising substations fed in a loop, protection can be at the head of the loop or by sections: 

Protection at the head of the loop


The circuit breaker at the head of each loop is equipped with an overcurrent protection device. A fault in a cable joining up 2 substations causes the opening of one of the two circuit breakers at the head, depending on the position of the loop opening. 

Protection is often completed by an automation system which: 
  • clears the fault with the power off by opening the devices located at the ends of the cable involved, after localization of the faulty cable (by cable fault detector), 
  • close the incomer circuit breaker that tripped, c closes the device which ensured the normal opening of the loop.


Loop section protection Each end of the cable is equipped with a circuit breaker, with severall protection solutions. 

Differential Protection Solution



In the differential protection solution, each cable is equipped with a differential line protection device and each substation is equipped with a busbar differential protection device. This type of protection is very quick but expensive. Also, if the neutral is resistance grounded, the sensitivity of the differential protections must cover phase-to-earth faults. This solution may be used in both open and closed loops.


Reference: 
  • Protection Guide and Control
  • Publisher: Merlin Gerin | Download

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