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).
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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
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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.
Related Article: Protection Systems Applied in Substation and Power Networks
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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