DIFFERENTIAL
PROTECTION
CONTENTS
DIFFERENTIAL
PROTECTION......................................................................
THE PRINCIPLE....................................................................................................
Circulating Current (cc) Principle...........................................................................
Balanced Voltage (bv) Principle..............................................................................
CIRCULATING CURRENT SYSTEM.................................................................
Voltage Distribution.................................................................................................
3-Phase Protection...................................................................................................
Differential Protection of a Transformer..................................................................
BALANCED VOLTAGE SYSTEM......................................................................
DIFFERENTIAL PROTECTION
THE PRINCIPLE
Differential
protection depends on a method of fault detection based on the principle that
the total current flowing into one part of a system is equal to the total
current flowing out of it unless there is some unintended alternative path for
it in between. This is just another
statement of Kirchoff's Law.
This type of
protection is used to guard against faults arising only within the protected
unit, ignoring those occurring outside it.
The unit itself then becomes the 'protected zone'. It is in some respects similar to restricted
earth-fault protection but should not be confused with it. REF guards only against earth faults in the
protected zone, whereas differential protection covers also phase-to-phase
faults within the zone. It does not
however deal with inter-turn faults within one phase - say in a generator -
since that will not cause differing currents at the two ends.
Differential
protection is insensitive to through faults - that is, to faults outside the
protected zone - because the same fault current flows through both ends of the
zone. It may therefore be used to
provide relatively sensitive protection for the equipment inside the protected
zone without its being affected by the discrimination scheme of the whole
network. The advantage of this is
particularly apparent in the case of generators and large bulk power
transformers, which may demand rapid and sensitive protection against internal
faults but which, because of their position at the high-level end of the power
supply system, would be among the last items to be tripped in the event of a
through-fault.
FIGURE 9.1 -
DIFFERENTIAL PROTECTION
The term 'differential protection' (symbol DIF) is used generally
throughout the offshore installations, but elsewhere it may be known by the
names 'Merz Price' (after the original inventors), 'Unit', 'Circulating
Current' or 'Balanced Voltage' protection.
All these terms may be met as well as such trade names as 'Translay' and
'Solkor' which introduce variations into the basic scheme.
Differential
protection is basically of two kinds, as shown in Figure 9.1. The two kinds are described in principle
below.
Circulating Current (cc) Principle
Figure 9.1(a) shows identical current
transformers connected at each end of any piece of electrical equipment - a
generator, a motor, or even a length of cable - through which a current is
flowing. A single-phase circuit has been
used for simplicity. The CT secondaries are
connected by 'pilot cables' in a loop as shown, and a voltage-sensitive relay
is connected across the pilots at about their mid-points.
Current flowing through the electrical unit
causes a secondary current through both CTs to circulate round the pilot
circuit without producing any current in the relay. A fault within the zone between the two CTs
(the protected zone) will on the other hand cause secondary currents of
differing values in the two CTs, and their difference current will flow through
the relay. If this difference is
sufficient, the relay will operate.
Balanced Voltage (bv) Principle
Figure 9.1(b) shows another arrangement where
the two current transformers are connected in opposition and the relay is in
series. With the same primary current
flowing through both, the secondary emfs oppose each other and no secondary
current flows in the pilot circuit - the voltages are balanced.
In the event of an internal fault causing
differing primary currents in the CTs, the two opposing secondary emfs will no
longer be equal, and current will flow round the pilot circuit, causing the
series relay to operate.
It should be noted that in the balanced
voltage system the CT secondary current flows normally, and the CTs are
effectively on open circuit, giving high voltages on the pilot lines. Moreover this condition would cause the
overburdened CTs to saturate and become inaccurate. Special CTs are used having an air-gap or
other non-magnetic gap to avoid saturation.
CIRCULATING CURRENT SYSTEM
Voltage Distribution
The simplified explanation of circulating
current protection as given above needs some further attention in order to
understand how it works in practice. In
particular the distribution of voltages round the secondary loop will be
described.
If the potentials at all points round the
secondary loop are plotted, beginning at O where the potential is zero, the
curve will be as shown in Figure 9.2(a).
From O to A the potential will rise due to the emf in the CT; from A to
B it will steadily fall due to the resistance of the pilot leg AB; from B to C
it will rise again within the CT; and from C to O it will fall once more to
zero due to the resistance of the leg CO.
FIGURE 9.2 -
CIRCULATING CURRENT PROTECTION VOLTAGE DISTRIBUTION
At a certain point P midway between the two
CTs the potentials of the two secondary lines (red) will be equal because of
symmetry. A voltmeter applied across
them there would read zero. If a relay
were connected across the lines at that point it would be unaffected.
If now a fault or leakage developed somewhere
inside the equipment, part (or all) of the 'go' current would be shunted into
the return line, so that the currents I1 and I2 on
either side of the equipment would be unequal.
So, therefore, would be the CT secondary voltages, and the potential
curves would be distorted as shown red in Figure 9.2(b), the voltage gradients
on the faulty side being greater than on the other. They are no longer symmetrical, and the
crossover voltage-balance point has moved from P to some other point Q. At P there is now a voltage difference
between the lines (P1-P2); and the relay (an attracted-armature instantaneous type) inserted
at that point would be energised. If the
relay setting were sufficient, it would operate to trip whichever
circuit-breakers it was necessary to open.
The relay setting range is typically 5 to 20% of normal full-load
current.
It has been shown that the relay must be
connected at the point in the pilot lines where, under normal conditions, the
voltages are equal. In practice such a
point is not easy to find.
FIGURE 9.3 - CIRCULATING CURRENT PROTECTION WITH
RESISTANCE
What is done is to insert resistances into
the pilot circuit so that most of the voltage drop in each line is concentrated
in the resistors. The crossover point is
then bound to be somewhere in the resistors themselves, so they are provided
with tappings, which can be adjusted until the balance point is found. By this means the crossover point, instead of
being at some unknown place far from the switchboards, is brought as a
'resistance box' right into the switchboard where the relay itself is
installed.
The resistances add to the burden on the CTs,
but this is acceptable.
For satisfactory operation it is essential
that the pairs of CTs be accurate and perfectly matched. Therefore they are usually of the special
class of accuracy (Class X) and are supplied as matched pairs.
Since differential protection operates only
over a limited zone, it does not form a step in the discrimination ladder. It is therefore instantaneous in operation
and the relay can be given a very low setting.
3-Phase Protection
Figures 9.1 to 9.3 show, for simplicity, a
single-phase system, but the principle can be applied - and usually is - to
3-phase systems.
Three carefully balanced pairs of CTs of high
accuracy are inserted, one pair into each of the three phases, and voltage
balance is measured between each secondary line and neutral by a 3-element
relay. A resistance box containing three
tapped resistors is used as described above.
This is shown in Figure 9.4.
The 3-phase system requires four pilot lines
between the sets of CTs, with further lines from the relay contacts to trip the
circuit-breaker. For long lines
variations of the system such as 'Translay' and 'Solkor' operate over only two
pilot lines and can initiate tripping simultaneously at both ends. It should be noted also that differential
protection will operate on both internal phase-to-phase and earth faults, and
in this respect it is superior to restricted earth-fault protection.
FIGURE 9.4 -
DIFFERENTIAL (CIRCULATING CURRENT) PROTECTION (3-PHASE)
Differential Protection of a Transformer
The differential protection so far described,
whether circulating current or balanced voltage, depends on identical and
matched current transformers at both ends of each phase. For most electrical units the incoming and
outgoing currents are, or should be, the same.
This applies to generators, motors and cables, but it is not true of
transformers.
The outgoing current in any phase of a
transformer differs, ideally, from the incoming in inverse proportion to the
voltage ratio. For example a 2000kVA,
6600/440V transformer (ratio 15:1) has a primary current of 175A but a
secondary current of 2625A (ratio 1:15).
Therefore, to achieve balance of the CT
secondaries, the CT ratios must be inversely proportional to the main
transformer voltage ratio, as shown in Figure 9.5 (a).
Most distribution transformers are delta/star
connected, and this affects the line current ratio in the individual phases by
a factor of 3. If the main transformer is
delta/star connected, then the three CT secondaries must be connected in the
opposite sense, namely star/delta. This
is shown in Figure 9.5(b).
FIGURE 9.5
DIFFERENTIAL (CIRCULATING CURRENT)
PROTECTION OF A TRANSFORMER
BALANCED VOLTAGE SYSTEM
As
stated in para. 9.1 the balanced voltage system is less used than the
circulating current type.
One
consequence of the high voltage on the pilot lines is that it can give rise to
appreciable shunt capacitive currents if the pilot cable is long; these can
lead to inaccurate operation unless special steps are taken to deal with them.
It
is for these and other reasons that the circulating current type of protection
is generally preferred. In the US the
balanced voltage system is referred to as 'transactor'.
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