Power System Protection Course- DIFFERENTIAL PROTECTION

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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