Differential protection is a relaying scheme used to protect an element of a power system from electrical faults. The differential protection scheme utilizes the vector difference of two or more similar electrical parameters and compares it to a preset value. If the preset value is exceeded it generates a trip signal to the circuit breaker’s trip coil. Thus tripping the breaker and clearing the fault.
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The basic principle of differential protection
A simple differential arrangement is shown in the figure.
Here the CT secondaries are interconnected and the coil of the relay is connected across these. The current I1 and I2 may be different provided both sets of CTs have appropriate ratios and connections.
Under normal load conditions or when there is a fault outside the zone of protection of the element, the secondary currents will circulate between the two CTs and will not flow through the relay coil. However, if a fault occurs between the two CTs, the fault current would flow toward the short circuit and through the relay coil. In these cases, the current in the relay coil will be proportional to the vector difference between the current that enters and leaves the protected element. If the current flowing through the relay coil exceeds the relay preset value, it will immediately cause a trip signal.
Types of Differential Protection Relays
SIMPLE or BASIC DIFFERENTIAL PROTECTION
In a simple differential protection scheme we use a simple differential relay. The simple differential relay is also called the basic differential relay. The relay is basically an overcurrent relay, the phasor difference of currents at the two ends of a protected element is carried by the operating coil of the relay. The relay operates when the phasor difference of secondary currents of the CTs at the two ends of the protected element exceeds a predetermined or preset value. The pilot wire circuit connects the two secondaries of the CT.
In the mid section of the pilot wire, the operating coil is connected. The simple differential protection scheme is also called the circulating current differential protection scheme of the Merz-Price protection scheme.
PERCENTAGE or BIASED DIFFERENTIAL RELAY
The disadvantage of the simple differential relay due to CT errors during heavy through faults is fixed by using a percentage differential relay which is also called a biased differential relay. It provides high sensitivity to light internal faults and high restraint for external faults which makes the differential protection scheme more stable.
This relay has two coils. One is known as restraining coil or bias coil which inhibits the operation of the relay. The other coil is the operating coil which produces the operating torque. When the operating torque is more than the restraining torque, the relay operates.
Settings of Percentage Differential Relay
The percentage differential relay has the following two types of settings.
Basic Setting or Sensitivity Setting
The basic setting is the minimum current in the operating coil only (zero bias) which will operate the relay. It is expressed as a percentage of the rated current. It is defined as follows:
Typical values of the basic setting might be 10 to 20% for the generator and 20% for the transformer.
Bias Setting
The bias of the relay, K is basically a ratio of the number of turns in the restraining coil (Nr) to the number of turns in the operating coil (N0). It can also be defined as the ratio of minimum current through the operating coil for causing the operation to the restraining current. It can be expressed as
Typical values of bias setting might be 10% for the generator and 20 to 40% for the transformer the higher bias values are used for tap-changing transformers.
BALANCED (OPPOSED) VOLTAGE DIFFERENTIAL PROTECTION
In this scheme, the relay functions normally during external faults. This is because the CT secondaries are so connected that the secondary currents remain in opposition cancelling each other so that their voltage remains balanced. Therefore, no current flows through the pilot wire. However, on the occurrence of an internal fault, a differential current proportional to I1 – I2 for a single-relay-fed system and proportional to (I1 + I2) for a double-relay-fed system, flows through the pilot wire. When this differential current flowing is higher than the pick-up value, the relays operate to trip the circuit breakers, thus protecting the protected equipment from internal fault.
Transformer differential protection
The differential protection system can protect a transformer effectively because of the inherent reliability of these relays, and also because they are highly efficient in operation. The CTs on the primary and secondary sides of the transformer are so connected that they form a circulating current system with equivalent ampere-turns. Terminal faults or winding faults within the protection zone of the relay create differential currents and are cleared quickly by the relay.
Most internal faults that occur in the windings involve ground or between it occurs between turns which results from insulation failure.
Differential protection can also detect and clear insulation faults in the transformer windings. The insulation faults are caused by arcing inside the bushing or tap changer. Thus differential protection can efficiently clear the phase-to-phase and phase-to-earth fault. However, the former between the windings of the power transformer is rare.
Basic considerations for differential protection in three-phase transformers:
To apply the principles of differential protection to three-phase transformers, the following factors should be considered:
Transformation ratio
Because of the transformation ratio of the transformer, the current in the HV and LV sides will be different. Therefore the current transformers are to be selected in such a way that they reproduce the actual current on either side in a proportional way.
For example, in a 132/33KV transformer, suppose the HV side has 100A, the LV side will have 400A (based on transformation ratio, say). The CT on the HV side should be sized to 100/1-A and the LV to 400/1-A. Both the CT secondaries will reproduce proportional 1Amps thus forming equal ampere turns.
When a transformer is connected in star/delta, there will be a phase current on one side and a line current on the other. This can be compensated by using a suitable delta CT connection on the primary and a star CT connection on the secondary side of the transformer. Otherwise, the CT secondary current connected to the delta side of the transformer has to be multiplied by √3 for balancing.
Also because of the vector group of the transformer, the secondary current will have a phase shift of a multiple of 30◦ relative to the primary. This shift can be compensated at the relay for the currents to be compared.
Tap changer
A Tap changer in a transformer can vary the transformation ratio. This variation will result in variation in current, the differential protection must be able to cope up and have a zone of tolerance where the relay responses can be altered. Therefore, a bias mechanism is essential in such protection systems.
Magnetization inrush
This occurs when a transformer remains de-energized for a long duration and is energized again. The magnetizing inrush creates a flow of current in the primary winding that is not transformed in the secondary winding. It thus produces the same effect as in the case of an internal fault and results in a differential trip, it is therefore necessary to have an arrangement that can discriminate between magnetizing inrush and fault current. These methods include:
- By using a unit that can vary the sensitivity and introduce a time delay for the initial inrush peak period.
- Use of a harmonic-restraint unit, in conjunction with a differential relay unit which blocks the inrush rich in 2nd or 5th harmonics.
- Limiting the differential relay function during the energizing of the transformer.
Line differential protection
It is a protection system used in high voltage transmission system for protection of the overhead lines. This system includes a set of CT at each end of the transmission line for measuring currents at both ends. The differential relay units in this system must be interconnected with a very high-speed connection. So that the measured currents can be compared and the relay can make a decision and issue both direct and remote trip signals.
The line differential protection is a very high-speed protection system capable of clearing faults in less than 1 cycle. It is very selective as only operates for internal faults, that are there in the overhead line. This relay does not operate for load encroachments and external faults.
The reliability of this protection system has increased drastically with the use of the optical fibre signalling channel.
Busbar differential protection
Busbar differential protection is based on the same principles as transformer differential protection. Under normal system conditions, the power that is entering and leaving the busbar is the same. As a fault is encountered inside the busbar, a current imbalance is generated, which also gets reproduced in the associated CT secondaries. The differential current flowing the operating coil of the relay due to the imbalance, sends the trip signal of all the breakers associated with that busbar.
FAQ’s
What are the types of differential protection?
It normally depends on the differential parameter upon which the relay works. The popular types are current differential, voltage differential, percentage, or biased differential relay.
How does a differential relay work?
A basic differential relay works by detecting the difference in current at two points in a circuit and generates a trip signal if the difference is greater than a preset value.
This article is a part of the Protection System, where other articles related to the protection of electrical equipments are discussed in details.
