Differential protection
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.
Table of Contents
The basic principle
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 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, K of the relay is the 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 secondaries of the CTs, are connected in such a way that during normal operation and during external through faults, the secondary currents of CTs on two sides oppose each other and their voltages are balanced. Hence no current flows in pilot wires and relays. During an internal fault, however, a differential current proportional to I1 – I2 in the case of a single-end-fed system and proportional to (I1 + I2) in the case of double-end, flows through the pilot wire into the relay coils. When this differential current flowing in the relay coils is higher than the pick-up value, the relays operate to isolate the protected equipment from the fault.
Transformer differential protection
The differential system can protect a transformer effectively because of the inherent reliability of the relays, which are highly efficient in operation, and the fact that equivalent ampere-turns are developed in the primary and secondary windings of the transformer. The CTs on the primary and secondary sides of the transformer are connected in such a way that they form a circulating current system. Faults on the terminals or in the windings are within the transformer protection zone and should be cleared as quickly as possible to avoid internal stress and the danger of fire.
Most internal faults that occur in the windings are to earth (across to the core) or between turns.
Differential protection can also detect and clear insulation faults in the transformer windings. The principal cause of these faults is arcing inside the bushings and faults in the tap changer. This type of protection not only responds to phase-to-phase and phase-to-earth faults but also in some degree to faults between turns. However, phase-to-phase faults between the windings of a three-phase transformer are less common.
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
The nominal currents in the primary and secondary sides of the transformer vary in inverse ratio to the corresponding voltages. This should be compensated for by using different transformation ratios for the CTs on the primary and secondary sides of the transformer.
Transformer connections
When a transformer is connected in star/delta, the secondary current has a phase shift of a multiple of 30◦ relative to the primary, depending on the vector group. This shift can be offset by suitable secondary CT connections. Furthermore, the zero-sequence current that flows in the star side of the transformer will not induce a current in the delta winding on the other side.
The zero-sequence current can therefore be eliminated from the star side by connecting the CTs in the delta. For the same reason, the CTs on the delta side of the transformer should be connected in a star. When CTs are connected in the delta, their nominal secondary values should be multiplied by √3 so that the currents flowing in the delta are balanced by the secondary currents of the CTs connected in the star.
Tap changer
If the transformer has the benefit of a tap changer it is possible to vary its transformation ratio, and the differential protection system should be able to cope with this variation. Since it is not practical to vary the CT transformation ratios, the differential protection should have a suitable tolerance range in order to be able to modify the sensitivity of its response to operation. For this reason, it is necessary to include some form of biasing in the protection system together with some identifying markings of the higher current input terminals.
Magnetization inrush
This phenomenon occurs when a transformer is energized, or when the primary voltage returns to its normal value after the clearance of an external fault. The magnetizing inrush produces a current flow into the primary winding that does not have any equivalent in the secondary winding. The net effect is thus similar to the situation when there is an internal fault in the transformer. Since the differential relay sees the magnetizing current as an internal fault, it is necessary to have some method of distinguishing between the magnetizing current and the fault current. These methods include:
Using a differential relay with a suitable sensitivity to cope with the magnetizing current, usually obtained by a unit that introduces a time delay to cover the period of the initial inrush peak.
Using a harmonic-restraint unit, or a supervisory unit, in conjunction with a differential unit.
Inhibiting the differential relay during the energizing of the transformer.
Line differential protection
The form of differential protection using only one set of relays is not suitable for long overhead lines since the ends of a line are too far apart to be able to interconnect the CT secondaries satisfactorily. It is therefore necessary to install a set of relays at each end of the circuit and interconnect them by some suitable communication link.
Pilot protection is an adaptation of the principles of differential protection that can be used on such lines, the term pilot indicating that there is an interconnecting channel between the ends of the lines through which information can be transmitted. There are three different types of interconnecting channels – pilot wires, current-carrying wires, and centimetric-wave systems.
The principle of operation of pilot differential protection is like the differential systems for protecting the transformers, but the relays have different settings because the breakers at the ends of the line are more widely separated and a single relay should not be used to operate two tripping circuits. This method of protection is ideal from a theoretical point of view as both ends of the line should open instantaneously for faults wherever they occur on the line. In addition, the system should not operate for faults outside the section and is therefore inherently selective.
Many of the operational difficulties with conventional schemes due to induced currents have been overcome by the use of fiber optics, which has greatly improved the reliability of this type of protection.
Busbar differential protection
Busbar differential protection is based on the same principles as transformer differential protection. Under normal system conditions the power that enters a busbar is identical to the power that leaves, a fault inside the differential circuit unbalances the system and current thus flows in the operating coil of the relay, which then results in the tripping of all the breakers associated with that busbar.
There can be many circuits connected to the busbar, which necessarily implies the connection of several CT secondaries in parallel. In busbar differential schemes that involve bushing-type CTs, six to eight secondaries can usually be connected in parallel without difficulty.
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 trip signal if the difference is greater than a preset value.
