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STL (Differential Relay) Protection Basics

DUNIAPEMBANGKITLISTRIK.COM - Differential relay is a relay whose working principle is based on a balance, which compares secondary currents of current transformers (CT) installed in secure equipment or electrical terminals. The use of differential relays as safety relays includes generators, power transformers, bus bars and transmission lines. Differential relays are used as main protection (main protection) on the power transformer which is useful for securing the transformer winding in the event of a disturbance. This relay is very selective and the system works very fast.

The Principle of Differential Relay Work

As mentioned above, a differential relay is a protective device that works very quickly and is very selective based on balance, which is the ratio of the current flowing on both sides of the power transformer through an intermediary, the current transformer (CT). Under normal conditions, current flows through secured electrical equipment (generators, transformers and others). The secondary currents of the current transformer, i1 and i2, emulate through the IA line. If a safety relay is installed between terminals 1 and 2, under normal conditions no current will flow through it. Can be seen in the image below :

Basic Treatment of Differential Relays

If an interruption occurs outside the electrical equipment the electrical equipment is secured (external fault), then the current flowing will increase, but the circulation will remain the same as in normal conditions, so that the safety relay will not work for external interference. If interference occurs inside (internal fault), the direction of current circulation on one side will be reversed, causing the balance under normal conditions to be disturbed, consequently ID current will flow through the safety relay from terminal 1 to terminal 2.

As long as the secondary currents of the current transformer are the same large, then there will be no current flowing through the working coil (operating coil) of the safety relay, but any disturbance (between phases or to the ground) that results in a disturbed balance system, will cause the current to flow through the Operating Coil of the safety relay, the safety relay will work and giving tripping orders to the circuit breaker (CB) so that the disrupted electrical equipment or installation can be isolated from the electric power system. Like the picture below :

Differential Relay Safety System


Overview of Some Problems With Differential Relays

1. CT characteristics

Differential relay in its operation that under normal circumstances or interference occurs outside the area of ​​safety current relay is zero. Because of this, the possibility of wrong work from a differential relay can occur, the current which can cause the wrong work relay is called an imbalance current. If a large current flows through a current transformer, the current in the secondary terminal is no longer linear to the primary current.

This is due to saturation at its core. In the differential transformer the current must be identical, but the saturation of the core cannot be the same. This is due to differences in the load of each of the current transformer.

2. Characteristics of Current Transformers in differential relays, as shown below :

Current Transformer (CT) Characteristics in Differential Relays


3. Change in Saddle Leads

At this time the transformer is generally equipped with a load tapping modifier where the tap input can be changed to obtain the desired output. The adjustment of the current transformers in the power transformer is set at the nominal voltage of the power transformer. Thus, if a disturbance occurs during the operation of the transformer, the voltage on the primary side must be changed so that the voltage on the secondary side remains. Therefore the transformer tap prices that have been set at their nominal voltage will no longer be correct. This is what causes an imbalance current that can make the relay work wrong.

4. The existence of Magnetising Inrush Current in the Transformer

If the power transformer is connected to a power source (network), then on the primary side there will be a transient process that is an increase in the current called the magnetizing inrush current which can reach 8 to 30 times the full load current that occurs in a relatively fast time. This event can have an influence on the work of a relay even though the security area does not occur error.


Differential Differential Relay

To overcome the problems (a) and (b) above, the differential relay is equipped with a work coil and a restraining coil (holding coil) or better known as a Differential Differential Relay (Differential Relay Bias). By updating the defferential relay based on the current circulation principle is to overcome the interference that arises outside of the difference in the ratio of the high value of the external short circuit current. Differential relays with percentages have additional damping coil connected to the pilot wire as shown below :

Differential Differential Relay (Differential Relay Bias)

In this relay the working coil is connected to the midpoint of the retaining coil (dampers), the total number of impedance windings in the dampening coil is equal to the number of amperes of windings that exist in both ½ parts of the coil namely I1N / 2 + I2 / N which gives an average damper current of (I1 + I2) / 2 in the N winding preform. For external disturbances I1 and I2 are getting bigger and hence the dampening coupling is getting bigger which can prevent operation errors.

The operating characteristics of such a relay are given in the figure below :

Operating Characteristics Of A Differential Relay

The average immersion current ratio of the percentage of the differential operating current can be determined, so the relay is called a differential relay with a percentage. The relay is also called a differential bias relay, because this relay is equipped with an additional flux. Percentage of differential relay bias has higher pick-up characteristics. Because the amount of current passing by is increasing, the reducing current is increasing.

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