How do you size a current transformer for protection?

Recommended is a ratio slightly beyond the measured / defined maximum load current (In). Example: In = 1,154 A; selected transformer ratio = 1,250/5. The nominal current can also be defined on the basis of the following considerations: Dependent on the mains supply transformer nominal current times approx.

Can we use metering CT for protection?

Protection Current Transformer (CT) and Metering CT are two different types of CT as per their requirement and hence design. Therefore, Protection class CT should not be used as Metering CT. Similarly, a Metering Current Transformer should not be used for protection purpose.

What are the requirements of protective relays?

Functional Requirements of Protection Relay

• Reliability. The most important requisite of protective relay is reliability.
• Selectivity. The relay must be operated in only those conditions for which relays are commissioned in the electrical power system.
• Sensitivity.
• Speed.
• Switchgear.
• Protective Gear.
• Station Battery.

What is a protection current transformer?

A protection current transformer is designed to operate well into the overcurrent range. This enables the protective relays to measure fault currents accurately, even in very high current conditions.

What is difference between metering current transformer and protection current transformer?

Current transformers are used for both metering and protection purposes. By reduce the power system primary current to lower value CT is used for measuring or metering purpose. By permit the use of standard current rating for secondary equipment CT is used for Protection purpose.

What is selectivity of protective relay?

Selectivity means the ability to determine the point at which the fault occur and select the nearest circuit breaker in the system to trip so that the fault is cleared with minimum or no damage to the system.

Do I need a current transformer?

A Current Transformer (CT) is used to measure the current of another circuit. In doing so, the current transformer reduces a high voltage current to a lower value and therefore provides a safe way of monitoring electrical current flowing in an AC transmission line.

What is a class PS transformer?

Class PS transformers are the special purpose protective current transformers. These are used where the required characteristics of the current transformers cannot be conveniently expressed in terms of the ordinary protection current transformers like Class 5P, 10P and 15P current transformers.

A protection current transformer is designed to operate well into the overcurrent range. This enables the protective relays to measure fault currents accurately, even in very high current conditions. The secondary current is used to operate a protective relay which can isolate part of the power circuit experiencing a fault condition.

Why do I need an external transformer for a macromatic relay?

The Macromatic line of current sensing relays come with a variety of current ranges. Since these ranges are not always large enough to work on most applications, an external transformer can be used to extend the range above the maximum value of each product.

How do I select and apply a current transformer relay?

Follow these steps to properly select and apply a current transformer relay with our current sensing relays: Typically, a standard 5A secondary current transformer is selected. Select a mounting style that fits your application.