Current transformer for protection question

In summary, the conversation is about determining the RR value for a relay system where the relay current should not exceed 20mA. The solution involves using the Merz-Price theorem and accounting for the resistances of CT2, RL2, and RS2. The purpose of RR is to prevent unwanted relay operation. However, without knowing the voltage and other resistance values, it is difficult to calculate the exact RR value.
  • #1
bizuputyi
42
1

Homework Statement



Determine RR value if the relay current has not to exceed 20mA. Neglect relay resistance.

2. The attempt at a solution

I believe the solution is behind Merz-Price theorem, where the voltage system is balanced. CT1 and CT2 are identical current transformers, however, RL2 (leads resistance) and RS2 (secondary winding resistance) upset the Merz-Price balance, therefore the need for RR I guess. The relay pulls in when current through it exceeds 20mA. RR is there to prevent unwanted relay operation (relay is to remain off under threshold current).
Now, without voltage and other resistance values I don't see the trick.

Is there a formula for this?

Thank you for your help.
 

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  • #3
I believe I sorted out. The thread may be closed. Thank you.
 

Related to Current transformer for protection question

1. What is the purpose of a current transformer for protection?

The main purpose of a current transformer for protection is to accurately measure the current flowing through a circuit and convert it to a lower value that can be safely measured by protective relays. This helps to protect the electrical system from overcurrent, short circuits, and other faults.

2. How does a current transformer for protection work?

A current transformer works by passing the current-carrying conductor through its primary winding, which creates a magnetic field that induces a current in the secondary winding. The secondary current is proportional to the primary current and is used to operate protective relays and other instruments.

3. What are the main types of current transformers for protection?

The main types of current transformers for protection are the traditional wound type and the newer electronic type. Wound type current transformers use a magnetic core and windings, while electronic current transformers use electronic circuits to measure and convert the current.

4. How do you select the correct current transformer for protection?

The correct current transformer for protection should be selected based on the system voltage, maximum current, and the type of protection required. It is important to also consider factors such as accuracy, burden rating, and the frequency of the system when selecting a current transformer.

5. Can a current transformer be used for both protection and metering?

Yes, current transformers can be used for both protection and metering. However, it is important to ensure that the current transformer is properly sized and rated for both purposes. In some cases, separate current transformers may be required for protection and metering to ensure accurate measurements and reliable protection.

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