EETC Transformers 4

Question 1

An instrument transformer used to measure high voltages safely by stepping them down to a standard 120V level is called a:

Accepted Answer

Potential transformer (PT)

Answer

A potential transformer (PT), also called a voltage transformer (VT), accurately steps down high voltage to a safe 120V level for metering and protection relays.

Question 2

The secondary circuit of a current transformer (CT) must never be open-circuited while the primary carries current because:

Accepted Answer

It will develop a dangerously high voltage across the open secondary terminals

Answer

With the secondary open, the primary MMF is unopposed, driving the core into saturation and inducing a very high dangerous voltage across the open secondary.

Question 3

In a three-phase transformer bank, what is the primary advantage of using a delta connection on the primary side?

Accepted Answer

Third-harmonic currents circulate within the delta, preventing harmonic voltage distortion

Answer

The closed delta path provides a circulating path for triplen (3rd, 9th, etc.) harmonic currents, keeping them out of the supply line.

Question 4

A transformer's short-circuit test (impedance test) is performed to determine:

Accepted Answer

Full-load copper losses and series impedance parameters

Answer

The short-circuit test is conducted at rated current with the secondary shorted to measure winding resistance, leakage reactance, and copper losses.

Question 5

What is the typical efficiency of a well-designed modern distribution transformer at full rated load?

Accepted Answer

97% to 99%

Answer

Modern distribution transformers achieve efficiencies of 97–99% because both core and copper losses are very small relative to the power transferred.

(EETC) Electrical Technician Certification Practice Test

(EETC) Electrical Technician Certification Practice Test

EETC Transformers 4