NETA Instrument Transformer Testing (CTs and PTs) — Questions and Answers
Question 1: What is the primary purpose of performing a ratio test on a current transformer (CT)?
- To verify that the CT accurately steps down current by its nameplate ratio (Correct answer)
- To measure the insulation resistance between primary and secondary windings
- To determine the CT's burden capacity in volt-amperes
- To check the polarity marking on the secondary terminals
Correct answer: To verify that the CT accurately steps down current by its nameplate ratio
The ratio test confirms that the CT's actual transformation ratio matches its nameplate rating (e.g., 600:5). An incorrect ratio means the metering or protective relays connected to the CT receive inaccurate current signals, leading to metering errors or relay misoperation.
Question 2: A CT with a 5A secondary is left open-circuited while the primary carries load current. What hazard does this create?
- The CT core saturates and the secondary develops dangerously high voltage (Correct answer)
- The primary fuse blows due to excessive current
- The CT ratio decreases, causing relay under-reach
- The insulation resistance drops to zero
Correct answer: The CT core saturates and the secondary develops dangerously high voltage
Unlike a power transformer, a CT is a constant-current device. When the secondary is open while primary current flows, all the magnetizing force drives the core into deep saturation, inducing voltages on the secondary that can reach thousands of volts—enough to be lethal and to destroy CT insulation.
Question 3: During a polarity test on a current transformer, what does a 'subtractive polarity' result indicate?
- Current enters the H1 terminal and exits the X1 terminal simultaneously (Correct answer)
- The primary and secondary windings are wound in opposite directions
- The CT secondary voltage is in phase with the primary voltage
- The CT has been connected with reversed primary leads
Correct answer: Current enters the H1 terminal and exits the X1 terminal simultaneously
Subtractive polarity (standard for most power CTs) means that when current enters the primary at the H1 terminal, it exits the secondary at the X1 terminal at the same instant. Knowing polarity is critical for differential protection schemes where CT secondaries must be correctly phased to prevent false tripping.
Question 4: What does the 'burden' of a current transformer refer to?
- The total impedance (in ohms or VA) connected to the secondary winding (Correct answer)
- The maximum primary current the CT can carry continuously
- The number of secondary turns relative to primary turns
- The dielectric strength of the CT's insulating medium
Correct answer: The total impedance (in ohms or VA) connected to the secondary winding
CT burden is the load placed on the secondary, expressed in ohms or VA at a rated secondary current. If the actual burden exceeds the CT's rated burden, the CT can saturate, causing ratio errors and inaccurate relay inputs. NETA procedures include measuring burden to confirm it is within the CT's rating.
Question 5: A potential transformer (PT) is being tested with an insulation resistance test. What minimum insulation resistance is generally acceptable per NETA standards?
- 100 megohms or greater (Correct answer)
- 1 megohm or greater
- 10 kilohms or greater
- 500 ohms or greater
Correct answer: 100 megohms or greater
NETA Acceptance Testing Specifications (ATS) require a minimum of 100 MΩ for instrument transformer insulation. Lower values indicate deteriorated insulation that could cause ratio errors, safety hazards, or dielectric failure under normal operating voltage.
Question 6: Which test is used to verify that a CT's accuracy class is maintained at elevated current levels, ensuring it does not saturate prematurely?
- Excitation (saturation) curve test (Correct answer)
- Contact resistance test
- Secondary injection test at rated load
- Dielectric withstand test
Correct answer: Excitation (saturation) curve test
The excitation (saturation) curve test plots secondary exciting voltage versus exciting current. The knee-point voltage on this curve indicates where the CT begins to saturate. For protective relaying CTs, a high knee-point voltage confirms the CT can accurately reproduce fault currents without saturating, which is essential for reliable relay operation.
NETA Instrument Transformer Testing (CTs and PTs)
What is the primary purpose of performing a ratio test on a current transformer (CT)?