Current transformers (CTs) and voltage transformers (VTs) convert the network signal to a level that is safe for relays and meters. If the ratio, polarity or accuracy class of a CT is wrong, even the best relay takes the wrong decision. CT/VT testing is therefore one of the first stages in commissioning any substation.
Metering vs protection CTs
- Metering CTs (class 0.2, 0.5, 1 etc.): high accuracy around rated current; they saturate early at fault current to protect the meter. An instrument security factor (FS) is specified.
- Protection CTs (such as 5P20 or 10P10): maintain accuracy up to several times rated current. In 5P20: composite error of at most 5 % up to 20 times rated current.
- Class PX / TPS (IEC 61869-2): defined by knee-point voltage (Vk) and winding resistance, used for differential and distance protection.
CT tests
1. Inspection and insulation
- Condition, nameplate reading and agreement with drawings
- Insulation resistance of primary and secondary windings with a megger (typically 500 to 1000 V for the secondary and 2.5 to 5 kV for the primary)
2. Ratio
Using primary injection (high-current test set) or the secondary-voltage method, the measured ratio is compared with the nameplate. Permitted error depends on the class.
3. Polarity
- Battery (DC kick) method: momentarily connect a battery to the primary and observe the direction of the galvanometer deflection on the secondary
- Or use a test set and compare the phase angle of primary and secondary currents
Wrong polarity causes mal-operation of differential and directional relays.
4. Excitation (saturation) curve
AC voltage is applied to the secondary and the excitation current measured with the primary open. The knee point per IEC is where a 10 % increase in voltage produces a 50 % increase in excitation current. The test shows that the CT will not saturate during faults:
where Rct is CT winding resistance, RL the loop lead resistance, Rb the relay burden resistance and k a safety factor (depending on the protection type).
5. Winding resistance and burden
- Measure the secondary DC resistance (Rct) and compare with the datasheet
- Measure the actual circuit burden at rated current injection; burden must not exceed the CT's rated VA: VA = I² × R
6. Secondary circuit review
- Secondary earthing at one point only
- Connections and test terminals; the secondary must never be open while the primary is live
Safety warning: the secondary of an energised CT must never be left open; the resulting voltage is lethal and destroys insulation.
VT tests
- Ratio and polarity: inject low voltage into the primary or secondary and compare
- Insulation resistance and winding resistance
- Burden: total relay and meter burden must not exceed the rated VA
- Protection: secondary fuses or MCBs and VT supervision operation
- Phase sequence and open-delta wiring for zero-sequence voltage
- For capacitor VTs (CVT), transient response and ferroresonance are checked.
End-to-end primary test
Injecting real current into the CT primary confirms that the whole path to the relay and meter is right: ratio, polarity, phase connections and the value shown in SCADA. This finds wiring faults that individual component tests miss.
Common mistakes
- Connecting to the wrong secondary tap
- CT saturation from heavy burden or long thin leads
- Earthed at two points simultaneously
- Mixing metering and protection cores
- Ignoring CT direction (P1/P2) relative to current flow
The test report must include ratio, polarity, excitation curve, burden and the test equipment with its calibration number.