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Instrument transformers used for measurement and protection purpose in substations are
Current transformer
Potential transformer
Power transformer
Both (a) and (b)
Both (a) and (b)
Instrument transformers, comprising Current Transformers (CTs) and Potential Transformers (PTs), are essential electrical devices used in substations to step down high currents and voltages to standard, safe, and measurable levels for protection relays and metering instruments.
Instrument transformers, comprising Current Transformers (CTs) and Potential Transformers (PTs), are essential electrical devices used in substations to step down high currents and voltages to standard, safe, and measurable levels for protection relays and metering instruments.
IsтАЛ=IpтАЛ├ЧNsтАЛNpтАЛтАЛ тАФ CT transformation ratio principle
VsтАЛ=VpтАЛ├ЧNpтАЛNsтАЛтАЛ тАФ PT transformation ratio principle
The CT operates on the principle of electromagnetic induction to transform high primary currents into a standard low secondary current (typically 1A or 5A). The PT acts as a step-down transformer to convert high line voltages to a standard secondary voltage (typically 110V). By providing galvanic isolation, they ensure that sensitive low-voltage instrumentation is protected from high-voltage circuits.
CTs are connected in series with the line to measure current.
PTs are connected in parallel with the line to measure voltage.
These devices provide electrical isolation between high-voltage power circuits and low-voltage secondary devices.
They reduce quantities to standard values compatible with secondary protection and metering equipment.
Safety for operating personnel and measurement equipment.
Enables the use of standard, mass-produced low-rating measuring instruments.
Provides physical isolation, preventing high voltage from reaching metering panels.
CT secondary must never be open-circuited while primary is energized (leads to core saturation and high voltage).
PT secondary must never be short-circuited (leads to high current and overheating).
Energy metering in HV/MV substations.
Relay protection systems for circuit breakers.
Power quality monitoring and fault detection.
| Feature | CT | PT |
|---|---|---|
Connection | Series with line | Parallel with line |
Primary winding | Few turns, thick wire | Many turns, thin wire |
Power transformers (Option C) are used to change voltage levels for bulk power transmission and distribution, not for measurement or protection signal acquisition.
Standard secondary current rating is typically 1A or 5A, while standard secondary voltage is typically 110V phase-to-phase.
D is correct тАФ Both Current Transformers and Potential Transformers are essential instrument transformers used for metering and protective relaying in electrical substations.
Always remember the safety rule: Never open the secondary of a current transformer under load, as the lack of secondary MMF results in high core flux, inducing dangerously high voltages.