- 2073 Bhadra · 4 marks
Distinguishing CT and PT
Short answer
CT primary is series-connected with line, reducing current to 1A/5A; PT primary is parallel-connected, reducing voltage to 110V. CT has few primary turns, many secondary turns, low secondary impedance (near short-circuit); PT has many primary turns, few secondary turns, high secondary impedance (near open-circuit). CT secondary open-circuit dangerous (high voltage); PT secondary short-circuit dangerous (high current). CT burden affects secondary voltage; PT burden affects secondary current.
What are the main facts to distinguish current transformer and potential transformer?
A current transformer reduces a large line current to a standard value (1 A or 5 A), while a potential transformer reduces a high voltage to a standard value (110 V). Their main distinguishing facts are:
| Point | Current transformer (CT) | Potential transformer (PT) |
|---|---|---|
| Connection | Primary in series with line | Primary in parallel (line to line or line to earth) |
| Primary quantity | Line current; not decided by CT burden | System voltage; fixed |
| Primary winding | Few turns (often a single bar) | Many turns |
| Secondary winding | Many turns | Few turns |
| Standard secondary | 1 A or 5 A | 110 V or 110/√3 V |
| Core flux | Varies with line current; low normally | Almost constant (voltage fixed) |
| Secondary load | Very low impedance (ammeter, relay current coil) – nearly short circuit | High impedance (voltmeter, relay voltage coil) – nearly open circuit |
| Secondary open circuit | Dangerous: very high voltage, core overheats | Normal (no-load) condition |
| Secondary short circuit | Normal condition | Dangerous: very high current, winding burns |
| Burden effect | Burden changes secondary voltage, primary current unchanged | Burden changes secondary current |
| Errors due to | Exciting current | Winding voltage drops and exciting current |
| Construction at EHV | Bar/ring/bushing type | Electromagnetic or capacitor (CVT) type |
Both provide isolation, standardise relay and meter inputs, and have ratio and phase angle errors.
✓ Answer checked by Claude Opus 5.5 (high effort)
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