---
title: "Causes of PT ratio and phase errors"
subject: "Switchgear and Protection"
chapter: 2
key: "semester_6/switchgear_and_protection:pt-ratio-phase-angle-error"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/pt-ratio-phase-angle-error
site: BCT Engineering
---

# Causes of PT ratio and phase errors

Switchgear and Protection · Chapter 2: Current and Potential Transformers

- Asked in: 2070 Magh, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/pt-ratio-phase-angle-error
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2

## Question

Discuss the causes of ratio error and phase angle error in PT.

## Short answer

Ratio error in a PT stems from voltage drops in winding resistances and leakage reactances due to load and no-load currents, worsening with higher burden. Phase angle error is caused mainly by magnetizing current and reactive voltage drops, with its magnitude dependent on burden power factor.

## Answer

In an ideal PT, V_s = V_p/K_n, exactly in phase opposition. In a practical PT, the voltage drops in the winding impedances and the no-load current cause **ratio error** and **phase angle error**.

#### Phasor relation
```text
 V_p = (reversed) E_p + I_p(R_p + jX_p)
 E_s = V_s + I_s(R_s + jX_s)
 I_p = I₀ + I_s/n   (I₀ = I_m + I_w, n = N_p/N_s)
```
So the reversed V_s differs from V_p in magnitude (ratio error) and angle (phase error).

#### Causes of ratio error
1. **No-load (exciting) current I₀** flowing through the primary resistance and leakage reactance produces a voltage drop.
2. **Load current** flowing through the primary and secondary resistances (R_p, R_s) and leakage reactances (X_p, X_s) causes further drop.
3. **Burden:** a higher burden (more VA) means more current and larger drops, so the ratio error increases.
4. **Burden power factor:** at lagging pf the reactive drops add more directly to the magnitude.
5. A supply voltage or frequency different from rated changes I₀.

```text
 Ratio error (%) = (K_n − R)/R × 100,  R = V_p/V_s
```

#### Causes of phase angle error
1. **Magnetising current I_m** and loss current I_w flowing through the winding impedance shift the phase of E_p relative to V_p.
2. **Resistive drops** (I·R) of the windings, especially at unity-pf burden.
3. **Leakage reactance drops** at lagging-pf burden.
4. Burden magnitude and power factor.

#### Methods to reduce errors
- Low flux density and good core material to reduce I₀.
- Low winding resistance and leakage reactance (thick conductors, interleaved windings).
- Turns compensation (slightly fewer primary turns).
- Keeping the burden within rating.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What causes PT errors?
   A. Voltage drops in winding impedances due to no-load and load current
   B. Saturation by fault current
   C. Open secondary
   D. Absence of burden
   Answer: A. Drops in R and X shift magnitude and angle. (Key point: Cause)

2. Effect of higher burden on ratio error:
   A. It decreases
   B. None
   C. It increases
   D. It removes phase error
   Answer: C. More burden means larger drops. (Key point: Burden effect)

3. Which contribute to phase angle error?
   A. Only core colour
   B. Magnetising and loss currents and resistive/leakage drops
   C. Only frequency
   D. Only primary insulation
   Answer: B. Per listed causes. (Key point: Phase error)

4. How are PT errors reduced?
   A. High flux density
   B. Thin conductors
   C. Larger burden
   D. Low flux density, low winding impedance, turns compensation, burden within rating
   Answer: D. These reduce I0 and drops. (Key point: Reduction)

## Related in Switchgear and Protection

- [CT purpose, phasor diagram and errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-phasor-ratio-phase-error) — Switchgear and Protection, chapter 2
- [Knee point voltage and CT errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-errors) — Switchgear and Protection, chapter 2
- [CT ratio for 33/6.6 kV star/delta (300:5)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-33-6-6kv-300) — Switchgear and Protection, chapter 7
- [CT ratio for 33/6.6 kV star/delta (400/5)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-33-6-6kv-400) — Switchgear and Protection, chapter 7
- [Merz-Price CT ratio for 66000/6600 V](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amerz-price-66kv-ct-ratio) — Switchgear and Protection, chapter 7
- [CT ratio and connection for 11/132 kV](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-11-132kv) — Switchgear and Protection, chapter 7

## Related in other subjects

- [Thread errors and effect of pitch error](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Ascrew-thread-errors-and-pitch-effect) — Metrology, chapter 12
- [Phase and frequency error in local oscillator (DSB and SSB)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aphase-freq-error-lo-dsb-ssb) — Communication Systems, chapter 2, asked 2×
- [Phase error in synchronous DSB-SC detection; synchronized receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aphase-error-dsbsc-costas) — Communication Systems, chapter 2
- [Costas loop detector and phase error correction](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acostas-loop-phase-error) — Communication Systems, chapter 2