Voltage Transformer (VT/PT)
Theory: voltage drop becomes error
Overview
After adding instruments to a substation VT, the energy meter reading seems slightly lower. Is that related?
Very much so. A VT steps high voltage down to 110 V at an accurate ratio, but when connected instruments draw current, winding resistance and leakage reactance drop the voltage. By that drop the secondary voltage falls below its ideal value and shifts in phase. The larger the instruments' consumption (burden), the larger the error, so adding instruments increases error. VTs have accuracy classes with error limits over a specified burden range; beyond it, the guarantee lapses.
Ratio error and phase displacement
$K_n$ is the rated ratio, $V_p$ and $V_s$ primary and secondary voltages. Phase displacement $\delta$ is usually expressed in minutes (1/60 degree).
Source of error
Drop across winding impedance from secondary current $I_s$, plus primary drop from magnetising current $I_0$, constitute the error. Burden $Z_b$ follows from rated VA $S_b$ and power factor.
Transformers underpinning power transactions
When AC transmission spread in the late 19th century, connecting instruments directly to high voltage was dangerous, so instrument transformers appeared. As electricity became a traded commodity, metering accuracy became billing accuracy, and standards set strict limits on instrument-transformer accuracy — a class 0.2 VT must keep ratio error within 0.2%. A 1% error adds up to a large sum on a big factory's annual bill. Recently, digital instrument transformers using optical sensors or dividers have begun to appear.
Worked example: a 6600/110 V VT
Rated burden 50 VA (0.8 lagging); referred to the secondary, winding resistance 0.4 Ω and leakage reactance 0.6 Ω on each side, magnetising reactance 20 kΩ, core-loss resistance 50 kΩ. Class 0.5 limits: ratio error ±0.5%, phase ±20 minutes:
| Burden | Ratio error (no compensation) | Class 0.5 | Ratio error (turns +0.3%) | Class 0.5 | Phase |
|---|---|---|---|---|---|
| 0% | −0.004% | Within | +0.296% | Within | 0.0 min |
| 25% | −0.144% | Within | +0.155% | Within | −1.7 min |
| 50% | −0.284% | Within | +0.015% | Within | −3.4 min |
| 100% | −0.563% | Outside | −0.264% | Within | −6.8 min |
| 150% | −0.840% | Outside | −0.543% | Outside | −10.1 min |
Just 0.3% more turns fits the whole error range into class 0.5.
Right. As burden goes from zero to rated, ratio error spans about 0.56%. Turns compensation lifts the whole band so its middle sits near zero. But the band itself doesn't shrink, so beyond rated burden even a compensated VT falls outside — that's the 150% row. When adding instruments, check that their total consumption stays within the VT's rated burden. Phase displacement is small, but in energy metering its effect grows at low power factor, so high-accuracy classes like 0.2 need designs with small leakage reactance.
Design and evaluation workflow
- Set the required accuracy class and total instrument burden.
- Compute error versus burden from winding resistance, leakage reactance and magnetising characteristics via the equivalent circuit.
- Center the error with turns compensation.
- Confirm leakage flux and core behaviour (saturation, loss) by electromagnetic analysis.
- Test over the voltage range (e.g. 80–120% of rated) and frequency variation.
“More instruments, smaller bills”
At a factory substation, several instruments were connected to the same VT for power monitoring, and the energy meter began reading about 0.5% lower. The instruments' total consumption far exceeded the VT's rated burden, lowering the secondary voltage. Since this affected utility billing, the monitoring instruments were moved to a separate VT and burden brought back within rating. VT error depends not only on the VT's quality but on how it's used.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Not checking total burden | Error exceeds class | Stay within rated burden |
| Too little burden | Compensation drives error positive | Check over the working range |
| Ignoring phase displacement | Metering error at low pf | Evaluate phase too |
| Ignoring lead resistance | Voltage drop to instruments | Include leads in burden |
| Ignoring ferroresonance | Overvoltage, burnout | Analyse system conditions |
I'd like to learn related topics.
Try the voltage-drop idea with the transformer voltage regulation calculator. Related pages include current transformers (CT), transformer equivalent circuit, leakage inductance, core loss and three-phase transformers.
Related Topics
Experience the theory firsthand with the interactive simulator for this field
All Simulators