A medium voltage PT (Potential Transformer) is an instrument transformer that steps down high distribution voltages to a standardized, safe secondary voltage for metering and protective relays. In a real circuit, it changes a lethal, unmeasurable primary voltage (like 13.8kV) into a proportional, isolated, low-voltage signal that standard 120V panels, PLCs, and microprocessor relays can read without destroying their input circuits or endangering personnel.
The Core Function: Isolation and Step-Down
Unlike a standard control transformer used to drop 480V to 120V for outlet power, a PT is engineered for extreme precision and phase-angle accuracy. Its job is not to deliver bulk power, but to deliver a flawless replica of the primary voltage waveform to secondary devices. According to the IEEE C57.13 Standard for Instrument Transformers, these devices must maintain strict ratio and phase-angle error limits, categorized into metering accuracy classes (like 0.3) and relaying accuracy classes (like 0.6 or 1.2).
The physical construction usually involves a primary winding rated for the line voltage and a secondary winding designed to output a standard voltage. The terminals are marked with polarity dots: H1 and X1 indicate the primary and secondary polarity marks. Getting these wrong won't stop a simple voltmeter from reading, but it will cause directional overcurrent (ANSI 67) or differential (ANSI 87) relays to trip falsely or fail to operate during a fault.
Worked Numeric Example: Calculating Primary Voltage and Burden
Let us look at a standard 13.8kV distribution feeder. The switchgear lineup is equipped with a PT rated at 13,800V primary to 120V secondary.
1. The Turns Ratio:
Ratio = 13,800 / 120 = 115:1.
If your multimeter reads exactly 120.0V on the secondary terminals, the primary line voltage is 120.0 × 115 = 13,800V. If the grid sags and your meter reads 114.5V, the actual primary voltage is 114.5 × 115 = 13,167.5V.
2. The Burden Calculation:
PTs are rated by their maximum burden in Volt-Amperes (VA) at a specific accuracy class. Suppose this PT is rated for 50VA at 0.3 metering accuracy.
We connect three devices to the secondary:
• Power meter: 15VA
• Protective relay: 20VA
• SCADA transducer: 10VA
Total connected burden = 45VA. Because 45VA is less than the 50VA rating, the PT will maintain its 0.3% accuracy. If we added a fourth device drawing 10VA, the total would hit 55VA, exceeding the rating and degrading the accuracy class.
Where You Meet This in Practice
You will rarely see a medium voltage PT on a residential bench, but they are ubiquitous in commercial and industrial power systems. You will encounter them in:
- Medium Voltage Switchgear: Inside metal-clad drawout breaker cells (like Siemens GM-SG or Eaton VCP-W), PTs are often mounted on roll-out trucks so they can be isolated and racked out for safe testing.
- Utility-Scale Solar and BESS: The point of common coupling (PCC) for a 5MW solar farm tying into a 33kV grid requires PTs to feed the utility-grade revenue meter and the anti-islanding protection relay.
- Padmount Transformers and Reclosers: Pole-mounted reclosers use built-in bushing PTs to sense line voltage for automatic reclosing sequences and fault location algorithms.
Real-World Scenario: The Burden Overload Trip
To understand what happens when theory meets jobsite reality, consider this actual commissioning failure on a 13.8kV microgrid tie-breaker.
The Setup: A new microgrid controller was being integrated into an existing 13.8kV switchgear lineup. The existing PT was an older cast-resin model rated for 50VA thermal and 50VA at 0.6 relaying accuracy.
The Numbers: The integration team added a new power quality analyzer (25VA), a modern microprocessor relay (10VA), and a PLC analog input isolation module (20VA). The existing electromechanical voltage relay drew 15VA. The total connected burden was 70VA on a 50VA-rated PT.
The Outcome: During primary injection testing, the primary voltage was raised to a nominal 13.8kV. However, the secondary voltage measured at the relay terminals was only 94V. The microgrid controller interpreted this 94V secondary reading as a primary voltage of 10,810V (94 × 115). Seeing a massive "undervoltage" condition, the ANSI 27 element tripped the main tie-breaker offline.
What Went Wrong: The PT core saturated due to the 140% burden overload. When an instrument transformer exceeds its VA burden, the internal voltage drop across its own winding impedance increases drastically, causing the secondary terminal voltage to sag under load. The fix required upgrading the PT to a 200VA unit and verifying the wire gauge (12 AWG copper) to ensure the lead resistance did not add hidden burden to the circuit. For a deeper look at how burden affects relay performance, the Electrical Engineering Portal's guide on instrument transformers provides excellent field calculations.
What People Commonly Confuse It With
On the jobsite, terminology gets sloppy. Here is how to distinguish a medium voltage PT from similar equipment:
| Device | Primary Function | Secondary Output | Danger Condition |
|---|---|---|---|
| Potential Transformer (PT/VT) | Steps down voltage for metering/relaying | 110V - 120V (Voltage Source) | Short-circuited secondary |
| Current Transformer (CT) | Steps down current for metering/relaying | 1A or 5A (Current Source) | Open-circuited secondary |
| Capacitive Voltage Transformer (CVT) | Steps down EHV (69kV+) via capacitive divider | 110V - 120V | Ferroresonance during switching |
| Control Transformer | Steps down voltage for control power/outlets | 120V / 24V (High VA capacity) | Overloading beyond breaker trip |
FAQ: Bench and Jobsite Questions
Do I need to ground the secondary of a medium voltage PT?
Yes. Per standard practice and NEC-style guidance, one side of the PT secondary (usually X2) must be solidly grounded. This ensures that if the insulation between the 13.8kV primary and the 120V secondary fails, the fault current has a path to trip the primary breaker rather than energizing the relay panel at 13.8kV.
Can I use a standard 13.8kV to 120V control transformer instead of a PT for metering?
What is the difference between a PT and a VT?
There is no functional difference. PT (Potential Transformer) is the traditional North American term, while VT (Voltage Transformer) is the preferred IEC and modern IEEE terminology. They refer to the exact same device.
