A Potential Transformer (PT), also known as a voltage transformer, is one of the most important instrument transformers used in electrical power systems. Its primary function is to step down high voltages to safe, measurable values, allowing meters, protective relays, and control devices to operate accurately without being exposed to dangerous high voltages.
Since modern power networks deal with kilovolts of transmission voltage, directly connecting meters or relays is impossible. This is where PTs come into play, providing a scaled-down replica of the system voltage with high precision.
What is a Potential Transformer?
A Potential Transformer (PT) is a step-down transformer designed to reduce high voltages (in kV) to standardized low voltages (usually 110 V or 120 V) for safe measurement and protection.
Key Points:
- The primary winding is connected to the high-voltage system.
- The secondary winding is connected to voltmeters, relays, or synchronizing equipment.
- The output voltage is proportional to the input voltage, with minimal error.
This ensures both safety and accuracy in power system operations.
Construction of Potential Transformer
The construction of a PT is similar to a conventional transformer but with special design considerations for accuracy and insulation:
- Magnetic Core – Made of laminated silicon steel to reduce hysteresis and eddy current losses.
- Primary Winding – High-voltage winding connected to the power line, designed with strong insulation.
- Secondary Winding – Provides standard low voltage output (usually 110 V).
- Insulation – Special insulation to withstand high system voltages.
- Tank & Bushings – For outdoor PTs, the windings are enclosed in an oil-filled tank with porcelain bushings.
Note: Indoor PTs are usually dry-type, while outdoor PTs are oil-immersed for insulation and cooling.
Working Principle of Potential Transformer
The working principle of PT is based on Faraday’s law of electromagnetic induction.
- When the primary winding is connected to a high-voltage line, it produces an alternating magnetic flux in the core.
- This flux induces a scaled-down voltage in the secondary winding.
- The secondary voltage maintains the same phase and is directly proportional to the primary voltage (within rated accuracy).
Thus, PTs allow measuring devices to safely read high system voltages without being exposed to dangerous levels.
Potential Transformer Symbol
The symbol of a potential transformer is represented as a transformer symbol with:
- Primary connected to high-voltage line.
- Secondary marked for measurement/control circuits.
Diagram of Potential Transformer
The diagram of a potential transformer shows:
- Primary winding connected to high-voltage lines.
- Secondary winding connected to voltmeters, relays, and synchronizing devices.
Connection of Potential Transformer
A potential transformer (PT) is always connected in parallel with the circuit whose voltage is to be measured.
Basic Parallel Connection of PTs with circuits
- Primary Connection:
- The primary winding is directly connected across the high-voltage line or between phase and ground.
- The primary terminal is rated for voltages ranging from 400 V to several thousand volts depending on the system.
- Secondary Connection:
- The secondary winding is magnetically coupled to the primary through the transformer core.
- It is always standardized at 110 V (sometimes 120 V) for safe connection to measuring instruments like voltmeters, wattmeters, energy meters, or protective relays.
- Transformation Ratio (Turns Ratio):
- The ratio of primary voltage to secondary voltage is called the transformation ratio.
- Example: A PT with 11 kV/110 V rating has a ratio of 100:1.
System-Level PT Connections
Depending on system requirements, PTs can be connected in different configurations:
- Single-phase PT
- Connected phase-to-ground or phase-to-phase.
- Common in low and medium-voltage systems.
- Three-phase PT Bank
- Uses three single-phase PTs connected together.
- Provides complete three-phase voltage measurement.
- Star Connection (Y-Connected PTs)
- All secondary windings connected in star, with neutral grounded.
- Supplies line-to-neutral voltages.
- Open-Delta (V-V Connection)
- Uses only two PTs instead of three.
- Supplies line-to-line voltages, reducing cost.
- Used where neutral measurement is not required.
In summary:
- Basic principle: PT is always connected in parallel with the circuit.
- System configuration: PTs can be connected as single-phase, star, or open-delta, depending on measurement needs.
Types of Potential Transformer
Potential transformers are classified into different types depending on construction and application:
1. Electromagnetic Potential Transformer (EM PT)
- Conventional PT with windings and a magnetic core.
- Used up to 33 kV systems.
2. Capacitor Voltage Transformer (CVT)
- Uses a capacitive divider with an auxiliary transformer.
- Commonly used in EHV and UHV systems (132 kV and above).
- Also supports power line carrier communication (PLCC).
3. Optical Voltage Transformer (OVT)
- Uses optical sensors and fiber optics.
- Provides digital signals for smart grid and SCADA applications.
- Immune to electromagnetic interference.
EMF Equation of Potential Transformer
Like any transformer, a PT works on Faraday’s law of electromagnetic induction. The alternating magnetic flux (Φm) linking the primary and secondary windings induces emf in both windings.
If,
- N₁ = Number of turns in primary winding
- N₂ = Number of turns in secondary winding
- f = Supply frequency (Hz)
- Φm = Maximum flux in the core (Wb)
Then the RMS value of induced emf is:
For the primary winding:
For the secondary winding:
The turns ratio (transformation ratio) is:
Where,
- E₁ ≈ Primary induced emf ≈ Primary applied voltage (Vp)
- E₂ ≈ Secondary induced emf ≈ Secondary terminal voltage (Vs)
Phasor Diagram of a Potential Transformer
The following figure illustrates the phasor diagram of a potential transformer.
Where:
- Vp – Primary terminal voltage
- Ep – Primary induced emf
- Ip – Primary current
- Rp – Primary winding resistance
- Xp – Primary winding reactance
- Vs – Secondary terminal voltage
- Es – Secondary induced emf
- Is – Secondary current
- Rs – Secondary winding resistance
- Xs – Secondary winding reactance
- Kt – Turns ratio
- Io – Excitation current
- Im – Magnetizing component of Io
- Iw – Core loss component of Io
- Φm – Main flux
- β – Phase angle error
The main flux (Φm) is taken as the reference in the phasor diagram of a potential transformer. In an instrument transformer, the primary current (Ip) is the vector sum of the excitation current (Io) and the reversed secondary current (Is/kt). The applied voltage at the primary terminal is denoted as Vp.
As the primary current flows through the winding, voltage drops occur across the primary resistance (IpRp) and primary reactance (IpXp). When these drops are subtracted from the applied primary voltage (Vp), the induced emf in the primary (Ep) is obtained.
This induced emf is transferred to the secondary winding through mutual induction, producing the secondary induced emf (Es). Due to the resistance (IsRs) and reactance (IsXs) of the secondary winding, part of this emf is lost. The remaining voltage appears at the secondary terminals, known as the secondary terminal voltage (Vs).
This diagram is useful for analyzing accuracy and error performance.
Burden of a Potential Transformer
The burden of a PT refers to the total load (in VA) connected across its secondary winding, including meters, relays, and wiring.
- Measured in Volt-Amperes (VA).
- PTs are rated for specific burdens (e.g., 25 VA, 50 VA).
- Excess burden increases errors and reduces accuracy.
Ratio and Phase Angle Errors of Potential Transformer
A potential transformer is designed to give an exact ratio of primary to secondary voltage, but in practice, errors occur due to core losses, winding resistance, and leakage reactance.
Ratio Error (Voltage Error): Defined as the percentage difference between the actual transformation ratio and the nominal (ideal) ratio.
Where,
- K = nominal ratio
- Vs = secondary voltage (measured)
- Vp = actual primary voltage
Example: If a PT is rated 11 kV/110 V, the ideal ratio is 100. But if actual ratio is 100.5, the small deviation is ratio error.
Phase Angle Error: Defined as the phase difference between the primary voltage (Vp) and the secondary voltage (Vs).
- Ideally, both should be in phase.
- Small lag occurs due to magnetizing current and leakage flux.
These errors affect the accuracy of meters and protective relays, which is why PTs are manufactured with accuracy classes like 0.1, 0.2, 0.5, 1.0 depending on application (metering or protection).
Accuracy Class of a Potential Transformer(PT)
The accuracy class of a Potential Transformer defines the permissible limits of ratio error and phase angle error under specified conditions of burden and power factor. It ensures that the PT delivers a secondary voltage that is a true scaled replica of the primary voltage.
- Common accuracy classes: 0.1, 0.2, 0.5, 1.0, 3.0
- For metering PTs: High accuracy is required (0.1 or 0.2 class).
- For protection PTs: Slightly lower accuracy is acceptable (1.0 or 3.0 class) because precise measurement is less critical during fault conditions.
In short: Accuracy class indicates how precise a PT is for measurement and protection purposes.
Applications of Potential Transformer
Potential Transformers are widely used in power systems for:
- Voltage Measurement: Safe measurement of high voltages using standard voltmeters.
- Protective Relays: Supply reduced voltage for fault detection and protection schemes.
- Synchronizing Generators: Assist in synchronization of alternators with the grid.
- Metering Systems: Provide input for energy meters and SCADA systems.
- Power System Automation: Essential for digital substations and smart grid applications.
Potential Transformer and Current Transformer
Both PTs and CTs are instrument transformers, but they serve different purposes:
| Feature | Potential Transformer (PT) | Current Transformer (CT) |
| Function | Steps down voltage | Steps down current |
| Primary | Connected in parallel to high-voltage lines | Connected in series to line carrying high current |
| Secondary | Standardized to 110 V | Standardized to 5 A or 1 A |
| Application | Voltage measurement, relays, synchronizing | Current measurement, relays, protection |
Together, PTs and CTs form the nervous system of power measurement and protection.
Potential Transformer and Power Transformer
Both PTs and Power Transformers work on the principle of electromagnetic induction, but their purpose and design are different:
| Feature | Potential Transformer (PT) | Power Transformer |
| Function | Steps down high voltage to a standard measurable value (e.g., 110 V) | Steps up or steps down voltage for power transmission and distribution |
| Primary Connection | Connected in parallel with the power circuit | Connected between transmission and distribution lines |
| Secondary Output | Standardized low voltage (usually 110 V) | Varies according to application (kV levels for transmission, LV for distribution) |
| Accuracy | Designed for high accuracy for measurement and protection | Designed for high efficiency and large power handling |
| Application | Used with voltmeters, wattmeters, relays, and synchronizing devices | Used in power plants, substations, and industries for bulk power transfer |
Conclusion
A potential transformer is a critical instrument transformer that ensures safe and accurate voltage measurement in electrical networks. By stepping down high voltages to manageable levels, PTs protect instruments, relays, and engineers from hazards.
We discussed the definition, construction, working principle, types, errors, burden, phasor diagram, applications, and symbol of PT. Along with current transformers, PTs play a vital role in the operation, protection, and automation of modern power systems.
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