Agenda
Current Transformers
Transformer Ratio
Polarity
IEEE Metering Accuracy
IEEE Relaying Accuracy
Accuracy Influencing factors
Burden Calculation
Rating Factor
Voltage Transformers
IEEE Accuracy Classes
Installation Guidelines
Required Information for Specifying CTs &
VTs
“Take Home Rules” for CTs & VTs
IEEE CT Metering Accuracy
Accuracy Class ( * )
Application
0.15 High Accuracy Metering
0.15S “Special” High Accuracy Metering
0.3 Revenue Metering
0.6 Indicating Instruments
1.2 Indicating Instruments
* All accuracy classes defined by IEEE C57.13 or C57.13.6
* Accuracy classes include both ratio & phase angle error
IEEE CT Metering Accuracy
Burden
Load connected to CT secondary
Includes devices & connecting leads
Expressed in ohms
Standard values = B0.1, B0.2, B0.5, B0.9, B1.8
E0.04, E0.2
All burdens defined by IEEE C57.13 or C57.13.6 for 60 Hz only
Basics of Current & Voltage Transformer Guidelines
Current transformers (CTs) and voltage transformers (VTs), also known as potential transformers (PTs), are essential components in electrical systems for measurement, protection, and control. They step down the high currents and voltages in power circuits to manageable levels that can be safely measured or used for protection relays. Understanding the basics and guidelines for selecting and using CTs and VTs ensures the reliability and accuracy of electrical systems.
1. Current Transformers (CTs)
Purpose of Current Transformers:
- Current Measurement: CTs scale down high current to a lower value for measurement by meters, indicating the actual current flowing through the circuit.
- Protection: CTs provide input to protective relays, allowing them to sense fault conditions (e.g., short circuits or overloads) and activate the necessary protection mechanisms.
- Isolation: CTs provide galvanic isolation between the high-voltage primary circuit and the low-voltage secondary circuit.
Basic Operation:
- A CT works on the principle of electromagnetic induction. It consists of a primary winding (usually a single conductor or busbar carrying the current) and a secondary winding (used to provide a proportional output for measurement or protection).
- The current ratio between the primary and secondary is determined by the turns ratio. The primary current flows through the primary winding, creating a magnetic field that induces a proportional current in the secondary winding.
Key Guidelines for CT Selection:
- Rated Current: Choose a CT with a secondary rating that matches the required current range for measurement or protection purposes (commonly 5A or 1A).
- Burden: The burden is the load connected to the secondary side (e.g., meters or relays). Ensure the CT is rated for the correct burden, as too high a burden may reduce the accuracy of the CT.
- Accuracy Class: CTs are available in different accuracy classes (e.g., 0.5, 1.0). Select the CT with the appropriate accuracy for the measurement or protection function.
- Saturation Point: The CT should not saturate at fault current levels. Saturation leads to incorrect current measurement and unreliable relay operation.
- Rated Short Circuit Current: Ensure the CT can withstand the fault current without damage. This is usually a multiple of the rated current.
- Frequency: CTs are typically designed for operation at power system frequencies (50Hz or 60Hz), but special CTs may be needed for non-standard frequencies.
Installation Guidelines for CTs:
- Ensure correct polarity when connecting CTs to meters or protection relays. Incorrect polarity can result in wrong measurements or incorrect relay operation.
- Install CTs around the primary conductor such that the magnetic field generated by the current flows correctly through the core of the transformer.
- CTs should be securely mounted to prevent mechanical stress that may alter their accuracy.
2. Voltage Transformers (VTs) / Potential Transformers (PTs)
Purpose of Voltage Transformers:
- Voltage Measurement: VTs step down high voltages to lower, safe levels for measurement, allowing voltmeters, meters, and control systems to monitor the system's voltage.
- Protection: VTs provide voltage signals to protective relays that can act in case of over-voltage or under-voltage conditions, ensuring the safety of electrical equipment.
- Isolation: Like CTs, VTs provide isolation between the high-voltage primary side and the low-voltage secondary side.
Basic Operation:
- VTs function based on electromagnetic induction. The primary winding is connected across the high-voltage circuit, and the secondary winding produces a scaled-down voltage proportional to the primary voltage.
- The turns ratio determines the voltage reduction between the primary and secondary windings. For example, a VT with a turns ratio of 1:100 will reduce the primary voltage by a factor of 100.
Key Guidelines for VT Selection:
- Rated Voltage: Choose a VT with the appropriate rated voltage to match the system’s operating voltage.
- Accuracy Class: The accuracy class of a VT (e.g., 0.2, 0.5) indicates how closely the secondary voltage reflects the actual primary voltage. Higher accuracy classes are necessary for precise measurement applications.
- Burden: The secondary burden is determined by the load (e.g., meter, relay). Ensure that the VT’s secondary burden rating matches the connected equipment.
- Insulation Level: VTs must be rated for the highest expected voltage (including transient overvoltages) to ensure adequate insulation.
- Frequency: Most VTs are designed for power frequencies (50Hz or 60Hz), but special VTs may be required for non-standard frequencies.
- Rated Short Circuit Current: Similar to CTs, ensure the VT can handle short-circuit conditions without damage, especially during fault conditions.
Installation Guidelines for VTs:
- Install VTs so that the primary winding is correctly connected across the high-voltage circuit, and the secondary side is connected to measurement or protection devices.
- VTs are typically installed in parallel with the electrical system to provide accurate voltage measurements.
- Proper grounding of the secondary side is essential to prevent dangerous voltage build-up and ensure safe operation.
3. General Guidelines for CT and VT Protection
- Protection Against Overloading: Both CTs and VTs must be protected from overload conditions that could damage them. Proper circuit protection, such as fuses or circuit breakers, should be installed in the secondary circuit.
- Continuous Monitoring: Both types of transformers should be monitored periodically for accuracy and to detect any signs of deterioration, such as insulation failure or excessive heating.
- Avoid Open Circuits in Secondary Windings: Never leave the secondary side of a CT or VT open-circuited under normal operation. This could cause dangerous voltage buildup, especially on the secondary of a CT, which can lead to electrical shock hazards or damage the transformer.
4. Safety Considerations
- Isolation: Both CTs and VTs provide isolation between high-voltage circuits and the measuring or protection equipment, which is critical for operator safety.
- Secondary Circuit Grounding: Proper grounding of the secondary circuits of both CTs and VTs is necessary to ensure safe operation and to prevent electrical shock hazards.
- Surge Protection: Both CTs and VTs may require surge protection devices to protect against voltage spikes, such as lightning strikes or switching surges, which can damage the transformers.
Conclusion
Current and voltage transformers play a crucial role in the measurement, protection, and control of electrical systems. Understanding their basic operations, selection criteria, and installation guidelines is essential for ensuring their reliable performance and the safety of the electrical system. Proper installation, maintenance, and adherence to manufacturer specifications will ensure that both CTs and VTs operate efficiently, accurately, and safely in various applications.
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