The zero-sequence impedance test of a transformer is an essential diagnostic method used in the power industry to assess the behavior of transformers under unbalanced fault conditions, such as a ground fault. By determining zero-sequence impedance, engineers can ensure the reliable and safe operation of power systems, especially in networks that depend on transformers for voltage regulation and power distribution.
At GlobeCore, a leader in electrical equipment solutions, we understand the importance of such tests for system stability. Our range of oil purification, regeneration, and transformer testing equipment complements these diagnostic processes, helping engineers achieve optimal transformer performance.
What is Zero Sequence Impedance?
Zero-sequence impedance refers to the impedance offered by a transformer to a set of three identical-phase voltages that are in phase with each other. Unlike positive and negative sequence components, which are associated with balanced operation, the zero-sequence component appears during unbalanced conditions, such as when a fault occurs between a phase and ground.
The zero-sequence impedance is mainly influenced by:
- Transformer core configuration
- Winding connections (star, delta, or zigzag)
- Magnetic flux paths in the transformer core
- Grounding conditions of the transformer and its neutral point
Why is Zero Sequence Impedance Important?
- Fault Analysis: The zero-sequence impedance test identifies the transformer’s behavior under phase-to-ground faults. Accurate impedance values help in designing protective relay settings.
- Transformer Grounding: It is vital for analyzing the grounding impedance of the transformer neutral and verifying that the ground fault current remains within safe limits.
- Harmonics and Unbalance: It determines the ability of transformers to handle zero-sequence currents caused by unbalanced loads or harmonics.
- System Reliability: Knowledge of zero-sequence impedance is crucial for power system modeling and stability analysis, especially in high-voltage transmission systems.
How is Zero Sequence Impedance Measured?
The zero-sequence impedance test involves applying a balanced three-phase voltage (or current) to the windings and measuring the impedance offered. The test procedure varies depending on the transformer configuration, especially the neutral grounding type and winding connections.
Here is the step-by-step process for performing the test:
1. Test Preparation:
- Isolate the Transformer: The transformer is disconnected from the power supply and other loads to prevent interference.
- Grounding the Neutral: If applicable, the neutral point of the star-connected winding is grounded to complete the current path.
- Equipment Setup: Use a low-voltage three-phase source, current injectors, and precision voltmeters to measure the applied voltage and resulting current.
2. Test Procedure:
- Short-Circuit Secondary Windings: The windings not being tested (typically the secondary side) are short-circuited.
- Voltage Application: Apply a low-voltage, balanced three-phase current to the primary winding.
- Measurement: Measure the voltage across the windings and the resulting zero-sequence current.
The zero-sequence impedance (Z0) is calculated using:

Where:
•  = Applied voltage
•  = Zero-sequence current
The test must be conducted carefully to avoid overheating or damaging the transformer windings.
Transformer Connection Configurations and Zero Sequence Impedance
Different winding configurations behave uniquely during a zero-sequence impedance test. The most common configurations include:
1. Star-Connected Transformer (with Neutral Grounded):
• In a grounded star connection, the zero-sequence current flows through the neutral point to the ground.
• The zero-sequence impedance is determined by the transformer winding and core.
2. Star-Connected Transformer (Neutral Ungrounded):
• Without a ground connection, zero-sequence current cannot flow, resulting in an infinite zero-sequence impedance.
3. Delta-Connected Transformer:
• In a delta configuration, the zero-sequence current circulates within the closed loop of the delta winding.
• This results in a low zero-sequence impedance but leads to internal heating due to circulating currents.
4. Zigzag-Connected Transformer:
• Zigzag transformers offer a unique advantage as they provide a low-impedance path for zero-sequence currents while balancing unbalanced loads.
Challenges and Precautions During Testing
While performing the zero-sequence impedance test, certain challenges and precautions must be considered:
1. Transformer Heating: Prolonged application of test current can cause winding overheating. Proper monitoring of temperature is necessary.
2. Voltage Limits: Low test voltages are used to ensure safety and prevent damage.
3. Test Accuracy: External factors such as residual magnetism or improper neutral grounding can affect test accuracy.
4. Safety Measures: Ensure that all high-voltage safety protocols are followed to protect personnel and equipment.
Practical Applications of Zero Sequence Impedance
The results of the zero-sequence impedance test have numerous practical applications in power systems:
1. Protective Relay Coordination: Relays are set based on the transformer’s zero-sequence impedance to quickly isolate ground faults.
2. Fault Current Calculation: Helps determine the magnitude of fault currents in case of single-phase-to-ground faults.
3. Neutral Grounding Design: Ensures the neutral grounding impedance is appropriately designed to limit fault currents.
4. Harmonic Mitigation: Helps identify transformers’ ability to manage unbalanced harmonics and zero-sequence currents.
GlobeCore Solutions for Transformer Maintenance
GlobeCore, a global leader in transformer equipment, offers a wide range of solutions to enhance transformer reliability and ensure optimal test results:
• Oil Regeneration Systems: Our oil regeneration units restore insulating oil properties, reducing the risks of overheating during tests.
• Transformer Oil Filtration Systems: Proper oil filtration ensures the dielectric strength of oil remains intact, critical for transformer testing.
• Testing Equipment: GlobeCore’s advanced transformer diagnostic tools support impedance testing, enabling precise fault identification and prevention.
By combining regular maintenance with accurate zero-sequence impedance tests, operators can prolong the transformer’s lifespan and minimize downtime.
Conclusion
The zero-sequence impedance test is a critical diagnostic tool for transformers, ensuring their performance under unbalanced fault conditions. By analyzing the zero-sequence impedance, engineers can design safer and more efficient power systems, improve relay coordination, and prevent catastrophic failures.
At GlobeCore, we remain committed to providing innovative solutions that support transformer diagnostics and maintenance. Our advanced oil purification systems, regeneration units, and testing equipment empower industries to achieve reliable transformer operation and meet the evolving demands of modern power networks.
Contact GlobeCore today to learn more about our comprehensive range of transformer maintenance and testing solutions!
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