The zero‑sequence current transformer (core balance current transformer)is an important measuring instrument in power systems, which is used to measure zero‑sequence current. A power system consists of three phases: Phase A, Phase B and Phase C, and each phase can operate independently. Zero‑sequence current occurs simultaneously in all three phases. By adopting zero‑sequence current transformers, we can measure such current to guarantee the normal operation of power systems.
Conventional current transformers are designed to measure load operating current, while zero‑sequence current transformers are specialized for zero‑sequence current measurement. Therefore, zero‑sequence current transformers require special structures and measuring capabilities to acquire accurate zero‑sequence current readings.
Zero‑sequence Current
Zero‑sequence current flows through three phases simultaneously in power systems. As an abnormal current, it may trigger system failures and equipment damage. Hence, measuring and monitoring zero‑sequence current is of great significance.
In a well‑balanced power system, Phase A, B and C carry equal voltage and current. When system faults occur or unbalanced loads are connected, zero‑sequence current will be generated. It originates from unpaired electric charges distributed across the system or inside ungrounded cables and equipment, which leads to three‑phase current imbalance.
Working Principle of Zero‑sequence Current Transformer
The zero‑sequence current transformer measures zero‑sequence current and outputs corresponding electrical signals.
Working principle: When current flows through cables passing through the transformer, a magnetic field proportional to the current magnitude will be produced. The magnetic field acts on the secondary winding and generates corresponding current signals.
Characteristics and Parameters of Zero‑sequence Current Transformer
Zero‑sequence current transformers are manufactured according to dedicated parameters, with key characteristics listed below:
Sensitivity: The amplitude of output signal shall be proportional to the magnitude of input zero‑sequence current.
Frequency Response: The effective frequency range for measurement. Zero‑sequence current in power systems is normally 50 Hz.
Linearity: The input and output signals shall maintain a linear relationship within operating range to ensure measurement accuracy.
Phase Difference: The phase offset between output and input signals. Ideally the phase difference should be zero or close to zero for high‑precision measurement.
Applications of Zero‑sequence Current Transformer
Power System Protection: Zero‑sequence current serves as fault criterion for fault detection and relay protection of power equipment.
Power Quality Monitoring: Detect hidden abnormalities and faults of power systems via zero‑sequence current monitoring.
Ground‑fault Detection: Ground fault is a frequent fault in power systems. Zero‑sequence current transformers can detect ground faults caused by unbalanced charges and improper grounding of equipment.
Conclusion
Zero‑sequence current transformer is critical detection equipment for capturing zero‑sequence current in power systems. Compared with ordinary current transformers, it adopts dedicated structures and special measuring performance to realize accurate zero‑sequence current measurement. Widely applied in power system protection, power quality monitoring and ground‑fault detection, it plays a vital role in power engineering.
