Zero Sequence CT

What is a Zero Sequence Current Transformer?

Zero sequence current transformers, also known as Core Balance Current Transformers (CBCT), are specialized electrical devices designed to detect and measure zero sequence currents, which are commonly associated with ground faults in power systems. These transformers play a critical role in maintaining system reliability and safety by facilitating swift fault detection and isolation.

Zero sequence CTs monitor currents flowing through industrial 3 phase electrical systems and provide a signal when there is a current imbalance between any of the 3 phases. In normal operating conditions, magnitude of currents in all three phase cables are similar and their phase angles are shifted by 120 degrees. In such conditions, the magnetic field produced by these phase currents cancels itself. The magnetic field flowing in zero sequence CT core is zero, so secondary current is also zero.

When a fault occurs, which could be phase to phase or phase to ground, the geometric sum of primary currents is not zero anymore and current in the secondary winding of the zero sequence CT is induced. This secondary current will give information to relays that fault has occurred and they will disconnect power in the faulty system.

By monitoring the zero-sequence current, these CTs enable protection relays to promptly identify and isolate ground faults, thereby minimizing damage to electrical equipment, reducing the risk of fire, and enhancing overall system stability. Moreover, zero sequence CTs contribute to improved power quality and energy efficiency by allowing operators to more precisely monitor and optimize the balance of electrical networks.

Polycast designs and manufactures a variety of indoor and outdoor zero sequence CTs cast in epoxy resin or finished with fiberglass tape. Product performance is measured throughout the manufacturing process to confirm compliance with design criteria. All Polycast zero sequence CTs are individually designed, manufactured, and tested to meet and exceed North American and International standards including CSA, IEC, and IEEE.