Working Principle of Core balance current transformer (CBCT)
The core balance method is based on primary current phasor addition or flux summation. The remaining zero-sequence component, if any, is then transformed to the secondary. The corebalance CT or sensor is the basis of several low-voltage ground-fault protective systems. (The core-balance CT is frequently called a zero-sequence sensor or window CT, but the term core balance is preferable because it more specifically describes the function of the CT.)
The principle of the core-balance CT circuit is shown in Figure-1. All load-carrying conductors pass through the same opening in the CT and are surrounded by the same magnetic core. Core-balance CTs are available in several convenient shapes and sizes, including rectangular designs for use over bus bars. This method can be more sensitive than the residual method because the sensor rating is large enough for the possible imbalance, not for the individual conductor load current.
Under normal conditions [i.e., balanced, unbalanced, or single-phase load currents or short circuits not involving ground (if all conductors are properly enclosed)], all current flows out and returns through the CT. The net flux produced in the CT core is zero, and no current flows in the ground relay. When a ground fault occurs, the ground-fault current returns through the equipment grounding circuit conductor (and possibly other ground paths) and bypasses the CT.
The flux produced in the CT core is proportional to the ground-fault current, and a proportional current flows from the CT secondary to the relay circuit. Relays connected to core-balance CTs can be made quite sensitive. However, care is necessary to prevent false opening from unbalanced inrush currents that may saturate the CT core or through faults not involving ground. If only phase conductors are enclosed and neutral current is not zero, the transformed current is proportional to the load zero-sequence or neutral current. Systems with grounded conductors, such as cable shielding, should have the CT surround only the phase and neutral conductors, if applicable, and not the grounded conductor.
By properly matching the CT and relay, ground-fault detection can be made as sensitive as the application requires. The speed of the relay limits damage and may be adjustable (for current or time, or both) in order to obtain selectivity. Many ground protective systems now have solid-state relays specially designed to operate with core-balance CTs.
The relays in turn open the circuit protective device. Power circuit breakers, MCCBs with shunt trips, or electrically operated fused switches can be used. The last category includes service protectors, which use circuit breaker contacts and mechanisms, but depend on current limiting fuses to interrupt the high available short-circuit currents. Fused contactors and combination motor starters may be used where the device-interrupting capability equals or exceeds the available ground-fault current.
Figure-2 shows a typical termination of a medium-voltage shielded cable. After the cable is pulled up through the core-balance CT, the cable jacket is removed to expose the shielding tape or braid. Jumpering the shields together, the connection to the ground is made after this shield lead are brought back through the CT. This precaution would have been necessary only if the shield had been pulled through the CT. Between multiple shield ground connections on a single conductor cable, a potential exists that drives a circulating current, often of such a magnitude as to require derating of the cable ampacity. When applying the core-balance CT, the effects of this circulating current should be subtracted from the measuring circuit.
