There isn’t any recommended solution; you must use similar line and neutral CT or low impedance (biased type) differential relay. Of course using of interposing CT for ratio matching can be an urgent/temporary solution, but other CTs mismatching such as internal resistance and magnetizing current can be caused protection instability against out of zone earth fault and generally it is not recommended. A number of design considerations must be taken into consideration when designing a high-impedance REF scheme. The most important considerations are described here: – The ratio of the phase and neutral CTs must always be the same. – In general, the CTs should have the same saturation characteristics. – The kneepoint voltage must be higher than the stabilization voltage for external faults. – The voltage across the relay and CTs (all in parallel) should be kept at safe levels while still being sufficiently high to allow operation of the relay when required. The magnetizing current of the CTs depends on the voltage across it, but too high a voltage results in higher magnetizing current that leads to a less sensitive scheme. – In most cases, a metal oxide varistor (MOV) or surge arrestor is connected across the parallel connection of the CTs and relay to clamp the voltage to a safe limit, without affecting relay operation. The MOV protects the relay against high voltages developed during inzone faults. Sufficient current still flows through the relay to ensure operation.