The HTS power cable consists of a conducting and a
shield layers for carrying the required current. The direction of
shield layer current reverses against the conducting layer current
and its value is over 95% of conducting layer current at the steady
state. However, if the fault current flows through the HTS power
cable, quench occurs in both conducting and shield layers of the
HTS power cable. The current distribution of conducting and shield
layers changes due to the above mentioned phenomenon caused
by the fault current [12]. In the Fig. 3, IC: current of conducting
layer, IS: current of shield layer, DI: induced current ratio of conducting
and shield layers, respectively. DI will be used as a reference
value of the protective relay for the HTS power cable
section, DIconstant: induced current ratio in steady state, DIfault: induced
current ratio in fault state. In the proposed fault detection
algorithm, the decision making of fault is done through the following
concept. The induced current ratio of HTS power cable in steady
state (DIconstant) is greater than or equal to 0.95 (95%). However,
if the fault current flows through the HTS power cable, the induced
current ratio of HTS power cable (DIfault) is suddenly decreased due
to the increased resistance of shield layer caused by quench. Consequently,
if the DIfault is less than the DIconstant, then the protective
relay detects the fault and outputs the circuit breaker operating
signal.