For residual voltage detection in three phase system, we need three independent one phase voltage transformer with delta connection in secondary side. However in normal three phase three limb V.T. when one phase experiences earth fault ( or disconnection) , the resultant flux due to two healthy lines returns through transformer limb of faulty line, including a heavy short circuit current in the related winding on the limb . Also voltage induced in secondary open – delta winding is not true residual voltage. To overcome this difficulty two separate limbs are provided in the magnetic circuit of three phase voltage transformer to form five limb voltage transformer as shown in Fig. below.
For similar discussion you can refer to Transformer Riddle No.1