Principle of High-Impedance Bus Differential Relay (87BB)
The high-impedance protection scheme includes the CTs with a high impedance to force the error differential current through the CTs instead of the relay-operating coil. The basic principles are shown in Figure 2.
During the external fault condition, the maximum voltage VR across the differential relay, will happen in case that the CT on the faulted circuit (i.e., No. 1) is completely saturated and the other CTs (i.e., No. 2 and 3) do not saturate. This can be considered as the worst-case scenario because, in reality, all CTs will not saturate on external light faults or will have varying degrees of saturation for the high fault current.
A practical margin with a safety factor is usually provided by the manufacturer to adjust the maximum voltage calculation for the proper relay setting. This calculation is performed for both conditions: the maximum symmetrical three-phase and phase-to-ground faults. The fault currents are different, and the lead resistance RL is RL for three-phase faults and 2RL for phase-to-ground faults.
VR is voltage across relay during the completed saturation of one CT
IF is the maximum external fault current
N is the CT ratio (at a particular tap)
RCT is the CT secondary winding and lead resistance up to the CT terminals
RLEAD is the one-way resistance of lead from junction points to the most distant CT
k is equal to 1 for three-phase faults and 2 for single phase-to-ground faults
For internal bus faults, the high-impedance ZR of the bus differential relay forces most of the secondary current through the CT exciting impedances. Therefore, the VR will be high to operate the relay and it is essentially the open-circuit voltage of the CTs. A varistor or similar protective device across ZR provides circuit protection by limiting the voltages to a safe level. A tuned circuit provides maximum sensitivity. The impedance between the junction and the relay RLR is usually negligible, compared with the high value of the relay ZR.
The scheme requires that the total resistance of the CTs and leads to the junction point (RS+RL) be kept low. Therefore, the bushing or toroidal wound CTs, where its secondary impedance is very low, can be used. They shall be interconnected together as near the CT locations as possible, preferably equivalent distance so that the RL values will be equal and low.
In addition, all CTs must have the same ratio and operate on the full winding. The operating at CT taps and the use of auxiliary CTs are not recommended.
As mentioned above, the various limitations are not too difficult to meet with the modern CTs and proper bus design, so this relay is a very effective and widely used for bus protection system. Typical operating times are in the range of 20–30 msec, and if a supplementary instantaneous unit is used for high-current internal faults, times of 8–16 msec can be achieved.
 J. Lewis Blackburn Thomas J. Domin “Protective Relaying Principles and Application Third Edition”
 Ken Behrendt, David Costello, and Stanley E. Zocholl “Considerations for Using High–Impedance or Low–Impedance Relays for Bus Differential Protection”