Monitoring a bushing set when one of them has a BPD installed

It is possible to use BM to monitor the capacitance progress of a bushing set even if one of them has a BPD (Bushing Potencial Device) installed in one phase of the whole set. To use BM at this scenario, its just needed to do some compensations.

The way BM works, to mesure the bushing capacitance progress it must measure and get the vectorial sum of the three set leakage currents.

To the sum doesn’t be unbalanced, the magnitude order  of these currents must be similar, as occur when the currents are measure without the BPD.

Considering that the impedance of C2 is too high to a significant amount of current pass through it , we can estimate a bushing leakage current I1  without BPD (left image) by adopting the following formula: I₁ = V_L.ω.C₁

At BPD output, using a coupling capacitor AC we can correct the phase and limit the I2 current in order to I2 be at the same magnitude order of I1. So on, we can use I2 to monitor the bushing.

I2 estimate current  is: I₂ = V_DPB.ω.C_A

Matching the currents we get: ?_?=(?_?/?_???).C₁

If, for an example: V_L=(525/√3) kV ; V_DPB = 115 V ; C₁ = 498,2 pF

The coupling capacitance will be: CA = 1.31 μF

The BPD inner circuit cause too much disturbance so that the tangent delta be measured, but proceeding this way, the capacitance of both three bushings will be monitored satisfactorily.