Short answer
A power factor controller parallel transformer arrangement needs careful CT placement, because each controller compensates only the load that its CT measures. On a switchboard with two transformer incomers and a bus coupler, the coupler’s position changes which loads each incomer CT sees. In an illustrative case, transformer 2 is out for maintenance and the coupler is closed, so all load flows through incomer 1. Controller 1 sees the total load but can switch only its own capacitor bank; controller 2’s CT, on the dead incomer, sees nothing and switches its bank off, leaving half the capacitors idle while the plant pays for low power factor. Alternatively, a controller that holds its stages in when its measurement disappears can overcompensate. Summation CTs, coupler-status inputs to the controllers, and drawings of compensation zones for each switching state solve this.
Compensation zones
| Item | Section A | Section B |
|---|---|---|
| Controller sees | 1,200 kW, ≈ 744 kvar | Nothing |
| Bank switched | 400 kvar | 0 |
| Net uncompensated | ≈ 344 kvar total | — |
Draw the switchboard with:
- Incomers I1 and I2, each with its CT.
- Bus sections A and B, joined by coupler C.
- Capacitor banks on A and B, each with its controller.
- Loads on A and B.
For each switching state, mark which loads each CT sees.
| State | CT I1 sees | CT I2 sees |
|---|---|---|
| Both incomers, coupler open | Section A loads | Section B loads |
| Both incomers, coupler closed | Share of total, split by transformers | Share of total |
| I2 out, coupler closed | All loads | Nothing |
| I1 out, coupler closed | Nothing | All loads |
Illustrative failure
Illustrative example. Sections A and B each with 600 kW at 0.85 power factor (about 372 kvar each); banks of 400 kvar on each section. Transformer 2 out; coupler closed. Values chosen for demonstration.
Controller A measures 1,200 kW and about 744 kvar demand, switches all 400 kvar of bank A, and still sees lagging power factor. Controller B measures zero and switches bank B off. Net uncompensated reactive power is about 344 kvar, although 400 kvar of capacitors sit idle.
Solutions
- Summation CT combining I1 and I2, feeding a master controller that commands both banks.
- Coupler status input: when the coupler is closed, controllers switch to a master–slave mode.
- CTs on section feeders rather than incomers, so each controller sees its section regardless of supply.
- Defined behaviour on loss of measurement: banks step out gradually rather than hold or drop abruptly.
Parallel operation with coupler closed
With both transformers in parallel, each CT sees a share of the total that depends on transformer impedances, not on section loads. Controllers may fight if their targets differ. A master controller avoids this. Transformer-out conditions also shift harmonic resonance; see harmonic amplification after one transformer is removed.
Overcompensation at low load is covered in transformer reactive demand at low load; step sizes in capacitor-bank step size; active filter CT placement, with similar issues, in active harmonic filter CT placement.
What this article does not cover
Tap-changer coordination between parallel transformers is a separate topic and is not covered here.
Where this reasoning stops. Values are illustrative; actual control behaviour depends on controller model.
Practical takeaway
Draw compensation zones for every switching state, give controllers coupler status or a summation measurement, define behaviour when a CT loses signal, and test each state at commissioning.
Reference framework
- IEC 61921, Power capacitors: low-voltage power factor correction banks.