Short answer
Active harmonic filter placement of current transformers decides what the filter measures and therefore what it corrects. With CTs on the source side, between the transformer and the point where the filter connects, the filter works closed-loop: it adjusts its output until the source current is clean. With CTs on the load side, it measures load current and injects the opposite, open-loop; anything between the CTs and the source is ignored. Capacitor banks complicate both. In an illustrative bus, a capacitor bank amplifies background harmonics and draws 40 A of 5th harmonic; load-side CTs never see it, while source-side CTs do but may lead the filter into interaction with the capacitor. Wrong CT polarity makes the filter add harmonics rather than cancel them. CTs must have range and bandwidth for harmonic content.
Source-side versus load-side
| Placement | Corrects | Leaves |
|---|---|---|
| Load side | 60 A load harmonics | 40 A capacitor harmonics |
| Source side | Net source harmonics | Possible interaction |
| Reversed polarity | Invalid direction information | May worsen harmonics or cause blocking, depending on the controller |
| Placement | Control | Sees | Risk |
|---|---|---|---|
| Source side | Closed loop | All current through transformer, including capacitor effects | Interaction with capacitors, stability |
| Load side | Open loop | Load current only | Misses capacitor and background effects |
Many manufacturers prefer source-side for accuracy; the choice must follow their documentation.
Capacitor bank position
Illustrative example. LV bus with nonlinear loads producing 60 A of 5th harmonic and a plain PF capacitor bank drawing 40 A of 5th harmonic from background distortion. Values chosen for demonstration.
- Load-side CTs (on load feeders only): the filter cancels 60 A from loads but the 40 A capacitor current still flows through the transformer.
- Source-side CTs (on transformer incomer, capacitor and loads downstream): the filter sees the net and acts on it, but if the capacitor and filter form a resonant loop, control may oscillate.
Plain capacitors on buses with active filters should be detuned; see capacitor overload from background harmonics and detuning factor.
Polarity and phase mapping
- Polarity: P1 toward source is the usual convention; a reversed CT can command compensation in the wrong direction; actual injection, alarms or blocking depend on the filter controller.
- Phase mapping: each CT on the correct phase input.
- Commissioning check: small injection, observe whether source harmonics fall.
Range and bandwidth
- CT ratio sized for maximum current including harmonics.
- Accuracy across the harmonic range the filter corrects, often up to the 50th.
- Saturation during faults or inrush can distort the filter measurements, and sustained saturation in normal service distorts control.
- Burden within the CT’s rating for the filter input and cables.
Multiple filters and banks
Where several active filters share a bus, CT arrangement must avoid each filter correcting the other’s output. Coordination with capacitor banks is covered in hybrid compensation with active filters. Power-factor controller CT placement has similar issues; see power-factor controller CT location with parallel transformer incomers.
What this article does not cover
Saturation of transformer differential protection CTs is a separate topic and is not covered here.
Where this reasoning stops. Control interaction depends on filter algorithms; follow manufacturer guidance.
Practical takeaway
Place active filter CTs as the manufacturer specifies, account for every capacitor bank between CTs and source, verify polarity by observing source harmonics during commissioning, and size CTs for harmonic range and bandwidth.
Reference framework
- IEEE 519, Standard for harmonic control in electric power systems.