Current Transformer C/K Adjustment

The C/k setting is the threshold that determines how precisely a reactive power control relay switches capacitor steps in and out. In short, it is the current of the smallest compensation step as reflected on the current transformer secondary. If it is set incorrectly, the bank either hunts continuously or the facility remains under-compensated and incurs reactive energy penalties.

Definition

The Current Transformer and C/k

A current transformer converts the high current flowing through the main line into a measurable secondary current (usually 5 A or 1 A). The reactive power control relay sees the facility’s reactive demand through this secondary current. The relay compares the change it measures in the reactive component with the change the smallest step would cause; this comparison threshold is the C/k value. If the threshold is chosen correctly, the relay switches steps only when a truly significant reactive difference occurs.

Secondary current5 A or 1 A
Typical accuracy classCl. 1 – Cl. 3
TerminalsP1-P2 / S1-S2
Healthy C/k range0.10 – 1.00
Calculation

C/k Calculation Formula

The C/k value is found by dividing the current drawn by the smallest step by the current transformer ratio. The formula is:

C/k = (Q × 1000) ÷ (√3 × U × k)Q: smallest step rating (kVAr)  •  U: line-to-line voltage (V)  •  k: current transformer ratio (primary ÷ secondary)

Worked example: If the smallest step in the capacitor bank is 12.5 kVAr, the supply voltage is 400 V and the current transformer is 400/5, then k = 80. Hence C/k = (12.5 × 1000) ÷ (1.732 × 400 × 80) = 12,500 ÷ 55,424 ≈ 0.23, and this value is entered into the relay.

Table

Ready-Made C/k Table for 400 V

The table below gives C/k values calculated for a 400 V network according to the smallest step rating and the current transformer ratio. Cells highlighted in blue fall within the 0.10 – 1.00 range in which relays operate reliably.

Scroll the table sideways to see all current transformer ratios.

C/k values for a 400 V network by smallest compensation step and current transformer ratio
Smallest step (kVAr) 100/5 200/5 300/5 400/5 600/5 800/5 1000/5 1500/5 2000/5
5 0.36 0.18 0.12 0.09 0.06 0.05 0.04 0.02 0.02
7.5 0.54 0.27 0.18 0.14 0.09 0.07 0.05 0.04 0.03
10 0.72 0.36 0.24 0.18 0.12 0.09 0.07 0.05 0.04
12.5 0.90 0.45 0.30 0.23 0.15 0.11 0.09 0.06 0.05
15 1.08 0.54 0.36 0.27 0.18 0.14 0.11 0.07 0.05
20 1.44 0.72 0.48 0.36 0.24 0.18 0.14 0.10 0.07
25 1.80 0.90 0.60 0.45 0.30 0.23 0.18 0.12 0.09
30 2.17 1.08 0.72 0.54 0.36 0.27 0.22 0.14 0.11
50 3.61 1.80 1.20 0.90 0.60 0.45 0.36 0.24 0.18

A value below 0.10 indicates that the current transformer is oversized, while a value above 1.00 indicates that the smallest step is too large. In either case, the compensation design should be reviewed.

Application

C/k Setting Step by Step

  1. Identify the smallest step. Note the rating of the smallest capacitor group in the bank in kVAr.
  2. Read the current transformer ratio. Divide the primary current on the nameplate by the secondary current to find k.
  3. Apply the formula. Calculate the C/k value or read it from the table above.
  4. Check the connection. Make sure the current transformer sees the entire facility load and that the P1-P2 direction and S1-S2 terminals are correct.
  5. Enter the value into the relay. Enter the calculated value in the relay’s C/k parameter; if the relay has automatic learning, compare its result with your calculation.
  6. Observe the behavior. Check that steps switch in and out without hunting and that the power factor stays at the target value.
Current transformers and S1-k-S2 secondary connection terminals inside a switchboard
Current transformer and secondary terminals: C/k accuracy starts with this connection.
Installation in the panel

A correct C/k value only means something with correct installation.

The current transformer is fitted on the main conductor with the P1-P2 direction toward the load; the secondary leads are taken from terminals S1 and S2 to the relay’s measuring circuit.Leaving the secondary circuit open under load causes dangerously high voltage in the current transformer. That is why the secondary terminals are short-circuited before any work on the measuring circuit.The transformer ratio is read from the nameplate and recorded; the C/k calculation is based on this ratio and the rating of the compensation step.Secondary wiring is kept short and direct; if the measuring circuit must share a duct with power cables, a gap is left between them, otherwise interference will affect the measurement.

Caution

Common Mistakes

  • Installing the current transformer at a point where it does not see the current of the compensation bank.
  • Connecting P1-P2 in reverse, so that the relay interprets the load as generation.
  • Leaving the current transformer secondary open under load, which produces dangerously high voltage.
  • Failing to update the C/k value after step ratings have been changed.
  • Calculating k with a 5 A assumption for transformers with a 1 A secondary.
  • Using capacitors without reactors in facilities with high harmonic loads, which distorts the measurement.

Is your power factor correction panel working properly?

An incorrect C/k setting shortens contactor and capacitor life and increases the risk of reactive energy penalties. Devpan verifies the current transformer selection, step plan and relay parameters of your power factor correction panels together with on-site measurements.Contact us for compensation support

Summary

  • Formula: C/k = (Q × 1000) / (√3 × U × k)
  • Q: Rating of the smallest capacitor step (kVAr)
  • U: Line-to-line supply voltage (usually 400 V)
  • k: Current transformer ratio (e.g. k = 80 for 400/5)
  • Typical range: values between 0.10 and 1.00 A are the healthy operating range
  • Example: 12.5 kVAr step, 400 V, 400/5 current transformer → C/k ≈ 0.23
Frequently Asked Questions

Frequently Asked Questions About Current Transformer C/k Setting

What is the C/k setting?
The C/k setting is the threshold that determines how sensitively a reactive power control relay switches steps in and out. It represents the current of the smallest compensation step as reflected on the current transformer secondary, and prevents unnecessary switching by the relay.

How is the C/k value calculated?
The formula C/k = (Q × 1000) / (√3 × U × k) is used, where Q is the rating of the smallest step (kVAr), U the supply voltage (V) and k the current transformer ratio. For example, with a 12.5 kVAr step, 400 V and a 400/5 current transformer, C/k ≈ 0.23.

How do I find the current transformer ratio k?
Divide the current transformer’s primary current by its secondary current. For a 400/5 current transformer, k = 400 / 5 = 80. For transformers with a 1 A secondary, the divisor is 1.

What happens if C/k is set too low?
The relay reacts to even very small reactive changes and keeps switching steps in and out. This hunting shortens contactor and capacitor life and leads to overheating and faults.

What happens if C/k is set too high?
The relay does not switch in steps even when they are needed. The facility remains under-compensated, the power factor drops and there is a risk of reactive energy penalties.

Where should the current transformer be installed?
The current transformer should be installed on the main incomer so that it sees the entire facility load, including the power factor correction panel. A position where it does not see the capacitor bank current causes the relay to measure incorrectly.

Why are the P1-P2 direction and S1-S2 connection important?
Correct connection of the current transformer’s primary direction (P1-P2) and secondary terminals (S1-S2) is essential for the relay to correctly detect the direction of current and the phase relationship. A reversed connection makes the relay see the load as generation.

Why must a current transformer secondary never be left open?
If the secondary of a current transformer is left open while current flows through the primary, a very high voltage builds up. This is dangerous for personnel and can permanently damage the transformer’s insulation. If it is to be taken out of service, the secondary terminals must be short-circuited.

Can modern relays set C/k automatically?
Yes. Current reactive power control relays can automatically learn the C/k value and the current transformer direction by trying the steps one by one. Even so, it is recommended to verify the value by calculation at initial commissioning.

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