What Is a Single-Phase Reactive Power Control Relay? Types and Selection Criteria
A single-phase reactive power control relay measures one phase through a single current transformer and switches power factor correction capacitor steps in and out.
A single-phase reactive power control relay measures the supply on one phase through a single current transformer, calculates the power factor and switches the capacitor steps in a power factor correction panel in and out through contactors. Because it measures only one phase, it is used in small and medium installations where the load is evenly spread across the phases. This is the difference from three-phase reactive power control relays, which measure each phase separately.
Single-phase or three-phase?
| Relay | Measurement | When to choose |
|---|---|---|
| Single-phase relay | One current transformer, one phase | Load evenly spread across the phases, steps are three-phase capacitors |
| Three-phase relay | One current transformer per phase | Load differs between phases, per-phase compensation needed |
The product data of the Entes RG-8T and RG-12T states "1 phase, 1 current transformer, 3-phase capacitor": the relay measures one phase and the steps are three-phase capacitors. If the phases are unbalanced, the unmeasured phases are under- or over-compensated and penalties become a risk; in that case choose a three-phase relay.
Selection criteria
- Number of steps: The number of steps is the number of capacitor groups. The catalog has 7-step (Tense RGM-07E), 8-step (Entes RG-8T) and 12-step (Entes RG-12T, Tense RGM-12E) models. Leave spare outputs for fine adjustment and steps to be added later.
- Current transformer: The transformer goes where the current of the whole load passes on the measured phase, before the capacitor connection. Its ratio is set in the relay; choose it from current transformers according to the installation's current.
- Supply and voltage input: The Entes RG-T models are supplied with 230 V AC. Check in the wiring diagram which phase the voltage input is taken from and which phase the current transformer goes on.
- Display and control features: The Tense RGM product data lists a 3×20 mm LED display, step and status LEDs, 100 mA current sensitivity and an automatic current transformer test.
- Panel size: The Tense RGM models are for a 144×144 mm panel door; take the cut-out size of the other models from the product page.
Common mistakes
- Fitting the current transformer in the wrong place: If it sits on the line after the capacitors, the relay does not see the effect of its own steps and cannot decide correctly.
- Mixing up transformer direction and phase: A reversed transformer or a voltage taken from a phase other than the diagram shows makes the relay calculate the power factor wrongly.
- Using a single-phase relay in an unbalanced installation: The inductive or capacitive penalty limit may be exceeded on the unmeasured phases.
Single-phase reactive power control relays at Welibaba
The Single-Phase Reactive Power Control Relays category has 4 active products, all listed with prices: Entes RG-8T and RG-12T, and Tense RGM-07E and RGM-12E. For a comparison of relay types, see the reactive power control relay guide.
Frequently asked questions
Can a single-phase reactive power relay be used in a three-phase installation?
Yes, if the load is evenly spread across the phases; the relay measures one phase and controls three-phase capacitor steps. If the load differs clearly between phases, a three-phase relay that measures each phase is needed.
How many steps should the relay have?
Determine the total compensation power and the smallest step; the step count must cover these steps and leave spare outputs. The catalog has 7-, 8- and 12-step models.
Where is the current transformer fitted?
On the measured phase, on the main line that carries the current of the whole installation and the capacitors, before the compensation connection.