Secondary injection tests what the relay can do; primary injection tests what reaches it. There is equipment where that difference decides: a recloser with current transformers and voltage sensors inside the tank can only be checked end to end by putting current and voltage through the primary. This guide explains what primary injection validates in that case, which list of points is verified and when another test is better.
Two tests, two questions
The general guide primary and secondary injection explains the basic difference: secondary injection tests the relay and its settings, and primary injection tests the whole chain starting from the power equipment. This guide adds a case where the chain cannot be separated: equipment with integrated sensors, such as a recloser. And it sets out the decision of when to do it. Further detail on commissioning and settings of the Tavrida Rec15/25 recloser: use, commissioning and settings guide for the Tavrida Rec15/25.
The list of points you will see below comes from primary injection protocols for reclosers that we consulted as a map of topics. We do not reproduce their formats or values.
Why the recloser is the typical case
In a solid-insulation medium-voltage recloser, the manufacturer builds the sensors into the tank. The NOJA Power product we have documented, for example, measures the voltage at each bushing with capacitively coupled voltage sensors and the current in each phase with current transformers, and delivers those measurements to the control. In its product data, the phase current measurement range reaches 800 A and the voltage and current accuracy is given as a percentage or absolute value.
That means the sensors are inside the tank and cannot be reached from outside like the CT of a panel. If you want to know that phase A of the primary shows up as phase A in the control, and with the correct scale, you have to inject through the primary. For the control logic, on the other hand, secondary injection is enough. The two tests complement each other.
See also recloser and NOJA RC10 and RC15 relay.
The list of points in a primary injection protocol
In the recloser protocols we consulted, primary injection is organized by equipment and by point. For each recloser:
- Contact resistance of the power pole.
- Current transformer of each phase (A, B, C): current is injected and compared with what the control shows.
- Voltage sensor of each phase on one side (A, B, C) and on the other (R, S, T): there are six sensors, because the equipment measures voltage on both sides of the contacts.
- Measuring core of the current transformer of each phase, when the connection point has a separate meter.
The reason for measuring voltage on both sides is the closing condition: to close with “live bus, dead line” or to verify synchronism, the control must know which side has voltage. See synchronism check (25).
What each point validates
| Point | What it checks |
|---|---|
| Contact resistance | That the closed pole conducts without abnormal resistance. See contact resistance |
| CT of each phase | Ratio, phase assignment and direction of the current the control sees |
| Voltage sensor of each phase | Scale and phase assignment of each side of the equipment |
| Measuring core | That the meter receives the current of the correct phase, with its ratio |
One possible error that only shows up on the primary: two phases swapped between the sensor and the control. On the secondary everything works, because the relay measures what is injected into it. On the primary, the phase that was injected appears in another column.
When to do it and when not
- Are the sensors built into the equipment, with no accessible terminals?
- Yes: Primary injection (the only end-to-end check before energizing)
- No: Was a CT or its wiring changed, or does the contract require it?
- Yes: Primary injection, or an equivalent CT and wiring test
- No: Secondary injection and on-load tests at energization
Three more things to consider:
- Available current. The equipment delivers a fraction of the fault current, so it serves for ratio and assignment, not for running the trip curves at high multiples. Those are tested by secondary injection.
- Access. The equipment must be de-energized and its terminals accessible.
- Safety. There are high currents, and a CT secondary with primary current is never left open. See test blocks and bay isolation.
After the injection
Primary injection does not replace verification with voltage and load. It completes it: on energizing, the control’s voltages and currents are compared with those of the meter and the direction of power is confirmed. See what is checked before energizing a substation.
What is attached to the protocol
For each point, a record is kept of the current or voltage injected, the control reading and the reference measurement, with the test equipment and its calibration. If the protocol changed during commissioning, it is noted as a modification. That way the test can be repeated or audited, and it serves as a baseline if a sensor or the control is changed later.
What we cannot state
The ranges and accuracies of the sensors change with each model; the 800 A figure comes from the product guide we have. For a specific unit, use its current datasheet. The acceptance criteria for each point are defined by the manufacturer or by the project’s protection study.
We do primary and secondary injection
At Tiza Group we test reclosers and medium-voltage bays, with primary and secondary injection. See testing of current and voltage transformers or write to us through contact.
Related guides
- Primary and secondary injection
- OMICRON CMC 356, CMC 430 and Test Universe test equipment
- Cold and hot commissioning
- First energization of a substation
- Tavrida Rec15/25: Use, Commissioning and Settings
References
- NOJA Power, Product Guide, OSM 15, 27 and 38 kV automatic circuit reclosers: voltage measurement at each bushing with capacitive sensors and current measurement with current transformers.
- Primary injection protocols for medium-voltage reclosers (consulted as a map of topics; not reproduced).
Frequently asked questions
What does primary injection validate that secondary injection does not?
That the equipment's sensors, their cables and the control measure what passes through the primary, on the correct phase and with the correct ratio. Secondary injection starts at the control cable and does not go through the sensors.
Why is it so important on a recloser?
Because its current and voltage sensors are inside the tank, built into the power equipment. The only way to put them to the test, besides factory verification, is to apply current or voltage through the equipment's terminals.
Is primary injection done on every bay?
No. It is decided by risk: it is done when the equipment allows no other check of its sensors, when a transformer or its wiring was changed, or when the contract requires it. In the rest, secondary injection and on-load tests are usually enough.