Data Center Relay Testing
Data center electrical systems are built around redundancy, but redundancy only works if the transfer and protection schemes operate in the sequence they were designed for. A relay that picks up correctly on a test bench can still leave a data hall unprotected if the ATS transfer logic, generator start signal, main-tie interlock or trip circuit does not behave as a system. Data center relay testing has to prove the sequence, not just the pickup value.
The electrical topology you are testing
A typical data center has two utility feeds, medium-voltage switchgear, transformers, a low-voltage main-tie-main arrangement, UPS systems, standby generators and ATS or STS units at the critical distribution level. Protection relays sit on the MV feeders, transformer primaries, LV mains and ties, generator breakers and feeder circuits. The ATS or STS controller sits above them and decides which source is live.
Each layer has a different test object. The relay test set injects current and voltage into the relay or controller, reads status contacts, and drives trip or close inputs. The pass criterion is the sequence the commissioning authority has specified: transfer time, return transfer delay, interlock behaviour, trip output and alarm annunciation.
What is different about data centers
- Two live sources. A dual-source ATS or STS needs three-phase voltage on both the normal and emergency inputs at the same time. A test set with only one three-phase voltage set cannot reproduce the phase relationship, sync-check window or closed-transition condition.
- Transfer time is a design parameter. The ATS controller is set to transfer within a defined time. The test has to measure the controller output and the status transition, not just confirm that the controller powers up.
- Generator start and load acceptance. The protection test proves the generator breaker and generator protection logic. The generator’s actual load-step performance is a separate test with a load bank and the generator controller.
- Selective coordination. Main-tie-main and feeder coordination depend on settings across several relays. Each relay can pass its own test while the pair still miscoordinates; the test plan has to include the pair.
- Documentation. Data center handover packages usually require test sheets per relay, settings files, trip-test records and a non-conformance log. The test set has to produce results that can be pasted into that package.
Test objects and what to prove
| Test object | What to prove | Injection and I/O | Pass criterion | Recommended |
|---|---|---|---|---|
| ATS / STS controller | Source-loss transfer, return transfer, closed transition, inhibit and interlock | Two three-phase voltage sets, status inputs, generator-start output | Transfer within the set time; no parallel closure outside the sync window; inhibit works | VAE-660 |
| Generator protection | 32 reverse power, 40 loss of field, 46 negative sequence, 51V, 64G, 81, 87G differential | Three or six currents, three or six voltages, trip and alarm outputs | Pickup, slope, timing and trip outputs within the settings | VAE-660 for 87G; VAE-430 for single-ended |
| Main-tie-main and busbar | 50/51, 50N/51N, 87B, interlocking, trip-circuit supervision | Three currents, three voltages, binary I/O | Selective trip; interlock prevents parallel sources | VAE-430; VAE-660 for 87B |
| Transformer and feeder relays | 50/51, 51N, 87T, 27/59, 81 | Three currents and voltages; six currents for 87T | Pickup, time curve, differential slope and harmonic restraint | VAE-430; VAE-660 for 87T |
| UPS bypass and static switch | Transfer logic, sync check, bypass inhibit | Voltage channels and status I/O | Transfer only within the allowed window; bypass status correct | VAE-430 or VAE-660 |
| Load shedding / PLC | Frequency and voltage thresholds, trip outputs, load profile logic | Voltage and frequency output, binary I/O | Correct stage trips at the set threshold and delay | VAE-430 or VAE-660 |
ATS transfer sequence: what to inject
The following sequence is a starting point for a dual-source ATS. The exact times and logic come from the commissioning specification and the ATS controller settings.
| Step | Simulate | Expected result |
|---|---|---|
| 1 | Normal source healthy, emergency source healthy | ATS remains on normal; no generator start; no alarm |
| 2 | Normal source fails (voltage and frequency collapse) | Generator start signal; transfer to emergency within the set time; status contacts change |
| 3 | Normal source returns | Return transfer after the set delay; no parallel unless closed transition is enabled |
| 4 | Both sources within the sync window | Closed transition allowed only inside the window; interlock blocks out-of-phase closure |
| 5 | Maintenance inhibit or upstream breaker open | Transfer inhibited; alarm correct; no unexpected trip |
Why four voltage channels are not enough for a dual-source ATS
The VAE-430 has four voltage channels: enough for one three-phase source plus a sync-check or auxiliary voltage. A dual-source ATS needs six voltage channels to keep normal and emergency three-phase voltages live at the same time. With four channels you can test one source at a time, or switch the voltage leads between steps, but you cannot reproduce the phase relationship and the sync-check condition in one pass. The VAE-660 provides six voltage and six current channels, so the normal source, emergency source and relay measurements stay connected through the sequence.
VAE-430 or VAE-660
| Requirement | VAE-430 | VAE-660 |
|---|---|---|
| Single-source feeder or transformer relay | Yes: 3 current, 4 voltage, 8 BI / 4 BO | Yes, with spare channels |
| Dual-source ATS / STS with simultaneous voltages | Limited: sequential or single-source only | Yes: 6 current, 6 voltage, 8 BI / 8 BO |
| Generator differential 87G | No: three current channels | Yes: six current channels |
| Main-tie-main with 87B | Limited | Yes |
| Portability | 18 kg | 21 kg |
What the test set does not do
The test set is a secondary-injection instrument. It does not perform a live transfer under load, it does not load the generator, and it does not certify the UPS or the ATS mechanical endurance. Those tests require a load bank, the ATS controller, the generator controller and the commissioning authority. It also does not test IEC 61850 GOOSE or Sampled Values; that requires a dedicated process-bus test set.
Frequently asked questions
Can the VAE-430 test an ATS?
It can test single-source logic, controller inputs and sequential transfer steps. It cannot hold both three-phase sources live at once, so a dual-source ATS with sync-check and closed transition needs the VAE-660.
Does the test set measure the mechanical transfer time?
It can time the controller output and status-contact transitions through its binary inputs. The mechanical transfer time at the switch is measured with the ATS controller’s own timer or a dedicated timing instrument.
Can it test the generator protection and the ATS in one sequence?
Yes, if the channels and I/O are planned together. The VAE-660 has six current and six voltage channels plus eight inputs and eight outputs, enough to keep the generator protection and the ATS status signals connected through the transfer sequence.
Does it support 60 Hz data centers?
Yes. The frequency range is 0–1000 Hz, with 50 Hz and 60 Hz both covered.
Can it test IEC 61850 GOOSE or Sampled Values?
No. The VAE series is an analog secondary-injection test set. Process-bus testing requires a dedicated IEC 61850 test set.