ATS and STS Controller Testing with a Relay Test Set
An automatic transfer switch is a controller problem before it is a switch problem. The contacts may be healthy, but if the controller does not recognise source loss, start the generator, wait for the right voltage and frequency, check synchronism and block an out-of-phase closure, the redundancy does not exist. Testing the controller means injecting the voltages it senses and reading the commands it issues. A relay test set does exactly that.
What the controller sees and what it commands
An ATS, STS or main-tie-main controller has two three-phase voltage inputs, a set of digital status inputs and a set of relay outputs. The inputs tell it whether each source is available and within limits. The outputs command the transfer, start and stop the generator, drive alarms and may inhibit a transfer.
| Controller terminal | What it expects | How the test set connects |
|---|---|---|
| Normal source voltage | Three-phase voltage, typically 100–120 V L-N or 380–415 V via PTs | Three voltage channels set to the PT secondary value |
| Emergency source voltage | Second three-phase voltage with independent frequency and phase | Three further voltage channels; six in total |
| Breaker and switch status | Potential-free auxiliary contacts | Binary outputs emulate open, closed and intermediate states |
| Generator start/stop | Controller output contact | Binary input records the command and its timing |
| Transfer and inhibit commands | Controller output contacts | Binary inputs record sequence and duration |
| Alarm and communication | Output contacts or bus signal | Binary inputs; bus protocols are outside the test set |
The direction matters. The test set sources the voltages and the status contacts. It reads the controller commands. Reversing the direction is the most common wiring mistake: the test then measures its own outputs and the controller appears to work.
What to prove
| Function | What is injected or emulated | Pass criterion |
|---|---|---|
| Source-loss detection | Normal source collapses in voltage and frequency | Controller declares source failure at the set undervoltage and underfrequency thresholds |
| Generator start | Source failure held for the start delay | Start output within the set time; no start on a transient shorter than the delay |
| Transfer to emergency | Emergency source healthy at rated voltage and frequency | Transfer command after the set delay; status contacts change in the correct order |
| Return transfer | Normal source restored and stable | Return after the set stability and return delays; no transfer on a short restoration |
| Synch-check and closed transition | Both sources live with adjustable phase angle and slip | Closure permitted only inside the voltage, frequency and phase window |
| Interlock and inhibit | Maintenance inhibit, upstream breaker open, manual mode | No transfer; correct alarm; no unexpected output |
| Load shedding and priority | Frequency or voltage threshold with delay | Correct stage output at the set threshold and time |
A test sequence for a dual-source controller
The times and thresholds below are placeholders. Take the real values from the commissioning specification and the controller settings file.
| Step | Source state | Status emulation | Expected |
|---|---|---|---|
| 1 | Normal and emergency both healthy, in phase | Normal closed, emergency open | No transfer, no generator start, no alarm |
| 2 | Normal collapses to 0 V and 0 Hz | Normal open | Source-failure alarm; generator start after the start delay |
| 3 | Emergency at rated V and f | Emergency closes after the transfer delay | Transfer output; status sequence correct |
| 4 | Normal returns at rated V and f | Normal available | Return transfer only after the stability delay |
| 5 | Both live, phase outside the sync window | Both closed request | Closed transition blocked; no parallel closure outside the window |
| 6 | Both live, phase inside the window | Closed transition enabled | Parallel closure permitted only inside the window |
| 7 | Normal healthy, inhibit active | Inhibit input active | No transfer; inhibit alarm correct |
Why four voltage channels are not enough
A dual-source controller needs normal and emergency three-phase voltages live at the same time. That is six voltage channels before any auxiliary or sync-check voltage is added. The VAE-430 has four voltage channels, enough for one three-phase source plus one auxiliary channel. It can test a single-source controller, a sequential transfer or the logic of a dual-source controller one source at a time, but it cannot reproduce the phase relationship between two live sources in one pass. The VAE-660 provides six voltage and six current channels, so both sources stay connected through the whole sequence.
Control power and coil voltage
Most ATS controllers are powered from a 24 V, 48 V or 110 V DC control battery. The VAE series provides a programmable DC source up to ±160 V and a fixed 110/220 V auxiliary supply rated at 1.5 A. The fixed supply can power a controller, but it is not a variable source, so it cannot be used to measure a 220 V DC coil’s pickup and dropout. Where the transfer switch uses 220 V DC operating coils, the coil test needs the station battery or a separate adjustable DC source.
What the test set does not do
- It does not transfer the load. The real transfer is proven with the switch, the load and the commissioning authority.
- It does not measure the mechanical transfer time at the switch contacts. It times the controller outputs and the emulated status transitions.
- It does not load the generator. Generator load acceptance is a load-bank test.
- It does not test the controller’s communication stack or its SCADA mapping.
Frequently asked questions
Can the VAE-430 test an ATS controller at all?
Yes for single-source logic, status input mapping, transfer delays and output timing. For a dual-source controller with synch-check and closed transition, the VAE-660 is the correct model because both three-phase sources must be live at once.
Can the test set emulate the generator as well as the mains?
It can emulate the generator voltage and frequency at the controller terminals, and it can read the start and stop commands. It does not emulate generator dynamics, excitation or governor response.
Does it measure transfer time in milliseconds?
The VAE-430 records binary events with 0.1 ms resolution; the VAE-660 with ±1 ms. That is sufficient for controller timing. Mechanical transfer time at the switch is a separate measurement.
Can it test 50 Hz and 60 Hz controllers?
Yes. The source covers 0–1000 Hz in PC mode and both 50 Hz and 60 Hz are standard test points.
What about three-source or main-tie-main schemes?
The VAE-660 covers two live three-phase sources plus six current channels. Three-source schemes usually need external switching of the voltage leads between steps, or a larger channel count than a portable test set provides.