When commissioning a transmission line or a 33kV feeder, testing each relay in isolation is simply not enough. To prove that a complete protection scheme works correctly, engineers must perform an End to End (E2E) test. This is the only method that verifies both the Local and Remote ends of a line act together as one unified protection system.
What Is an End to End Test?
An End to End (E2E) test is a synchronized functional verification of line protection schemes — primarily differential protection (87L) and distance protection (21) — between two substations.
To achieve this, engineers use GPS-timed current injection equipment, most commonly the OMICRON CMC 356 with the CMGPS 588 GPS module. This combination ensures both ends of the line receive injection signals at precisely the same microsecond, with a timing accuracy of 1 millisecond.
In short, E2E testing validates the entire protection chain: the relays, the communication links (fiber optic or PLC), and the tripping logic — all at the same time.
Why E2E Testing Is Essential
Standard secondary injection proves that a single relay responds correctly. However, it does not prove that the overall system works. This is exactly why E2E testing is non-negotiable on transmission lines and 33kV cables.
Specifically, E2E testing addresses four critical areas:
- Communication integrity — Does the fiber optic (FOC) or PLC link exchange data without delay?
- Time synchronization — Are both relays measuring the power cycle at the same instant?
- Stability — Will the relay remain stable during external (through) faults and avoid a false trip?
- Sensitivity — Will both relays trip instantly for an internal fault, even when fault current flows from one side only?
Without verifying all four areas together, no commissioning is truly complete.
Protection Schemes Covered in E2E Testing
Line Differential Protection (87L)
The differential scheme works on a simple principle from Kirchhoff’s Current Law. The relay at Substation A continuously compares its measured current vector with the vector received from Substation B over the communication link.
- During normal load or an external fault, the two vectors cancel each other out, so the differential current (I_diff) remains near zero. As a result, the relay stays stable.
- During an internal fault, however, the two vectors add up instead of cancelling. Consequently, I_diff rises above the set threshold, and both relays issue a trip command.
Distance Protection (21) with Communication Aid
In addition to differential protection, E2E testing also covers communication-aided distance schemes such as POTT, PUTT, and Blocking.
For these schemes, the test verifies that a fault at 90% of the line length — the far-end reach — triggers an instantaneous permissive trip rather than waiting for the Zone 2 time delay. This is a critical distinction in transmission line protection.
Equipment Required for E2E Testing
To carry out an E2E test professionally, the following tools are needed at each substation:
- Injection unit — OMICRON CMC 356 or Megger SVERKER 900
- GPS synchronization module — CMGPS 588 (essential for both sites)
- Relay software — Siemens DIGSI 5, ABB PCM600, or equivalent vendor tool
- Communication — Coordinated phone or radio contact between the two test teams
The CMGPS 588 is, without doubt, the most critical item. Without reliable GPS lock at both ends, the test results are invalid.
Step-by-Step E2E Field Procedure
Phase 1 — Communication and Signal Mapping
Before injecting any current, first verify the communication “handshake” between both relays.
- Channel check: Disconnect the fiber patch cord. The relay must immediately raise a “COM ALM” or “Link Failure” alarm.
- Signal mapping: Confirm that signals such as “87CH Healthy,” “CB Service,” and “Remote Trip Received” appear correctly on the SCADA screen and local relay LEDs.
This phase is often overlooked, but it is the foundation of a successful E2E test.
Phase 2 — Stability Test
The stability test confirms that the protection scheme does not trip for an external fault or normal through-load current.
- Inject current at both substations simultaneously using GPS synchronization.
- Set Substation A at 0° and Substation B at 180° (out of phase).
- With this setting, the currents oppose each other, and therefore: I_diff = |I_Local + I_Remote| ≈ 0
Expected result: The relay must remain stable — no trip, no alarm. If the relay trips during this test, immediately check for reversed CT polarity, which is the most common cause of failure.
Phase 3 — Sensitivity and Trip Timing Test
Next, the sensitivity test confirms that the scheme does trip correctly for an internal fault.
- Set both substations at 0° (in phase) to simulate an internal fault condition.
- Inject current equivalent to 50% of the line impedance to simulate a mid-line fault.
- Expected result: Both relays must issue a TRIP command. Furthermore, the inter-trip signal exchange must complete within 10 milliseconds.
This timing requirement is what separates line differential protection from slower backup schemes.
Acceptance Criteria
After completing all phases, the following results confirm a successful E2E test:
| Test Parameter | Expected Result |
|---|---|
| FOC / Communication Health | All alarms clear |
| Stability at 2× rated current | I_diff < 0.1 × I_n |
| Sensitivity trip | Relay trips at set threshold |
| Remote trip signal | High / Pulsed confirmed |
| GPS lock status | 3D Lock, 4+ satellites |
Key Safety Practices
Before starting any E2E test, always follow these safety steps:
- Use test switches (such as ABB FT-1) to isolate tripping circuits. This prevents accidental grid trips during the simulation.
- Check CT polarity before the GPS injection phase. Reversed polarity is the number one cause of E2E failure.
- Enable Trip Inhibit via binary input during initial logic checks. This protects the circuit breaker from unnecessary mechanical wear.
Final Summary
In conclusion, the End to End test (E2E) is the gold standard for commissioning transmission line and 33kV cable protection. By combining GPS-synchronized injection with thorough binary I/O mapping, engineers can prove that both differential and distance protection schemes will perform correctly under real fault conditions.
Whether you are working with Siemens SIPROTEC 5, ABB Relion, or GE relays, the fundamental approach remains the same: Synchronize, Inject, and Verify.
E2E testing is a GPS-synchronized functional verification of line protection schemes — typically 87L (differential) and 21 (distance) — carried out simultaneously between two substations at opposite ends of a feeder or transmission line.
Unlike standard secondary injection, which only proves that a single relay functions, E2E testing validates the entire protection chain: the relays at both ends, the communication link (fiber optic or PLC), and the inter-tripping logic — all working together as a single unified defense.
Line Differential (87L / 87CH): Both ends inject current simultaneously. The relay compares the local current vector with the remote vector received over the communication link. During a stability test the vectors cancel (I_diff ≈ 0); during a sensitivity test they add up, producing a trip.
Distance Protection (21)—aided schemes: E2E verifies that communication-aided modes like POTT, PUTT, or blocking operate correctly. A fault simulated at the remote end (e.g., 90% reach) must trigger an instantaneous permissive trip instead of waiting for the Zone 2 time delay.
The CMGPS 588 locks the OMICRON injection unit to GPS satellite time, giving a timing accuracy of ≤1 ms. Without this synchronization, the two injection units would operate on independent clocks and introduce an artificial phase shift between the local and remote current vectors.
This would make a properly wired and stable differential scheme appear to have a differential current—producing either a false trip or a misleading pass. For the test result to be meaningful, the GPS lock must show ” 3D Lock” with 4 or more satellites at both ends before any current is injected.
For the stability test (through-fault simulation), the injection angles are set as follows:
Substation A: 0°
Substation B: 180°
The 180° phase opposition is intentional. It mimics current flowing into the line at one end and out at the other—exactly what happens during a normal load or an external fault. Under this condition, Kirchhoff’s current law dictates that the vector sum equals zero: I_diff = |I_Local + I_Remote| ≈ 0. The relay must remain stable (no trip). This is also called Case 2 (GPS-Simulated), used when primary load current is not available.
For the sensitivity test (internal fault simulation), both ends inject at the same angle:
Substation A: 0°
Substation B: 0°
With currents in phase, they add rather than cancel: I_diff = |I_Local + I_Remote| >> I_pickup. This simulates an internal fault where current flows into the protected zone from both ends. The expected outcome is a TRIP command from both relays with the inter-trip signal exchanged within less than 10 ms. A fault at 50% of the line impedance is a typical test point.
Case 1 (Live primary current): If the feeder is energized and carrying load, primary current already provides a real through-load condition. Both relays see actual system currents, and the differential element must stay stable without any test injection.
Case 2 (GPS-simulated): Used when the feeder is de-energized or the primary current is unavailable. The OMICRON units inject at 0°/180° to simulate through-load artificially, providing the same functional verification without live primary current.
FOC health: All communication alarms clear with no latency warnings.
Stability @ 2×In: Differential current I_diff < 0.1 × I_n — no false trip during motor starts or external faults.
Sensitivity trip: Both relays trip at the configured pickup setpoint.
Remote trip signal: The Remote Trip Received” binary output shows High or Pulsed—confirming inter-station logic is functional.
GPS lock quality: “3D Lock” with 4 or more satellites at both ends before and during the test.
Inter-trip timing: Signal exchange confirmed within <10 ms.
Test switches: Use ABB FT-1 or equivalent test terminal blocks to isolate the relay’s trip output from the actual circuit breaker trip coil during the initial logic verification stages.
Trip Inhibit binary input: Activate the relay’s “Trip Inhibit” binary input during all binary I/O mapping checks. This allows the injection and logic to be verified without energizing the trip circuit, preventing unnecessary CB wear or an unintended trip.
Staged verification: Only remove isolation and inhibit functions once signal mapping, communication, and polarity have been confirmed at both ends.