In the high-stakes environment of power system commissioning, testing components in isolation is a recipe for disaster. To ensure a 33kV feeder or a high-voltage transmission line is truly secure, engineers rely on End to End Testing (E2E).
This is the only method to prove that the “Local” and “Remote” ends of a protection scheme act as a single, unified defense against faults.
What Is End to End (E2E) Testing in Line Protection?
End to End testing (E2E) is a synchronized functional verification of line protection schemes (87L & 21) between two substations. Using GPS-timed injection (Omicron CMGPS 588), it validates the entire chain: relays, communication links (FOC/PLC), and tripping logic to ensure stability during through-faults and instantaneous clearing during internal faults.
⚡ 1. Why E2E Testing is Non-Negotiable
Standard secondary injection proves a relay works, but E2E testing proves the system works. It targets four critical areas:
- Communication Integrity: Is the fiber optic (FOC) or PLC link exchanging data without lag?
- Time Synchronization: Are both relays “seeing” the power cycle at the exact same microsecond?
- Stability: Will the relay remain “blind” to external faults (through-faults)?
- Sensitivity: Will it trip instantly for internal faults, even if fed from only one side?
⚙️ 2. Core Protection Schemes Covered
Line Differential Protection (87L / 87CH)
This scheme relies on Kirchhoff’s Law. The relay at Substation A compares its current vector with the vector from Substation B.
- Stability Logic: During normal load, the vectors cancel out (I_{diff} |approx 0).
- Fault Logic: During an internal fault, the vectors sum up, triggering a trip.
Distance Protection (21)
E2E testing is vital for communication-aided distance schemes (POTT, PUTT, or blocking).
- Zone 1 & 2 Reach: Verifies that a fault at the “far end” of the line (90% reach) correctly triggers an instantaneous “permissive trip” rather than waiting for a Zone 2 time delay.
🧰 3. The Professional Tool Kit
To match the precision required by vendors like Siemens (SIPROTEC 5) or ABB (Relion), you need the following:
- Injection Units: OMICRON CMC 356 or Megger SVERKER.
- GPS Sync: CMGPS 588. This is the “heart” of the test, ensuring 1 ms timing accuracy between sites.
- Software: Siemens DIGSI 5 or vendor-specific IED tools to monitor real-time I_{diff} and I_{rest} values.
🧾 4. Step-by-Step Field Procedure
Phase 1: Communication & Mapping Verification
Before injecting current, you must verify the “handshake” signals identified in your Binary I/O (BI/BO) and LED tables:
- Channel Check: Disconnect the fiber patch cord. The relay must immediately flag a “COM ALM” or “Link Failure.”
- Signal Mapping: Verify specific signals like “87CH Healthy,” “CB Service,” and “Remote Trip Received” are mapping correctly to the SCADA and local LEDs.
Phase 2: The Stability Test (Case 1 vs. Case 2)
- Case 1 (Site Stability): If primary current is available, use it to confirm the relays see “through-load.”
- Case 2 (GPS-Simulated): Inject current at both ends simultaneously.
- Settings: Substation A at 0 degree | Substation B at 180 degrees.
- Requirement: The differential current must satisfy the following:
I_{diff} = |I Local + I Remote| approx 0 - Goal: The relay stays stable (no trip).
Phase 3: The Sensitivity & Timing Test
- Settings: Substation A at 0 degree | Substation B at 0 degree (in-phase).
- Distance Test: Simulate a fault at 50% line impedance.
- Requirement: Both relays must issue a TRIP command.
- Timing: The signal exchange (inter-trip) must be confirmed within <10 ms.
📊 5. Key Indicators for Acceptance
Based on actual field commissioning formats, your results should align with this table:
| Test Parameter | Expected Result | Significance |
| FOC Health | Alarms Clear | Comm link reliability. |
| Stability @ 2 I_n | I_{diff} < 0.1 \times I_{n} | No false trips during motor starts/external faults. |
| Sensitivity Trip | Relay Trips @ Setpoint | Fast fault clearing. |
| Remote Trip Signal | High/Pulsed | Inter-station logic is working. |
| GPS Lock | “3D Lock” / 4+ Satellites | Accuracy of the test data. |
🧠 6. Safety & Best Practices
- Isolation: Use test switches (like ABB FT-1) to ensure you don’t accidentally trip the entire grid during the simulation.
- Polarity Check: Reversed CT polarities are the #1 cause of E2E failure. Always perform a wiring check before the GPS test.
- Inhibit Tripping: Use the “Trip Inhibit” binary input during initial logic checks to protect the circuit breaker from unnecessary wear.
Final words:
E2E testing is the gold standard for 33kV and transmission line protection. By combining GPS-synchronized injection with rigorous binary I/O mapping, engineers ensure that the distance and differential schemes provide maximum reliability. Whether you are using Siemens, ABB, or GE relays, the physics 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.