What Is This Blog About?
If you work with Auto transformer testing protocols, you already know that before a transformer is switched ON for the first time, it must pass a series of strict protection relay tests. Skip these tests, and you risk a transformer explosion, equipment failure, or worse, a grid blackout.
This blog explains exactly how we tested the 250 MVA Auto-Transformer T-1 (ATR T-1) at ABC Grid Station (220/132 kV) for a client contracted by Siemens Pakistan Engineering Co. Ltd.
I will walk you through Auto transformer testing protocols, what we tested, why we tested it, and what values we were looking for.
About the Transformer (ATR T-1)
Before diving into the tests, here is a quick summary of the transformer we were testing:
| Parameter | Value |
|---|---|
| Rating | 250 MVA |
| Voltage Levels | 220 / 132 / 11 kV |
| Vector Group | YNa0d1 |
| Impedance (%Z) | 15.57% |
| Tap Changer | OLTC ±10%, 33 taps |
| Location | ABC Grid Station |
| Contractor | Siemens Pakistan |
All tests were done using secondary injection—meaning the transformer was completely de-energized, isolated, and earthed before any test began. Safety first, always.
Why Are These Tests So Important?
A 250 MVA transformer costs millions of dollars. The protection relays around it are its “immune system.” If those relays are not set and tested correctly, they will either:
- Not trip when a real fault happens (dangerous — can destroy the transformer), or
- Trip unnecessarily during normal operation (causes grid outages and financial loss)
This is why every relay is tested before the transformer is energized, Auto transformer testing protocols.
The Tests We Performed — Explained Simply
1. Pre-Test Safety Checks (Visual & Insulation Tests)
Before touching any relay, we completed 15 visual checks. These included:
- Verifying the transformer nameplate (250 MVA, 220/132 kV, YNa0d1)
- Checking that all relay wiring matched the approved Siemens drawings
- Confirming CT (current transformer) polarity was correct
- Testing that all earth connections were below 1 Ohm resistance
- Checking DC supply voltage was within ±10% of rated
We also did Insulation Resistance (IR) tests using a 500V DC megger on every circuit—CT circuits, trip circuits, DC supply, and relay chassis. The minimum acceptable reading was 100 MΩ. Any reading below this meant a wiring fault that had to be fixed before moving forward.
Why this matters: A low insulation reading can cause a relay to operate falsely or fail to operate when needed.
2. Differential Protection Test — 87T (The Most Critical Test)
The differential relay (Siemens 7UT6131) is the main protection for this transformer. It compares the current going INTO the transformer against the current coming OUT. If there is a big difference, it means there is an internal fault, and the relay trips the circuit breakers.
We had two sets of differential protection (Set-1 and Set-2) for redundancy.
Pickup Test: We injected current into the HV side (220 kV) and LV side (132 kV) separately. The relay must trip when the current difference exceeds 25% of rated current.
| Side | CT Ratio | Rated Current (Secondary) | Pickup Setting |
|---|---|---|---|
| HV (220 kV) | 1200/1A | 0.552 A | 0.138 A |
| LV (132 kV) | 2400/1A | 0.456 A | 0.114 A |
The relay must trip in less than 30 milliseconds — almost instantly.
Stability Test: We injected a balanced current on both sides simultaneously (simulating normal through-load). The relay must NOT trip in this condition. If it trips here, it means the CT polarity is wrong and needs rewiring.
Slope Characteristic Test: This test checks how the relay behaves during heavy fault currents when CTs may saturate.
- Slope 1 (normal load zone): Relay trips if differential current > 30% of bias current
- Slope 2 (CT saturation zone): Relay trips if differential current > 60% of bias current
Harmonic Restraint Test: When a transformer is first switched on, it draws a large inrush current that looks like a fault to the relay. The relay uses 2nd harmonic filtering (>15%) to block unnecessary trips during energization.
3. Overcurrent & Earth Fault Tests (HV and LV Sides)
These are the backup protection relays (Siemens 7SJ8021). They trip if the differential relay fails.
LV Side (132 kV) Overcurrent:
- Pickup set at 144% of rated current (1.44A secondary)
- Uses IEC Normal Inverse (IDMT) curve with TMS = 0.135
- At 5× the pickup current, the relay must trip in approximately 1.01 seconds
Neutral Earth Fault Protection:
- Pickup at 22% of rated current (0.22A secondary)
- Uses Definite Time — trips in exactly 1.5 seconds regardless of how large the fault current is
Tertiary Winding (11 kV) Protection:
- The 11 kV tertiary winding rated current is 3,937 A primary (75 MVA)
- Overcurrent pickup at 145% of rated with 1.5-second trip time
- Earth fault pickup at 46% of rated with 1.5-second trip time
4. Restricted Earth Fault (REF) Protection
REF protection (Siemens 7SJ8042) detects earth faults inside the transformer winding — faults that the differential relay may miss at low fault current levels. It is highly sensitive and operates within 50 milliseconds.
5. Breaker Failure Protection
If a circuit breaker fails to open after a relay trip command, the Breaker Failure Relay (Siemens 7VK6111) sends a backup trip to other breakers to isolate the fault. We tested both the HV (220 kV) and LV (132 kV) breaker failure relays.
6. Thermal Overload & Over-Flux Protection
- A thermal overload relay monitors winding temperature and trips if the transformer is continuously overloaded
- An over-flux (V/Hz) relay protects the transformer when voltage is too high or frequency drops—a common condition during grid disturbances in Pakistan
Key Standards Followed
All tests of Auto transformer testing protocols were performed in accordance with the following:
- IEC 60255 – Protection relay testing standard
- IEC 60076 – Power transformer standard
- IEC 60255-187-3 – Transformer differential protection
- ANSI/IEEE C37.90 – Relay systems for power apparatus
- NGC/WAPDA Protection Testing Guidelines
Frequently Asked Questions (FAQs)
Q: What is secondary injection testing for a transformer? Secondary injection means injecting test currents directly into the relay terminals (not through the transformer itself), with the transformer safely de-energised. It verifies relay settings without putting high voltage on the transformer.
Q: Why does a 250 MVA transformer need two sets of differential protection? For reliability. If one differential relay fails or is taken out for maintenance, the second set keeps the transformer protected. NGC requires this redundancy for critical assets.
Q: What is the IDMT relay trip time formula? For an IEC normal inverse curve: t = [0.14 ÷ (M^0.02 − 1)] × TMS, where M = injected current ÷ pickup current.
Q: What happens if insulation resistance is below 100 MΩ? Testing stops immediately. The faulty circuit must be found and repaired before any relay injection testing continues.
Final Thoughts
Testing a 250 MVA autotransformer is not a simple job. It involves dozens of relay tests, hundreds of data points, and strict adherence to international standards. Every test result must be witnessed by the client (NTDC) and signed off by the project manager before the transformer is allowed to energize.
The complete testing documents for ATR T-1 at Chakdara Grid Station are available as downloadable Excel files below.
📥 Download the Full Testing Protocol Sheets (Excel Part-1)
Download the Full Testing Protocol Sheets (Excel Part 2).
Written by Fazli Wahid — Test Inspector, NGC Pakistan | 26 years of substation field experience Published on Substationfaults.com—Your expert resource for electrical substation fault diagnosis and protection relay testing
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