Stability Test of Power Transformer (Differential Protection Stability Test)

The stability test of a power transformer is done to verify that the transformer differential protection relay remains stable (does not operate) during 

• Normal load conditions, 
• External faults and 
• Through fault currents.

In simple terms the test confirms that the differential relay does not trip when the currents entering and leaving the transformer are equal.

Purpose of Transformer Stability Test

The stability test ensures that:

• The differential protection relay does not trip for the external faults.

• The CT ratios and polarity are correct.

• The relay settings are coordinated with the CT ratios.

• No false differential current determined during load conditions.

This is an important verification step during:

• Transformer commissioning,
• Protection relay testing and
• Substation maintenance.

Principle of Differential Protection

Transformer differential protection works on the principle of Kirchhoff’s Current Law.

Concept

I_{diff ​}= I₁​−I₂​

Where

I₁​ − Current entering transformer

I₂​ − Current leaving transformer

If:

I₁​ = I₂​

Then:

I_diff  ​= 0

So the relay should be remain stable (no trip).

If an internal fault occurs the currents become unequal:

I_diff​ > Setting

The relay operates and trips the breaker.

Types of Transformer Stability Tests

There are 3 types of transformer stability tests:

1). Through Fault Stability Test

2). Load Stability Test

3). CT Polarity and Ratio Stability Test

1). Through Fault Stability Test

Through fault stability test checks relay stability when a large current flows through the transformer during external faults.

2). Load Stability Test

Load stability test ensures that the relay does not operate during normal load current.

3). CT Polarity and Ratio Stability Test

CT polarity and ratio stability test confirms that the current transformers (CT) are correctly connected.

Test Equipment Required

Typical equipment utilized are:

• Primary current injection test set,

• Secondary injection test kit,

• Clamp meter,

• Multimeter,

• Protection relay test kit (OMICRON / Megger / Doble),

• CT test leads and

• Wiring diagram of transformer protection scheme.

Stability Test Procedure

Step-1: Isolate Protection System

• Ensure transformer is isolated or under test condition.
• Disable trip circuit to avoid breaker operation.
• Inform control room.

Step-2: Verify CT Connections

Check:

• CT ratio,

• CT polarity,

• CT secondary wiring,

• CT star point connection.

Incorrect CT polarity is the most common cause of the differential protection maloperation.

Step-3: Inject Equal Current in Both Sides

Using a primary injection kit inject current such that:

I_primary = I_secondary x (CT ratio/Transformer ratio)

The relay should see balanced current.

Example

Transformer: 10 MVA, 110/33 kV

CT ratios:

HV side = 200/1 A

LV side = 600/1 A

Injected current should produce equal current at relay input.

Step-4: Observe Differential Current

Monitor relay parameters:

• Differential current,
• Bias current,
• Relay trip status.

Expected result:

Differential current ≈ 0 A

Relay should not trip.

Step-5: Increase through Current

Gradually increase injected current.

Example test levels:

• 25% rated current
• 50% rated current
• 75% rated current
• 100% rated current

The relay must remain stable for all currents.

Step-6: Record Relay Readings

Record:

Test Current	HV Current	LV Current	Differential Current	Relay Status
25%	Value	Value	≈0	Stable
50%	Value	Value	≈0	Stable
100%	Value	Value	≈0	Stable

Acceptance Criteria

The transformer differential protection is considered stable if:

• No relay trip occurs.
• Differential current remains within relay threshold.
• Bias characteristic remains within limits.

Typical acceptable value:

I_diff < 0.2I_n

It depends on relay setting.

Causes of Stability Failure

If the relay operates during the stability test the possible reasons include:

1). Wrong CT Polarity

The opposite polarity creates false differential current.

2). CT Ratio Mismatch

Example:

HV CT = 200/1

LV CT = 400/1

The improper compensation creates imbalance.

3). Incorrect Vector Group Compensation

Example:

Transformer vector group:

• Dyn11
• Yd1

The relay must compensate for 30° phase shift.

4). CT Saturation

During high through faults CT may saturate causing:

I_diff = 0

Importance of Stability Test

Transformer stability testing ensures:

• Protection reliability,

• Avoidance of the false tripping,

• Correct CT (Current Transformer) and relay configuration,

• System security during external faults.

Without proper stability testing a healthy transformer may trip unnecessarily causing the power outages and system instability.

Difference between Stability Test and Operation Test

Parameter	Stability Test	Operation Test
Purpose	Check relay does NOT trip	Check relay trips
Fault Type	External fault	Internal fault
Current Condition	Balanced	Unbalanced
Result	Relay stable	Relay operates

The stability test of power transformer verifies that the differential protection relay may remain stable during balanced current conditions and external faults without any unwanted tripping.