Floating Neutral Point Measurement Test of a Transformer: Step-by-Step Procedure

The floating neutral point measurement test is a critical low-voltage electrical test conducted on three-phase transformers (specifically those with star-connected winding) to verify the transformer’s neutral point remains properly grounded and detect any unbalanced winding conditions or short-circuited windings.

This test is performed by applying three-phase AC supply (typically 415V) across the high-voltage (HV) or intermediate-voltage (IV) windings while the transformer neutral is disconnected from ground.

In a healthy transformer with balanced windings, the voltage between the floating neutral and ground should be zero or negligible. However, if appreciable voltage appears at the neutral point, it indicates possible winding faults, inter-turn short circuits, or manufacturing defects.

What is Floating Neutral Point?

The neutral point of a star-connected transformer winding (also called star-point or star-winding neutral) is the common center point where all three windings meet. Under normal operating conditions, this neutral point is permanently grounded to earth, maintaining a reference potential of zero volts with respect to ground.

Floating Neutral Concept

Floating neutral refers to a condition where the neutral point of a transformer is disconnected from ground (earth), making its potential undefined or “floating.” When the neutral floats:

• Its potential is no longer fixed at 0V relative to ground
• The voltage of the neutral point becomes a function of the transformer’s internal impedance and the balance of loads on the three phases
• In a perfectly balanced transformer with ideal symmetric windings, the neutral voltage should remain at zero even when floating
• However, if winding imbalances or faults exist, voltage appears at the floating neutral point

Principle Based on Kirchhoff’s Voltage Law

According to Kirchhoff’s Voltage Law (KVL), the sum of voltages around a closed loop must equal zero. For a balanced three-phase system:

\(V_R+V_Y+V_B=0\)

When the neutral is disconnected and the transformer is healthy, the three-phase voltages maintain this relationship, resulting in zero voltage at the neutral point. If windings are unbalanced or faulted, this equation is violated, causing neutral point voltage to appear.

Why Floating Neutral Point Measurement Test is Important

1. Detection of Short-Circuited Windings: Short-circuited or partially short-circuited windings (inter-turn short circuits) create winding imbalances.
2. Detection of Unbalanced Winding Impedance: Manufacturing defects can create unequal impedance between phases.
3. Detection of Winding Faults During Service: The test helps identify gradually developing faults.
4. Quality Assurance During Commissioning
5. System Stability and Protection: Floating neutral conditions in actual systems can cause Unbalanced three-phase voltages, Unreliable operation of protection systems, Damage to sensitive loads, Ground fault detection failure etc.
6. Compliance with Standards: Standard testing procedures (IEC, IEEE, NETA) require this test as part of transformer commissioning, ensuring compliance with industry requirements and manufacturer recommendations.

Floating Neutral Point vs. Grounded Neutral

Normal Operation (Neutral Grounded)

When neutral is connected to ground:

• Neutral point potential: 0V (fixed reference)
• Phase voltages are stable and balanced
• Unbalanced loads or faults create measurable currents through neutral
• Protection relays can reliably detect faults
• No neutral voltage displacement

Floating Neutral Condition (Neutral Disconnected)

When neutral is disconnected from ground:

• Neutral point potential: Unknown (floating)
• Neutral voltage depends on transformer internal impedance
• In a balanced, fault-free transformer: Voltage ≈ 0V (negligible)
• In an unbalanced or faulted transformer: Voltage = Appreciable (indicates fault)
• Neutral point “floats” to a position determined by the unbalance

Equipment and Apparatus Required

Step-by-Step Test Procedure for Floating Neutral Point

Step 1: De-energization

• Ensure transformer is completely isolated from all power sources
• Open and lock all circuit breakers
• Use voltage tester to verify zero voltage on all terminals
• Ground HV terminals together temporarily for safety
• Wait 5 minutes for residual charge dissipation

Step 2: Identify Neutral Point

• Locate neutral bushing on HV winding (marked “N” or “0”)
• Verify transformer has star-connected winding (delta transformers don’t have accessible neutral)
• Inspect neutral terminal for corrosion or loose connections

Step 3: Set Autotransformer to Zero

• Verify autotransformer is OFF at main disconnect
• Manually set slider to ZERO position
• Test with multimeter to confirm zero output

Step 4: Disconnect Neutral from Ground

• Locate neutral earth/ground connection at transformer neutral bushing
• Carefully remove the earth connection from neutral terminal
• Verify disconnection with multimeter (should show open circuit to ground)

Why disconnect? When neutral is grounded, it provides a return path that bypasses transformer impedance, resulting in incorrect measurements. Disconnecting allows full winding impedance to be reflected, revealing any imbalances.

Step 5: Connect Three-Phase Supply

• 415V supply → Autotransformer input
• Autotransformer R-phase → Transformer HV R-terminal
• Autotransformer Y-phase → Transformer HV Y-terminal
• Autotransformer B-phase → Transformer HV B-terminal
• All connections made with transformer de-energized

Step 6: Apply Three-Phase Voltage

• Close autotransformer breaker to connect 415V supply
• Slowly increase voltage from 0V to 415V:
  • Increase to 50V, stabilize 10 seconds
  • Continue to 100V, 150V, 200V, etc.
  • Reach full 415V gradually
• Watch for abnormal sounds, sparks, smoke, or burning smell
• STOP IMMEDIATELY if any abnormality is observed

Step 7: Record Measurements

Once voltage is stable at 415V for 30-60 seconds measure the following voltages and record:

Measurement notes:

• Record readings taken simultaneously or in rapid succession while voltage is stable
• V1, V2, V3 should be approximately equal (around 240V for 415V three-phase input)
• V_NG is the critical measurement – should be zero or negligible
• Readings should be stable and not fluctuating before recording
• Repeat measurements 2-3 times to verify consistency

Step 8: Analyze Neutral Voltage Reading

Evaluation of V_NG (Neutral to Ground Voltage):

Case 1: V_NG ≈ 0V (or Negligible <1-5V)

• Result: BALANCED – Transformer is healthy
• Interpretation: Windings are symmetric, no short circuits detected
• Action: Transformer APPROVED for commissioning

Case 2: V_NG = 5-20V

• Result: MARGINAL – Investigate further
• Interpretation: Possible minor winding imbalance or manufacturing tolerance issue
• Action: Repeat test multiple times at different voltage levels (200V, 300V, 415V) to check consistency
• Further testing: Perform winding resistance test and insulation resistance test
• Decision: Consult manufacturer with detailed measurements before approval

Case 3: V_NG > 20-50V

• Result: UNBALANCED – Potential winding fault detected
• Interpretation: Significant winding imbalance, inter-turn short circuit, or loose winding connection possible
• Action:
  • Stop test and de-energize immediately
  • Do NOT apply further voltage
  • Perform additional diagnostic tests (SFRA, winding resistance, insulation resistance)
  • Consult manufacturer
  • Transformer likely unsuitable for service until repaired

Case 4: V_NG > 100V or approaching phase voltage (>200V)

• Result: SEVERE FAULT – Critically unbalanced or severely faulted
• Interpretation: Major winding fault, severe short circuit, or winding open circuit
• Action:
  • De-energize immediately
  • Mark transformer as “NOT SERVICEABLE”
  • Do not energize further
  • Return to manufacturer for repair or rewinding
  • Investigate cause (manufacturing defect, damage in transit, etc.)

Step 9: De-energize and Stabilize

• Gradually reduce autotransformer voltage to ZERO (reverse the ramp-up process)
• Spend 10-15 seconds at each 50V step during reduction
• Open autotransformer main breaker when voltage reaches zero
• Wait 5-10 minutes for transformer to cool and residual charge to dissipate
• Verify with multimeter that voltage is zero before touching anything

Step 10: Reconnect Transformer Neutral to Ground

• Clean the neutral terminal if oxidation has occurred
• Reconnect the earth connection to the neutral terminal
• Tighten connection bolt securely with appropriate wrench
• Verify connection is tight
• Test with multimeter between neutral and ground to confirm continuity
• Multimeter should show near-zero resistance (typically <1 ohm)

Also, check Polarity Test of a Transformer, Magnetic Balance Test of a Transformer, Vector Group Test of a Transformer.

Acceptable Limits

Acceptable Neutral Voltage Limits

For a healthy transformer, when 3-phase 415V is applied:

Phase-to-Neutral Voltage Balance

The three phase-to-neutral voltages should also be examined:

\(V_{Phase\,Balance}=\frac{V_{max}⁡−V_{min}⁡}{V_{average}} \times 100 \%\)

Acceptable balance:

• Each phase should read approximately 237-243V (for 415V three-phase input)
• Deviation between phases should be <2%
• If V1, V2, V3 differ significantly, indicates winding asymmetry

Example:

• V1 = 240V, V2 = 238V, V3 = 242V
• Max = 242V, Min = 238V, Average = 240V
• Balance = (242-238)/240 × 100% = 1.67% ✓ ACCEPTABLE

Floating Neutral Measurement for Three-Phase Bank Assembly

When combining three single-phase transformers into three-phase bank:

1. Individual tests: Test each single-phase transformer’s floating neutral
2. Consistency check: Verify all three transformers have similar characteristics
3. Bank test: If feasible, test assembled bank as three-phase unit
4. Phase sequence verification: Confirm R-Y-B phase sequence is correct
5. Neutral grounding: Verify neutral of assembled bank is properly grounded at single point

Safety Precautions

⚠️ CRITICAL SAFETY POINTS:

1. De-energize completely: Transformer must be isolated from all power sources
2. Neutral disconnection: Always disconnect neutral from ground BEFORE energizing
3. Gradual voltage ramp: Never jump directly to 415V; increase slowly to detect faults early
4. Emergency stop ready: Have procedure to quickly de-energize if abnormality occurs
5. Reconnect immediately: Restore neutral to ground AFTER test completes (do not delay)
6. PPE mandatory: Wear insulated gloves (1000V rated), safety goggles, appropriate footwear
7. Qualified personnel only: Only trained, qualified engineers should conduct this test
8. Second person present: Always have at least one other trained person present
9. Never leave floating neutral: Unattended transformer with floating neutral is dangerous
10. If fire/shock occurs: Immediately de-energize, call emergency services, provide first aid

Conclusion

The floating neutral point measurement test is a critical diagnostic procedure for three-phase transformers that verifies winding balance and detects internal faults before transformer energization. This test is fundamental to transformer commissioning, ensuring system reliability and personnel safety.