Warm Neutral Link and High Neutral Current on a Mixed LT Feeder
An 11/0.4 kV, 250 kVA distribution transformer supplies a 300 m long three-phase four-wire LT feeder. The feeder serves 22 single-phase shop connections and four small three-phase motors. During evening load, a lineman records phase currents of 128 A, 96 A and 62 A, and a neutral current of 88 A with a clamp meter. Line-to-neutral voltages at the far end are 218 V, 207 V and 226 V. The neutral conductor is 50 sq mm aluminium, while the phase conductors are 95 sq mm aluminium. The transformer neutral link feels warm, and several shopkeepers report flickering lights and slow fans. The transformer oil temperature is 71 °C and the ambient temperature is 38 °C. The lineman has a true-RMS clamp meter, an infrared thermometer and a low-voltage detector, but no power-quality analyser at that moment. The area has many LED drivers and computer power supplies in the shops.
1. Using the recorded currents, explain why the neutral current is high and whether the 50 sq mm neutral is necessarily the root cause.
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First, do not add the phase currents arithmetically. In a four-wire system the neutral current is the phasor sum of the three phase currents. For a rough check, assume the three currents are 120° apart and purely resistive:
I_N ≈ √(128² + 96² + 62² − 128×96 − 96×62 − 62×128) = √(16384 + 9216 + 3844 − 12288 − 5952 − 7936) = √3268 ≈ 57 A.
The measured 88 A is much higher than 57 A, so there is either phase-angle spread from different power factors or triplen harmonic current adding in the neutral. The average phase current is about 95 A; the maximum deviation is 33 A, roughly 35% of average. That is a badly unbalanced feeder. A 50 sq mm aluminium neutral has an ampacity around 110–120 A in free air, but joints, enclosure and high ambient derate it. At 88 A it is not clearly overloaded by fundamental current, yet the warm neutral link points to a loose joint, high harmonic content, or both. Therefore the neutral size is not proven to be the root cause; the evidence points first to load unbalance plus harmonics, with a possible bad neutral connection.
2. What immediate actions should be taken before the evening peak, and which measurements would separate unbalance from harmonic current?
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Treat the neutral link as live and do not open it under load. Use the infrared thermometer to compare the neutral link, phase links and cable terminations. Measure neutral-to-earth voltage at the transformer and at the far end; a rise above a few volts indicates neutral impedance or a loose connection. Clamp each phase and the neutral with the true-RMS meter, then switch off one shop group at a time and watch which phase and the neutral change. That helps separate unbalance from harmonic current. Redistribute single-phase shop loads from the 128 A phase to the 62 A phase, but only after switching off and locking the concerned circuit. Reduce non-essential load if the neutral link is hot. Record all readings. If the link is discoloured or the N-PE voltage is high, arrange a shutdown, retorque or replace the link, and check the transformer neutral earthing. Because no analyser is available, record the neutral current with phase currents nearly balanced; if it remains high, harmonics are likely.
3. Recommend a permanent remedy and state why simply replacing the neutral with a larger conductor may not solve the problem.
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A permanent remedy has three parts. First, rebalance the single-phase loads across R, Y and B so the maximum deviation is much smaller; this reduces neutral current and voltage unbalance. Second, check and correct the neutral conductor, link and all joints. If the neutral current remains high after balancing because of LED drivers and SMPS, use a neutral rated for harmonic current, or install a suitable harmonic filter or isolating transformer for the largest shops. Third, improve the transformer neutral earthing and record earth resistance. Simply replacing the 50 sq mm neutral with a larger conductor may reduce heating, but it will not fix the 207 V phase, the flicker, or the transformer loading. A larger neutral also does not help if the link is loose or if the neutral is open. Fix the cause first, then size the neutral for the corrected load and expected harmonics.