Agricultural Pump at the End of a 620 m LT Line: Diagnosing a 15% Voltage Depression
A 63 kVA, 11 kV/415 V distribution transformer at the edge of a village feeds a three-phase four-wire LT line built with 95 sq mm aluminium conductor. A 15 HP (11 kW) agricultural pump set is connected through a service connection at the far end, 620 m from the transformer. During a joint inspection the following readings are recorded with the pump running at full load: 405 V at the transformer LT bus, 352 V at the pump starter terminals, and a line current of 24 A on each phase. With the pump switched off, the voltage at the pump terminals rises to 408 V. A shopkeeper connected 40 m from the transformer reports no voltage problem. The pump motor runs hot and its contactor chatters during starting. The assistant lineman proposes replacing the 95 sq mm conductor over the whole 620 m with 150 sq mm conductor at a cost of about Rs 4 lakh.
1. Calculate the voltage regulation at the pump terminals and the deviation from nominal voltage, and state whether the installation complies with the permissible LT voltage band.
Read the worked answer
Take the no-load voltage at the pump terminals (408 V) as the reference for regulation, because it already includes the transformer's own regulation and tap position:
Regulation = (V_no-load − V_full-load) / V_no-load × 100 = (408 − 352) / 408 × 100 = 13.7 %.
Deviation from nominal = (352 − 415) / 415 × 100 = −15.2 %.
The permissible variation at an LT consumer terminal is ±6 % of declared voltage. On either basis - 13.7 % regulation or −15.2 % from nominal - the installation is grossly non-compliant, and the motor is running at about 85 % of rated voltage.
Judgement: this is not a marginal case for a tap change. A 15 % depression explains both the hot motor (current rises for the same load) and the chattering contactor, whose coil is also undervoltaged and drops out during starting. The source of the drop must be found before any hardware is changed.
2. Estimate the voltage drop that the 95 sq mm conductor itself would produce at 24 A over 620 m, and use the result to decide whether the proposed full-length replacement is justified.
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For 95 sq mm aluminium, R ≈ 0.32 Ω/km at 20 °C and about 0.37 Ω/km at operating temperature. For 0.62 km, R ≈ 0.23 Ω.
Balanced three-phase drop = √3 × I × R = 1.732 × 24 × 0.23 ≈ 9.6 V, about 2.3 %, plus a small reactance term - say 11–13 V in total.
But the measured drop is 405 − 352 = 53 V from the transformer bus (56 V from the pump's no-load value). So roughly 40–45 V, about 80 % of the loss, is not distributed along the conductor at all. It is concentrated at a high-resistance point: a pole-top joint or jumper, a corroded or partly cut service conductor, an undersized tap, or a loose neutral/contact.
Replacing the whole 620 m at Rs 4 lakh would remove only about 3 V more and would leave the real fault in place, so it is not justified. Locate and repair the high-resistance point first; consider augmentation only if the calculated drop afterwards still exceeds 6 %.
3. List, in order, the checks you would carry out to locate the actual source of the excessive drop, and state the corrective actions.
Read the worked answer
Sequence:
- Measure voltage at the transformer LT terminals and at the first pole; a drop across the DT terminal-to-cable lug points to the termination itself.
- Walk the line measuring phase-to-neutral voltage pole by pole; the fault lies in the span where the reading steps down sharply.
- Measure the drop across individual joints (line side to load side) under load, or compare adjacent pole voltages; confirm hot joints with a clamp meter or thermal imager.
- Clamp phase and neutral currents at several poles: unbalanced phase currents with a low or zero neutral current indicate a broken or shared neutral, which also causes voltage swings.
- Check the service jumper and pump cable size, then measure voltage at the starter line side against the motor terminals to separate the service drop from the motor-cable drop.
Corrective actions: remake the faulty joints with proper aluminium compression connectors and bimetallic lugs, cleaned and correctly torqued, and re-test; replace the undersized service jumper with a conductor adequate for 24 A and rebalance the single-phase loads across the LT phases; re-measure the full-load voltage. Only if regulation is still above 6 % after this should conductor upgradation, a second transformer or a shifted tap be planned.