Checking a Flour Mill's Energy Bill, CT Ratio and Power-Factor Penalty
A flour mill is a low-tension three-phase consumer with a sanctioned load of 20 kW. Its meter is connected through three 100/5 A current transformers, and the CT multiplier has been entered as 20 in the billing system. Over a 30-day billing period the meter register advances from 04512.0 to 04896.0 units; the recorded average power factor is 0.84 and the maximum demand indicator shows 44 A in the worst phase. The applicable tariff is a fixed charge of ₹150 per kW of sanctioned load per month, an energy charge of ₹7.50 per unit, and a power-factor penalty of 1% of the energy charge for every 0.01 by which the average power factor falls below 0.90 (no penalty and no incentive at or above 0.90). The owner has been quoted ₹9,000 for a switched capacitor bank, including contactor and installation, and asks two things: whether the quoted bill is correct, and whether fitting the capacitor bank, and replacing the CTs, are worth the money.
1. Calculate the energy consumed and the components of the monthly bill, and check whether the consumption is plausible against the maximum demand.
Read the worked answer
Meter advance = 04896.0 − 04512.0 = 384 meter units. With a 100/5 A CT the multiplier is 100/5 = 20, so energy = 384 × 20 = 7,680 kWh. Energy charge = 7,680 × ₹7.50 = ₹57,600. Fixed charge = 20 kW × ₹150 = ₹3,000. Power factor 0.84 is 0.06 below 0.90, i.e. six steps of 1%: penalty = 6% × ₹57,600 = ₹3,456. Bill = 57,600 + 3,000 + 3,456 = ₹64,056 (before electricity tax or any demand/excess-load charge). Plausibility: 7,680 kWh/720 h = 10.7 kW average, reasonable for a mill of this size; average current = 10,700/(√3 × 415 × 0.84) = 17.7 A, consistent with a maximum demand of 44 A. Note that 44 A corresponds to √3 × 415 × 44 = 31.6 kVA, which at pf 0.84 is 26.6 kW - above the 20 kW sanctioned load, so a demand or excess-load charge should be checked separately, and it is not part of this bill.
2. Calculate the capacitor bank required to reach a power factor of 0.92, and evaluate the payback.
Read the worked answer
Average P = 7,680/720 = 10.67 kW. At pf 0.84, φ = 32.9°, Q1 = 10.67 × tan 32.9° = 6.90 kvar. At pf 0.92, φ = 23.1°, Q2 = 10.67 × tan 23.1° = 4.55 kvar. Compensation = 6.90 − 4.55 = 2.35 kvar → select a standard 2.5 kvar bank. Because 0.92 is above 0.90, the penalty disappears entirely: saving ≈ ₹3,456 per month. Against ₹9,000 installed cost, payback is about 2.6 months - clearly worthwhile, provided the mill keeps running near this average load. Checks before ordering: the bank must be switched with the load by a contactor, otherwise at light load the installation becomes leading and may attract a new penalty under a kVARh tariff; the capacitor current is about 2.5 kvar/(√3 × 0.415 kV) ≈ 3.5 A, so the protection and cable must be sized for it and for the switching inrush; two steps (1 + 1.5 kvar) are better than one block where the load varies widely.
3. Assess whether the 100/5 A CTs and the multiplier are appropriate, and what you would recommend.
Read the worked answer
Average current is 17.7 A, so the secondary current with a 100/5 CT is only 17.7/20 = 0.88 A, i.e. about 18% of the 5 A rated secondary; the maximum demand of 44 A gives 2.2 A (44%). An accuracy-class 1.0 or 0.5S meter is only guaranteed over roughly 5–120% of rated current, so at 18% the error can be significant, and if the mill ever grows past 100 A the CT saturates and under-records - a billing dispute waiting to happen. The multiplier of 20 is mathematically correct for 100/5; common errors are entering 5 (the secondary rating) or 100, which would change the bill by a factor of 4 or 5, so verify the CT/PT ratio setting on the meter against the CT nameplate and the seals. Recommendation: replace with 50/5 or 75/5 class 1.0 CTs (average secondary then 1.8 A or 1.2 A) with a matching meter, reseal and re-verify the multiplier. Also address the demand side: 44 A = 31.6 kVA = 26.6 kW at 0.84 pf exceeds the 20 kW sanctioned load, so either the sanctioned load must be revised or the load pattern corrected.