Selecting 6.6 kV or 400 V Gensets for a Cement Mill

Use 6.6 kV generation as the screening preference because the largest individual load is the 2.8 MW, 6.6 kV mill motor. This arrangement supplies the motor bus without a step-up transformer and transforms only the 400 V auxiliary load. Confirm the choice with load, fault-level, transformer, protection, grounding, and lifecycle-cost studies before specifying the gensets.

Load and Operating-Mode Definition

1. Confirm whether the motor's 2.8 MW rating is electrical input or mechanical shaft output. If it is shaft output, calculate electrical input as P_input = 2.8 MW / efficiency using the motor datasheet efficiency.
2. Record the motor power factor at the intended operating point. Calculate running apparent power as S_motor = P_input / power factor. Do not size the generator or transformer from MW alone.
3. Measure the 1.5 MW standing load at the 400 V bus and record its kVA, power factor, harmonic content, largest load step, and expected growth.
4. Define the utility-parallel operating mode. Record the generator base-load target, permitted utility import or export, required spinning reserve, and the response required after a generator trip.
5. Confirm that the utility remains connected while the mill starts. Utility support can absorb the starting transient, but the generators must still carry their assigned steady-state kW and kvar after acceleration.

Do not move on until the load schedule distinguishes running MW, running kvar, motor starting demand, intermittent loads, and contingency loads. The present real-power total is 2.8 MW + 1.5 MW = 4.3 MW before motor-efficiency correction, transformer losses, reserve, or future capacity.

Generator Capacity Branch

1. If installed MW is below the corrected running demand, reject that combination.
2. If MW passes, compare the summed generator kVA rating with the calculated motor and auxiliary kVA. Check each alternator's allowable kvar loading on its capability curve.
3. Repeat the calculation with one generator unavailable if the process must continue after a unit trip. A three-unit plant that requires all three units for normal demand has no generator redundancy.
4. Check the load-sharing controller and utility interface for stable real- and reactive-power sharing. Base-load control governs kW; excitation and power-factor or kvar control govern reactive loading.

400 V Current and Fault-Level Branch

At 400 V, megawatt-class generators impose very high continuous current before fault duty is considered. Assuming three-phase output and unity power factor only for a lower-bound current calculation:

Actual current is higher when power factor is below unity. This current drives busbar size, breaker frame size, cable quantity, termination layout, heat loss, and installation space.

1. Obtain each alternator's subtransient reactance and grounding configuration from the selected manufacturer.
2. Calculate the contribution from every generator that can be paralleled, plus the utility and connected motors. Evaluate phase and ground faults for each operating configuration.
3. Compare calculated making, breaking, short-time, and peak duties with every switchboard and breaker rating. A preliminary estimate of about 85 kA was identified for a solidly grounded 400 V line-to-ground case, but it is not a design value; the selected alternator data and network model decide the duty.
4. If the 400 V switchboard cannot interrupt and withstand the calculated fault, reject a common LV bus or redesign the architecture with separated buses, generator transformers, current-limiting impedance, or another engineered arrangement.

Placing each LV generator directly on its own transformer and locating common switchgear on the 6.6 kV side can remove the large common 400 V switchboard. It does not remove the need to rate generator leads, transformer connections, protection, and grounding for their local fault duties.

Transformer Architecture Branch

1. Calculate transformer kVA from the corrected load kW and power factor, then add cooling, ambient, harmonic, load-cycle, and growth requirements defined by the project.
2. For the 6.6 kV option, evaluate the proposed 2 MVA, 6.6 kV/415 V transformer against the measured auxiliary kVA and required reserve.
3. For the 400 V option, evaluate the proposed 4 MVA step-up transformer against motor running kVA and operating duty.
4. Resolve the proposed 400 V/6.9 kV ratio before procurement. The stated motor voltage is 6.6 kV; select the ratio and taps from the actual bus-voltage study rather than carrying the 6.9 kV value into the specification.
5. Compare transformer impedance, losses, voltage regulation, inrush, vector group, neutral treatment, protection zones, and energization sequence. Transformer impedance changes both motor-bus voltage performance and fault current.

Protection, Arc-Flash, and Cost Branch

Purchase price alone does not identify the lower-cost system. Compare complete installed architectures over the same operating period.

1. Price generators, transformers, switchgear, bus duct or cables, terminations, protection relays, metering, grounding equipment, civil space, ventilation, commissioning, and spares.
2. Calculate losses at the expected base-load point. The 6.6 kV arrangement avoids transforming the 2.8 MW motor branch, while the 400 V arrangement avoids transforming the 1.5 MW auxiliary branch.
3. Perform protection coordination for utility-parallel, generator-only, transformer-energization, motor-starting, and generator-trip states. Verify that normal current and transformer inrush do not overlap the required protection operating regions.
4. Complete arc-flash calculations for both voltage levels using the final fault current and clearing time. Lower voltage does not by itself guarantee lower incident energy; clearing time and available energy can dominate the result.
5. If both designs pass technically, select the lower lifecycle cost. A preliminary comparison may assign a premium to a medium-voltage genset, but manufacturer quotations and installed-system costs decide the branch.

Resolving Procedure and Final Verification

1. Freeze the corrected motor input kW, motor kvar, 400 V bus kVA, load steps, and contingency requirement.
2. Reject generator combinations that fail either MW capacity, alternator kVA, capability-curve, or unit-outage requirements.
3. Model both one-line diagrams: generators at 6.6 kV with a 6.6 kV-to-400 V auxiliary transformer, and generators at 400 V with a 400 V-to-6.6 kV motor transformer.
4. Run load flow, motor-starting, short-circuit, grounding, protection-coordination, harmonic, and arc-flash studies for every credible operating state.
5. Select 6.6 kV generation when both options pass and its reduced LV current, smaller transformed load block, and installed lifecycle cost remain favorable. Select 400 V only when its common bus or individual-transformer arrangement passes the same studies and produces the better total result.
6. During commissioning, parallel the gensets and utility, raise generator loading to the specified base-load target, start the mill with utility support, and record bus voltage, frequency, generator kW, kvar, current, power factor, transformer loading, and utility interchange. Do not accept the system until every reading remains within the approved equipment capability and study limits after the motor reaches steady speed.

Frequently Asked Questions