Downhill Conveyor Regeneration: Specify the Transformer and Grid Interface

1. Define the duty

Size the downhill conveyor transformer from the current and reactive-power envelope in both motoring and regenerative operation, not from net energy imported over a shift. Negative active power reverses energy flow but does not reverse copper loss. The drive, transformer, feeder, protection and utility connection must support the same operating envelope. ABB identifies downhill conveyors as electrical braking applications; whether energy reaches the grid or a braking resistor depends on the actual converter arrangement. A conveyor that descends geometrically is not necessarily regenerative at every load and speed.

Primary references — ABB power regeneration

2. Required design inputs

Obtain the belt elevation profile, throughput range, rolling and mechanical losses, acceleration ramps, empty-belt duty, emergency stopping philosophy and simultaneous drives. From the converter supplier obtain signed P-Q points, RMS line current, harmonic spectrum and permitted supply-voltage range for each state. State the sign convention at the point of connection, because drive screens and utility meters may use opposite signs. Include auxiliary loads on the same transformer and identify whether export is allowed when other mine consumers are unavailable.

3. Illustrative engineering example

Illustrative fundamental-frequency point: the transformer transfers −1.20 MW active power with 0.40 Mvar reactive power at 6.6 kV. The negative sign denotes export from the conveyor bus. The apparent-power and current results below are positive magnitudes. They describe a single balanced operating point, not a nameplate recommendation. Converter losses, harmonic current, ambient temperature, altitude and overload duration must be added through the agreed supplier model.

4. Compare failure cases

Compare loaded downhill running, empty-belt motoring, controlled stop and grid loss separately. A regenerative front end needs a receiving electrical system; a network outage removes that path. Agree the drive’s response with the mechanical braking and process safety designers. A resistor or DC-link energy strategy, if provided, has its own finite duty and cannot be assumed to absorb indefinite downhill production. Transformer differential protection remains a fault function; directional feeder or reverse-power logic needs explicit interpretation of normal export.

5. Procurement evidence

The RFQ should include continuous and short-duration P-Q envelopes, frequency, winding connection, impedance, harmonics, tap range, cooling availability and grid export conditions. Request a transformer loss assessment using actual converter current, rather than a single motor shaft kW. An active front end may reduce some harmonics but its product claims do not remove the requirement to examine the installed network, parallel converters and capacitor banks.

Primary references — ABB harmonics

6. Commissioning and operation

Commission the metering sign convention and the transitions between motoring and generation under an approved plan. Compare recorded P, Q, voltage and current with the declared envelope, then verify export-control and drive-trip interfaces by the specified simulation or test method. Preserve settings and disturbance records. If the mine later adds a generator or islands a bus, revisit whether that source can receive energy; its kVA rating alone does not prove regenerative absorption capacity.

7. Frequently asked questions

Does export cool the windings? No. Current magnitude and spectrum still produce losses.

Can a generator always receive regenerated power? No. Check the generator, controls and energy balance.

8. References and related guidance

• Primary references — ABB power regeneration
• Primary references — ABB harmonics
• Primary references — IEC 60076-7:2018

Related engineering guidance:

• Regenerative transformer duty
• Harmonic impedance study