Mining Operation Ensures Power Quality With Point-on-Wave Controlled Closing

Challenge

Southern Peru Copper Corporation, one of the largest copper producers in Peru, operates a transmission network that connects several substations. At one of the company’s substations, several 138 kV lines converge, and two 120 MVA, 138/34.5 kV power transformers operate in parallel.

Because the company’s rock crushing mills must work around the clock throughout the year, the transformers are rarely de-energized. When they’re re-energized after maintenance, the most important objective is to keep the mills in service.

The company’s existing control device for point-on-wave closing was malfunctioning, causing high currents and voltage drops each time one of the paralleled transformers was energized and resulting in the loss of sensitive loads, including the rock-crushing mills. 

Solution

Because it was impractical to isolate voltage disturbances by islanding the transformers during energization, Southern Peru Copper Corporation needed a new controlled switching solution. The company decided to evaluate an SEL-352 Breaker Failure Relay, which provides point-on-wave switching technology, as a replacement.

SEL engineers used the 1 MHz and 10 kHz three-phase voltage and current recording capabilities of an SEL-T400L Time-Domain Line Protection relay to measure the closing and opening times of the circuit breaker and determine the exact moment at which the breaker poles closed.

Then, having determined the optimal moment for closing, they set the new controller to perform the point-on-wave closing operation at the correct time.

Results

Testing performed during commissioning showed that the point-on-wave closing control provided by the SEL-352 Breaker Failure Relay reduced inrush current from 2.5 pu to less than 1 pu upon transformer energization. Voltage drops were minimal, well within the ride-through parameters of the rock crushing mills’ undervoltage protection.

The improved power quality has kept the mining company’s rock crushing mills online, preserving operations availability and preventing production and revenue losses.