October 21, 2002

S&C’s Power Systems Services Performs Load Test on Large Rectifier Unit.

A manufacturer of large rectifier assemblies recently turned to S&C’s Power Systems Services to perform a load test on a large rectifier assembly. This unit was larger than their typical rectifier assemblies and the electrical service at their facility would not provide enough line current to perform the load test in-house. The load test was required to confirm that the vital components of the system were functioning properly prior to shipment to an overseas customer.

The output of the rectifier was capable of delivering up to 20,000 amperes in the current-regulate mode. Input voltage to the unit was dual rated at 360 VAC, 50 Hz, 3-phase or 480 VAC, 60 Hz, 3-phase. Forced water provided cooling to the SCR’s, secondary windings, and a heat exchanger.

What was monitored during the test?

Prior to testing, four 350 kcmil copper conductors per phase were connected to the rectifier input circuit breaker. In addition, a full-size ground was connected from the frame of the rectifier to the ground bus in a 480 VAC, 3Ø, 4W, 1600 ampere switchboard that served as the source for this test. Each set of conductors passed through window type current-transformers to measure line currents during the test; three (3) single-phase instrument transformers measured the source voltages. The output signals from the current-transformers and instrument transformers were connected to a data acquisition system. The output voltage and current of the rectifier as well as a host of other signals were also recorded by the data acquisition system during the test.

What tests were performed?

The main power transformer in the unit had a delta primary winding; the secondary windings were configured in a double-wye arrangement with an inter-phase reactor. Two SCR’s were connected in each of the secondary windings, yielding a total of twelve SCR’s for rectification. During normal operation, two of the SCR’s conduct simultaneously. In order to ensure that each of the SCR’s were properly conducting prior to the high-current test, one-half of the SCR’s were disabled by disconnecting the gate leads. The power supply was energized and six output pulses per line cycle were observed on the data acquisition system. The gate leads were reconnected and the other six SCR’s were disabled by lifting their gate leads. The procedure was repeated again and six output pulses were observed from the companion SCR’s.

Next, a current regulation test was performed by placing a fixed resistance across the output terminals of the rectifier. The power supply was energized and the operator was able to manually regulate the output current of the power supply through the simulated load. A computer-based controller will be utilized by the end-user for precise current regulation.

The final test consisted of a high-current test where eleven sets of 600 kcmil copper conductors were placed across the output terminals of the rectifier. The unit was energized and the output of the rectifier was ramped up to its maximum output of 20,000 amperes; there, it remained running until the temperature of the cooling water rose to the point where it no longer provided adequate cooling and sensors in the rectifier assembly automatically shut-down the unit.

Results

The load testing in S&C’s Laboratory verified that the vital components of the system were functioning properly and the unit was ready for shipment to the customer. Engineers from Power Systems Services provided a comprehensive test report that included the test program, a one-line diagram, test photographs, along with current and voltage snapshots. In addition, a harmonic analysis of the line scurrent was included in the report.