Automatic current transformer testing and certification

To improve competitiveness, Stadtwerke Düsseldorf AG intends to expand its efforts in the service sector in all its energy divisions. This also applies to the test centre for electrical measuring instruments, EH11, which is supported by the utility. Until now, the centre has been mainly concerned with random testing and certifying of measuring instruments. Since current transformers are such an important element of a metering and billing system, however, it is essential to test them all before installation, as well as during the service period on a sampling basis. The new service requirements mean that this task will increase continuously.

Stadtwerke Düsseldorf AG knew that it was no longer economical to use the existing test facility for these additional tasks, and decided to invest in a new, fully automatic transformer test system.

ZERA GmbH in Königswinter was awarded the contract to build the new, fully automatic test facility. In October 2000, ZERA supplied an automatic CT testing facility to the EH11 test centre, to meet the following specifications:

  • Calibrate all marketable low voltage bar type current transformers up to 1,500A from various manufacturers.
  • Minimise lead time for calibrating and setting up transformer test facilities.
  • Operate the transformer test facilities in a user-friendly manner.
  • Achieve rapid transformer test throughput, e.g. calibration test (certification) of 40 units/hour.

The utility reports that it can already see the practical implementation of this project. The number of calibrations of low-voltage transformers doubled in the year 2000.

Stadtwerke Düsseldorf AG expects an amortisation period of three to four years for the new facility, based on the anticipated figures for annual low-voltage transformer calibrations. This relatively short period fully justifies the economics of the investment. The utility is thinking of extending its test services to other utilities.

High-voltage, accuracy and interturn tests can be carried out on the system. Five current transformers can be tested simultaneously during a test cycle. The operator selects the appropriate type and specification of the unit under test from a test table, which can be generated for individual requirements and stored in the system. The tables form the basis for simple, reliable implementation of automatic test and calibration processes. After start-up, secondary contacts are automatically set to the correct distances.

In the next step, the tester begins the high-voltage test. The automatic test process creates pneumatic and electrical contacts and also inserts the format filling primary bar. The high voltage is then applied, and after the time specified in the test table the results are saved for use in the test report.

This is followed by an interturn insulation test and an accuracy test which is carried out at the second test compartment in the system. To facilitate handling, the magazine containing five test units placed in fixed slots is moved on rollers mounted on the platform of the machine.

The test process follows the test rules for current transformers as laid down by the German national testing and certification authority. 

Advantages and features of the test system at a glance:

  • Fully automatic test process.
  • High accuracy measurement and testing.
  • Test time reduced to about 4 seconds per measuring point.
  • Test currents up to 3,000A.
  • Configurable current slopes.
  • Easy to use through automatic, menu- controlled test process.
  • Automatic identification of pass/fail units.
  • Comprehensive statistical analyses.
  • Automatic data transfer to central computer.

The test system can also be equipped with label printers, higher currents (up to 5,000A), a manual workbench for individual tests, and tests up to 3 secondary windings.