Power Transmission Case Study

Safety is always the primary concern when it comes to designing and implementing test solutions for electrical and electronic products. The test operator must of course be shielded from potential harm, but thought must also be given to protecting the unit under test (UUT) and the test equipment itself from damage should there be a fault in the product, or indeed the test system. For products used in the power generation and distribution industry, the requirements for ensuring safety are somewhat amplified. When ATE Solutions was chosen to provide test equipment for a large National Grid supplier, the implications of handling both high voltages (many kV) and high currents were uppermost in the design considerations. The products to be tested were part of a Voltage Source Converter, a type of High Voltage Direct Current (HVDC) power transmission system that converts AC to DC for transmission, and back again. Naturally, as a National Grid project, security and confidentiality were also key project requirements. ATE Solutions FLEX test platforms provide some inherent design features that help safeguard the operator, equipment and UUT as standard. For example, the mains supply to the system has to be ‘armed’ to switch power on to the test instruments. An ‘emergency stop’ switch is also provided alongside an unswitched mains supply within the system. This allows power supplies and instruments to be controlled safely, for example by allowing the control PC to remain on whilst shutting down supplies to the UUT in event of an emergency stop. These features are all included in the system’s Power Management Unit (PMU). For this project, some of the power to the UUT was provided by a separate High Voltage (HV) supply in the ATE, with additional safety interlocks that ensured operation only if the test fixture was properly engaged and the test programme operational. Whilst the ATE system was configured with the resources required to test several kinds of product, each type required a separate test ‘application’ comprising a fixture and software. The sequencing of the tests in the application software was very important. For most functional tests, it is usual to perform checks such as continuity (opens and shorts) first to ensure that it is safe to then apply power to the UUT. For high power products of course, this is even more important. Very careful control of current limits and monitoring of voltages was required as power was applied to, and measured from, the UUT. Part of the test specification required fault simulation to ensure that protection circuits, safety switches and alarms were activated within the required time of the ‘fault’ occurring. Again, this required very precisely timed switching and measurement due to the fast reaction times required. Consideration was also given to discharging the product and test system so the UUT could be safely removed from the tester, and the system prepared to a known state for the next test. Bespoke circuitry was required in the fixture to ensure rapid, controlled and safe discharge after testing. When switching diverse levels of currents and voltages, different types of relays are required. For example, for low voltage sensing, relays were chosen with minimal wetting currents to ensure reliable activation and measurement. The relays selected to switch large currents would have been inappropriate for sensing due to the relatively high wetting currents required for operation. Power distribution systems are not entirely about high voltages and currents. For this project, fibre optics were used as the main communications interface to the product due to their immunity to electrical interference. The test system was designed such that the control PC within it could communicate through the test system interface to the application fixture and then on to the UUT. The ATE Solutions FLEX system uses Virginia Panels Corporation (VPC) interfaces that provide a highly reliable and flexible interface between the test system and application fixture. Within a range of different connector housings, a wide variety of power and high-integrity signal contacts (pins) are available including some for fibre optic connections, as well as those suitable for high voltages and currents. This provided the ideal interface solution for these applications.

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