Drive system upgrade saves energy for low-voltage equipment lab

The Institute for International Product Safety GmbH (I ²PS GmbH) carries out tests and trials in the electrical engineering, electronics and environmental specification fields in accordance with national and international standards. Based in Bonn, Germany, the test laboratory uses the most modern testing equipment and measurement techniques and holds DIN EN ISO/IEC 17025 accreditation. Testing services include the areas of industrial low-voltage devices and systems, the safety of electrical equipment and electromagnetic compatibility (EMC), as well as environmental tests. The testing of equipment for low-voltage technology with short-circuit and internal arc tests with currents up to 300kA are also included in the services.

The high energy required for the experiments is provided by a surge generator with a separate excitation system. A special control device allows the adjustment of the impulse excitation generated by the exciter, even during short-circuits. In this way, a constant short-circuit current is available with short current flow times in the range of 100ms and with longer current flow times up to a few seconds. This makes it possible to reproduce the short circuit conditions required by the standards to test the generator or transformer.

The exciter of the surge generator was driven by a slip ring motor with oil-cooled starting resistors. These oil-cooled resistors heat up markedly during start-up; similarly, when the motor decelerates, the braking energy heats up the oil. Before restarting the motor, the machine operator had to wait a while to allow the oil to cool down. To save this time, often the surge generator was kept running during longer conversion phases of the experimental set-up. However, this wasted a lot of energy. On top of this, the slip ring rotor motor was already over 40 years old; so the maintenance and procurement of the necessary spare parts were becoming more difficult and very expensive.

For these reasons, I ²PS was on the search for a modern drive system. It was determined that the new system must fulfil the following criteria: it should function reliably, be energy efficient and allow restarts without long waits. Moreover, in the event of a breakdown or malfunction, a sufficient repair service level must be provided by the supplier.

Active Front End

Against this background I ²PS developed, in collaboration with Eaton, a system that replaces the slip ring motor with an asynchronous one. This is driven by an electronic drive unit consisting of a frequency converter and regenerative braking unit (AFE, Active Front End).

Both components belong to Eaton's 9000X series; this extensive range of DC bus drives includes several regenerative units and frequency converters with power ratings from 0.55 to 2750kW at 460 and 690V. They follow a modular compact design concept and providing versatile features, including quick start, flexible communication and extensive configuration options. Five slots are available for optional I/O and communication cards to accommodate customer-specific requirements. Various communication protocols are supported to allow connection to a wide range of automation systems based on Modbus/TCP, Modbus/RTU, Profibus DP and Ethernet/IP. In addition, the modular separation of the power and control units simplifies installation and reduces the need for spare parts.

In the case of the I ²PS system, the SPI300A0-4A3N1 inverter unit and the air-cooled regenerative SPA300A0-4A3N1 unit are used. The AFE SPA is a bi-directional voltage converter for the front-end of a common DC bus. It converts alternating current or voltage in direct current or voltage. The power is transmitted from the grid into the DC bus or vice versa.

When used on the exciter of the surge generator, the Eaton-Regenerative unit supplies the kinetic energy that is released during braking back into the grid. Previously, when the exciter was shut down it came to a complete standstill after about 30 minutes; with the SPA-regenerative unit, however, standstill is reached within 2 minutes. Equally, from a standstill, it's back up to normal speed within 2 minutes.

The upstream LCL (inductance-capacitance-inductance) filter corrects any waveform distortions on the output voltage, thus producing clean power with low harmonics that is suitable to be returned to grid. The Total Harmonic Distortion (THD) is below 5 per cent. For comparison, normal six-pulse frequency converters have a THD of approximately 35-40 per cent. With power regeneration, the system does not need large oil-filled resistor banks for removing excess energy. Eliminating the resistors increases energy savings while simplifying design.

Theoretically, a braking chopper can be used as an alternative to speed up the deceleration of the exciter. However, this would convert the kinetic energy into dissipated heat again. This type of braking system would be generously sized and therefore requires a lot of space. By contrast, the approach favoured by I ²PS is not only more energy efficient, but also more space saving than this quite common, but significantly more complex technology.

Regenerative operation

The SPI inverter used is a bi-directional DC-fed drive for controlling AC motors. The inverter has a DC bus coupling and also allows the user the regenerative operation of a drive system. In this manner, the braking energy of the drives can be directly transmitted to a motor driven drive via an intermediate circuit coupler or, as in the case of I ²PS, fed back into the grid. As with the regenerative unit the air cooled inverter also uses IGBT technology as well as an alphanumeric keypad and corresponds to the EMC class T (EN 61800-3 for IT networks), as well as the safety standards CE/UL.

The upgraded drive system of the exciter is now part of a surge generator plant with a peak short-circuit power of 298MVA. Short circuit tests for in the low voltage range with test currents up to 300kA are therefore possible. In the case of lengthy test setups, it's now possible to stop the surge generator to save energy, without losing time. Before modernisation, this was only possible a maximum of three times a day due to the oil-cooled starting or braking resistors. The necessary cooling phases required for the intensely heated oil are eliminated thanks to the new drive system. The braking energy of the exciter motor is now fed back to the grid. Therefore, the system can be stopped easily in the case of time-consuming conversion work of the experimental set up, and then quickly restarted. In parallel, the flexibility of the testing laboratory has increased.

Klaus Heidelberg, manager Energy/Switching Power, I ²PS, drew the following conclusion: "The ability to stop and start the surge generator as and when needed, is a decisive advantage of the modernisation for us. Moreover, the new drive solution is much more energy efficient than the old slip ring motor. The total energy saving is thus achieved in three ways - the energy efficient drive system, the energy saved by switching off the surge generator during set up and the return feed of the braking energy into the grid, which was previously lost in the form of heat. In addition, the Eaton customer service provided during the installation and commissioning phases was indispensable."

For more information, visit www.eaton.com.