Andy Pye discovers how a German test house uses an asynchronous motor with frequency converter to increase energy efficiency of its excitation generator.
The Institute for International Product Safety GmbH (I²PS 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 laboratory uses 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 (which boasts one of the world’s most efficient surge generator 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 conducted .
The surge generator features a special control device, which 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 original 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 wastes energy. The slip ring rotor motor was already over 40 years old, so maintenance and spare parts were becoming issues.
An alternative system needed to function reliably, be energy efficient and allow restarts without long waits. In collaboration with power management company Eaton, I²PS developed an asynchronous replacement for the slip ring motor. It is driven by an electronic drive unit consisting of a frequency converter and regenerative braking unit.
In this instance, 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.
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; now, standstill is reached within two minutes, while from a standstill, normal speed is restored within two minutes.
The upstream LCL (inductance-capacitance-inductance) filter corrects any waveform distortions on the output voltage, thus producing clean power with low harmonics. The Total Harmonic Distortion (THD) is below 5%. For comparison, normal six-pulse frequency converters have a THD of approximately 35 to 40%. With power regeneration, the system does not need large oil-filled resistor banks for removing excess energy.
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 also 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 I²PS, fed back into the grid.
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 in the low voltage range with test currents up to 300kA are therefore possible. In the case of lengthy test setups, it is 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.
“The ability to stop and start the surge generator as and when needed, is a decisive advantage of the modernisation for us,” said Klaus Heidelberg, manager Energy/Switching Power, I²PS. “Moreover, the new drive is much more energy-efficient than the old slip ring motor. The total energy saving is 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.”
Seven common EMC immunity tests
Magnetic Field Testing: simulates the effects of a magnetic field. Typical devices requiring this type of immunity testing include CRT monitors and electrodynamic microphones.
Voltage Drop Testing: tests the impact of a sudden voltage dip or other power interruption on a piece of equipment. This replicates the effects of brownouts, as well as the normal fluctuations in the AC mains network.
Surge Immunity Testing: electromagnetic surges can be caused by indirect lightning strikes and routine power switching events. Can lead to arcing, cabling breakdowns and motor damage. Requires specialised EMC test equipment.
Conducted Immunity Testing: simulating potential disturbances from other devices powered by the same power network, or inductively coupled onto its I/O lines. Uses CDNs, BCI probes and direct voltage injection equipment.
Radiated Immunity Testing: evaluates functioning of a device when exposed to different electrical field sources, such as mobile phones, microwaves and Wi-Fi routers.
Electrostatic Discharge (ESD): occur as a result of built-up static electricity. Can cause IC port damage, communication failures and damage to LCD screens. Performed with EMC test equipment that creates a short burst of energy, typically at 4kV and 8kV.
Electrical Fast Transient (EFT): disturbances caused by inductive load switching on the power grid. A burst generator is used to mimic the effects of electrical switches, motors and relays and fluorescent lamp ballasts.