Drive system upgrade saves energy in low-voltage equipment test lab

The Institute for International Product Safety GmbH (I²PS) in Bonn, Germany specialises in the testing of low-voltage switchgear and electric power systems. Its testing facility boasts one of the world’s most efficient surge generator systems. During the latest system upgrade, the company decided to also replace the existing exciter generator with an asynchronous motor with variable frequency drives from Eaton. This not only increased the energy efficiency of the system, but also its overall flexibility.

Background

I²PS carries out tests and trials of electrical and electronic equipment in accordance with the latest national and international safety and environmental standards. Its DIN EN ISO/IEC 17025-accredited test lab is based in Bonn, Germany, and relies on cutting-edge testing equipment and measuring techniques. The company provides testing services for industrial low-voltage devices and systems, the safety of electrical equipment and electromagnetic compatibility (EMC), as well as environmental tests. It also offers short-circuit and internal arc testing of low-voltage equipment with currents of up to 300 kA.

A surge generator with a separate excitation system provides the high levels of energy required for testing the equipment. And a special control device makes it possible to adjust the surges generated by the exciter, even whilst the short-circuit current is still present. In this way, a constant short-circuit current is available, both for shorter periods in the range of 100 ms and for periods up to a few seconds. This makes it possible to reproduce the short-circuit conditions required for testing generators or transformers in line with international standards. Customer success story I²PS - Institute for International Product Safety GmbH Market segment Machine building

Challenge

Previously, the exciter of the surge generator was driven by a slip-ring motor with oil-cooled starting resistors. These oil-cooled resistors heated up considerably during start-up, whilst the energy released during braking was also transferred to the oil. The motor could therefore only be restarted after a long cooling phase. To save time, the surge generator was thus kept up and running even during periods when the test configuration in the lab was being modified. This, however, wasted a lot of energy. In addition, the slip-ring motor was already more than 40 years old, meaning it required a lot of maintenance and the procurement of the necessary spare parts was becoming increasingly expensive.

For these reasons, I²PS was looking for a more modern drive solution. The new drive system had to meet the following three requirements: it needed to be reliable and energy-efficient and enable restarts without long waits. Moreover, in the event of a breakdown or malfunction, the supplier had to guarantee an adequate level of support. 

Solution

In collaboration with Eaton, I²PS therefore decided to replace the old slip-ring motor with an asynchronous one. The new motor is driven by an electronic drive that consists of a frequency inverter with Active Front End (AFE) regenerative braking unit.

Both are devices from Eaton’s comprehensive 9000X series, which includes regenerative units and drives with power ratings from 0.55 kW to 2,750 kW, at 460 V and 690 V, respectively. The devices combine a compact, modular design with versatile functionalities, including a quick start wizard and a wide range of communication and configuration options. Five slots are available for I/O and communication cards to accommodate customer-specific requirements. The devices support various communication protocols in order to facilitate the connection to automation systems based on Modbus/TCP, Modbus/RTU, Profibus DP or Ethernet/IP. In addition, the modular separation of the power and control units simplifies installation and reduces the need for spare parts.

The I²PS system relies on the SPI300A0-4A3N1 drive and the air-cooled SPA300A0-4A3N1 regenerative unit. The SPA AFE unit is a bi-directional voltage converter for the front end of a common DC bus. It converts AC voltage/current to DC voltage/current. Power is transferred from the mains to the DC bus or vice versa.

When used to drive the exciter of the surge generator, the Eaton regenerative unit feeds the kinetic energy that is released during braking back into the grid. Previously, when the exciter was shut down it only came to a complete standstill after about 30 minutes. With the SPA regenerative unit, the exciter now comes to a stop within two minutes, and will reach its nominal speed again in the same period of time.

The upstream LCL filter corrects any waveform distortions on the output voltage, thereby producing clean power with low harmonics that can be returned to the grid. The total harmonic distortion (THD) is below 5 %. For comparison, normal six-pulse frequency inverters have a THD rate of approximately 35 % to 40 %. Thanks to power regeneration, the system no longer requires large oil-filled resistor banks for removing the excess energy. Eliminating the resistors not only increases energy efficiency, but also simplifies the design.

Theoretically, a brake chopper could also be used as an alternative to speed up the deceleration of the exciter. However, this would convert the kinetic energy into heat, which would again have to be dissipated. This type of braking system would also need to be quite large and thus take up a lot of space. As a result, the system installed at I²PS is not only more energy-efficient, but also saves space compared to this common but more complex technology.

The SPI inverter is a bi-directional, DC-fed drive for controlling AC motors. The drive has a DC bus coupling and also makes it possible to operate the drive system in generator mode. In this way, the braking energy of the drives can either be transferred directly to a motor drive via an intermediate circuit coupling or, as is the case at I²PS, fed back into the grid. Like the regenerative unit, the air-cooled drive also comes with IGBT technology and an alphanumeric keypad. It complies with EMC class T (EN 61800-3 for IT networks), as well as the CE and UL safety standards.

Results

The upgraded exciter drive system forms part of a surge generator application with a peak short-circuit voltage of 298 MVA. This enables short-circuit testing in the low-voltage range with test currents up to 300 kA. In the event of time-consuming test set-ups, it is now possible to stop the surge generator to save energy, without losing time. Prior to the upgrade, the generator could only be brought to a stop three times a day on account of the oil-cooled starting/braking resistors. With the new drive system, the long cooling phases necessitated by the oil-based technology are a thing of the past. The braking energy of the exciter motor is now fed back to the grid. In the event of time-consuming modifications of the test set-up, the system can thus be easily stopped and quickly restarted. For the test lab, this offers the added advantage of greater flexibility.

Klaus Heidelberger, Head of Energy/Switching Capacity at I²PS, sums up the benefits as follows: “For us, a decisive advantage of the upgrade is the ability to stop and restart the surge generator as often as required. What’s more, the new drive system is much more energy-efficient than the old slip-ring motor. Three different factors contribute to the overall energy savings: the energy-efficient drive system, the energy saved by switching off the surge generator during start-up, and the ability to feed the braking energy, which was previously lost as heat, back into the grid. In addition, Eaton provided indispensable customer service during the installation and commissioning phase.”