Drives & Controls Magazine September 2023

40 n MAINTENANCE, SERVICE AND REPAIRS September 2023 Drives freeze out grid failures at Omani meat plant Screw compressors are increasingly used in refrigeration applications because of their high efficiency and reliability compared to other types of compressor. However, one user of such systems – a frozen meat products facility in Oman – was facing serious issues with its installation due to its inadequate power grid capacity. In facilities of this type, failure of the refrigeration system can be catastrophic, with potentially devastating losses. Even a short period of downtime can render whole batches of product unusable – leading to increased waste, safety worries and additional costs. According to a study by Enterprise Strategy Group, equipment downtime can cost food manufacturers €30,000 to €50,000 per hour. The Omani plant was using soft-starters to start its heavy-duty screw compressors on the main process line, rated at around 400kW and 355kW. Because of the soft-starters’ high starting currents, they were stressing the main power line. Because one soft-starter was started after the other, this was causing power supply blackouts due to the inadequate capacity of the power grid. This, in turn, was causing was resulting in major financial losses for the facility. The industrial refrigeration specialist which had supplied the installation approached the drives and motors supplier WEG for help with finding an answer to the problem. A team of engineers visited the site to perform a site study and to analyse the soft-starter capacity, panel design and available space. Following this, a power system study was performed to understand the grid capacity and the nature of other loads connected to the network. This revealed that the issues were wider than at first thought. It was not simply a problem related to the current rising at startup, but current rises were also observed during overloads. Because screw compressors are heavy-duty applications, the grid was being exposed to potential problems whenever there was a rise in load demand after starting the motors. A report was prepared, based on the investigation. The ideal way to solve the issues would have been to upgrade the grid capacity, but this would have required a huge investment. The end-user was not ready for such an upgrade and wanted a more costeffective answer to the problem. So WEG suggested replacing the soft-starters with variable-speed drives (VSDs) with a current ramp technology that limits the current rise within specified values not only during starting, but also during peak load demands. This allows the drive to control and manipulate the current ramp so that the overload demand is still taken care of, without the current rising, thus solving the blackout issues. WEG CFW11 drives were chosen. “Aside from the core technical challenge of mitigating the current rise, the facility wanted a compact solution,” explains Darshan Shejekar, development sales manager for low-voltage automation at WEG Middle East. “An additional challenge was the VSD input harmonics. Harmonics must be taken into consideration when dealing with VSDs and are an especially critical consideration when powered by a generator. This was typically the case here, because the client relied occasionally on a diesel generator set. To combat this, we considered installing a suitable input harmonic filter at the input to comply to IEEE 519 and to limit the VSD harmonics. This was equally important to help the main grid during normal operation.” A further challenge was that compressor motor was not suitable for VSD operation. This was solved by using sinusoidal filters on the output side, thus ensuring a long motor life, avoiding winding and bearing failures, and mitigating issues related to the long cable distances involved. This allowed the new drives to be used with the existing system. WEG was involved at all stages of the project, including consultation, commissioning, testing and installation. It also performed factory acceptance tests, followed by commissioning and site acceptance tests. The facility’s power quality issues were solved cost-effectively and quickly, increasing the refrigeration system’s reliability and efficiency. Two months after the first order, the facility ordered another four panels with input harmonic filters and output sine filters. n If critical equipment such as refrigerators fail at food plants, the losses can be huge. This was the issue faced by an Omani meat processor when soft-starters on its screw compressors caused its power supply to trip. The problem was solved by installing drives incorporating a current ramp technology. Upgrading the equipment at the Omani meat plant has ended the problems it had been suffering with its power supplies