Driving down operating costs with medium voltage drives

It’s no question that mining companies are constantly looking for ways to lower operational costs while still increasing productivity and efficiency.

This can be achieved by using medium voltage drives (MVDs).

According to Delta Energy Systems’ regional manager (industrial automation) David Bolt, MVDs have heavier lifting capabilities and further economies of scale in large industrial applications for use with HV motors.

They reduce the cost per kilowatt by operating at higher supply voltages, therefore reducing current flows for a given power output (according to the formulae power = volts x current). This also provides an additional benefit of using smaller cabling due to the lower current output, significantly reducing cable costs.

“We can lower the cost per kilowatt and maximise the dollar per tonne ratio especially in oil and gas, and mining applications where efficient bulk transportation is key,” Bolt told Australian Mining.

Apart from voltage, one of the other ways MVDs minimise the cost per kilowatt is through reducing input current harmonics, which have the potential to cause overheating and overloading of transformers and conductors.

“The more harmonics you have the more it’s going to take away from your usable current and voltage which makes it more inefficient,” he said.

MVDs reduce the input current harmonics and meet the IEEE 519 standard of attaining harmonic control in power systems to keep below five per cent (THDi).

“The way it does this is our drives make use of a cascade topology which incorporates a phase shift transformer. This eliminates the requirements for upstream power quality technology and mitigates the harmonics down below five per cent to meet that industry standard of IEEE 519,” Bolt said.

“So when you combine our integrated phase shift transformer in the drive and the general benefits of using a higher supply voltage as explained, the MVD basically then can reduce your total cost of ownership to lower energy consumption and increase application efficiency.”

This contrasts with low voltage drives which usually have a separate harmonic filter in the upstream of the drive which incur additional costs to both procure and install.

Monitoring and maintenance

Proper asset management ensures the longevity of mining equipment. The modular technology structure of MVDs, compared to low voltage variable speed drives (LV VSDs), provides easier replacement and overall maintenance.

“If you’re using a large LV VSD (>500kW), if something happens to it, most likely there’s a single point of failure,” Bolt said.

“Usually a lot of the time you have to replace quite a significant part of the LV VSD or replace the whole LV VSD itself depending on age.”

Bolt described how Delta’s MVDs use modular power cells which minimise operating costs.

“In the Delta MVD architecture, we use modular power cells with an inbuilt and patented semiconductor power cell bypass as standard, not a contactor bypass to ensure type testing in the drive is maintained and fast bypass of the cell,” Bolt said.

“If a power cell fails we can automatically bypass the ‘bad’ cell and run at a de-rated output to keep the system online. When available, the faulty power cell is then quickly replaced and the system reactivated. So, that really helps costs going forward.”

Although MVDs incur a higher capex, they return greater benefits in the long term by increasing the overall life of the drive.

In addition, MVDs offer redundancy compared to low voltage drives, which keep processes going in the event of an unexpected failure. Bolt explained that Delta MVDs have an option to add an additional power cell in each phase to maintain a 100 per cent power output without having to de-rate the system.

“By using this N+1 redundancy, if a power cell fails then the redundant power cell is activated and no de-rating occurs so full system output power is maintained,” he said.

“So for critical applications where derating the output power cannot be undermined basically the N+1 redundancy creates a redundant power cell and it keeps the output of whatever’s happening going.”

 Adopting medium voltage drives

Bolt said the medium voltage market is worth around $180 million each year, suggesting a growing adoption of the drives.

“Previously companies avoided them [because] they were very high in cost compared to low voltage, and the cost to install and get the skilled labour to put them in place was also expensive,” Bolt said.

“So what has happened in the last few years is that the cost of medium voltage drives has significantly gone down due to both changing market conditions but material costs coming down of critical components.

“This, coupled with the redundant options of our cascade topology, inbuilt harmonic mitigation and a lower holistic life cycle cost is ensuring the full CAPEX and OPEX decision is balanced, and both expenditures are taken into account when seeking to adopt an MVD in applications requiring more than 500kW power output.”

Bolt added that through the Delta alliance, the company has partnered with I S Systems, a Newcastle-based engineering solution integrator that addresses the need to reduce labour costs and increase engineering skill while putting the drives in place.

It means for the first time ever, customers no longer deal directly with the manufacturer, further increasing affordability, service time and overall customer focused support.