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Direct current to the rescue

27 April 2018

Food Processing finds out about a new approach to energy efficiency and system cost optimisation through the use of a DC grid. 

While the use of variable speed drives (VSDs) has produced a significant energy saving, and improved speed control further improvements in energy efficiency and sustainability are still a goal for many. A bold solution would be to implement of a direct current (DC) grid within industrial premises, such a solution would offer the potential to reduce operating costs and take advantage of renewable energy sources.

Since January 2017, all new three-phase motors sold in Europe with rated power from 0.75 to 375 kW have been required to conform to energy efficiency class IE3, or alternatively IE2 for use in frequency inverter operation. These efficiency classes are specified for three-phase asynchronous motors operating at nominal speed and nominal torque. However, experience has shown that an energy efficiency regulation of a component can only sustainably reduce energy in certain operating modes.

With this in mind, a three-year DC-INDUSTRIE project, initiated by the German Electrical and Electronic Manufacturer's Association (ZVEI) is looking at the use of direct current networks to support energy transition and energy efficiency.

Inefficiencies in speed control
The advantage of using a frequency inverter is the continuous adaptation of the motor speed to the actual need, which also leads to energy savings. A frequency inverter is supplied with the alternating current, which is first converted into direct current using a rectifier. The direct current is then converted into alternating current with variable frequency and voltage through a voltage feed inverter in order to electronically change the speed of a three-phase motor.

However, if the three-phase motor is operating in the braking mode the energy flow changes and this energy cannot be fed back into the grid by the frequency. Instead it must be dissipated via the direct current voltage circuit of the frequency inverter.

Reducing harmonics
The use of frequency inverters to control motor speeds has led to problems with mains effects, causing harmonics. If frequency inverters or other devices with power electronics are increasingly installed, grid effects will increase. So, a further increase in the use of inverters for the flexible control of electric motors is often necessary to improve production processes and energy efficiency. However, line perturbation due to harmonics and equipment costs limit the increase.

To achieve significant progress in energy efficiency and system cost optimisation, new approaches are needed. To enable energy efficiency, energy transition and Industry 4.0, new grid structures are required.

The new network structure is based on an alternating current supply, which provides the direct current power supply for production plants via a central rectifier. Active grid filters are integrated into the central rectifier to ensure the voltage quality harmonic requirements.

The direct supply of the frequency inverter with direct current means that all decentralised energy conversion is no longer needed. Since central energy conversion (from AC to DC) is more efficient, conversion losses are significantly reduced.

Through the direct supply of electric motors via a frequency inverter with direct current power supply, installed motors are connected via a common direct current voltage grid. Further, a direct current voltage network essentially only causes ohmic transmission losses. Compared to an alternating voltage network, the capacitive and inductive line losses are eliminated.

In addition, the central direct current voltage network offers the possibility of integrating photovoltaics directly at the direct current voltage level. In this case also, conversion from DC to AC does not need to be handled by an inverter. This grid infrastructure would optimise the purchase of energy and stabilise the grid.

By eliminating of the input rectifier and the grid filter frequency inverters could be designed more cost-effectively and more compactly. Variable speed motors allow for a reduction in variants and energy savings. They provide status signals from all DC-fed drivers, which are important for flexible and safe production control.

Grid management makes it possible to optimise operational management in terms of energy costs. The accessible information enables preventive production control measures to significantly increase the availability of production. This is a prerequisite for Industry 4.0.

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