SPECIALIST EQUIPMENT GET UP TO SPEED on VSDs
The risks related to earth-leakage current are well known, but designers and engineers have so far lacked a comprehensive overview of possible solutions with VSDs. Tino Wyman presents a simple and eff ective ‘best-practice’ approach
G
rowing use of variable speed drives (VSDs) in underground mining applications has introduced two new safety challenges that did not exist with
the previous fi xed-frequency equipment. First is the need for earth-fault protection at variable frequencies; second is high-frequency coupling through leakage capacitances in long cables. Before drives were used, earth-fault
protection was rather simple; all you did was watch the fundamental 50Hz frequency. However, when you introduce a VSD, earth- leakage currents then cover frequencies varying over a wide spectrum, including high frequencies. Previous conventional solutions do not work, as they specifi cally ignored these low- and high-frequency regions. When using drives, simply installing a
diff erent relay is not a complete solution. Instead, ABB suggests a more comprehensive best practice. T is includes four key steps:
● Select the right relay ● Set up a protective zone ● Keep input stray capacitances small ● Install high-impedance earthing on the output side of the drive
The right relay A novel, patented method for measuring DC and AC up to 90Hz using a standard current transformer (CT) was developed by Littelfuse. To accomplish this, the relay injects a signal into the CT secondary, and monitors output from the CT on the same terminals. T is method provides a signifi cant benefi t: a single CT with a sensitive pick-up level can be used to monitor current from 0-90Hz, including the typical VSD output frequencies, DC and ignoring high-frequency leakage currents. A second CT input capable of AC
measurements from 20 to 6,000Hz, with user-selectable fi ltering ranges and an
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independent pick-up level, is also provided. T is is critically important, allowing the user to accommodate diff erences in each installation, and prevent nuisance trip events.
Protective zone against cross coupling Cross coupling with the rest of the network (under normal and fault conditions) is hard to predict and thus hard to control. Having multiple drives connected to one transformer often leads to high earth-leakage currents under normal operation. Especially in underground mining, high earth-leakage currents are unacceptable and a protective zone must be created. T is is accomplished for medium- voltage VSDs via an input transformer to facilitate galvanic isolation.
ACS1000i medium voltage variable frequency drive from ABB
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