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Operation of Automatic Voltage Controls (AVCs)
in Manual and/or Below Sparkover

By TRK Engineering Services, Inc.

We were recently on a site where many of the automatic voltage controls were found to be operating in the MANUAL mode. This is not a desirable mode of operation because many of the control and protection features of the control are not available in this mode. Additionally, most of the AVC controls were found to be operating at reduced power levels. It is evident that Plant personnel are in the habit of adjusting power levels to below sparkover to discourage it's occurrence. Operated in this manner (MANUAL, or in AUTO but manually adjusted below sparkover), the control is not able to automatically respond to changes in operating conditions that are likely to occur with changes in fuel, load and/or temperature. The sparkover limit will change with any of these occurrences and the control needs to be readjusted in order to maintain optimum power levels. This was not being done. Control operation should be able to respond automatically to a reasonable level of sparking and changes in operating parameters to maintain the highest average power levels. Operation personnel need to understand that when a voltage controller is sparking, it has achieved the highest power level for the existing operating conditions. For best ESP performance, all the voltage controls should be operating at either a TR voltage or current limit, or sparking. If adjustments are required, it should be to try to control the level or magnitude of sparking, not to totally eliminate it. If that is not possible, an electrician, or someone familiar with control operation, should be called to investigate why. Guidelines for adjusting levels of sparking should be to allow around 30-60 sparks per minute (SPM) in the inlet fields, and less sparking towards the outlet (1-10 SPM).

The other problem with operating at manually reduced power levels is that as power levels are reduced, material is being passed further and further into the ESP, disrupting the normal pattern of collection and rapping from inlet to outlet. This creates the potential for fouling of the downstream field and can lead to rapper related material reentrainment.  This can also sensitize the ESP to process upset conditions that can cause opacity excursions.

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Last updated: May 10, 2009.
Copyright 1997 TRK Engineering Services, Inc. All rights reserved.
For more information contact: TRK Engineering Services - 95 Clarks Farm Road - Carlisle, MA 01741 - Telephone: 978-287-0550 - Fax: 978-287-0569 - email: