Electricity + Control January 2018

DRIVES, MOTORS + SWITCHGEAR

This load shift was realised only through manipu- lating the VSD speed of the pump during certain times of the day according to the values shown in Table II. From Figure 8 it is clear that by operating ac- cording to the calculated set points, a 2,29 MW evening peak load reduction was realised.The nec- essary a comeback load was implemented during the off peak periods. This resulted in no water transfer deficit for the day and financial savings of about R10 000 per weekday during the high de- mand season and R3 000 during the low demand season. No savings are realised over weekends due to the need for additional comeback capacity during the cheaper weekend energy tariffperiod. Conclusion and recommendations In South Africa, water transfer schemes convey water over long distances. The high electrical en- ergy demands of bulk water pumping stations and storage capacity provides scope for ESCOs to implement DSM strategies on these pumping sta- tions. From the study it was determined that load management through the use of VSDs is possible. The VSDs actually provide much more variable control possibilities, proven by the fact that a load management intervention could be implemented on only one operating pump. Load management on only one pump is not readily accepted where conventional pump control methods are used such as on-off control. This is especially true for large pumping stations where standby pumps are not available. The main constraint that inhibited the successful implementation of the control philoso- phy was the unavailability of pumps due to break- down of VSDs and mechanical seal failures. During this study one of the main reasons for the breakdowns was identified as inadequate maintenance of VSDs and pumping infrastructure. Further studies are however needed to determine other causes, such as the effect of using hard wa- ter as a coolant for the mechanical seals. Lastly, widening the study to include additional pumping stations as case studies will provide more data and

other constraints. From this, the viability of install- ing VSDs on large scale pumping schemes can be determined. This will provide good insight into the limitations and other considerations regarding the installation of VSDs on current or planned pumping systems, such as the financial payback period and maintenance costs and plans. References [1] Department of Energy South Africa, Annual Report 2012/13, 2013. [2] Eskom Holdings SOC Limited. Load shedding: Understand- ing the facts. 2015. [3] Duvenhage DF. Integration of DSM interventions into bulk water supply strategies. M.Eng Thesis, NWU-Potchef- stroom Campus, 2015. [4] US Department of Energy. Energy service companies. Avail- able: http://energy.gov/eere/femp/energy-service-compa- nies-0 [5] Muller M. Inter-basin water sharing to achieve water securi- ty – a South African perspective. Pretoria, Department of Water Affairs and Forestry, South Africa, 2002. [6] Chambers K, Creasey J, and Forbes L. Design and operation of distribution networks, in Safe Piped Water: Managing Microbial Water Quality in Piped Distribution Systems. Ainsworth R. Ed. London: IWA Publishing, 2004. pp. 38– 68. [7] Grundfos, Pump Handbook, Grundfos. Available: http://net.grundfos.com/doc/webnet/mining/_downloads/ pumphandbook. pdf [8] Jacobsen CB. The Centrifugal Pump. Bjerringbro, Denmark, 2008. [9] The Engineering Toolbox. Angular motion - power and torque.[Online]. Available: http://www.engineeringtoolbox. com/angularvelocity-acceleration-power-torque-d_1397. html. [10] The Engineering Toolbox, Radian. Available: http://www.en- gineeringtoolbox.com/radian-d_942.html. [11] Karassik IJ, Messina JP, Cooper P, Heald CC. Pump Drivers. Pump Handbook, 4 th ed., New York: McGraw Hill Profes- sional, Access Engineering, 2008. [12] ABB, Application guide No. 2: Using variable speed drives (VSDs) in pump applications, Helsinki, Finland. 2006. [13] Deysel JA, Prof. Kleingeld M, Kriel CJR. DSM strategies to reduce electricity costs on platinum mines. Industrial and Commercial Use of Energy. pp.89 - 96, 2015. Acknowledgement This topic was presented at the 15 th Industrial and Commercial Use of Energy Conference at the Cape Peninsular University of Technology (CPUT).

Mr. J.P. De Wet holds a Masters degree in Mechanical Engineering from the North-West University. Dr J.F. van Rensburg is a registered professional engineer and holds a PhD in electrical engineering from the North-West University. He is a lecturer at the North- West University. Dr W Schoeman holds a PhD in Engineering from the North-West University. Enquiries: Email wschoeman@rems2.com

Electricity + Control

JANUARY 2018

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