Load management through the utilisation of VSDs on water transfer schemes
De Wet, Johannes Petrus
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South Africa is a water scarce country and due to this, large water transfer schemes were built to supply major parts of the country with raw and potable water. The Department of Water and Sanitation directly control some major water transfer schemes in South Africa, with some others being managed by regional water boards. Water transfer schemes are high energy consumers due to large pumps utilised to pump water over great distances with significant static head. With the current electricity situation in the country, Eskom invested in demand-side management interventions, among others, to ease the load on the electrical grid during peak hours. The pump stations that form part of water transfer schemes were identified by energy services companies as possible viable sites for the Eskom funded demand-side management interventions. A pump station that utilises Variable Speed Drive technology for pump control was identified for possible load management. The selected pump station supplies key water consumers such as Eskom and Sasol. Therefore, any water transfer deficit due to load management interventions must be avoided, making a load shift intervention the only viable option. To develop an optimised control philosophy that utilises the Variable Speed Drive technology, tests were conducted on the pump station. Data gathered from these tests was used for the verification of a simulation built to test the optimised control philosophy. The data was also used to calculate adjusted flow set points used as control inputs for the optimised control philosophy. From this it was concluded that the utilisation of Variable Speed Drives for load management on this large pump station is possible in this particular pump station. After the proposed strategy was simulated and optimised, control and maintenance constraints limited the implementation of the optimised strategy. The simulation however, indicated that an annual electricity cost savings of around R 3.3 million can be achieved. Due to these constraints, it was not possible to implement the load management initiative for a significant part of 2016, resulting in about half of the possible financial savings being lost. The optimised control philosophy was however, implemented on one operational high-lift pump set and three low-lift pumps. A daily peak period saving of 2.29 MW was achieved on the pump station with a daily cost saving of R10 000 during the high demand season. The transfer deficit due to the peak period cutback was successfully neutralised through an off-peak comeback load. The dam levels were kept at the required set point by matching the flow of the high and low-lift pumping stations. This was all achieved by only manipulating the speed of the pumps through the Variable Speed Drives, according to pre-calculated station flow set points. Load management through the utilisation of Variable Speed Drives on water transfer schemes is thus a viable demand-side management intervention. The optimised control philosophy can easily be incorporated into the existing control of pumping stations equipped with Variable Speed Drives.
- Engineering