Electricity + Control January 2018

Load Management Through Utilisation of VSDs on Water Transfer Schemes

JP de Wet, W Schoeman, JF van Rensburg

Pumping stations across the country utilise energy intensive electrical pumps to convey water over distances and from low to high geographical areas to ensure the stable supply of water.

I n South Africa, water transfer schemes convey water over long distances. This water is used by numerous consumers such as those in ag- riculture, municipalities and electricity generation. Some of these consumers are situated lengthy distances from water sources, be they natural or constructed. The need to supply water to users from these distant sources necessitates water resource management. The management of the water sector is governed by the South African De- partment of Water and Sanitation (DWS). They op- erate numerous pumping stations, reservoirs and large capacity dams, to ensure the stable supply of water. These pumping stations utilise energy in- tensive electrical pumps to convey water over dis- tance and from low to high geographical areas [1]. Over recent years, the electricity sector in South Africa experienced periods where the de- mand nearly reached the supply capacity. Such an event can lead to an overload of the national power grid, and subsequently to a national black-out. To prevent such an event, the primary electricity sup- plier in South Africa introduced initiatives such as DSM [2]. One of the aims of these initiatives is to reduce the peak period demand of large electricity consumers. These initiatives are implemented by Energy Services Companies (ESCOs) on behalf of, and with funding from, the power utility [3] [4]. The high electrical energy demands of bulk water pumping stations and storage capacity pro- vides scope for ESCOs to implement DSM strate- gies on these pumping stations.

Background to load management andVSDs Inter-basin transfer scheme infrastructure and functionality South Africa’s key centres for economic and social development are generally situated where natural occurring water sources are scarce. Due to this, Inter-basin Transfer schemes (IBTs) were developed to convey water from areas with a relative abun- dance to areas where water is relatively scarce [5]. For applications where gravity is not sufficient for supplying water, pumps are installed [6]. The most commonly used pump for conveying liquids is the centrifugal pump.These pumps utilise a simple prin- ciple of flinging liquid entering the hub of an impeller towards the edge of the impeller [7]. The operation principle of a centrifugal pump is shown in Figure 1.

Take Note!

Water transfer schemes convey water over long distances. VSD control is the most energy efficient method for flow control on cen- trifugal pumps. In this study, the installed VSDs at the pump station provided an excellent op- portunity to investigate and implement a load management strategy.

1

2

3

Direction of rotation

Impeller

Outlet

Inlet

Outlet Impeller Impeller blade

Inlet

Figure 1: Operation principle of a centrifugal pump [8].

Several driving mechanisms exist for centrifugal pumps. The pump station in this study utilises electrical induction motors to drive the pumps. The power (P) for these motors with a specific torque (T)

16 Electricity + Control

JANUARY 2018