Electricity + Control January 2018

DRIVES, MOTORS + SWITCHGEAR

abbreviations DSM – Demand-side Management

VSD – Variable Speed Drive IBT – Inter-basin Transfer PC – Peak Clip ESCO – Energy Services Company

the x- and y-axes and the operating point of the pump. The relative power consumption of each method then calculates to [12] [13].

requirement is defined by (1) [9]. It is clear from this equation that the angular velocity (_) of the rotor, also contributes to the power required by the motor. P = T · ω Equation (1) Angular velocity of 1 rad/s converts to a rotation- al speed 9,55 RPMs [10]. The rotational speed at which a squirrelcage induction motor operates is lower than the synchronous speed by a specific slip factor [11].The synchronous speed (N) of a mo- tor is defined by (2), with (f) the line frequency and (p) the number of poles. Equation (2) shows that the speed of a motor, with a set number of poles, will change as the frequency supplied to the motor varies. This variation of an in- duction motor’s speed is widely used on pumps for flow control, and the most commonly used method to achieve this is through the utilisation of VSDs [13]. Use of VSDs on pumping systems Varying flow rates are often a requirement for pumping systems. Several methods of flow con- trol are available on pumping systems of which the most commonly used are throttling, bypass- ing, on-off control and VSD control. The resultant power consumption for each of these methods dif- fer. In Figure 2 , the arrows indicate the effect that each flow control method has on the pump char- acteristics curve.The square represents the power consumption of the pump after the corresponding flow control method is applied. N = --------------- f x 60 x 2 p Equation (2)

P = 15 x 15 P = 12 x 17,3 P = 9,5 x 17,1

= 225

No Control: Throttling: Bypassing:

= 207,6 = 162,5

P = (60% x 225,0) + (40% x 0,0) = 135,0

On-off control:

P = 9 x 8,3

= 74,7

VSD Control:

From the above results, it is clear that VSD control is the most energy efficient method for flow con- trol on centrifugal pumps. As the main goal of load management interven- tions is to focus on utilising existing infrastructure up to its optimum capability [13], the installed VSDs at the pump station provided an excellent opportunity to investigate and implement a load management strategy. Control philosophy development Confidentiality agreements with the client prohib- it the use of the actual pump station and transfer scheme names in this article. The pump station will therefore be referred to as Pump Station A and the water transfer scheme asWater Transfer Scheme A. Investigation of a water transfer system The investigation of a water transfer scheme is important to determine plant layout and current operation philosophy. This information is required to determine possible constraints and variables that influence the scheme’s operations. Through a thorough understanding of the water transfer scheme’s operations, any scope for possible load management initiatives can be identified. During the investigation it was determined that Pump Station A consists of a High-lift (Section-C) and Low-lift pumping station (Section-A), connected by a Balancing dam (Section-B).This layout is present- ed in Figure 3.

Load management through the use of VSDs in pumping systems is possible.

Throttling

Bypassing

H

H

H = 17,3

P = 225

15

15

H = 9,5

Q

Q

15

15

Q = 12

Q = 17,1

On-off Control

VSD Control

H

H

Pump runs 60% of the time P = 15 15 = 225

15

To Diversion Structure

15

DamA

H = 8,3

Pump is off 40% of the time P = 0

Q

Q

Section-A Section-B Section-C

15

15

LEGEND – GENERAL

Q = 9

Dam Pump

Balancing Dam Valve

System Curve

Pump Curve

Figure 2: Power consumption for flow control methods. (Adapted from [12]).

Figure 3: Pump Station A Layout.

The power consumption (P) of each method can be calculated by multiplying the flow (Q) with the head (H). This corresponds with the area between

The electrical consumption is billed by Eskom in accordance with their Megaflex Time-of-Use based tariff structure. It essentially divides each day of the

Electricity + Control

JANUARY 2018

17