Electricity + Control January 2018

DRIVES, MOTORS + SWITCHGEAR

Through optimisation of Pump Station A’s control philosophy, the above savings can be realised. The simulation uses a controller that mimics the con- trol available through the VSDs installed at Pump Station A. The VSDs allow a load management interven- tion that does not require switching pumps on and off. This allows for a unique approach to load man- agement on large energy consumers when they are utilising VSD technology. Implementing the simulated load management strategy would be the next step. Pump Station A can however, only implement the load management initiative if it has at least three of its four highlift pump sets available. The operators of Pump Station A however, struggle to maintain this availability. Implementation of optimised control philosophy and constraints for success The utilisation of Pump Station A was calculat- ed for the years 2014, 2015 and the first half of 2016. This relates to the availability of the pumps, as they utilise all the available pump sets up to a maximum of three. Figure 7 shows the utilisation of Pump Station A over the last two years. The line indicates a monthly utilisation of above 65%, which is required before any load management in- tervention is possible.

calculated R3 300 000 annual saving opportunities being lost. On Pump Station A, the main problem for this low utilisation during 2016 was VSD fail- ures and pump mechanical problems. Improved pump availability can be realised through the in- troduction of good preventative and predictive maintenance schedules. These schedules can be incorporated into the optimised control philosophy by alternating between the four pumps over a pe- riod of four weeks. During the month of August 2016, load manage- ment on one high-lift pump set was approved on specific days. It was used to determine the viabil- ity of introducing a control philosophy that utilises the VSD technology to realise load management. To realise the load shift, flow set points for each time of day were calculated for one pump set. The set points for each time period are given inTable II.

Table II: RPM set points for load shifting one High Lift pump set.

Morning Peak

Afternoon Peak

Off Peak

Standard

Weekend

RPM

1300 1300 1000 1350 1310

The set points in Table II were calculated from data retrieved during the RPS test. Certain constraints were set up in the MS Excel Solver Add-In to deter- mine the required transfer target for each time inter- val.This transfer target was then used to determine the necessary speed set points as indicated. The baseline for this load shift intervention was scaled to be energy neutral instead of transfer neutral. The reason for this is that Eskom only focus on evening peak demand reduction. Figure 8 represents a day where the load management intervention was real- ised on one available high-lift pump set.

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

Utilisation

Jul 15

Jul 16

Jan 15

Jan 16

Apr 15

Apr 16

Jun 15

Jun 16

Oct 14

Oct 15

Feb 15

Feb 16

Sep 14

Sep 15

Aug 14

Aug 15

Aug 16

Nov 14

Nov 15

Mar 15

Mar 16

Dec 14

Dec 15

May 15

May 16

Month

Utilisation 65% Utilisation

Figure 7: Pump Station A utilisation.

Demand profile for Pump Station-A forWednesday 31 August 2016 with a saving of 2,29 MW

During the above period, at 15 months a utilisation of less than 65% was shown.This equates to 60% of the total months where no load management initiatives could be implemented. Lost load man- agement opportunities directly influence possible financial savings on the annual operational costs for Pump Station A. Focusing on the first part on 2016, it is clear that all possible cost savings were lost due to low pump availability.This equates to almost half of the

8000 7000 6000 5000 4000 3000 2000 1 000 0

Demand (kW)

0:00

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

9:00

Time (Hours) 10:00 11:00 12:00

17:00

13:00

14:00

15:00

16:00

19:00

18:00

20:00

21:00

22:00

23:00

Scaled Baseline Power Profile

Figure 8: Pump Station-A load shift on one HL pump set.

20 Electricity + Control

JANUARY 2018