THE EAST AFRICAN POWER THE EAST AFRICAN POWER

INDUSTRY CONVENTION HELD ON 5INDUSTRY CONVENTION HELD ON 5THTH

-- 9TH SEPTEMBER 20119TH SEPTEMBER 2011

EXCITATION EXCITATION SYSTEM UPGRADE SYSTEM UPGRADE AT NALUUBALE AT NALUUBALE POWER STATIONPOWER STATION

By ANGELLA BAMULESEYOBy ANGELLA BAMULESEYOEskom Uganda Limited.Eskom Uganda Limited.

Background

• Eskom Uganda Limited, which is owned by Eskom holdings South Africa took over the operation and maintenance functions of both Nalubaale and kiira power stations in Jinja, Uganda on the 1st April 2003 under a 20 concession agreement.

• Nalubaale power station has 10 generators rated at 20MVA, generating at 11kV and a p.f of 0.9 with an installed capacity of 180MW.

• Kiira power station has 5 generators rated at 46MVA, generating at 11kV and a p.f of 0.9 with an installed capacity of 250MW.

AERIALAERIAL VIEW OF NALUBAALE VIEW OF NALUBAALE

AND KIIRA POWER STATIONSAND KIIRA POWER STATIONS

Nalubaale power station excitation system upgrade

The station has lived through two types of excitation systems and yet to go through a third type;

• The rotating brush-type exciter systems

• The static exciter system and

• The static exciter system with a micro processor based controller (UNITROL 6080).

Functions of excitation system

• To maintain the generator terminal voltage.

• To provide fast response in case of system disturbance.

• Maintain generator operation in synchronous mode and within its operating limits.

• Help stabilize power oscillations

Nalubaale power station rotating brush -type exciter system(1954-1991/96)

240VDC BATTERY

SUPPLY

DC GENERATOR(Aux. Exciter)

ROTOR

MAIN EXCITER

OPERATOR CONTROL SYSTEM

CONTROLLED EXCITER SIGNALS

DC MOTOR

AMPLIDYNE

Nalubaale power station rotating brush -type exciter system Cont’d

Advantages.• These require a small input to produce a bigger output to

the main exciter.

• Easy to appreciate technology.

• Very reliable and easy to trouble shoot.

Disadvantages• They have a very slow response to system disturbance.

• Obsolete technology with spares availability not supported by manufactures.

• They take up much space and are not easy to install.

Nalubaale power station static exciter system (1990/96 -2011/12)

11kV

410V

Synchronous generator

Excitation transformer

EXCITATION SYSTEM

ROTOR

Nalubaale power station static exciter system (1990/96 -2011/12)

Advantages• Fast response to system disturbances

• Less maintenance costs (labour costs) compared to rotating exciters

• Minimum space and foundation costs

Disadvantages

• Relatively higher maintenance costs (labour costs) compared to micro processor based exciters

Nalubaale power station micro processor based static exciters (2011/12-)

11kV

415V

Synchronous generator

Excitation transformer

EXCITATION SYSTEM

ROTOR

ENGINEERING STATION

Nalubaale power station micro processor based static exciters (2011/12-)

Advantages• Less maintenance costs (labour and spares costs)

• Better supervision and monitoring especially with the MMI installed

• Easier to change and set parameter.

• Increased internal supervision, diagnostic and self diagnositic

Disadvantages• Initial installation cost is very high.

Common Nalubaale Exciter system problems

• Microgem failure; this is used to control the generator voltage build up sequence

Common Nalubaale Exciter system problems cont’d

• Erroneous excitation trips

Common Nalubaale Exciter system problems cont’d

• Hard to get expensive and critical spares

• System surges causing unnecessary excitation trips

Options considered

• Replace the micro gem with a modern PLC (programmable logic controller).

• Go on servicing and replacing the welded electromechanical relays with new ones.

• Replace the regulator cubicle only and retaining the converter cubicles.

• Replace the entire exciter unit

Old Vs New exciter system

Old Vs New exciter system

Benefits

• Better system supervision and monitoring.

• Less down time due to fast diagnostic capabilities of the system.

• Less maintenance costs due to reduced maintenance man hours.

• Improved reliability and availability of plant.

• Faster response in case of system disturbances.

Likely Challenges Of The New Exciter System

• Skills gap ; with new technology comes with new skills acquisition.

• Workers are forced out of their comfort zone.

• New operating procedures have to be set up.

Cost Benefit Analysis (CBA)

Factors considered in the calculation of CBA.

• Cost of IMW (13.68USD), one unit is averagely loaded at 16MWh.

• Procurement lead time is 11 months

• Micro gem repair costs

• Cost of critical spare parts

Cost Benefit Analysis (CBA)

Old exciter New exciter Remarks(Saved money)

Component failure

1,733,490USD 1,175,480USD 558,010 USD

Micro gem repair costs

2400 USD N/A 2400 USD

Spare parts cost

30,695 USD 5,900 USD 24,795 USD

TOTAL 1,766,585 USD 1,181,380 USD 585,205 USD

Conclusion

• The new exciter system will enable Nalubaale power station reduce its maintenance costs by 585,205 USD annually.

• At the same time, plant reliability and availability will be improved due to less down time.

END