6. Balance Of Plant

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BALANCE OF PLANT (BoP)

Apart from the turbine, generator, and generator step-up transformer, the balance of plant system controls the entire of plant. It is a complex system designed to manage regulate the rest of the plant to ensure reliable and efficient power. The BoP system controls and monitors complimentary devices, values, gauges, fuel levels and pumps, electronically using a number of different systems, including SCADA (Supervisory Control And Data Acquisition). It works by measuring current running condition (e.g. rate of fuel injection into the combustion cans), comparing the values with pre-set reference values and altering the system in charge (e.g. fuel valve and pump system) to ensure it matches the reference exactly (or within pre-set tolerance levels).

There are two parts of balance of plant system, the Electrical Balance of Plant (EBoP) and Mechanical Balance of Plant (MBoP).

The electrical balance of plant system is made up of High Voltage (or Primary) Systems

1. TRANSFORMERS: used to step-up (or down) voltage.

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2. GAS / AIR INSULATED HIGH VOLTAGE SWITCHGEAR: switchgear is a collection of electrical devices (e.g. fuses, disconnect switches etc.) used for switching, protecting, control, monitoring and measurement of electrical equipment such as transformers, large motor pumps, fans etc. The switchgear is also used to isolate these components whenever the need arises. They can be insulated with either a specific type of gas or surrounding air.

3. DISCONNECT SWITCHES: are used to completely de-energize electrical equipment for maintenance and repairs. These are usually opened after interrupting current by a suitable device (e.g. a circuit breaker).

4. CIRCUIT BREAKERS: are used to interrupt high voltage current safely and quickly in the event of an electrical fault or for maintenance.

5. INSTRUMENT TRANSFORMERS: are used to transform high current and voltage to lower, more manageable values.

6. SURGE ARRESTER: are electrical devices used to divert lighting, which could cause damage, safely away from the other systems, and

Low/Medium Voltage Systems:1. MEDIUM/LOW VOLTAGE SWITCHGEAR: are used for

low to medium voltage electricity. It works the same way as the high voltage switchgear.

2. BUS BARS: are used to transport generated electricity from the generator, to the generator step-up transformer.

3. DRIVES & MOTORS: Drives are used to ensure safe, effective running of DC and AC motors. The drives ensure smooth start, precise speed and torque control and efficient process control. Motors are used

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to drive various components (e.g. fuel pumps) across the plant.

4. MOTOR CONTROL CENTER (MCC): is an assembly of enclosed sections mainly containing control units for motors around the plant.

5. VARIABLE SPEED (OR FREQUENCY) DRIVES: are used to vary the running speeds of motors when there is need for speed adjustment.

6. AUXILIARY TRANSFORMER: supplies low voltage electricity to AC systems (e.g. lighting, heating etc.).

7. MV/LV CIRCUIT BREAKERS: are used to interrupt medium to low voltage current safely and quickly in the event of an electrical fault or for maintenance.

8. STATIC FREQUENCY CONVERTER: is used to change the frequency of AC current from one to another. It works by passing the input AC power through a sine-wave rectifier (which makes the negative half of the AC current positive), then to a DC link (which connects the rectifier to the inverter) and final through an AC sine-wave inverter (which converts the DC current back into an AC current, but with a different frequency).

Sine-wave rectifier input to output

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Sine-wave inverter input to output

9. EXCITATION EQUIPMENT: is used to provide variable DC current to the generator’s rotor windings.

Components of the MBoP system, on the other hand, depend largely on the type of plant (i.e. the MBoP for a Nuclear power plant is different from that of a Hydro or CCGT power plant). We are looking at a CCGT power plant and its MBoP is made up of:

1. COOLING WATER SYSTEM: cools condensed spent steam from the steam turbine so it can be fed back to the HRSG. It is also used the cool the lube oil systems, flame detectors, turbine supports, liquid fuel shaft pump and atomizing air systems.

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2. DRAINAGE SYSTEM: used to remove unwanted water from the different systems.

3. BLOW DOWN SYSTEM: occasionally removes water from the boiler racks to avoid the build up of impurities.

4. EMERGENCY DIESEL: Emergency diesel is used to run the emergency generator used if there is loss of power to the plant. The emergency generator is used to run utilities (e.g. lighting, heating and air conditioning etc.).

5. FIRE PROTECTION SYSTEM: is used to monitor the turbine, especially the combustion cans, to ensure low concentration of the combustive agent. The system not only detects fires automatically, but also trips the system and extinguishes the fire quickly and effectively.

6. HEATING, VENTILATION, AIR CONDITIONING SYSTEMS: These systems are used to remove hot air and heat the enclosures during downtime. The system also protects potentially dangerous systems (e.g. gas turbine, gas valves, load gear, etc.) from gas leaks as they cause to fires or explosions.

7. OIL TREATMENT SYSTEM: ensures clean oil is supplied where it is needed. In use oil is flushed out of components, like gas-insulated transformers, as fresh oil is pumped in. The difference in density between the used oil (as a result of dirt and other impurities) and new oil makes it easy to flush used oil out the bottom of each unit, and pump new oil in through the top. Used oil is then sent through the oil treatment system to recover as much oil as possible, to reduce wastage, and the dirt from the spent oil is discarded.

8. RAW, POTABLE, AND DEMINERALIZED WATER SYSTEMS: are responsible for providing water for the

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power plant. Raw water is pumped, usually from a natural source (e.g. a river) into reservoirs. This water is then converted to portable water for consumption and demineralized water. Demineralized (or de-ionized) water is water with most mineral ions removed. Water has to be demineralized to ensure water fed to the HRSG for boiling, is free from impurities and boils as close to 100o as possible, this reduces heat used during boiling.

9. FUEL FORWARDING SKID: is used to provide the gas turbine’s combustion cans with fuel that is at the optimal pressure and temperature. The unit usually includes an electric heater (for fuel heating and/or cooling), pressure control valve (for pressure control) and a flow meter (to measure the fuel flow).

10. BACK UP POWER: there are occasions where the plant losses power. To avoid serious damage being caused to the plant equipment, there are a few power loss prevention and control system employed.

a. UNINTERRUPTIBLE POWER SUPPLY (UPS): is usually located between the power source and the load and is used to provide power in the event of power interruptions. The UPS also protects the supply network from surges and other electrical problems. The UPS gives plant operators time to react to sudden power outages.

b. EMERGENCY GENERATOR: is used to provide power to the plant for lighting, heating and other basic tasks in the event of a power outage. The emergency generator does not power the gas and steam turbines or generator.

c. BACK-UP GENERATOR: is used the start the gas turbine in the event of a black start. In the event where there is no power to start, as soon as the turbine is running at full power, the back-up generator is tripped.

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11. GAS CONDITIONING SKID: is used to treat gas before it enters the gas turbine. This treatment process removes liquid droplets and solid particles from the gas ensuring only gas is injected into the gas turbine. The skid also adjusts gas temperature and pressure to injection requirements.

12. OIL-WATER SEPARATOR: is used to separate oil and other solids from water. The system uses gravity to perform the separation, wastewater (including oil and particles) is pumped into a large tank, since oil is denser than water, it floats and the solid particles, which are denser than water, sink. Oil is skimmed off the top of the water, via an oil skimmer, while the water is drained, and the solid particles settle at the bottom of the tank. See figure below.

13. STORAGE TANKS & VESSELS: are metal units of varying strengths and sizes used as reservoirs for water, oil and fuel (e.g. diesel). Gas is usually not stored in large volumes, rather it is supplied directly from the source, the gas storage facility is used to hold gas before it enters the gas conditioning skid.

14. SEWAGE TREATMENT SYSTEM: is used to ensure waste discharged from the plant is treated. The system separates dirt and waste from water via a system of filters, aeration chambers and sediment tanks. After separation, microbes from the atmosphere are introduced to breakdown

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biodegradable materials. The microbes digest sludge to produce methane, carbon dioxide and traces of other gases.

15. COMPRESSED AIR SYSTEM: is used to provide valves, pumps and other equipment high-pressure air for operation.

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2. Swapanbasu. "Balance of Plant - Introduction." Scribd. Swapanbasu, 23 Sept. 2010. Web. [Accessed 20 June 2014].

3. "Standby Generator." Wikipedia. Wikimedia Foundation, 07 May 2014. Web. [Accessed 08 July 2014].

4. "Emergency Power System." Wikipedia. Wikimedia Foundation, 07 May 2014. Web. [Accessed 08 July 2014].

5. "Fact Sheet." Chemical & Engineering News 47.10 (1969): 101A. Web.

6. "Voith | Balance of Plant Mechanical Systems." Voith | Balance of Plant Mechanical Systems. N.p., n.d. Web. [Accessed 08 July 2014].

7. "Electrical Balance of Plant :: Power Delivery Projects :: GE Digital Energy." Electrical Balance of Plant :: Power Delivery Projects :: GE Digital Energy. N.p., n.d. Web. [Accessed 08 July 2014].

8. "API Separator" by Original uploader was Mbeychok at en.wikipedia - Originally from en.wikipedia; description page is/was here.. Licensed under

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Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:API_Separator.png#mediaviewer/File:API_Separator.png

9. "Parallel Plate Separator" by Original uploader was Mbeychok at en.wikipedia - Originally from en.wikipedia; description page is/was here.. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Parallel_Plate_Separator.png#mediaviewer/File:Parallel_Plate_Separator.png