Power Plant Engineering: Load Change

 

LOAD CHANGE

Load change is the variation in the power demand imposed on a power generation system or electrical grid. Whenever the demand for power increases or decreases, the power produced in the power plant (or load) must change accordingly to meet that demand.

‘Power produced must always equal demand’

Changing the supply of power to meet demand is known as load change. To do this, operators pay close attention to several indicators in the control room. Among different parameters, there are four important flow rates that operators pay close attention to: steam flow, feed water flow, fuel flow and combustion air flow.

Figure 1. Main flows used to vary the power produced (load).

How is this accomplished?

In most systems, a load change is initiated when the dispatcher requests the operator to vary the load.

Figure 2. The dispatcher contacts the control room operator
to execute the proper adjustments.

The operator then begins the load change by operating the turbine control system -the governor- to increase or decrease (depending on which way the controller is moved) the amount of superheated steam at the inlet to the turbine. This is done by regulating the valve openings.

Figure 3. Adjusting the control valves openings.

Bear in mind that not a single controller can make the whole change by itself; rather, there are separate control systems for the turbine feedwater, fuel and combustion air. This arrangement can vary from plant to plant; thus, it is important to get acquainted with the system your own facility deals with, but, in general, once the operator changes the load on the turbine, the other systems automatically follow. 

Figure 4. A control system arrangement.

A load change must be accomplished slowly so that operating limits on plant equipment are not exceeded, therefore it is not unusual for a load change rate to be a certain amount of MW per minute (e.g., 

4 or 5 MW per minute), meaning it would take 20 minutes or more to complete a 100 MW change.

The operator must pay attention to the four main flows to be sure that all systems are making the proper adjustments.

It is of great importance that if one flow is changing, the others must also change accordingly; for example, steam flow and feedwater flow must be kept equal. This means that if steam flow through a unit is increased x-times, feedwater flow must also increase x-times (e.g., doubled) to keep the systems in balance and maintain the correct levels in the boiler drum and in the main condenser.

Figure 5.Steam drum.

Figure 6. Basic feedwater supply system.

Figure 7. Steam flow adjustments over time.

PRACTICAL EXAMPLE

NORMAL OPERATION:

Let's take a look at a typical example. Specific numbers are used to make the load change easier to follow. 

Unit (rated power) = 200 MW

Let's say that at the start of the shift, demand is relatively low and the unit is only required to produce 100 megawatts. But it's capable of producing an additional 100 megawatts if necessary. 

To produce 100 megawatts, the steam flow through the unit is around 

680,000 lb./per hour. Since feed water flow and steam flow must be balanced, the feedwater flow is also 680,000 lb./per hour.

Figure 8. Steam and feedwater flows under normal conditions.

FUEL & COMBUSTION AIR

The fuel requirement for this unit producing 100 megawatts is around 37 tons of coal per hour. To burn 37 tons of coal per hour, the operator has to mix it with about 1.5 million pounds of combustion air per hour. 

Typically, when the demand for power increases, the operator will get a call from the dispatcher to increase the load a certain amount. The change for this example requires the operator to increase the output from 100 megawatts to 200 MW. In other words, the output of the unit must be doubled.

Figure 9. Amount of fuel and combustion air under normal conditions..

Figure 10. Power output under normal conditions.

INCREASE IN DEMAND

The operator begins by adjusting the turbine governor and opening the control valves slowly while watching the steam flow indicator so he won't exceed the manufacturer's specifications.

• To produce 200 MW of power, the steam flow needs to double to 1,360,000 pounds per hour. 
• As the steam flow increases, it causes a change in steam pressure. The control systems for the unit sense the changes in steam flow or steam pressure and adjust their systems accordingly.
• The feedwater control system responds by adjusting feedwater flow so that it matches the increase in steam flow. 
• In this way, the steam and feedwater flow stay pretty close during the increase in load.
• The automatic control systems for fuel and air sense the decrease in steam pressure and begin adjusting the fuel flow and combustion air flow to restore steam pressure to its desired level.
• By the time the unit is producing 200 MW, the fuel flow will have doubled to 74 tons per hour, while the combustion air flow will have doubled to 3,000,000 pounds per hour.
  
  
   

Figure 10. Steam and feedwater flows under new conditions (200 MW).

Figure 11. Amount of fuel and combustion air under new conditions (200 MW).

Figure 12. New power output (from 100 MW to 200 MW).

NOTE

While the basic principle of turbine governors remains unchanged, modern systems have evolved significantly with advancements in technology. Most modern plants now use electronic or digital governors, often integrated into Distributed Control Systems (such as Delta V), providing a faster and more precise responses compared to traditional mechanical systems.

REFERENCES

NUS Training Corporation

National Power Training Institute (NPTI, India)