Hot Topic: Automatic Generation Control

Hot Topic: Automatic Generation Control

Automatic Generation Control

In today’s world, we rely heavily on the power grid. Power is required for lighting up the streets as you drive home from a party at night. It is required to keep you safe and warm in your home while a winter storm passes by. We often take it for granted and expect there to be a reliable source of power at all times.

Ideally, we want a power system that has a constant frequency while providing enough power to supply loads such as our homes, offices, hospitals and other facilities in the area. The load demand changes constantly as we turn on or off (or change) power to devices in an area. The power generation units must include necessary controls to ensure that the electrical power generation satisfies the load demand.

AGC Theory

With primary frequency control for the generating units, variation in load demand is satisfied at the expense of frequency. Figure 1a shows the block diagram of a two area system with primary frequency control. In the absence of secondary frequency control, the reference power given to the turbine (Pref) remains unchanged (i.e.: ∆Pref = 0).  The primary frequency control (or governor control) adjusts the turbine output by –∆f/R in response to a change in load demand, ∆P. When the load demand increases, the frequency decreases and vice versa.

 

acg-a

(a)

 

Figure 1: (a) Simplified representation of a two area network with primary frequency control (with ∆Pref = 0), and (b) Simplified AGC which adjusts Pref to satisfy load demand.

Automatic Generation Control (AGC), shown in Figure 1b, is a secondary frequency control which has the following functions [1]:

1.) AGC regulates the frequency of the system to its nominal value at steady state by changing the reference power to the turbine in order to supply dynamic load demand.

2.) For multiple control areas, AGC regulates the tie line power, ∆Ptie, to its scheduled value to ensure that changes in the load demand of one control area does not affect the power transfer to other connected areas at steady state.

3.) When there are multiple generating units in an area, AGC allocates the desired changes in Pref amongst the generating units that participate in secondary frequency control.  This is determined by performing an economic dispatch considering the cost of generation.

Importance of real time simulation for AGC

Real time simulation is a key component of system testing. Before a device, such as an AGC controller, is commissioned in a system, it is important to understand the behaviour of the system in real time to various contingencies and operating conditions. Using the RTDS simulator, the behaviour of the AGC to various changes in load demand can be effectively modelled.

Simulation Results

In this article, the two area network proposed in [1] is used as the base case to demonstrate the operation of AGC. This case demonstrates the AGC concept; however, no effort was made to optimize the controller parameters used in this case. The system response to an increase in load demand in area 1 from 967 MW to 1000 MW is shown in Figure 2a. A load decrease from 967 MW to 900 MW in area 1 is shown in Figure 2b. The plots in Figure 2 are obtained using the ‘MultiPlot’ feature available in RSCAD, which is the user interface of RTDS.

Primary Frequency Control:
The red curves in Figure 2a shows that for primary frequency control, the frequency (W1) decreases as a result of an increase in load demand and there is an undesirable steady state frequency error. In this case, the tie-line power is not regulated. The generators in both area 1 and area 2 have identical ratings. As a result, the tie-line power transferred to area 2 drops by ∆P /2 (which is 16.5MW) as shown by the red curve in Figure 2a.

For a decrease in load demand, the frequency increases with a constant steady state error as shown in the red curve in Figure 2b. The results show that the tie-line power increases by 33.5MW (∆P /2 = 33.5MW) in response to the load change. Ideally, we do not want a load change in one area to affect the other area (i.e.: the tie-line power should be maintained constant at steady state).

AGC without Tie-Line Power Control:

Although the frequency changes during a transient due to the fast primary frequency control, the PI-control in AGC ensures that the steady state frequency settles to its nominal value, as shown by the green curves in Figure 2. Zero frequency error is possible due to the AGC which adjusts the reference power to the turbine in response to the change in load demand. It must be noted that the tie-line power is not regulated and changes in response to the load demand as described above.

AGC with Tie-Line Power Control:

AGC including tie-line power control, shown by the blue curves in Figure 2, regulates the system frequency at its nominal value to ensure the steady state frequency error is zero. During the transient, both areas respond to the change in load demand; however, with the tie-line power control, the steady state tie-line power is regulated at its scheduled value. Although interconnection of areas improves the stability of power system, a constant tie-line power is desired at steady state.

ACG

(a) Increase Load demand in area 1 from 967MW to 1000MW

ACG

(b) Decrease in load demand in area 1 from 967MW to 900MW

To obtain an example RTDS case that demonstrates the operation of AGC on the Two Area Network, please contact: udeesha@rtds.com.

Author: Udeesha Samarasekera
April, 2017

References

[1] P. Kundur, “Control of Active and Reactive Power”, in Power System Stability and Control, Mc-Graw Hill, 1994