10
Frequency Regulation Using Energy Capacitor System
As the use of energy capacitor systems (ECSs) and other energy storage devices increases worldwide, there is a rising interest in their application in power system operation and control, especially in power system frequency regulation.1–6 Frequency regulation as an ancillary service associated with AGC systems is becoming increasingly important for supplying sufficient and reliable electric power, in keeping high power quality under the deregulated power systems, including independent power producers.7,8 As mentioned in Chapter 2, to ensure satisfactory area frequency, interarea tie-line power is one of the main requirements for frequency regulation. The tie-line bias control (TBC) through the supplementary feedback loop plays a significant role to meet the mentioned requirement.9,10
This chapter presents a coordinated frequency regulation scheme between the conventional AGC participating units and a small-sized ECS to improve the frequency regulation performance. The ECS consists of electrical double-layer capacitors. One of its specific features is the fast charging/discharging operation with a high power level. In the proposed scheme, the charging/discharging operation is performed on the ECS to balance the total power generation with the total power demand. The ECS can absorb the rapid variation of load power for balancing the generation with the demand. However, a small-sized ECS is assumed in this study; therefore, the continuous discharging and charging operations are not available on the ECS because the stored energy level hits its lower or upper limit. To perform the continuous frequency regulation on the small-sized ECS, coordination by the conventional AGC units has been proposed to keep the stored energy level on the ECS within the prespecified level. The power generation from the AGC units increases whenever the stored energy level on the ECS decreases. On the contrary, the power generation from the AGC unit decreases whenever the stored energy level on the ECS increases. By the proposed coordination, the frequency regulation performance is highly improved. To demonstrate the efficiency of the proposed control scheme, nonlinear detailed simulations have been performed for a two-area interconnected system with a practical size in the MATLAB/ Simulink environment. The simulation results clearly indicate the advantages of the proposed control scheme.
10.1 Fundamentals of the Proposed Control Scheme
According to the frequency and tie-line power changes, the total demand of the additional generation is determined. To determine the additional generation, the area control error, ΔACE, as a linear combination of frequency deviation, Δf, and tie-line power change, ΔPtie, is defined (Equation 2.11).
In this study, an innovative frequency regulation scheme has been proposed considering the coordination between the ECS and the conventional AGC units. The basic configuration of the proposed controller for the ECS is shown in Figure 10.1. The charging/discharging level, Uecs, is specified by using the ΔACE monitored in the study area, for an output setting of ECS.
From the monitored area control error, the charging/discharging level on the ECS is determined to balance the total generation with the total demand. Following the control signal Uecs from the PI control loop, the charging/ discharging operation is performed on the ECS for the frequency regulation. Because of the specific feature of the ECS dynamics, the fast charging/ discharging operation is available on the ECS. Therefore, even the rapid variations of demand can be efficiently absorbed through the charging or discharging operation on the ECS. Since small-sized ECS is considered in this study, the regulation power from the conventional AGC units is required to keep the stored energy level of the ECS unit in a proper range and not to prevent the AGC action on the ECS.
FIGURE 10.1
Control block for the proposed regulation scheme on the ECS unit.
FIGURE 10.2
Coordinated control structure for the AGC unit.
Figure 10.2 illustrates the configuration of the coordinated controller for the conventional AGC units. In the proposed control scheme, the ECS provides its main function and the AGC units provide its supplementary function, not to prevent the charging/discharging operation on the ECS. The coordinated frequency regulation between the ECS and AGC units has been considered to balance the power demand and total power generation. Namely, the power generation from the AGC units is regulated to maintain the stored energy level on the ECS for the uninterruptible frequency regulation on the small-sized ECS. In Figure 10.2, Wr and WECS are defined as target stored energy and current stored energy, respectively. The PECS is the produced power by ECS, and Uagc-unit represents the required regulation power from the AGC unit for coordination with ECS. The power regulation command Uagc-unit is utilized for the power regulation from the AGC units.
10.1.1 Restriction of Control action (Type I)
The charging/discharging operation is performed by the control signal Uecs determined through the PI control block as shown in Figure 10.1. Whenever the stored energy reaches its upper or lower limit, the tracking of the load variation will stop until the stored energy level comes back to its prespecified operation range and the charging/discharging operation starts again on the ECS. Namely, there exist some impacts to the study system when the stored energy level hits its upper or lower limit.
To overcome this situation, restriction of the control signal Uecs has been proposed. Figure 10.3 illustrates one of the restrictions as type I to prevent the impact when the stored energy reaches its limits. The detailed restriction shown in Figure 10.3 is mathematically expressed as depicted in Table 10.1. The upper or lower limit of the control signal Uecs is modified when the stored energy level reaches its limit to prevent the unnecessary control action. In this figure, [–dWess_max +dWess_max] represents the range of acceptable operation.
FIGURE 10.3
Restriction of the control signal to prevent excessive control action (type I).
TABLE 10.1
Restriction of Control Signal (Type I)
10.1.2 Restriction of Control action (Type II)
In the aforementioned type of restriction (type I), there still exist some impacts when the stored energy level reaches its upper or lower limit, because the control signal Uecs is set to zero from some existing nonzero signals in such conditions. Therefore, an alternative restriction has also been proposed for the further improvement to prevent unnecessary control actions when the stored energy level approaches its limits. Then, the upper or lower limit of the control signal Uecs is gradually modified to zero without giving any significant impact to the study system when the stored energy level reaches its upper or lower limit. The graphical description of this concept is shown in Figure 10.4. Table 10.2 gives the mathematical expression of the type II restriction for the upper or lower limit of the control signal Uecs. Here, SP shows the switching point. Y1 and Y2 in Table 10.2 are defined as follows:
10.1.3 Prevention of excessive Control action (Type III)
When the control action is restricted by applying a type I or type II restriction, there still exist some impacts caused by the accumulation of the area control error through the integral control loop shown in Figure 10.1. To overcome this situation, the control area itself should be modified to ΔACE* according to the size of the control signal Uecs* after having type I or type II restriction, as shown in Figure 10.5. The mathematical expression for this restriction is shown in Table 10.3.
FIGURE 10.4
Restriction of the control signal to prevent excessive control action (type II).
TABLE 10.2
Restriction of Control Signal (Type II)
10.2 Study System
Here, a two-area interconnected system, shown in Figure 9.1, is used as a study system. Utility X and utility Y are interconnected through the 500 kV tie-line. Utility X consists of four subareas, A to D. Subareas A, B, C, and D have eight, five, seven, and three thermal units, respectively. In addition, four and two nuclear units are operating in subareas C and D, respectively. Four hydro units are also included in subarea D. Five and three AGC units are in subareas A and B, respectively. The ECS is set in area A of utility X. Some nonlinear detailed simulations have been performed to demonstrate the efficiency of the proposed control scheme in the Matlab /Simulink environment.
FIGURE 10.5
Restriction of the area control error to prevent excessive control action (type III).
TABLE 10.3
Restriction of Area Control Error (Type III)
Condition |
Restriction |
Uecs_upper_limit ≤ Uecs* |
AACE* = (AACE > 0) |
Uecs_lower_limit < Uecs* < Uecs_upper_limit |
AACE* = AACE |
Uecs* ≤ Uecs_lower_limit |
AACE* = (AACE < 0) |
10.3 Simulation Results
To evaluate the efficiency of the proposed coordinated AGC-ECS frequency regulation scheme, several nonlinear simulations have been performed following a random load change in subarea A and a ramp load change of 200 MW with a duration of 100 s. The capacity of the ECS is set to 1.6 MWh, and the maximum charging/discharging power is fixed at 120 MW.
Figure 10.6 illustrates the conventional frequency regulation performance. In the figure, the load change in subarea A, the load change in subarea B, the frequency deviation in subarea A, the deviation of the tie-line power, the power from the ECS, the deviation of the stored energy on the ECS, and the power change of the AGC (load-frequency control (LFC)) units in subareas A and B are illustrated from the top to the bottom. A relatively large fluctuation of the tie-line power of nearly 200 MW is caused by the random load change in subarea A.
Figure 10.7 shows the proposed frequency regulation performance with the control constraints of type I and type III. Figure 10.8 shows the proposed control system performance with the control constraints of type II and type III.
FIGURE 10.6
Performance of the conventional frequency regulation system.
The proposed control scheme is highly improved, especially when considering the type II and type III restrictions, as shown in Figure 10.8. Namely, the variations of the frequency deviation and the tie-line power deviation are small compared with those shown in Figure 10.6, where the conventional AGC is applied to the study system.
As shown in Figures 10.7 and 10.8, the stored energy level can be kept within the prespecified range through the coordination of the conventional AGC units. Therefore, a continuous AGC is available on the small-sized ECS. Furthermore, the AGC action is rocked for quite a short time range considering the restrictions of type II and type III to prevent the excessive control action, as shown in Figure 10.9.
FIGURE 10.7
Performance of the proposed frequency regulation system (type I + type III).
10.4 Evaluation of Frequency Regulation Performance
In order to evaluate the AGC performance, the following performance index, J, is defined by using the area control error of area A.
The above performance index is evaluated under various situations on the capacity of the ECS and also on the maximum charging/discharging power to or from the ECS. The calculated performance index is normalized based on the index value obtained by the conventional frequency regulation scheme. Figure 10.9 illustrates the profile of the reduction of the performance index by applying the proposed frequency control scheme, where type I and type III restrictions are considered.
FIGURE 10.8
Performance of the proposed frequency regulation system (type II + type III).
In Figure 10.9, the region of 0 to 20 indicates that the performance index is less than 20% of the conventional case. The region of 20 to 40 indicates that the performance index is greater than 20% and less than 40% of the conventional case, and so on. When considering type II and type III restrictions, the evaluated performance index is shown in Figure 10.10. The aspect of improvement is quite consistent according to the ECS capacity and the maximum power of the ECS. Namely, the improvement of the frequency regulation performance is further increasing for the larger ECS capacity and the larger power output of the ECS. From comparison of Figures 10.9 and 10.10, it is clear that the proposed control scheme with type II and type III restrictions is superior to achieve the better frequency regulation performance even on the small-sized and low-power ECS.
FIGURE 10.9
Evaluation of the frequency regulation performance (type I + type III).
FIGURE 10.10
Evaluation of the frequency regulation performance (type II + type III).
FIGURE 10.11
Effect of size of the load change on frequency regulation performance.
Figure 10.11 shows the variation of the performance index after changing the size of the ramp load in subarea B, where the capacity of the ECS is 1.6 MWh and the maximum power of the ECS is 120 MW. Here, it must be noted that all the control parameters have been determined for the ramp load change of 200 MW in subarea B. The proposed control scheme is highly robust when the restrictions of type II and type III are incorporated into it. The percentage of the performance index is less than 5% for the step load change, up to 225 MW.
10.5 Summary
A coordinated frequency regulation has been proposed for the small-sized and high-power energy capacitor system and the conventional AGC units. To prevent unnecessary excessive control action, two types of restrictions have been proposed for the upper and lower limits of the control signal as well as for the area control error. The simulation results clearly demonstrate the advantages of the proposed frequency regulation scheme. The control performance is highly improved through the proposed frequency regulation scheme.
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