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Performance analysis of automatic generation control of interconnected power systems with delayed mode operation of area control error

2015; Institution of Engineering and Technology; Volume: 2015; Issue: 4 Linguagem: Inglês

10.1049/joe.2015.0044

2051-3305

J. Nanda, Dushyant Sharma, Sukumar Mishra,

Physics of Superconductivity and Magnetism

The Journal of EngineeringVolume 2015, Issue 4 p. 164-173 ArticleOpen Access Performance analysis of automatic generation control of interconnected power systems with delayed mode operation of area control error Janardan Nanda, Janardan Nanda Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, IndiaSearch for more papers by this authorDushyant Sharma, Corresponding Author Dushyant Sharma dushyantnitrkl@gmail.com Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, IndiaSearch for more papers by this authorSukumar Mishra, Sukumar Mishra Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, IndiaSearch for more papers by this author Janardan Nanda, Janardan Nanda Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, IndiaSearch for more papers by this authorDushyant Sharma, Corresponding Author Dushyant Sharma dushyantnitrkl@gmail.com Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, IndiaSearch for more papers by this authorSukumar Mishra, Sukumar Mishra Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, IndiaSearch for more papers by this author First published: 30 April 2015 https://doi.org/10.1049/joe.2015.0044Citations: 5AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract This study presents automatic generation control (AGC) of interconnected power systems comprising of two thermal and one hydro area having integral controllers. Emphasis is given to a delay in the area control error for the actuation of the supplementary controller and to examine its impact on the dynamic response against no delay which is usually the practice. Analysis is based on 50% loading condition in all the areas. The system performance is examined considering 1% step load perturbation. Results reveal that delayed mode operation provides a better system dynamic performance compared with that obtained without delay and has several distinct merits for the governor. The delay is linked with reduction in wear and tear of the secondary controller and hence increases the life of the governor. The controller gains are optimised by particle swarm optimisation. The performance of delayed mode operation of AGC at other loading conditions is also analysed. An attempt has also been made to find the impact of weights for different components in a cost function used to optimise the controller gains. A modified cost function having different weights for different components when used for controller gain optimisation improves the system performance. 1 Introduction In interconnected power systems power is exchanged between utilities over tie lines by which they are connected. The desired operating conditions include nominal frequency, voltage profile and load flow configuration. These quantities are controlled by controlling the active and reactive powers. Automatic generation control (AGC) plays an important role in maintaining desired system frequency and tie line flow during normal operating conditions and also under small perturbations. Many investigations in the area of AGC of isolated and interconnected power systems have been reported in the past, mostly pertaining to suitable design of secondary controller. Various controller optimisation techniques for design of secondary controller such as classical, optimal, bacterial foraging, genetic algorithm, artificial neural networks, fuzzy logic, particle swarm optimisation (PSO) and so on have been reported in the literature [1-20]. However, so far in the studies that have been reported, there is no mention in the literature regarding the effect of any delay in the area control error (ACE) to actuate the secondary controller of a governor and its impact on dynamic responses of the system. Generally, the ACE command that goes to the speeder gear to regulate the real power generation is present all the time. What happens to the function of the governor and the corresponding dynamic responses if the ACE is just delayed before it actuates the supplementary controller? The motivation of delaying the ACE is that once there is a delay in ACE, the total working of the secondary controller from the instant of initiation of the perturbation (i.e., from the instant when frequency deviates from the desired value) to the steady state will be reduced and consequently the life expectancy of the controller will be increased. The governor secondary control (regulator) works only when it is actuated by ACE. If ACE is absent for some interval of time, then the governor secondary control does not work for that interval of time and thus the additional mechanical movements of governor settings and associated linkage mechanisms and so on of the governor, because of secondary control, remain absent, thus reducing wearing and tearing of the equipment for the interval. These aspects are examined by the authors for a power system consisting of two thermal and one hydro system having generation rate constraints (GRCs) and working under different loading conditions. The frequency control device is required to be fast acting and this aspect is very well taken care of by the fast acting inherent or primary control of the governor. It may be mentioned here that a governor has two controls, one the inherent or the primary control which is pretty fast, and another the slow secondary or supplementary control and this control is actuated by the ACE. Furthermore, any undesired triggering of under frequency relays is taken care of by fast acting primary control, thus giving us scope to appropriately delay the ACE to actuate the secondary control. The main concept is to explore the allowable delay to actuate the supplementary controller in order to take advantage of reduced wear and tear and more life for the governor while maintaining good dynamic response. Sensitivity analysis is also performed to see if the selected delay is acceptable for different loadings by observing the response at different loading conditions. Moreover, the robustness of the controller gains has been examined for changes in the delay in the ACE. It has been shown in the literature that optimum proportional integral (PI) controller gives same behaviour as an optimum integral (I) controller in operation of AGC of interconnected power systems. In view of the above discussion, the authors have investigated the AGC problem of a three area system provided with I controllers and with a time delay circuit for the ACE. Delay in the secondary control loop provides reduced wear and tear in the governor valve assembly linkages and also mitigates the possibility of the controller entering into saturation which leads to undesirable responses. 2 System investigated Investigations have been carried out on a three area system consisting of two thermal and one hydro system considering GRC. A step load perturbation (SLP) of 1% of rated capacity of each area has been considered in all the areas simultaneously which is justified as all the areas in general undergo load perturbations at all points of time. For the load frequency control, I controller is considered. All details for the system are given in Table A1 in Appendix. A typical value of GRC for thermal units as 3% per minute for both ramp-up and ramp-down is considered which is the case in most power plants in India. Modern turbines with GRC of 10% per minute are now operational with super critical boiler parameters. System response with GRC of 10% per minute is also observed. In normal mode of operation, ACE goes to supplementary controller all the time following a load change in the electrical power system. There is continuous change in load in the electrical network and hence ACE is generated all the time. In delayed mode of operation, the ACE is allowed to go to supplementary controller only after some time delay from the occurrence of the load change in power system as shown in Fig. 1 (encircled in dotted black). Fig. 1Open in figure viewerPowerPoint Transfer function block diagram of three area power system with delayed ACE The power system parameters are taken from [4, 10, 19, 20]. Results are obtained by giving an SLP of 1% in all the areas. The analysis carried out is based on 50% nominal loading condition in all the areas. GRC has been taken as 3% per minute and 10% per minute for the system investigated. Speed regulation parameter is taken as 4%. Detailed model of operation of AGC of interconnected power system is depicted in Fig. 1. The system is analysed for various delays in the ACE and the changes in frequency (ΔF) and tie line power (ΔP tie) of the three areas are observed corresponding to each delay. Controller gains are optimised using integral square error (ISE) criterion, where the objective is to minimise the cost function given by (1) Fig. 2 shows the modulation of ACE of the three areas for the system under study for a time delay of 0, 5, 10, 15 and 20 s with GRC of 3% per minute in the thermal areas and 1% SLP in all the three areas. 0 s delay is the normal without delay operation while for other time delays the ACE is absent to the controller for a time periods of 0–5, 0–10, 0–15 and 0–20 s. Fig. 2Open in figure viewerPowerPoint Modulated ACE of the three areas for different time delays for 1% SLP in all the areas considering GRC of 3% per minute in the thermal areas a Area 1 b Area 2 c Area 3 3 Results and analysis 3.1 Responses for equal load perturbations in the three areas Figs. 3-5 show dynamic responses for frequency and tie line power change in the three areas for 1% SLP considering GRC of 3 or 10% per minute in the thermal systems and a speed regulation parameter (R) of 4%. Optimum I controller has been used considering different delays of ACE in steps of 5 s. The controller gains are optimised by using PSO. The optimised controller gains for the three areas for 3% GRC are given in Table A2 in Appendix. Fig. 3Open in figure viewerPowerPoint Dynamic responses for frequency deviations in the three areas at various time delays for 1% SLP in all areas considering GRC of 3% per minute in the thermal areas and a speed regulation parameter (R) of 4% a Area 1 b Area 2 c Area 3 Fig. 4Open in figure viewerPowerPoint Dynamic responses for tie power deviations at various time delays for 1% SLP in all areas considering GRC of 3% per minute in the thermal areas and a speed regulation parameter (R) of 4% a ΔP tie12 b ΔP tie23 c ΔP tie31 Fig. 5Open in figure viewerPowerPoint Dynamic responses for frequency and tie line power change at various time delays for 1% SLP in all areas considering GRC of 10% per minute in the thermal areas and a speed regulation parameter (R) of 4% a ΔF 1 b ΔP tie12 It is observed in Figs. 3 and 4 (for 3% per minute GRC in the thermal area) the frequency deviation responses show slight deterioration in terms of peak undershoot but appreciable improvement is observed in peak overshoot and settling time when a delay is applied in the controller. The peak overshoot for a normal (without delay) operation is about 0.06 Hz in area 1 (Fig. 3 a) which reduces with increase in delay and is found to be even <0.01 Hz when delay is 20 s. The frequency oscillations are also reduced when a delay is introduced. Undershoot of 0.04 Hz in the second oscillation is also maximum under normal (without delay operation). Similar results are seen in the other areas as well (Figs. 3 b and c). Looking at the responses for change in tie line power flow (Fig. 4), we can see that there is considerable improvement in peak deviation for an operation of the system with delay introduced in the AGC operation while the settling time is almost same. The same is true for all the three tie lines. As it can be seen in Fig. 4, the peak deviation in tie line power flow is least for 10 and 15 s delay, and we can clearly say that a delay of about 10–15 s is welcome for ACE actuation owing to improvement in settling time and peak deviations in the responses of frequency and tie line power changes. The deviations in peak undershoot in frequency response are small as a result of the governor primary or inherent control. Moreover, as the secondary controller is slow acting because of I action, its contribution during the small time duration when ACE is absent is small and hence slight deviations are seen in peak undershoot. Similar to the results observed in Fig. 3, the results with GRC of 10% per minute (Fig. 5) show improved responses up to an ACE delay of 10 s and then the deviations in the tie line power increase with further delay in the ACE actuation. Fig. 5 a shows the frequency deviation in area 1 and Fig. 5 b shows the tie power deviation in line connecting area 1 and area 2. Similar results are obtained for other areas and lines as well. Although the frequency and the tie power response for 10 and 15 s delay are quite similar as in Figs. 3, 4; there is considerable deterioration in tie power response when delay is increased from 10 to 15 s (Fig. 5 b). Thus a higher value of GRC reduces the permissible ACE delay for retaining the quality of dynamic responses. It can thus be recommended that irrespective of 3 or 10% per minute GRC, an ACE delay of about 10 s can be comfortably accepted in practice without hampering dynamic responses. The choice of appropriate time delay stays with the utility. A larger delay may also be accepted depending on the acceptance of the deviations and the settling times. 3.2 Responses with un-optimised gains We have seen that a delay in ACE can be acceptable in a power system. The studies performed so far have been made by optimising gains for each delay. For existing systems the gains may have been tuned for in-practice operation (i.e. without delay operation). Even with these gains a delay in the controller actuation shows better response as shown in Fig. 6. Taking R 1 = R 2 = R 3 = 4% and taking a time delay of 10 s, the frequency and tie power deviations response obtained are compared with normal without delay operation at gains optimised for normal operation (i.e. 0 s delay). Fig. 6Open in figure viewerPowerPoint Comparison of dynamic responses for frequency and tie line power change in the three areas for 1% SLP in all areas considering GRC of 3% per minute in the thermal areas at 0 s delay (normal operation) and 10 s delay at gains optimised at 0 s delay a ΔF 1 b ΔP tie12 Looking at the responses it can be observed that with 10 s delay the responses are improved (similar to Figs. 3 and 4). Fig. 6 a shows that other than a slight deterioration in peak undershoot the response for delayed mode operation is better as there are less oscillations and peak overshoot is reduced. The settling time remains the same for both the cases. In Fig. 6 b, it is observed that the peak deviation is reduced to 0.032 pu in delayed mode operation from 0.04 pu in normal mode of operation. Therefore from the above responses it is eminent that a 10 s delay (even without optimising the gains) can be accepted for the system investigated without hampering the system performance in terms of peak deviation or settling time. 3.3 Responses for multiple load changes The delayed mode AGC operation is also analysed when there are multiple load changes within the period of ACE delay. A 1% load perturbation is followed by 2% load change at 4 s. Delayed mode operation at 10 s is compared with normal without delay operation and the result is shown in Fig. 7. The results obtained earlier hold good in this case too. There is a slight deterioration in frequency undershoot in delayed mode operation. The peak undershoot for 0 s delay is −1.16 Hz which is slightly increased to −1.19 Hz. The settling time remains the same for the two operating scenarios. Thus even for multiple load changes, the delayed mode operation of ACE can be accepted. Tie power deviations and frequency deviations in other areas also match the previous findings. Fig. 7Open in figure viewerPowerPoint ΔF1 when a delay of 10 s is used in AGC operation and there are multiple load changes within 10 s 4 Sensitivity analysis The system investigated has been analysed for 50% nominal loading condition in all the areas. To check the acceptance of the delay at different loading conditions, the system is analysed at 80% and 30% loadings for R = 4%, GRC = 3% per minute in the thermal areas and 1% SLP in all the areas. Similar to Section 3.2 the gains are optimised only at one condition which is the nominal 50% loading and 0 s delay. The performance with 80% loading for 10 s delay is compared with performance without delay in Fig. 8, and the performance with 30% loading for 10 s delay is compared with performance without delay in Fig. 9. The frequency deviation in area 1 and tie power deviation in one line is shown in these figures; the others show similar results. Fig. 8Open in figure viewerPowerPoint Comparison of dynamic responses for frequency and tie line power change in the three areas for 1% SLP in all areas considering GRC of 3% per minute in the thermal areas at 0 s delay and 10 s delay at 80% loading with gains optimised at nominal 50% loading and 0 s delay a ΔF 1 b ΔP tie12 Fig. 9Open in figure viewerPowerPoint Comparison of dynamic responses for frequency and tie line power change in the three areas for 1% SLP in all areas considering GRC of 3% per minute in the thermal areas at 0 s delay and 10 s delay at 30% loading with gains optimised at nominal 50% loading and 0 s delay a ΔF 1 b ΔP tie12 The results in both the cases are similar as obtained in the analysis so far. The delayed mode AGC operation enhances the frequency as well as tie power deviations, except for slight deterioration in peak undershoot in the case of frequency response. 5 Cost function optimisation An attempt has been made to change the cost function used for optimisation in order to achieve better system performance. As per the normal cost function given in (1), the contributions of tie line power change in the total cost are much less as compared with the contributions of the frequency deviations. Therefore it is justified to put more weights to the tie line deviations in the cost function so that their contributions are also reflected. Such a cost function is given in (2) below (2) Controller gain optimised using such a cost function will enhance the tie power responses of the system. The weights w 1, w 2 and w 3 are to be judiciously decided and optimised depending on the contributions in the original case. In this paper, we have made this analysis by setting all the three weights w 1, w 2 and w 3 to 50. The system response is compared with original response (obtained by choosing cost function given in (1)) in Fig. 10. Responses are shown for one area and one tie. Similar results are observed with other areas and lines as well. From Fig. 10 the results obtained at two different cost functions show that the frequency deviation response shows slight deterioration in peak undershoot but considerable improvement is observed in peak overshoot, oscillations and settling time. The settling time with original cost function is 130 s which reduces to 80 s with modified cost function. Peak overshoot in tie power with original cost function is 0.04 pu which is reduced to just 0.018 pu in the latter case. This shows that more participation of the tie power deviations in the cost will lead to improved responses. In the above analysis, the weights are all set to 50 and further optimisation of the weights may give even better performance and can further enhance the delayed mode AGC operation performance. With properly optimised weights of the cost function and then properly selecting gains accordingly, the performance of delayed mode operation of secondary controller can be made more acceptable. Fig. 10Open in figure viewerPowerPoint Comparison of dynamic responses for frequency and tie line power change in the three areas for 1% SLP in all areas considering GRC of 3% per minute in the thermal areas at 0 s delay (normal operation) at gains optimised by different cost functions a ΔF 1 b ΔP tie12 6 Conclusions A properly chosen delay in the ACE to actuate the secondary controller of the governor with optimum I controller provides dynamic responses better than without delay operation of ACE prevalent in practice. A delay in the ACE should be recommended, as wear and tear of the moving parts and linkages of governing system is reduced significantly and thus increasing the life of the governor. Sudden loading and unloading which may lead to the mal-operation of the safety valves can be avoided to a great extent in the presence of delayed ACE. The optimum I gains with nominal loading condition and operating conditions are robust and can be used under wide changes in the loading conditions and time delays. Selection of optimum weights in the cost function is also an important aspect in the AGC performance. Controller gains optimised using properly weighted cost function gives better tie line and frequency responses. 8 Appendix See Tables A1 and A2. Table A1. Nominal system parameters Description Symbol Value type of units in each area area 1 thermal area 2 hydro area 3 thermal rated capacity of each area P r1 2000 MW P r2 1000 MW P r3 4000 MW nominal frequency f 60 Hz inertia constant of the three areas H 1, H 2, H 3 5 s thermal unit hydraulic governor time constant T g 0.08 s thermal unit reheater steam turbine parameters T r 10 s T t 0.3 s K r 0.5 hydro unit mechanical governor time constant T 1 48.75 s hydro unit turbine parameters T R 5 s T 2 0.513 s Tw 1 s system loading 50% of rated capacity of each area nominal speed regulation parameters R 1, R 2, R 3 2.4 Hz/pu MW load frequency parameter D 1, D 2, D 3 0.00833 pu MW/Hz power system gains KP 1, KP 2, KP 3 120 power system time constants TP 1, TP 2, TP 3 20 synchronising coefficient T 12, T 23, T 13 0.544 GRC thermal ramp-up 3% per minute and 10% per minute ramp-down 3% per minute and 10% per minute GRC hydro ramp-up 270% per minute ramp-down 360% per minute Table A2. 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