Experience with Automatic Generation Control (AGC) Dynamic Simulation in PSS E

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1 Semens Industry, Inc. Power Technology Issue 113 Experence wth Automatc Generaton Control (AGC) Dynamc Smulaton n PSS E Lu Wang, Ph.D. Staff Software Engneer lu_wang@semens.com Dngguo Chen, Ph.D. Staff Engneer dngguo.chen@semens.com Introducton Ths artcle s wrtten to beneft PSS E customers wth ther applcaton of Automatc Generaton Control (AGC) dynamc smulaton n PSS E. Snce the publcaton of the authors artcle Automatc Generaton Control (AGC) Dynamc Smulaton n PSS E [1] and the authors paper Extended Term Dynamc Smulaton for AGC wth Smart Grds [2], the authors have been contacted by researchers, consultants and engneers worldwde for answers to some how-to questons. In ths artcle, the authors share ther experence n AGC dynamc smulaton wth PSS E customers. User-wrtten AGC Model The extended-term dynamc smulaton module n PSS E s used by the authors. The authors wrote a user-wrtten AGC model whch works very well. In a user-wrtten model, many nternal varables and arrays are accessble by the model wrters. If you are famlar wth PSS E nternal varables and arrays, you know that user-wrtten models are preferred. The authors user-wrtten AGC model s a generator model wth zero MW and Mvar njectons. All the AGC functons are mplemented n the generator model. Although PSS E does not convert a zero njecton generator to ts Norton equvalent crcut, t s suggested that the generator nternal mpedance (ZR and ZX) n the raw fle must be very bg so that the generator has no sde effect on the system. A sample generator record s shown below: 101,'GC', 0.000, 0.000, 0.000, 0.000, , 0, , E10, E10, E+0, E+0, ,1, 100.0, , 0.000, 1, If the study system s small, you can use state space dynamc smulaton n PSS E. It s requred that the ntegraton tme step must be small, e.g., s. For a large power system and long study perod, a small ntegraton tme step makes the smulaton process very long. In ths case, extended term dynamc smulaton s preferred. In case you are not famlar wth PSS E nternal varables and arrays, the Python AGC model s recommended. PSS E provdes varous Python APIs for you to mplement the AGC model. However, the model wrters are subject to the lmtatons of avalable Python APIs. In ths artcle, only the user-wrtten model and extended term dynamc smulaton are dscussed. Supervsory Control The authors used governor model GGOV3 n the AGC dynamc smulaton. Model GGOV3 s not ncluded n the PSS E standard lbrary yet. However, t s smlar to GGOV1. The block dagram of GGOV1 s shown n Fgure 1. The supervsory control nput s Pmwset whch s stored n VAR(L+6). Updatng VAR(L+6) to the setpont calculated by AGC mplements the supervsory control of the governor. If you use other types of governor models, turbne load controller model LCFB1 has to be used. The supervsory control nput Pmwset s stored n array LCREF. Updatng LCREF to the setpont calculated by AGC mplements the supervsory control of the governor.

2 Fgure 1 - Block Dagram of Governor Model GGOV1 Integraton Tme Step and Smulaton Perod An ssue wth a small ntegraton tme step s related to sngle precson float number. Sngle precson float numbers are used n PSS E to defne some nternal varables and arrays. There s no ssue at all f the smulaton perod s short. However, for long-term dynamc smulaton, ssues may arse. In PSS E dynamc smulaton, most quanttes (voltages, state varables, etc.) have reasonable value lmts. There are two exceptons: smulaton perod and generator angles (dscussed later). Assume t s the cumulatve tme value. Consder the followng update: t 1 t Δt Where t s the ntegraton tme step. Both t and t are stored n sngle precson float numbers. The above update has no ssue f the smulaton perod s short. If the smulaton perod s long, however, the round-off error n (1) wll accumulate and may be ntolerable when reaches a bg number. As an example, assume starts from zero and the half cycle ntegraton tme step s s used ( Δ t = s); the results of update n (1) are shown n Fgure 2. (1) Page 2

3 Cumulatve Tme (Tme Step = s) Cumulatve Tme (seconds) Tme (seconds) Accurate Cumulatve Fgure 2 - Cumulatve Tme ( Δ t = s) It can be seen that the cumulatve tme has ntolerable round-off error at the early stage of smulaton. Actually, the cumulatve tme has round-off error when 2. The round off error accumulates very quckly. After (tme = s), t loses sgnfcance n (1) and t becomes flat. In order to elmnate the round-off error, the followng ntegraton tme steps are suggested (n seconds): 2.0, 1.0, 0.5, 0.25, 0.125, , , , and As an example, assume starts from zero and the ntegraton tme step s s used ( Δ t = s); the results of update n (1) are shown n Fgure 3. Page 3

4 Cumulatve Tme (Tme Step = s) Cumulatve Tme (seconds) Tme (seconds) Accurate Cumulatve Fgure 3 - Cumulatve Tme ( Δ t = s) It can be seen that update n (1) has no round-off error at all untl (tme = s). After (tme = s), t loses sgnfcance n (1) and t becomes flat. So the maxmum smulaton perod s seconds or about 36.4 hours. Table 1 shows the maxmum smulaton perod for dfferent ntegraton tme steps. Integraton Tme Step (seconds) Maxmum Smulaton Perod (hours) Table 1 - Maxmum Smulaton Perod for Dfferent Integraton Tme Steps Page 4

5 If the smulaton perod s wthn the maxmum smulaton perod for the chosen ntegraton tme step, there s no round-off error n cumulatve tme. The authors prefer 0.25 second and larger as the ntegraton tme step. Usng 0.25 second, the maxmum smulaton perod s greater than 1165 hours (whch s more than enough for AGC dynamc smulaton) and also the smulaton process s reasonably fast. Asde from numercal ntegraton stablty requrement, system dynamc behavor requrement, SCADA cycle and AGC cycle, s there any other upper lmt on ntegraton tme step? The answer s YES. But why and how? The readers are encouraged to answer the questons. Another artcle (or paper) addressng ths s planned by the authors. Generator Angle Adjustment Generator angles n PSS E are stored n sngle precson float arrays. Consder the followng update: δ 1 δ Δδ Where and are generator angle and angle correcton respectvely, and both are stored n sngle precson float numbers. Update n (2) wll have a problem f δ >>. Ths may happen when the system s under frequency (or over frequency) for a long tme and then becomes normal. If a small ntegraton tme step Δ t s used, the stuaton s even worse because Δ δ s much smaller for small Δ t even wth dramatc system dsturbances. A small ntegraton tme step makes the smulaton process much longer. AGC dynamc smulaton s generally used to smulate slowly changng power systems, so a large ntegraton tme step s preferred n order to speed up the smulaton process. Even wth a large ntegraton tme step, update n (2) may stll cause a problem because there s no lmt on δ. Fortunately, addng 360 degrees to δ or subtractng 360 degrees from δ doesn t affect smulaton results n PSS E. So t s suggested to keep δ between -200 degrees and 200 degrees. If δ >200, subtract 360 from δ. If δ <-200, add 360 to δ. It s a lttle trcky to do so because a generator angle s generally a state varable: several nternal arrays should be updated accordngly. As an example to show the results from generator angle adjustment, a three-bus system (a regular generator, a wnd farm and a load) s used to run AGC dynamc smulaton. Fgure 4 shows the angle of the regular generator wth and wthout angle adjustment. Fgure 5 shows the MW output of the regular generator wth and wthout angle adjustment. Fgure 6 shows ACE (Area Control Error) (one hour tme span from 10000s to 13600s) wth and wthout generator angle adjustment. Fgure 7 shows CPS1 (Control Performance Standard 1) wth and wthout generator angle adjustment. The smulaton results show that the generator angle adjustment s correct. Although the results wth angle adjustment and the results wthout angle adjustment are matched n the smulaton, t can be concluded that smulaton results wth angle adjustment are more accurate f there are msmatches. (2) Page 5

6 Fgure 4 - Generator Angle Fgure 5 Generator MW Page 6

7 Fgure 6 - ACE Fgure 7 - CPS1 Page 7

8 Conclusons PSS E provdes customers wth the capablty to smulate AGC dynamcally under the condton of ntermttent generaton and varyng load. Ths smulaton can help the power system planner to study and understand AGC behavor when a renewable energy resource s planned. The nformaton provded n ths artcle clarfes some confuson and wll help PSS E customers to do ther AGC dynamc smulaton or long-term dynamc smulaton. Reference [1] L. Wang and D. Chen, Automatc Generaton Control (AGC) Dynamc Smulaton n PSS E, Semens PTI enewsletter Issue 107, Feb [2] L. Wang and D. Chen "Extended Term Dynamc Smulaton for AGC wth Smart Grds," IEEE Power and Energy Socety General Meetng, 2011, /PES Page 8