Določitev regulabilnih parametrov elektroenergetskega sistema za povišanje meje tranzientne stabilnosti z uporabo direktnih metod

KOPŠE, DAMIJAN (author), Mihalič, Rafael (mentor)

Pričujoča doktorska disertacija obravnava področje analize tranzientne stabilnosti elektroenergetskih sistemov (v nadaljevanju EES). Na tem področju so bili z leti razviti številni pristopi in metode, med katerimi so pomembno mesto dobile t. i. direktne metode. Te temeljijo na opisu določenega EES s funkcijami, ki imajo posebne lastnosti (govorimo o t. i. funkcijah Ljapunova in energijskih funkcijah), s čimer je mogoče stabilnost EES oceniti brez reševanja kompleksnega sistema nelinearnih diferencialnih enačb, ki skupaj z algebrajskimi enačbami opisujejo model EES. V disertaciji je predstavljena uporaba direktnih metod pri analizi tranzientne stabilnosti ter izvedbi regulacijskih strategij v EES, nato pa so prikazani še pristopi k modeliranju in vrednotenju rezultatov dinamičnih simulacij v EES. V prvem sklopu disertacije je predstavljena praktična uporaba direktne metode za določitev tranzientne stabilnosti EES. Razvili smo programsko orodje, ki omogoča izvedbo ocene področja stabilnosti z direktno metodo (uporabili smo t. i. metodo PEBS) v standardnem programu za simulacijo prehodnih pojavov v EES. Na ta način lahko uporabimo že izdelane oz. standardne modele EES poljubnih velikosti ter se izognemo razvoju osnovnih orodij za digitalno simulacijo EES. Glavna prednost uporabe direktne metode je možnost hitre ocene področja stabilnosti EES. Omeniti je potrebno, da je točnost metode v primerjavi s simulacijsko metodo nekoliko slabša zaradi določenih temeljnih značilnosti direktnih metod in zaradi uporabe poenostavljenega modela EES, na podlagi katerega je izračunana energijska funkcija EES. Za metodo PEBS je v literaturi pogosto izpostavljeno, da so v določenih primerih ocene področja stabilnosti optimistične (primer, ki je s simulacijsko metodo ocenjen kot nestabilen, je z metodo PEBS ocenjen kot stabilen). V obravnavanih testnih sistemih so bili takšni primeri relativno redki in v večjem deležu teh primerov je bilo pozitivno odstopanje kritičnih časov odstranitve motnje dokaj majhno. Statistične analize rezultatov so pokazale tudi, da je ocena področja stabilnosti v določenih primerih preveč pesimistična, tako da bi bile potrebne dodatne raziskave v smeri izboljšave algoritma metode. Poleg analize tranzientne stabilnosti EES so se direktne metode dodobra uveljavile tudi pri razvoju regulacijskih strategij za regulacijo nelinearnih sistemov, med katere štejemo tudi EES. V disertaciji je predstavljen izvirni pristop za izboljšanje tranzientne stabilnosti in dušenje nihanj s pomočjo večparametrične regulacijske naprave. Regulacijska strategija temelji na energijski funkciji EES, cilj regulacije pa je doseči čim hitrejše zmanjšanje energije sistema, pridobljene med motnjo. Odvod energijske funkcije je izražen kot funkcija moči in odvodov rotorskih kotov generatorjev v celotnem modelu EES. Na ta način dosežemo globalno optimalno (v Ljapunovem smislu) regulacijsko strategijo. Karakteristika odvoda energijske funkcije v odvisnosti od regulacijskih parametrov naprave UPFC je določena z numeričnim postopkom tekom simulacije dinamike. Na podlagi določene karakteristike so parametri UPFC regulirani na tak način, da minimiziramo odvod energijske funkcije in s tem dosežemo učinkovito evakuacijo moči generatorjev med prvim nihajem in dušenje po nastopu večje motnje v EES. Regulacijsko strategijo smo preizkusili s pomočjo računalniških simulacij sistema generator–toga mreža in večgeneratorskih testnih modelov. V disertaciji je predstavljen tudi postopek verifikacije dinamičnega modela slovenskega EES s pomočjo sistema WAMS. V ta namen smo ustvarili dinamični model dela ENTSOE omrežja s pomočjo dveh programskih orodij za simulacijo dinamike EES, ki za matematično rešitev modela uporabljata različne pristope. Rezultate simulacij smo primerjali med seboj ter z meritvami pridobljenimi s sistemom WAMS. Slovensko prenosno omrežje je zelo dobro pokrito z meritvami PMU-enot, kar smo izrabili pri prilagoditvi simulacijskega modela. Na ta način smo prilagodili tudi model preostalega ENTSO-E omrežja in potrdili ustreznost rezultatov ter podali fizikalno razlago določenih razlik med rezultati simulacij in meritvami. Pri prilagoditvi modela smo uporabili meritve WAMS-sistema za različne dogodke, ki so se v slovenskem EES zgodili med letoma 2010 in 2011, tako da smo dobili dobro ujemanje modela z meritvami sistema WAMS. Poudariti je potrebno, da so po končanih prilagoditvah modela, parametri modela ostali enaki za vse obravnavane primere, seveda pa je bilo potrebno od primera do primera prilagoditi začetno stacionarno obratovalno stanje glede na meritve.V zadnjem sklopu disertacije je predstavljeno področje sprotne analize dinamične stabilnosti, ki postaja vse bolj zaželeno orodje pri obvladovanju različnih kritičnih situacij, s katerimi se soočajo operaterji prenosnih omrežij. Sprotni analizi dinamične stabilnosti je v zadnjem času namenjene veliko pozornosti. Zasnovana naj bi bila tako, da upravljavcu EES nudi bistvene informacije o stabilnosti sistema ob prehodnem pojavu, pri čemer je bistveno, da je ocena stabilnosti izvedena znotraj časovnega okvira, ki še omogoča pravočasno sprejemanje odločitev, ki bi preprečile, da bi EES prešel mejo stabilnosti. Področje sprotnega določanja stabilnosti EES smo navezali na predstavljene direktne metode, tako da smo uporabili verificiran dinamični model Slovenskega EES in ga analizirali s pomočjo PEBS-metode, s čimer smo skušali prikazati koncept uporabe direktnih metod pri sprotni analizi tranzientne stabilnosti. Zaradi natančnosti ocene področja stabilnosti je uporabnost rezultatov omejena, tako da bi bilo direktni pristop smiselno uporabiti v kombinaciji s simulacijsko metodo (npr. za hitro analizo potencialno nevarnih motenj).

• Language: Slovenian
• Keywords: elektroenergetski sistem, tranzientna stabilnost, direktna metoda, regulacija, FACTS-naprave, verifikacija dinamičnega modela
• Work type: Doctoral dissertation
• Organization: FE - Faculty of Electrical Engineering
• Year: 2015
• PID: 20.500.12556/RUL-30680
• Publication date in RUL: 31.03.2015

Secondary language

• Language: English
• Title: DETERMINATION OF POWER-SYSTEM CONTROLLABLE PARAMETERS FOR TRANSIENT STABILITY ENHANCEMENT USING DIRECT METHODS

Abstract

This doctoral thesis deals with the electric power systems (EPS) transient stability analysis. Many approaches and methods have been developed over the years and important place among them goes to the so-called direct methods. These methods are based on representing the EPS using functions exhibiting special properties (i.e. Lyapunov function and transient energy function). Direct methods allow one to assess the EPS stability level without addressing the complex system of nonlinear differential equations which along with the algebraic equations describe the EPS model. This thesis presents the application of direct methods for transient stability analysis and implementation of EPS control strategies. Approach to modelling and EPS dynamic simulation results evaluation is also presented. In the first part of the thesis practical application of the direct EPS transient stability assessment is presented. A software tool was developed for this purpose that allows one to assess the EPS stability via direct method (we used the well-known PEBS method) within a standard software for the EPS dynamic simulations. In this way already available (standard) EPS models of an arbitrary size can be used. Besides, development of the basic EPS simulation tools was avoided in this manner. The main advantage of the direct approach is the possibility of forming a relatively fast recognition of the EPS transient stability boundary. It should be noted, however, that the accuracy of the direct method is slightly worse compared to the simulation method due to the certain fundamental properties of the direct methods and the use of a simplified EPS model, which is a basis for the transient energy function calculation. In the literature it is often pointed out that PEBS method is in certain cases known to produce optimistic estimates of the stability region (i.e. contingency which is determined as an unstable using the time-domain simulation appears as stable using the PEBS method). Such cases appeared relatively seldom in the analysed test systems. In a larger proportion of these cases the positive deviation of critical clearing times was relatively small. The statistical analysis of the results also showed that the assessment of the stability region was too conservative in some cases, which means that further research and additional improvements of the developed method are required. In addition to the EPS transient stability assessment, direct methods are also well established in the field of control strategies for the non-linear systems, which also include EPS. In this thesis we present a novel approach to improving the transient stability and power oscillation damping using a multi-parametrical control device. The control strategy is based on the use of an EPS energy function. The aim of the control is to achieve a rapid dissipation of the energy gained during the disturbance. The energy-function time derivative is expressed as a function of the generator’s electrical powers and the rotorangle derivatives of the entire power system. In this way a globally optimum control strategy, in the Lyapunov sense, is achieved. A characteristic of the energy-function time derivative relative to the UPFC control parameters is numerically determined using an online procedure during the dynamic simulation. Based on the determined characteristic the UPFC parameters are controlled in such a way as to minimize the energy-function derivative and achieve an efficient power evacuation during the generator’s first swing and the power oscillation damping after the large disturbance in the EPS. The control strategy was verified with computer simulations for Single-Machine-Infinite-Bus (SMIB) and multi-machine test-system models, and the results are in line with the theoretical considerations. In this thesis a validation of a dynamic model of a Slovenian EPS using WAMS is also presented. For this purpose a dynamic model of a part of the ENTSO-E system was constructed by applying two professional dynamic-simulation software tools that use different approaches to obtain mathematical solutions for the model to solve the problem. The simulation results were compared with each other as well as with the measurements gathered by the WAMS. Namely, Slovenia has almost full coverage of PMU measurements at the high-voltage level (220- and 400-kV) buses. This fact has been used to adapt the model, especially for the representation of the rest of the ENTSO-E network, as well as for the validation of the results and explaining the physical background for the existing differences. WAMS measurements for several different events occurring during 2010 and 2011 were used to adapt the model, which then exhibits a good match in all situations recorded by the WAMS. The physical background for the resulting deviations is presented. However, it should be pointed out that, after the model was finalized, for all the validated cases the system element parameters remained unchanged. Of course, depending on the simulated case, the initial steady-state conditions were set according to the measurements. In the last section of the thesis a field of an online dynamic security analysis (DSA) is presented. Tools for an online DSA are very useful for the management of critical situations in a transmission system operation. Online DSA has received a significant attention in the last years. It should be designed to give the essential information about the EPS stability during contingencies to the transmission system operator, whereby it is essential that the stability assessment is carried out within the time frame that enables taking appropriate measures which could prevent EPS reaching the stability region limit. The online DSA was linked to the direct method by using the dynamic model of the Slovenian EPS verified with the WAMS and analysed using the PEBS method as a reference. Due to the variable accuracy of the stability region assessment, the applicability of the results is limited, so it would be reasonable to apply the direct approach in a combination with the time-domain simulation method (e.g. for the fast scan of potentially dangerous contingencies).

• Keywords: power system, transient stability, direct method, power system control, FACTS devices, dynamic model validation