Digital protection of power transformers against internal faults

U ovoj doktorskoj disertaciji predstavljeni su algoritmi za digitalnu zaštitu energetskih transformatora (ET-a) od unutrašnjih kvarova. Predstavljeni algoritmi za zaštitu od kratkih spojeva namotaja ET-a i za zaštitu ET-a od zemljospojeva baziraju se na primeni digitalne fazne komparacije u vremenskom domenu. Digitalni fazni komparator ne računa direktno fazni pomeraj između signala koje poredi, već proračunava integral proizvoda dva signala na intervalu dužine polovine osnovne periode signala. Ispitana je mogućnost primene fazne komparacije primarne i sekundarne struje za detekciju unutrašnjih kratkih spojeva u ET-u. U normalnom režimu rada i tokom kvarova van zone zaštite struje primara i sekundara su u fazi, dok se u slučaju unutrašnjih kratkih spojeva pojavljuje fazni pomeraj između ove dve struje. U disertaciji je pokazano da fazni pomeraj između struja primara i sekundara tokom unutrašnjih kratkih spojeva zavisi kako od procenta kratkospojenih navojaka, tako i od parametara mreže i radnog režima koji je prethodio kvaru. Korišćenjem fazne komparacije primarne i sekundarne struje mogu se detektovati unutrašnji kratki spojevi. Zbog postojanja opadajuće jednosmerne komponente u struji kvara algoritam je proširen digitalnim filterom opadajuće jednosmerne komponente, a zbog postojanja viših harmonika digitalnim Fourier-ovim filterom. Na ovaj način poboljšane su performanse algoritma. Zatim je predstavljen algoritam koji koristi novi pristup u rešavanju problema nepotrebnog reagovanja diferencijalne zaštite ET-a usled kvarova van štićene zone praćenih zasićenjem strujnih transformatora (ST-a). Novi pristup se zasniva na faznoj komparaciji primarne i sekundarne struje i služi za indikaciju prisustva kvara van štićene zone. Prikazani rezultati potvrđuju da algoritam može da identifikuje prisustvo spoljašnjeg kvara i aktivira signal za blokadu pre nego što ST-i uđu u zasićenje. Njegovom primenom uspešno bi se moglo eliminisati nepotrebno delovanje diferencijalne zaštite i na taj način povećala njena sigurnost u radu. U nastavku je predstavljen algoritam za zemljospojnu zaštitu energetskog transformatora koji se bazira na faznoj komparaciji sume struja na priključcima ET-a i struje neutralnog provodnika. Prikazani algoritam predstavlja novo rešenje za realizaciju zemljospojne zaštite energetskog transformatora. Primenom predstavljenog algoritma eliminisalo bi se nepotrebno reagovanje konvencionalne zemljospojne zaštite ET-a bazirane na diferencijalnom principu tokom spoljašnjih kvarova i uključenja neopterećenog ET-a praćenih zasićenjem ST-a. Algoritam koristi signale koji se ne filtriraju digitalnom Fourier-ovom transformacijom. Takođe, algoritam ne koristi blokadu po drugom harmoniku što ima prednost u brzini formiranja signala reagovanja. Prikazano je da se dodatnim usrednjavanjem vrednosti indeksa usmerenosti povećava otpornost algoritma na oscilacije indeksa koje nastaju usled zasićenja ST-a. Svi algoritmi koji su predstavljeni u ovoj doktorskoj disertaciji su testirani pomoću signala koji su snimljeni u laboratoriji na fizičkim modelima jednofaznog i trofaznog energetskog transformatora i pomoću kompjuterski generisanih signala. Prednost algoritama, koji su predstavljeni u ovoj doktorskoj disertaciji, za primenu u zaštiti energetskog transformatora čini njihova jednostavnost, kratko vreme odziva, pouzdanost i sigurnost u radu. Predstavljeni algoritmi se lako mogu implementirati u već postojeće jedinice za zaštitu energetskog transformatora. Zaštita energetskog transformatora najčešće obuhvata nekoliko jedinica, među kojima su diferencijalni releji i zemljospojni releji. Za rad pomenutih jedinica vrši se merenje struja na priključcima transformatora i struje neutralnog provodnika. Tako da su u već postojećim jedinicama za zaštitu ET-a, signali potrebni za rad predloženih algoritma obezbeđeni. Na osnovu razvijenih algoritama za zaštitu energetskog transformatora kreiran je softverski alat pomoću koga su korisniku slikovito i jasno približeni principi rada tradicionalne diferencijalne zaštite i zemljospojne zaštite ET-a, i predstavljen je rad algoritama za zaštitu koji se baziraju na faznoj komparaciji u vremenskom domenu. Softverski alat omogućava da se algoritmi za zaštitu testiraju učitavanjem realnih signala koji su snimljeni u laboratoriji ili korišćenjem signala koji su generisani kompjuterskim simulacijama. Kreirani softverski alat se može koristiti u edukativne svrhe.Algorithms for digital protection of power transformers (PT) against internal faults have been presented in this doctoral dissertation. The presented algorithms for protection against short circuits of PT windings and for protection against ground-faults are based on time domain digital phase comparison. The digital phase comparator does not calculate the phase displacement between the signals which are being compared, but calculates the integral of the product of two signals over one basic half-period of the signals. The possibility of applying the phase comparison of the primary and secondary currents aimed at detecting internal faults in PTs has been investigated. In normal operating regimes and during faults outside the protection zone, the primary and secondary currents are in phase, while in case of internal short-circuit faults a phase shift appears between these two currents. In this dissertation, it has been shown that the phase shift between the primary and secondary currents during internal short-circuits depends both on the percentage of the short circuited windings and the network parameters as well as operating regime prior to the fault. It is possible to detect internal short-circuit faults by utilizing the phase comparison of the primary and secondary currents. The algorithm has been expanded by adding a digital filter for the dc component due to the existence of a decreasing dc current component in the fault current; in addition to this, the algorithm has been extended by adding the digital Fourier filter due to the existence of higher harmonics. In this way, the algorithm performance has been enhanced. Next, the algorithm that uses a new approach to solve the problem of misoperation of the transformer differential protection due to faults outside the protected zone accompanied by current transformer (CT) saturation has been presented. The new approach is based on the phase comparison of the primary and secondary currents, and is used as indication of an external fault presence. The presented results confirm that the algorithm can identify an external fault presence and activates the blocking signal before CTs reach saturation. Its application would successfully eliminate an unnecessary operation of the differential protection and so increase its security. In the part that follows, an algorithm for restricted earth fault protection of power transformers has been presented based on the phase comparison of the sum of phase currents and the neutral conductor current. The presented algorithm represents a new solution for realization of restricted earth fault protection of power transformers. By applying this algorithm one would eliminate false tripping of the conventional restricted earth fault protection of PTs based on the differential principle during external faults or energization of an unladen PT accompanied by CT saturation. The algorithm utilizes the signals that are not filtered by means of the digital Fourier transform. Moreover, the new method does not make use of second harmonic blockade which results in a high speed of making trip decision. It has been shown that an additional averaging of the directional index values increases robustness of the algorithm to oscillations of the directional index caused by CT saturation. All algorithms presented in the thesis have been tested by the laboratory recorded signals using physical models of a single-phase or three-phase PT and also by computer generated signals. The advantage of the algorithms, which have been presented in this doctoral dissertation, in the domain of their application in PT protection lies in their simplicity, short response time, reliability and security. The presented algorithms can easily be implemented in already existing PT protection units. The PT protection usually requires several units, these being differential relays and ground fault relays. The phase currents at transformer terminals and current of the neutral conductor are measured with the aim of enabling proper function of these units. In such a way, the already existing PT protection units require signals necessary for the proposed algorithms’ functionality. On the basis of the developed PT protection algorithms a software tool has been created with the help of which the principles of operation of the traditional differential protection and restricted earth fault protection of power transformers have been brought closer to an end user in order to understand them visually and clearly. In addition to this, the performance of new protection algorithms that are based on time domain phase comparison has been presented. The software tool enables the protection algorithms to undergo testing by loading the genuine signals, recorded in the laboratory, or by means of computer simulation generated signals. The created software tool can also be used for educational purposes