measurements of the distorted no-load current of a 20_60kv 6mva yy0 power transformer

1、Measurements of the distorted no-load current of a 20/60kV 6MVA Yy0 power transformerKim Søgaard Aalborg University Aalborg, Denmark soegaard@ieee.org Wojciech T. Wiechowski Aalborg University Aalborg, Denmar

2、k wtw@iet.auc.dk Claus Leth Bak Aalborg University Aalborg, Denmark clb@iet.auc.dk ABSTRACT This paper presents the results from measurements of the distorted no-load current of a 6MVA 20/60kV transformer. In the

3、execution of the measurement, the supply setup during the measurements has been changed. The changes imply different connections of the 0.4/20kV supplying transformer. The secondary winding of the supplying transform

4、er are connected in D, wye and YN, and for each connection, the measurements are repeated. The measurement is performed with constructed current and voltage transducers with an amplitude accuracy better than

5、77;1%. It has been shown that the harmonic contents in the no-load current the 6MVA 20/60kV transformer Thrige is as predicted by [6]. 1. INTRODUCTION Due to the lack of measurements of the distorted no-load current

6、 of large power transformers published in the literature, this paper is prepared. The main reason for the measurements is to validate a new power transformer model based on a procedure presented by [3]. The Ph.D. stu

7、dent Wojciech Wiechowski implements the Transformer model in DIgSILENT Powerfactory. The origin of the Ph.D. project is that the Danish transmission company ELTRA has experienced operational malfunctions of s

8、ome of the measuring and protection equipment. The conclusion after a preliminary investigation was that the reason for that were harmonic voltages and currents that propagate in the transmission grid. To invest

9、igate and map the harmonic propagation, ELTRA has offered this Ph.D. project. The Contribution of this Ph.D. project is to build a computer model of the entire transmission system in the area supplied by the Danish t

10、ransmission company ELTRA so it will reflect the real system behaviour at harmonic frequencies, up to 1-2 kHz. Special focus shall be paid on the nonlinearities present on the transmission level, which means HVDC lin

11、ks and transformer core nonlinearities. In the collaboration between Aalborg University and the Danish distribution company HEF Net, a 6 MVA 20/60 kV transformer 20/60 kV Thrige are at disposal for the author. The

12、original setup for the transformer 20/60 kV Thrige includes voltage and current transformers intended to measure the fundamental frequency component of voltages and currents. These original voltage and current transf

13、ormers do not measure with an acceptable accuracy in the frequency range dc up to 2 kHz. To measure more accurately a measuring setup has been designed and constructed. The measuring setup consists of three current an

14、d voltage transducers and a data acquisition device. The current transducer is a LEM current clamp. The voltage transducer is made of two capacitors that form a capacitive voltage divider. Originally, the 20/60 kV Th

15、rige transformer is supplied from the 60 kV bus at the 60/20 kV station Aggersund in northern Jutland. However, to ensure that malfunction of the measuring setup does not interfere with the rest of the 60/20 kV stati

16、on, this 20/60 kV transformer Thrige is supplied through the local 0.4 kV network in Aggersund. 0.4 kV supplying voltage is transformed to 20 kV through a 100 kVA transformer connected on the high voltage side in Y o

17、r YN or a 160 kVA transformer connected on the high voltage side in D. A schematic diagram of the supply setup for Thrige is shown on Figure 1, with the tree different connection on the high voltage side of the 0.4/2

18、0kV transformer. Figure 1. Schematic diagram of the supply setup for 20/60kV transformer Thrige in Aggersund. 2. MEASUREMENT SETUP 2.1. CURRENT TRANSDUCER The current transducer is a LEM PR30 current probe. The curre

19、nt transducer transforms the measured current to a voltage with a precision that is present in Table 1 next to the demands from IEC 61000-4-7[5]. Table 1. Precision for the current transducers and demands from IEC 610

20、00-4-7. Accuracy Current transducer IEC 61000-4-7 Amplitude ± 1 % ± 5 % Phase < 2 º - Frequency range DC to 100 kHz DC to 2.5 kHz The precision for the current transducer is obtained from the

21、 relevant datasheet [4]. The LEM current transducer 20/60 kV Thrige is supplied from a Delta connected transformer. The no-load current of power transformers is harmonically distorted, because is related to the

22、(sinusoidal) supplying voltage with a nonlinear characteristic of the transformer iron core. In case of 1- ph transformers, this current will contain odd harmonic, including the triplens (3,5,7,9, etc.) In the case

23、of 3-ph transformers, this natural harmonic content of the magnetising current may be altered, depending on the connection arrangement of the primary winding. If it is a grounded star (yn), then all triples harmonics

24、 (3’th, 9’th, etc.) will be still present in the current, since they will have a return path provided. In the case if we connect this primary winding of investigated transformer in not-grounded star, all the triples

25、(which in balanced systems are of zero-sequence) will be blocked- they will not flow. However, it is important to realise that not only the Y or D connection of the primary winding of this investigated transformer wil

26、l block triplens, but also the connection of the source matters. If the source is connected in not- grounded star (Y) then, again, the triplens will not have the return path provided. In our case, the secondary windin

27、g of the supplying transformer is our source, therefor the way it is connected (Y, YN or D) will have an impact on the triplen harmonics that flow out of our investigated transformer. The only return path

28、for the zero sequence current components is through the voltage transducer. The impedance of the voltage transducer for the third harmonic is 890k?. The zero sequence component of the current is for that reason expec

29、ted to be roughly zero. Since in balanced conditions, the 3’rd harmonic component is of zero sequence, it should not be present in the noload current of the Thrige transformer, since it does not have any return path.

30、 Figure 5. Measurement setup with the 6MVA transformer supplied from a delta connected 160kVA transformer. C1L1, C2L1 is the voltage transducer for phase L1. Omicron is measuring the voltage across C2L1. This is carr

31、ied out for all three phases. The currents IL1, IL2 and IL3 are converted to a voltage which the LEM current probes. The voltage generated by LEM current transducer is measured with Omicron. Next, the supply setup is

32、 changed, so the 20/60 kV transformer Thrige is supplied from the rebuild Wye connected transformer with grounded neutral. This establishes a return path through the ground for the zero sequence components. The setup

33、 is illustrated on Figure 6. Figure 6. Measurement setup with the 6MVA transformer supplied from a Wye connected 100kVA transformer with grounded neutral. The return path for the third harmonic is through the ground a

34、nd the zero sequence impedance of the wye- connected transformer. This transformer was originally a wye-connected transformer without the neutral point available; it was rebuild to make it available for the measuremen

35、ts. The zero sequence impedance is unknown but since it is a three limb transformer it is significantly smaller than the open-circuit impedance and higher than the short-circuit impedance [1]. The open-circuit impedan

36、ce and short-circuit impedance is calculated to 240.44k? and 72.11? from the data presented in Table 3. This makes the flow of the third harmonic current possible. The measurement setup in Aggersund is shown on Figu

37、re 7. Figure 7. The measurement setup in Aggersund, northern Jutland. 2.5. TRANSFORMER DESCRIPTION A Danish company Thomas B. Thrige produced the 6MVA 60/20 kV transformer. On August I 1955, the transformer was del

38、ivered. The available data are listed in Table 3. The delta connected 160kVA 20/0.4 kV ynD transformer essential data are in the lack of test report estimated [2]. The estimated results are listed in Table 3.

39、On the 0.4 kV side there is a tap changer so the output voltage on the 20kV side is adjustable in the discrete values [19.2, 20, 20,8]kV. The star connected 100kVA transformer 20/0.4kV znYN is originally without avail