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ИССЛЕДОВАНИЕ МИКРОПРОЦЕССОРНОЙ ТОКОВОЙ ЗАЩИТЫ ТРАНСФОРМАТОРА В РЕЖИМАХ КОРОТКИХ ЗАМЫКАНИЙ
Магистр техн. наук ЛОМАН М. С.
ОАО «Белэлектромонтажналадка» Е-mail: mihail.loman@gmail.com
INVESTIGATION OF MICROPROCESSOR TRANSFORMER CURRENT PROTECTION IN SHORT CIRCUIT FAULT MODES
LOMAN M. S.
JSC "Belelektromontazhnaladka "
Представлено исследование токовой защиты трансформатора с блокировкой от броска тока намагничивания в режимах коротких замыканий. Показано, что предлагаемый алгоритм блокировки от броска тока намагничивания может быть применен в микропроцессорных токовых защитах трансформаторов.
Ключевые слова: блокировка от броска тока намагничивания, токовая защита.
Ил. 5. Библиогр.: 3 назв.
The paper presents an investigation on transformer current protection with blocking against magnetizing inrush current in short circuit fault modes. It has been shown that the proposed magnetizing in-rush current blocking algorithm can be implemented in microprocessor current protections of transformers.
Keywords: magnetizing in-rush current blocking, current protection.
Fig. 5. Ref.: 3 titles.
The paper presents investigation of transformer current protection with inrush blocking algorithm during fault processes. Results of the previous series of experiments [1] showed that proposed inrush blocking algorithm provides the higher sensitivity to inrush currents than classical inrush blocking algorithm which is based on the presence of the second harmonic into the phase currents.
The investigation was performed by computer simulation method using mathematical models of transformer and overcurrent protection. The mathematical model of transformer was validated on the basis of the comparison with the results obtained in a field experiment [2].
Investigation includes simulation of various fault types on the 10 and 110 kV bushings of the 6.3 MV-A reducing transformer. The overcurrent stage with proposed inrush blocking algorithm was realized in 1-millisecond cycle for the reason of a detailed study.
The inrush blocking algorithm for overcurrent protections [1] estimates the ratio of the positive sequence current of the second harmonic I2h-pos to the negative sequence current of the first harmonic I1h-neg when the negative sequence current of the first harmonic I1h-pos is high enough. The blocking
signal is generated if the positive sequence current of the first harmonic does not exceed the value of the inrush current.
During the computer simulation experiments the fault oscillograms (Fig. 1-5) were obtained. The oscillograms contain the following signals:
• IA, IB and IC - secondary currents of current transformers (CT) which are installed in the 110 kV side of the power transformer in the A, B and C phases correspondingly;
• signals SG_A, SG_B and SG_C show that the value of the second harmonic current is higher than 15 % in A, B and C phases correspondingly;
• signal SG_po shows that ratio I2h-pos/I\h-neg is higher than 15 %;
• signal Kop forbids inrush blocking when I\h-negH\h-pos is lower than 25 %;
• signal Kdb forbids inrush blocking when I1h-pos is higher than 8 rated currents of the power transformer;
• Blk - inrush blocking signal of the current protection;
• MU_A, MU_B and MU_C - current protection measurement units of the А, В and С phases correspondingly. Signal shows that phase current is higher than 1.5 rated current of the power transformer;
Наука итехника, № 3, 2014
• Trip I > A, Trip I > B h Trip I > C - overcur-rent protection tripping operation of the A, B and C phases correspondingly. Overcurrent protection tripping signal is formed when two consecutive measurement unit signals occur and there is no blocking signal Blk.
During single-phase fault at the 110 kV transformer bushings (Fig. 1) CT in the A phase operates in a saturation mode which leads to the second harmonic appearance (signal SG_A) into the current IA. The ratio I2h-poJIih-neg has the value higher than 15 % and signal SG_po is formed. During single-phase fault the current I1h-neg has a high level and the signal Kop is not formed. A high level of the fault current determines the appearance of the signal Kdb which forbids the blocking signal Blk formation.
IA, A 50 0
-50 -100 ib, a ■ 50 0
-50 ■ -100 ic, A ■ 50
1 —--- i
0, 11 0,[ 2 0,03 0,04 0,05 0,06 0,07 0,1 S 0,09 0,10 t: s
-100
SG_A SG_B SG_C SG_po Kop Kdb Blk MU_A MU_B MU_C Trip I > A Trip I > B Trip I > C
1 --T-- i-—- i
0, )1 0,0 2 0,03 0,04 0,05 0,06 0,07 0, 8 0,09 0,10 t, s
0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 t, s
Fig. 1. Single-phase fault at the 110 kV transformer bushings
The overcurrent protection determines the fault mode, forms signals of the power transformer tripping (Trip I > A, Trip I > B and Trip I > C). Over-current protection operating time is 11 ms. The presence of the currents in the healthy phases B and C
is caused by short-circuit point contribution through a grounded transformer neutral.
During two-phase ground fault at the 110 kV transformer bushings (Fig. 2) CTs in the A and B phases operate in a saturation mode which leads to the second harmonic appearance (signals SG_A and SG_B) in the currents IA and IB. Current IB is less distorted, the second harmonic component of the current IB is around 15 % and the signal SG_B is unstable.
-100
/ I / I / \ / \ I 1
0, V 2 0,03VeÎ4 0,05\ ^ / it 0, 1 ]7\ y À 0,09 \ 0, / 1 b t: s
u
L_A4I
n i \ ! N / \ \ 1
0, 31 0,( 2\ 0, p 0,04\ 0, p 0,0 A 0, 1 t o, 1 A o, f 39 0, 1 1 o\ t, s
Ic, a -
0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 f. s
SG_A SG_B SG_c SG_po Kop Kdb Blk MU_A MU_B MU_c Trip I > A Trip I > B Trip I > c
-n
0,01 0,02 0,03 0,04 0,05 0,06 0,07
0,09 0,10 t, s
Fig. 2. Two-phase ground fault at the 110 kV transformer bushings
The signal of the blocking criterion SG_po is generated 10 ms after the beginning of the fault. Signal SG_po interrupts for a few milliseconds but during the most part of the fault process the signal SG_po is equal to the logical «one» which could result to overcurrent protection blocking. A high level of the fault current determines the appearance of the signal Kdb which forbids the blocking signal Blk formation.
The overcurrent protection determines the fault mode, forms signals of the power transformer tripping (Trip I > A, Trip I > B and Trip I > C). Over-
HayKa «TexHMKa, № 3, 2014
A
0
b
0
0
100
0
50
current protection operating time is 11 ms. The presence of the current in the healthy phase C is caused by short-circuit point contribution through the grounded transformer neutral.
During three-phase fault at the 110 kV transformer bushings (Fig. 3) CTs in the A and C phases operate in a saturation mode which leads to the second harmonic appearance (signals SG_A and SG_C) in the currents IA and IC. The signal of the blocking criterion SG_po is generated 10 ms after the beginning of the fault. The current I\h-nez is close to zero and the signal Kop must be in logical «one» status in three-phase fault mode with sinusoidal phase currents. But the distortions of the currents of the phases A and C cause unstable signal Kop status. However the appearance of the blocking signal Blk is prohibited by signal Kdb. The overcurrent protection determines the fault mode, forms signals of the power transformer tripping (Trip I > A, Trip I > B and Trip I > C). Over-current protection operating time is 11 ms.
Ib, а -
0, ]1 0,0 2 № УЗ 0,04 X te 0,06 £ T? 0, ]8 L Л 0,10 /
\ \ / t-. s
0, 2 0, W o;c 4 0, s o;fl6 o, rs o; ]8 0,0/ 0Д0
\ / \ t- 5
/
0, 2 \ 0,03 0,04 0,05 0,06 0, П ол as 0,09 O/TK
\J t, s
SG_A SG_B SG_C SG_po Kop Kdb Blk MU_A MU_B MU_C Trip I > A Trip I > B Trip I > C
H H 1 H 1
0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 t, s
Fig. 3. Three-phase fault at the 110 kV transformer bushings
Phase currents have sinusoidal forms during two-phase fault at the 10 kV transformer bushings (Fig. 4). The signals SG_A, SG_B, SG_C and SG_po
occur at the initial period of the fault that is caused by digital filter characteristic [3]. The digital filter uses a 20 ms window for the calculations which includes at the initial period both the pre-fault and fault modes. The blocking signal Blk is reset when the signal SG_po goes into logical «zero» status, the overcurrent stage generates signals to trip the power transformer (Trip I > A, Trip I > B and Trip I > C). Overcurrent protection operating time is 18 ms.
10 0 -10
Ib, а 10
0
-10
Ic, а 10
0
-10-
SG_A SG_B SG_C SG_po Kop Kdb Blk MU_A MU_B MU_C Trip I > A Trip I > B Trip I > C
0, :i 0,02 л 1 / L 3 0,04 ,-Ш 1 / 5 0,06^^ V 1 / 37 0,08y^6v \ 1/ 39 0,10 /Л V 1/ I
\ / \ V \ V \ V vy t.
],01 0,02 0,03 0,04 0,05 0,t
0,05 0,10 t. s
Fig. 4. Two-phase fault at the 10 kV transformer bushings
Phase currents have sinusoidal forms during three-phase fault at the 10 kV transformer bushings (Fig. 5). The current I1h-neg has a low value which is caused by the errors of the CTs, analog and digital filters. This means that the appearance of the signal SG_po is caused by low level of the signal I1h-neg but not caused by a high content of the second harmonic. This disadvantage of the I2h-pos/I1h-neg criterion is eliminated by signal Kop generation. Additionally the high level of fault current determines the appearance of the signal Kdb. The blocking signal Blk is not formed and the overcur-rent stage generates signals to trip the transformer (Trip I > A, Trip I > B and Trip I > C). Overcurrent protection operating time is 11 ms.
Наука итехника, № 3, 2014
Ia, а
Ia, а
0
0
c
0
ia, а ■ 100 -10 -20
10 /л
0 / v |/ \f \ | / } | /}
10 0, л o,o 2\_p3 0,04\j/5 0,0б\0д7 0,08\0,л9 0,icl\j/
20
Ia, а 10 0
-10-20 -
SG_A SG_B SG_C SG_po Kop Kdb Blk MU_A MU_B MU_C Trip I > A Trip I > B Trip I > C
CZb
0,01 0,02 0,03 0,04 0,05 0,[
0,03 0,10 t, s
Fig. 5. Three-phase fault at the 10 kV transformer bushings C O N C L U S I O N S
1. The analyzed blocking criterion does not retard transformer overcurrent protection tripping in fault modes. The proposed blocking algorithm provides necessary sensitivity to inrush current modes only and insensitivity to fault modes due to using the unlatch signal of the overcurrent protect-
tion (signal Kdb) and control of the content of the negative sequence current of the first harmonic (signal Kop).
2. The overcurrent stage with the proposed inrush blocking algorithm was realized in 1-milli-second cycle for the reason of a detailed study. Overcurrent protection operating time is 18 ms. Typically the protection functions into microprocessor devices are implemented in 10-millisekond cycle. Thus overcurrent protection operating time will increase to 28 ms.
3. The proposed inrush blocking algorithm can be implemented in the microprocessor current protection of transformers.
R E F E R E N C E S
1. Romaniuk, F. A., Loman, M. S., & Gvozditskiy, A.
S. (2014) Investigation of Blocking Algorithm for Transformer Current Protections in Magnetizing Inrush Current Modes. Izvestiia Vysshikh Uchebnykh Zavedenii i Energeticheskikh Ob 'edinenii - Energetika. [Proceedings of the Higher Education Institutions and Power Engineering Associations - Power Engineering], 2, 5-10.
2. Romaniuk, F., Novash, I., Loman, M., W^gierek, P., & Szrot, M. (2014) Validation of Mathematical Model of Differential Protection. Przeglqd Electrotechniczny, 3, 187-190.
3. Romaniuk, F. A., & Loman, M. S. (2012) Formation of Orthogonal Controlled Value Components in MicroProcessor Protection of Power Reducing Transformer. Izvesti-ia Vysshikh Uchebnykh Zavedenii i Energeticheskikh Ob 'edinenii - Energetika. [Proceedings of The Higher Education Institutions and Power Engineering Associations - Power Engineering], 4, 5-9.
Поступила 11.03.2014
Наука итехника, № 3, 2014
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