Control of Power Transformer Windings during Electrodynamic Test Using High- Voltage Thyristor Valve

Control of Power Transformer Windings during Electrodynamic Test Using HighVoltage Thyristor Valve for Regulation the Angle of the Initial Inclusion Alexander Yu. Khrennikov1, Alexei Kuvshinov A.2, Vladimir Mazurov O.3 , AlexeiNesterchuk K.4 Division of electrical equipment and transmission lines, Scientific and Technical Center of Federal Grid Company of United Energy System, Moscow 1

Togliatti State University, Togliatti

2

Moscow Energy Institute (Scientific Research University), Moscow

3

Moscow Energy Institute (Scientific Research University), Moscow

4

ak2390@inbox.ru Abstract Authors consider principles of construction and functioning of high-voltage thyristor valve when carrying out electrodynamic test of power transformers. A procedure of structure determining of high-voltage thyristor valve which provides safety of carrying out short-circuit experiments for power thyristor has been developed. The authors show that high-voltage thyristor valve rules out the possibility of occurrence of abnormal and no normalized modes for potential transformer during shortcircuit experiments.

Keywords High-Voltage Thyristor Valve; Thyristor Cell; Power Transformer; Electrodynamic Test

Introduction The theme of electrodynamics tests arrangements in Russia for improving the reliability of transformer and reactor equipment (TRE), the existing testing facilities and opportunities of the TRE tests in the future are consideredin [1].

This article is a continuation of this important research.

Currently there are independent testing centers operating in Europe which are integrated into the International Association of test centers (STL, Shortcurrent Testing Laboratory). In last USSR, there were 5 major independent testing facilities for testing high voltage equipment 110 - 1150 kV. In modern Russia, such leading centers providing a complete cycle of testing electrical equipment are not available. However, testing is the only way to obtain reliable information about the characteristics, properties and behavior of high-voltage electrical equipment, especially new one in various conditions including emergency mode. Among the most complicated technical implementation, but also mostly demanded nowadays, there is electromagnetic testing of power transformers for resistance to short-circuit (SC), that conduct is only possible on a dedicated test bench [1].

Testing of power transformers for electrodynamic stability involves the deliberate creation of short-circuit at a predetermined time interval [2,3]. This test is created with the generated high-voltage thyristor valve (HVTV), which is able to switch on the specific time and to maintain the conductive state, and predetermined number of periods of mains voltage.

It should be noted that HVTV has been widely used as a part of power converter stations and inserts DC [4], static thyristor compensators of reactive power [5], setting a controlled series compensation on power lines AC [6], rectifier converters for superconducting inductive energy storage [7]. However, during the electrodynamic test, the nature of electrical interference on the HVTV is significantly different from that in the conditions of functioning as 58

International Journal of Automation and Control Engineering, Vol. 4, No. 2—October 2015 2325-7407/15/02 058-9 Š 2015 DEStech Publications, Inc. doi:10.12783/ijace.2015.0402.01

Control of Power Transformer Windings during Electrodynamic Test Using High-Voltage Thyristor Valve for Regulation the Angle of the Initial 59

a part of these systems, but in the literature this question has not been studied and requires separate consideration.

Such a task is extremely urgent, because nowadays there is an urgent need for electrodynamic testing of power transformers, with power more than 40 MVA, which is considered in this article[8-14].

Structural Scheme of HVTV Fig.1 shows a simplified structural scheme of HVTV (fig. 1a), the equivalent circuit to determine the current impact on power thyristors (fig.1b) and the temporary diagrams of HTV functioning at electrodynamic test (fig. 1, c).

HVTV power circuits during the presented by two opposite directed strings of series-connected power thyristors VS1(1)...VS1( n) and VS 2 (1)...VS 2 (n) . The control cabinet (СС) at a signal "Start" generates a series of short control pulses UI1, UI2 that along optical fibers are transferred to the HVTV potential of power thyristors VS1(1)...VS1( n) and

VS 2 (1)...VS 2 (n) and determine the amount of positive and negative power half cycles in a short circuit tests. At the end of a series of control pulses, HVTV continues to be in a conducting state until the reduction of the anode current of switched on power thyristors (Fig. 1), these thyristors) to zero, during the half cycle of the supply voltage.

At creation of HTV for electrodynamic tests, the most promising technical solutions can be used which are designed for DC power Ekibastuz - Center and have been proven experience of many years of service on DC link 330 kV substation Vyborg, North-West. These technical solutions are:  

 

modular design for indoor installation, allowing tobuild HVTV by series and parallel connection of standardized modules, each of which contains the same amount of power thyristors; tracking principle of management of power thyristors, which provides the formation of the control pulses at a predetermined range of conductivity (in this case interval experience of short circuit) at the moment of positive anode voltage; light transmission system of control pulses from the CC to power thyristors; diagnosis of each power thyristor by the presence of the anode voltage and light signaling system for monitoring potential "ground" in the CC [15-16].

a)

b)

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Alexander Yu. Khrennikov, Alexei Kuvshinov A., Vladimir Mazurov O., AlexeiNesterchuk K.

c) FIGURE.1. A SIMPLIFIED BLOCK SCHEME OF HTV (A), THE EQUIVALENT CIRCUIT (B) AND TEMPORARY DIAGRAMS OF HVTV FUNCTIONING(C), WHERE: HVTV - HIGH-VOLTAGE THYRISTOR VALVE; CC -CABINET OF CONTROL; DAV1, DAV2 - DIVIDERS OF ANODE VOLTAGE; DP1, DP2 - THE DRIVING PULSES.

Anode Voltage of HVTV The required number of series-connected power thyristorsisdefined by n

2  EC  kO  kU  k N  k I U DRM

(1)

where E s - the current value of EMF phase mains supplying the test bench; U DRM - Repetitive pulsed voltage power thyristor in the closed position; kO  1,1 - Coefficient taking into account the possibility of voltage fluctuation on the tire test stand; kU  (1,6  1,8) - The level of external constraints (lightning and switching) surge provides non-linear overvoltage limiters FV (see Fig. 1, a); k N  (1,1  1,2) - Coefficient of uneven division voltage power thyristors; ki  (1,05  1,1) - Coefficient of redundancy that providing the ability to work in the breakdown of the HVTV (510)% of power thyristors.

For example, for testing from the network of part 500 kV, HVTV should consist of n  140 serially connected power thyristors T273-1250 ( U DRM  4,2kB ) made of PC"Elektrovypryamitel".

During the experiment of short circuit voltage at the tire of test, rig will reduce to the value U S  ES 

xtr2  rtr2

xs  xtr 

2



 rtr2

xtr , xs  xtr

(2)

as xtr / rtr  24,2 for power transformers with power S nom  40MVA .

As it is seen from (2), the ratio between the values of the equivalent inductive reactance of network xs  ES2 / SS C 2 / 100  S nom determines the depth of the and inductive resistance of short-circuitof powertransformer xtr  u sc% U nom

Control of Power Transformer Windings during Electrodynamic Test Using High-Voltage Thyristor Valve for Regulation the Angle of the Initial 61

voltage dip U S during the short circuit experiment. U S 

ES  U S 1 ,  ES 1  u s c%  ( S s c / S nom ) / 100

(3)

where S S C - the short-circuit power mains; u s c % - Short-circuit voltage of the transformer ; U nom - Rated phase voltage AC transformer winding (you can take U nom  Es ).

Equation (3) allows following allowable depth of a voltage dip U S in the tests of the most powerful of power transformer

S H max to determine the required short-circuit power and the allowable amount of equivalent inductive

reactance of network S S C  100  S H max 

(1  U S ) U S , xS  xtr  . u s c %  U S (1  U S )

(4)

For example, to test the electrodynamic power transformers with a capacity up to S Hmax  1000MVA (u s c  13%) at U S  0,2 electrical networkis required with SS c  30,7GVA and xS  0,25  xtr . With the known value of the expression (2)  (4), the basic operational parameters of a short-circuit experience aredefined and baseline data to determine the current impact on power thyristors of HVTVare obtained [17-18].

Current Actions on Power Thyristors during Electrodynamic Tests The estimated value of the short circuit current, as the equivalent scheme that shown in fig. 1b, is determined by the equatation:





is-c (t )  I km  sin(t     )  sin(   )  e t /  ,

(5)

where I km  2  EC / Z  - the amplitude of the steady-state short-circuit current on the high voltage side of the under test transformer; Z  

xS  xtr 2  rtr2

- full resistanceof a short-circuit module; х S - The equivalent

inductive resistance of the network; xtr , rtr - Inductive and active components of short circuit inductance and resistance during the test of the power transformer;   arctg[( xS  xtr ) / rtr ] - Phase shift angle with respect to short circuit current EMF phase electric network;   ( xS  xtr ) /   rtr  tg /  - Time constant of the short-circuit scheme;   2 / T, T - The angular frequency and the period of the mains voltage, respectively; start of the HVTV (fig.1c).

 - The angle of the initial

For power transformers rated power S nom  (40  1250)MVA and rated voltage windings network (110500) kV inductive attitude and active components of short circuit resistance is within xtr / rtr  24,2  68,3 and the relative  value of the time constant  tr   tr / T  xtr / 2  rtr  3,85  8,84 (higher values are for power transformers of greater

power) [8, 16-20]. The amplitude I km of the steady-state short-circuit current can be in the range of 1,42 кА up to 25,4 кА , complete attenuation of the aperiodic component of fault current will take (12 15) to (26  35) periods of the mains voltage and phase shift angle short-circuit current can be assumed to be equal    / 2 .

A significant impact on the value of short-circuit current, as seen from (5), has a DC component and thus the angle of the initial inclusion of HVTV. When      / 2 and DC component is zero, and the test transformer is exposed only to establish the short-circuit current. Power thyristors VS1(1)...VS1( n) and VS 2 (1)...VS 2 (n) (fig. 1,a) are the same current influences, and conduction intervals of each are equal.

When the DC component of short circuit current is maximum, there will be significantly heavier impact on test power transformer, and the expression (5) takes the form

is c (t )  I Km  [e t /   cos t ] .

(6)

In HVTV, chain of power thyristors exposed the most severe impacts, which comprises a first, for example. In the first period of the experience of the short-circuit, power thyristors are exposed to surge current with an amplitude

62

Alexander Yu. Khrennikov, Alexei Kuvshinov A., Vladimir Mazurov O., AlexeiNesterchuk K.



(1) 1 / 2 IVS )  (1,88  1,95)  I Km 1  I Km  (1  e m

and the duration of the period close to the mains voltage. For example, if    8,84 , the conduction interval of (1) power thyristors VS1(1)...VS1( n) will be VS 1  347 electrical degrees and power thyristors VS 2 (1)...VS 2 ( n) after first (1) (1) (1) inclusion - only VS 2  27 electrical degrees. ( VS 1  0,5  VS 2  360 electrical degrees).

On the experience of the l -th period short-circuit current amplitude and duration of the conduction interval of power thyristors VS1(1)...VS1( n) amount (l ) IVS 1  I Km  (1  e m

1  (l  ) /   2 ),

(l )  (l 1) /  VS 1  2  [arccos( e



    l /   arccos( e )],  

(7)

and for the power thyristors VS 2 (1)...VS 2 (n) respectively  (l ) I VS  I Km  (1  e l /  ),   2m  (l ) l /   VS 2  2  arccos( e ).  

Start and finish of the

(8)

l  th conduction interval of power thyristors VS1(1)...VS1( n) , defined with respect to the

experience of a short circuit is given by    arccos( e (l 1) /  ),      1   l  T   arccos( e l /  ).   

t1(Hl )  (l  1)  T  t1(kl )

1

(9)

Using (6) and (9), we can write an expression for the instantaneous current of power thyristors VS1(1)...VS1( n) on the experience of the l-th period short-circuit. 

(t t1H ) /  (l ) iVS  cos  (t  t1(Hl ) )]  I Km  [k1(l )  e t /   cos(t  arccos( e (l 1) /  ))] , 1 (t )  I Km  [e (l )

(10)



where k1(l )  exp[ (l  1) /    arccos( e (l 1) /  ) / 2  ] - factor, taking into account of the decreased aperiodic component of short circuit current at the beginning of the l-th conduction interval of power thyristors VS1(1)...VS1( n) [19-20].

Likewise, the moments of the beginning and the end of the l-th conduction interval    arccos( e l /  ),      1  l  T   arccos( e l /  ),    

t 2(lH)  l  T  t 2(lk)

1

(11)

and the instantaneous values of current power thyristors VS 2 (1)...VS 2 (n) in the l-th period of experience short-circuit 

(l ) (l ) t /  iVS  cos(t  arccos( e l /  ))] , 2 (t )  I Km  [k 2  e

(12)



where k 2(l )  exp[ (l /   )  arccos( e l /  ) / 2  ] - factor, taking into account of the reduction of the aperiodic component of short circuit current at the beginning of the l-th conduction interval of power thyristors VS 2 (1)...VS 2 (n) .

Expressions (7)  (12) allow us to determine all the parameters of the current interactions on HVTV to reflect changes in magnitude and duration, occurring as it flows experience a short circuit needed to assess the thermal regime of power thyristors [19-24].

Control of Power Transformer Windings during Electrodynamic Test Using High-Voltage Thyristor Valve for Regulation the Angle of the Initial 63

Thermal Mode of Power Thyristors Emergency overload capacity of power thyristors is characterized by non-recurring impact shock in the open position I TSM , whose reference data aregiven in the form of the time dependence I TSM  f (t ) [9]. For example, for a power thyristorТ273-1250 I TSM  28kA at 10 ms, and only when I TSM  15kA at 20мС.

However, the working conditions of power thyristors during electrodynamic test differ significantly from the reference and the initial temperature of the semiconductor structure, which is determined by the temperature of the environment (in the summer can reach 40С and more), and duration of current flow (more than 100msec.).

Mode of operation HVTV during electrodynamic test is short, since the duration of the experience of a short circuit (0,1  0,2)sec. significantly has less time to establish the thermal regime in power thyristor semiconductor structure, which is characterized by transient thermal resistance "structure - the case" (about 10 sec. for power thyristors type Т273-1250). Under these conditions, the overload capability of power thyristors determined by the maximum allowable temperature of the semiconductor structure (usually 125С 150С), must not be exceeded by the time the short-circuit experiment. Pause between tests short-circuit must be sufficient to cool the structure of power thyristors to ambient temperature.

Previously, using the reference value I TSM for the duration of the current pulse of 20msec. estimated the required number of parallel-connected power thyristors for the most severe test mode with a maximum value of DC component: m

(1) I VS 1m

I TSM ( 20)

 k HT ,

(13)

where k HT  (1,1  1,2) - factor, taking into account of the uneven division of power between the parallel-connected power thyristors.

Given the expressions (10) and (13) determined by the instantaneous value of the current most loaded power thyristor in the l-th period of short-circuit shot: iT(l ) (t ) 

k HT (l )  i (t ) . m VS 1

(14)

(l ) Integrating (14) в within the limits defined by the expressions (9), calculating the average forward current I FAV , the (l ) current direct current I FRMS and power loss in power thyristors in the l-th period of short-circuit shot:

(l ) (l ) PT(l )  U TO  I FAV  rD  ( I FRMS )2 ,

(15)

where U TO , rD - threshold voltage and dynamic resistance of the power thyristor, respectively.

The total power loss in power thyristors for all time will make the experience a short circuit L

PT 

P

(l ) T

,

(16)

l 1

where L - the number of periods of experience short-circuit.

Then the temperature of the semiconductor structure of power thyristor in the end the experience of a short circuit reach [9]

T j  Ta  PT  Z (th)tja ,

(17)

where Ta - ambient temperature; Z (th)tja - transient thermal resistance corresponding to the duration of the experiment short circuit and determined from the dependence Z (th)tja  f (t ) for the type of refrigerant, which provides for standard cooling in the data sheet.

The criterion for the correct choice of the number of parallel-connected power thyristors is the ratio of T j 

64

Alexander Yu. Khrennikov, Alexei Kuvshinov A., Vladimir Mazurov O., AlexeiNesterchuk K.

T jm ,which shows that the temperature of the semiconductor structure of even the most loaded power thyristor does not exceed the limit value T jm (for example, T jm  125 С for power thyristor type Т273-1250). If T j  T jm , then value m , predetermined by the expression (13), needing to be adjusted upward and repeat the calculation of the thermal regime of the most loaded power thyristor, using relations (14)  (17) [19-24].

Conclusion 1. The use of HVTV for electrodynamic testing of power transformers provides, unlike conventional switching equipment, high precision job experience short circuit duration and magnitude of the valuation aperiodic component of short circuit current.

2. HVTV with tracking management principle provides continuous power thyristors, without pauses, the current flow through the windings of the power transformer under test during the test short circuit, as well as multiple repeatable experiments largest short-circuit DC component, including experiments with only the steady shortcircuit current.

3. The most severe impacts of current power thyristors are exposed in the first and subsequent odd half-periods experience of a short circuit when the HVTV with zero angle of initial engagement. It needs for align the current loads between power thyristorsHVTV each subsequent experience short in the wire with pre-modified to 180el.gr. by the angle of initial start.

4. The required number of parallel-connected power thyristors previously is estimated by the ratio of the surge current in the first half-cycle experience of a short circuit and shocks nonrecurring power thyristor current pulse duration 20msec., and then refinesthe highest temperature of the semiconductor structure, which by the end of the experience of the short circuit should not exceed the maximum permissible values.

5. Duration experiments of short circuit aremuch smaller than those of the thermal time setting mode in the semiconductor structures of power thyristors, however HVTV structure can be substantially simplified by the use of cooling, if the time between experiments short circuit will be sufficient to cool the thyristors to ambient temperature.

REFERENCES

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[7] Khrennikov, A.Yu. (1999) Short-circuit performance of power transformers. Test experience at Samaraenergo Co and at Power Testing Station in Togliatti, including fault diagnostics. CIGRE Study Commitee 12 Transformers. Hungary, Budapest, 14-17 June.

[8] Khrennikov, A.Yu. (2000). Power transformer's fault diagnostics at SAMARAENERGO Co, including FRA/LVI method.

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[10] Khrennikov, A.Yu., Skladchikov, A. A. (2014). Control and analysis of breakdowns of overhead transmission lines (6 – 500 кV) in the Middle Volga region. International Journal of Engineering Research & Management (IJERM), Volume 1, Issue 6, September.http://ijerm.com/ar_download.php?archive=Vol.+1+Issue+6&&archive_id=10 [11] Khrennikov, A.Yu.,Goldshtein, V.G., Skladchikov, A.A. (2010). The analysis of a condition of overhead lines of power transmission 6 – 500 кV”, Power plants, № 5.

[12] Khrennikov, A.Yu.,Goldshtein, V.G. (2007). The technical diagnostics, damages and resource of power and measurement transformers, reactors.Energo-Atomizdat( 319 p.) (in Russian).

[13] Khrennikov, A.Yu. (2009) Complex diagnostic modeling of technical parameters of power transformer- reactor electrical equipment condition, PhD Thesis, The Samara University (in Russian).

[14] Khrennikov, A.Yu. (2004). Short-circuit performance of power transformers. Test experience, including fault diagnostic, Reports оf Advanced Research Workshop on Modern Transformers (ARWtr 2004),Vigo – Spain.

[15] Khrennikov, A.Yu. (2006). Diagnostics ofelectrical equipment’sfaults, defects and weaknesses, Reports оf Conference on Condition Monitoring and Diagnosis (CMD 2006), Korea.

[16] Bertagnolli, Giorgio. (1998). Short-Circuit Duty of Power Transformers// ABB Transformatori,Legnano (Milano) – Italy. (p.

197).

[17] Khrennikov, A.Yu. (2012). Monitoring information-measuring system for detecting power transformer faults, FRA and LVItesting diagnostics experience, Reports оf Asia-Pacific Power and Energy Engineering Conference (APPEEC 2012), Shanghai, China.

[18] KhrennikovA.Yu. New «intellectual networks» (Smart Grid) for detecting electrical equipment faults, defects and weaknesses//Smart Grid and Renewable Energy, February, 2012, Volume 3, Issue 1.

[19] Khrennikov, A.Yu. (2013). Smart Grid technologies for Detecting Electrical Equipment Faults, Defects and Weaknesses.

Workshop on Mathematical Modelling of Wave Phenomena with applications in the power industry, Linnaeus University, Växjö. http://lnu.se/mmwp [20] Khrennikov, A.Yu.,Shakaryan, Y. G., Dementyev, Y. A. (2013). Shortcurrent Testing Laboratories. Short-Circuit Performance of Power Transformers, Transformer Testing Experience. International Journal of Automation and Control Engineering (IJACE), Volume 2, Issue 3. (pp. 120-127). http://www.seipub.org/ijace/AllIssues.aspx [21] Khrennikov, A. Yu. (2014). Transformer Testing Experience by LVI/FRA Methods. Short-Circuit Performance of Power Transformers.

Current

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www.vkingpub.com/VkUpload/201406131101418881.pdf [22] Khrennikov, A.Yu.,Mazhurin, R.V. ( 2014). The SF6 high-voltage electrical equipment: analysis of accident rate and service experience.

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http://www.vkingpub.com/Journal/CAIER/245.html [23] Khrennikov, A.Yu.,Mazhurin, R.V. (2014). High-voltage electrical equipment with the SF6 (sulfur hexafluoride) gas insulation: analysis of accident rate and service experience. International Journal of Engineering Research & Management (IJERM), Volume 1, Issue 5. (pp. 31-36). http://www.ijerm.com/ar_download.php?archive=Vol.1+Issue+5&&archive_id=9 [24] Khrennikov, A.Yu., Mazhurin, R.V., Radin P.S. (2015). Diagnostic Methods for Detection of Electrical Equipment’s Faults and Defects Including with Sulfur Hexafluoride Gas Insulation. Energy and Power Engineering Science, EPES 2015, 2(1): pp. 13-19. http://manuscript.sciknow.org/uploads/epes/pub/epes_14139061671.pdf Alexander Yu. Khrennikov was born in Bratsk, Russia, in 1964. He received Philosophy Doctor Degree in Electrical Engineering from Samara City University of Technology in 2009 in the field of diagnostic modeling of technical parameters of power transformer-reactor electrical equipment.

Now, he works as senior expert of Division of electrical equipment and transmission lines, Scientific and

66

Alexander Yu. Khrennikov, Alexei Kuvshinov A., Vladimir Mazurov O., AlexeiNesterchuk K.

Technical Center of Federal Grid Company of United Energy System, Russia.

The author has more than 200 scientific and technical publications. His main research interests concentrate in the field of Transformer Short-circuit testing, Transformer winding fault diagnostic, Frequency Response Analysis, Smart Grid and Information-measuring systems.

He is CIGRE member and Prof. of Moscow Energy Institute, Russia.

Alexei A. Kuvshinov was born inTogliatti, Russia, in 1953. He is a professor of Togliatti State University now.

He received Philosophy Doctor Degree in Electrical Engineering from UlyanovskState University of Technology in 2004.The author has more than 195 scientific and technical publications.

Vladimir O.Mazurov is a student ofMoscow Energy Institute (Scientific Research University), Moscow.

AlexeiNesterchuk K. is a student ofMoscow Energy Institute (Scientific Research University), Moscow.