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Analyses of vacuum circuit breaker switching transients in medium voltage networks with respect to LC filters of solar converters T. Kuczek, M. Florkowski, W. Piasecki ABB Corporate Research Center 13A Starowislna St., Krakow, Poland [email protected], [email protected], [email protected] Abstract: This paper presents possible transient states related to vacuum circuit breaker operation in PV power plant. Case study of transformer de-energization under no-load conditions was analyzed. The variation with respect to up-to-date studies is related to determination of solar converters LC filters influence on multiple arc re-ignitions occurrence during vacuum circuit breaker operation. Research covers both laboratory measurement as well as PSCAD simulations. key words: vacuum circuit breaker, transients, switching, LC filters, solar converters, simulations verification I. INTRODUCTION The motivation for this study was driven by significant development and expansion of photovoltaic (PV) power plants in modern electrical power systems. Photovoltaic market has grown by a factor of hundreds of percent [1] during last 13 years. It covers both rise of small household installations as well as large photovoltaic plants with the peak power capability of 500 kW p and more [2]. Photovoltaic cells generate power that is dependent on solar irradiation and ambient temperature [3, 4]. Voltage and current at DC are converted by means of power electronic inverter to the AC side and then transferred to the medium voltage level by LV/MV transformer [6]. Such circuit may be subjected to de- energization operation that is usually performed by vacuum circuit breakers (VCB). At certain network conditions such operation may result in high peak of the transient recovery voltage, which can lead to significant overvoltages generation at the transformer terminals. Such effects were analyzed for example in [7] and [8], but for other applications like no-load transformer or arc furnace transformer, not for PV related. This paper is focused on determination of influence of solar converter’s LC filters on transformer de-energization during no-load conditions. Research covered both laboratory experiments as well as PSCAD numerical simulations. II. PV PLANT LAYOUT OF CONCERN The layout of concern in this study is presented in Figure 1. PV panels generate DC voltage and current, which are dependent on solar irradiance and ambient temperature. Manufacturers of PV panels provide nonlinear characteristics of generation with respect to above mentioned factors [3], [4]. The DC power has to be converted into AC by means of DC/DC and DC/AC power electronic converters. For DC/DC boost converters are most commonly utilized since DC voltage has to be appropriately adjusted in order to transfer the power by the inverter from PV panels into the external grid. The LC filters are required in order to limit the ripple and the THD in voltage and current [5]. They are designed according to desired output current, voltage, switching frequency and allowable limits of peak-to-peak ripple and THD. For example, 5% value can be found in the literature [6]. PV LV VCB external grid DC DC DC AC MV MV cable LC Figure 1. Equivalent circuit of grid connected PV plant As mentioned earlier, LV/MV transformer may be subjected to switching off operations by means of vacuum circuit breaker. This phenomena is well described in literature [7, 8, 9]. However, this paper focuses on influence of converter’s LC filters on switching conditions during vacuum circuit breaker operation. This is due to the fact that those filters may change the total impedance as seen from operated vacuum circuit breaker’s terminals, thus influencing the natural frequency of the circuit being switched. Based on above clarification, several laboratory tests of switching off operations were conducted in network prepared on AGH University of Science and Technology in Kraków, Poland. Measurement results allowed to develop PSCAD model that was utilized for experimental results verification as well as further analyses. III. VACUUM CIRCUIT BREAKER OPERATION Vacuum circuit breakers use vacuum as a quenching medium for electrical arc suppression, which appears across breaker’s contacts during any switching operations. Thanks to this method, the dielectric withstand between circuit breaker contacts is approximately 10 times larger than in air at atmospherical pressure. As a result, it is possible to decrease the gap between contacts inside the vacuum chamber of the circuit breaker. Several conditions have to be fulfilled in order

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Page 1: Analyses of vacuum circuit breaker switching transients in ...cld.persiangig.com/dl/qpTNK/T9kNGTkaUo/Analyses of... · switching may result in significant overvoltage hazards for

Analyses of vacuum circuit breaker switching

transients in medium voltage networks with respect to

LC filters of solar converters

T. Kuczek, M. Florkowski, W. Piasecki ABB Corporate Research Center

13A Starowislna St., Krakow, Poland

[email protected], [email protected], [email protected]

Abstract: This paper presents possible transient states related

to vacuum circuit breaker operation in PV power plant. Case study of transformer de-energization under no-load conditions was analyzed. The variation with respect to up-to-date studies is related to determination of solar converters LC filters influence on multiple arc re-ignitions occurrence during vacuum circuit breaker operation. Research covers both laboratory measurement as well as PSCAD simulations.

key words: vacuum circuit breaker, transients, switching, LC filters, solar converters, simulations verification

I. INTRODUCTION

The motivation for this study was driven by significant

development and expansion of photovoltaic (PV) power plants

in modern electrical power systems. Photovoltaic market has

grown by a factor of hundreds of percent [1] during last 13

years. It covers both rise of small household installations as

well as large photovoltaic plants with the peak power

capability of 500 kWp and more [2]. Photovoltaic cells

generate power that is dependent on solar irradiation and

ambient temperature [3, 4]. Voltage and current at DC are

converted by means of power electronic inverter to the AC side

and then transferred to the medium voltage level by LV/MV

transformer [6]. Such circuit may be subjected to de-

energization operation that is usually performed by vacuum

circuit breakers (VCB). At certain network conditions such

operation may result in high peak of the transient recovery

voltage, which can lead to significant overvoltages generation

at the transformer terminals. Such effects were analyzed for

example in [7] and [8], but for other applications like no-load

transformer or arc furnace transformer, not for PV related. This

paper is focused on determination of influence of solar

converter’s LC filters on transformer de-energization during

no-load conditions. Research covered both laboratory

experiments as well as PSCAD numerical simulations.

II. PV PLANT LAYOUT OF CONCERN

The layout of concern in this study is presented in Figure 1.

PV panels generate DC voltage and current, which are

dependent on solar irradiance and ambient temperature.

Manufacturers of PV panels provide nonlinear characteristics

of generation with respect to above mentioned factors [3], [4].

The DC power has to be converted into AC by means of

DC/DC and DC/AC power electronic converters. For DC/DC

boost converters are most commonly utilized since DC voltage

has to be appropriately adjusted in order to transfer the power

by the inverter from PV panels into the external grid. The LC

filters are required in order to limit the ripple and the THD in

voltage and current [5]. They are designed according to desired

output current, voltage, switching frequency and allowable

limits of peak-to-peak ripple and THD. For example, 5% value

can be found in the literature [6].

PV

LV

VCB

external

grid

DC

DC

DC

AC

MV

MV cableLC

Figure 1. Equivalent circuit of grid connected PV plant

As mentioned earlier, LV/MV transformer may be subjected

to switching off operations by means of vacuum circuit breaker.

This phenomena is well described in literature [7, 8, 9].

However, this paper focuses on influence of converter’s LC

filters on switching conditions during vacuum circuit breaker

operation. This is due to the fact that those filters may change

the total impedance as seen from operated vacuum circuit

breaker’s terminals, thus influencing the natural frequency of

the circuit being switched.

Based on above clarification, several laboratory tests of

switching off operations were conducted in network prepared

on AGH University of Science and Technology in Kraków,

Poland. Measurement results allowed to develop PSCAD

model that was utilized for experimental results verification as

well as further analyses.

III. VACUUM CIRCUIT BREAKER OPERATION

Vacuum circuit breakers use vacuum as a quenching

medium for electrical arc suppression, which appears across

breaker’s contacts during any switching operations. Thanks to

this method, the dielectric withstand between circuit breaker

contacts is approximately 10 times larger than in air at

atmospherical pressure. As a result, it is possible to decrease

the gap between contacts inside the vacuum chamber of the

circuit breaker. Several conditions have to be fulfilled in order

Page 2: Analyses of vacuum circuit breaker switching transients in ...cld.persiangig.com/dl/qpTNK/T9kNGTkaUo/Analyses of... · switching may result in significant overvoltage hazards for

to successfully interrupt the current, which happens when the

instantaneous current value drops below a threshold level

referred to as chopping current. Due to this feature VCB

switching may result in significant overvoltage hazards for

switched devices, such as electric machines, transformers or

shunt reactors. Several factors have influence on overvoltages

generation. The simplified circuit that represents external

network, vacuum circuit breaker and switched off transformer

is illustrated in Figure 2. It can be easily adopted to switching

off operations of transformers, shunt reactors or motors.

Figure 2. Transformer switching off – simplified single line diagram;

U – network source voltage, LZ, CZ – network inductance and capacitance,

LP1, LP2 – inductance of connections at both sides of vacuum circuit breaker W,

C0, R0, L0 – equivalent capacitance, inductance and resistance of transformer

Opening operation results in Transient Recovery Voltage

(TRV) that appears between the VCB’s contacts. The natural

frequency fn of its oscillations is determined by the inductance

and capacitance of L0-C0 circuit according to formula:

002

1

CLfn

(1)

During the contacts movement, TRV arises parallel to the

dielectric withstand UR of the vacuum gap. Most common

approach is to represent it as linear function in time domain:

UR = A(t – topen)+B (2)

where:

A – Rate of Rise of Dielectric Strength (RRDS),

B – initial dielectric withstand,

topen – opening time instant.

The RRDS depends on velocity of contacts movement as

well as the condition of vacuum inside breaking chamber.

Typically it is reported that RRDS value accounts between

2 kV/ms to 50 kV/ms [10]. Understanding of two above

mentioned curves (TRV and dielectric withstand) is essential

for description of entire process of current breaking. When

contacts start to open an electric arc is initiated. Once it is close

to its natural zero crossing, it is suddenly chopped – in modern

VCBs at approximately 3-5 Amps [11]. It results in TRV rising

– every time, when the TRV exceeds the dielectric withstand,

an arc is re-ignited and then chopped again. The process is

repetitive and it lasts until the dielectric withstand exceeds the

TRV. This effect may be hazardous to transformer’s insulation,

since at certain condition it may lead to significant overvoltage

escalation with high peak values as well as steepness, which

may exceed maximum permissible values.

IV. LABORATORY MEASUREMENTS

A. Test Stand Description

The laboratory tests covered measurement of overvoltages

arising during de-energization of 20 kVA distribution

transformer (Fig. 3). It was supplied from low voltage supply

network through step-down autotransformer connected in

series with 250 kVA distribution transformer. The

autotransformer was operating at 0.1 kV in order to set the

voltage at the 20 kVA transformer to 6 kV. Utilized vacuum

circuit breaker was rated at 12 kV and 1250 A of current. It

was motor spring operated. Electrical parameters of all

components utilized in the system are listed in Table I.

0.1 kVVCB

supply

network

6 kV

85 m MV

cable

voltage

6 kV 0.4 kV

1 m wire

connection

L

C

20 kVA250 kVA

Figure 3. Laboratory measurement test stand equivalent circuit

TABLE I COMPONENTS OF LABORATORY TEST STAND

Component Parameters Value

250 kVA

transformer

UP/US 15.75 kV / 0.4 kV

I0 0.25 %

uk% 4.5 %

20 kVA

transformer

UP/US 6 kV / 0.4 kV

I0 4.23 %

uk% 4.3 %

winding

capacitances

Cp_g = 3 nF

Cp_ph_ph = 0.2 nF

Cp_s = 1 nF

MV cable length 85 m

impedance Z 50 Ω

Cp_g – capacitance between primary winding and ground

Cp_ph_ph – capacitance between phases of primary winding

Cp_s – capacitance between primary and secondary winding

B. Measurement Results Three separate configurations of switching off operations

were performed, namely:

configuration 1: LC filters not connected, configuration 2: L = 200 µH, C = 1 µF (wye ungrounded),

configuration 3: L = 200 µH, C = 25 µF (wye ungrounded).

Voltage was measured for each scenario at the 20 kVA

distribution transformer’s primary terminals. Several switching

off operations were performed in each scenario in order to

eliminate statistical disorders. Representative waveforms are

illustrated in Figure 4 to Figure 6.

At first Figure 4 was analysed, since this gives direct input to

PSCAD simulations (section V). First configuration presents a

typical scenario, when transformer is subjected to switching off

operation under no-load conditions. It results in generation of

multiple arc re-ignitions. Based on measured waveforms, the Rate of Rise of Dielectric Strength can be easily determined.

As marked in Fig. 4, during the tests it was equal to 4.5 kV/ms.

When analysing further figures, one may see several effects of

LC filters presence.

Page 3: Analyses of vacuum circuit breaker switching transients in ...cld.persiangig.com/dl/qpTNK/T9kNGTkaUo/Analyses of... · switching may result in significant overvoltage hazards for

0 5 10 15 20-10

-5

0

5

10

t [ms]

Up

[kV

]

Figure 4. Laboratory test, configuration 1, LC filters not connected

0 5 10 15 20-10

-5

0

5

10

t [ms]

Up

[kV

]

Figure 5. Laboratory test, configuration 2, L = 200 µH, C = 1 µF

0 10 20 30 40 50-10

-5

0

5

10

t [ms]

Up

[kV

]

Figure 6. Laboratory test, configuration 3, L = 200 µH, C = 25 µF

It is visible that LC filter decreases the overall natural

frequency of the disconnected circuit (including transformer

and MV cables). For configuration with 1 µF capacitance the

overall effect is less visible than for 25 µF. It comes out of the

fact that the higher the capacitance of the filter, the lower is the frequency of oscillations, according to formula (1). In each

case, after successful current breaking, voltage oscillations are

damped within dozens of miliseconds, depending on the

capacitance of the filter. Damping comes out of resistive losses

of transformers and cables. For configuration 3 multiple arc re-

ignitions are almost totally eliminated – some are visible, but

those are negligible. Summarized measurement results were

presented in Table II. It compares maximum overvoltage peak

values, du/dt of a single spark and number of multiple arc re-

ignitions.

TABLE II LABORATORY TESTS SUMMARY

Configuration

Up du/dt fn no of arc

re-ignitions

[kVp] [kV/µs] [Hz] [maximum

per phase]

1 without LC filters 8.8 27.1 108 9

2 L = 200 µH, C = 1 µF 7.8 15.9 76 5

3 L = 200 µH, C = 25 µF 5.9 4.8 40 1

V. PSCAD SIMULATIONS

A. Model Description The model was based on the network diagram presented in

Figure 3. Parameters of the circuit were based on nominal

ratings as well as achieved measurement results, as mentioned

earlier (Fig. 4). PSCAD diagram is presented in Figure 7.

R=0

3 [n

F]

cable_85...#1#2 #1 #2

0.2

[nF

]

0.2

[nF

]

0.2

[nF

]

1 [nF]

200 [uH]

25

[uF

]

25

[uF

]

25

[uF

]

0.1

[oh

m]

VCB

0.1

[oh

m]

0.2 [nF]

0.2 [nF]

0.2 [nF]

A B C

LV_5_m_ca... 3 [n

F]

1.8 [nF]

4.8

[nF

]

LV_10_m_c...#1#2

Figure 7. PSCAD diagram

Distribution transformer models were represented by means

of “3-phase 2-winding transformer” component from PSCAD

masters library. The no-load current was measured during

laboratory experiments and it was equal to 4.23% (81 mARMS).

It determines the equivalent inductance of the transformer at

no-load state. Phase-to-phase, winding-to-ground and winding-to-winding capacitances were added, too. Magnetization of the

core was also included with knee point at 1.2 p.u. More

attention was paid to the 85 m MV cable model. During the

numerous sets of preliminary simulations it was determined

that the best convergence in terms of overvoltage waveforms is

achieved when cable was modelled as surge impedance. Wave

propagation speed as well as high frequency resistance for

damping were also established in the same way. The goal was

to achieve the same overvoltage peak values, TRV’s steepness

and frequency of oscillations after successful current breaking.

The following final data was used for cable modelling:

surge impedance Zc = 50 Ω,

wave propagation speed v = 200 m/μs,

per unit resistance R0 = 0.05 mΩ/m.

Finally, the vacuum circuit breaker model was prepared

according to principles described in section III. The controlling

algorithm of vacuum circuit breaker was based on PSCAD

CSMF: Continuous System Model Functions. The “A”

parameter from formula (2) was set to 4.5 kV/ms (measured).

Entire process of model verification is explained in Figure 8.

experimental results:

Up_L , fn_L , duL/dt

switching off

no

switching off

yes

no

yes

model verified VCB

switching off

fn_L = fn_S

Up_L = Up_S

fitting of:

cables: v, R0

transformers: capacitances, saturation

VCB: opening time instant

fitting of:

VCB: RRDS

Figure 8. PSCAD model fitting process

1/fn

RRDS = 4.5 kV/ms

Page 4: Analyses of vacuum circuit breaker switching transients in ...cld.persiangig.com/dl/qpTNK/T9kNGTkaUo/Analyses of... · switching may result in significant overvoltage hazards for

B. Simulation Results Based on clarification presented above, numerous sets of

simulations were conducted. All system aspects were taken

into consideration. Source voltage phase shift with respect to

time zero was also adjusted in order to achieve the best

possible convergence with measurement results. Calculated

simulation results are presented in Figure 9, Figure 10 and Figure 11.

0.0500 0.0550 0.0600 0.0650 -10.0

-5.0

0.0

5.0

10.0

sec

U [ k

Vp]

Voltage at 20 kVa transformer primary terminals

Figure 9. PSCAD simulation, configuration 1, LC filters not connected

0.0500 0.0550 0.0600 0.0650 -10.0

-5.0

0.0

5.0

10.0

sec

U [

kV

p]

Voltage at 20 kVa transformer primary terminals

Figure 10. PSCAD simulation, configuration 2, L = 200 µH, C = 1 µF

0.040 0.050 0.060 0.070 0.080 0.090 -10.0

-5.0

0.0

5.0

10.0

sec

U [ k

Vp]

Voltage at 20 kVa transformer primary terminals

Figure 11. PSCAD simulation, configuration 3, L = 200 µH, C = 25 µF

VI. SUMMARY AND CONCLUSIONS

The research conducted herein covered both laboratory

measurement as well as numerical simulations in PSCAD

software package. The following most important conclusions

and observations can be pointed out:

1) during de-energization of unloaded transformer without any

additional LC filters on the low voltage side, multiple arc re-

ignitions are well visible,

2) connection of LC filters has significant influence on

switching off conditions, since capacitance of the filter affects

the overall natural frequency of the transformer, 3) increase of LC filter’s capacitance results in decrease of

maximum overvoltage, steepness, number of strikes and

natural frequency of oscillations,

4) with 25 µF capacitance, multiple arc re-ignitions are almost

eliminated (only one singular arc re-strike per phase is visible)

5) in this particular case inductance of the filter is negligible

since it is several orders less than the inductance of the

transformer itself,

6) regarding the real application with photovoltaic installation

(Fig. 1) it can be concluded that from the point of view of

switching off operations’ strategies, de-energization of the transformers under no-load conditions is less hazardous thanks

to presence of capacitance of the filter,

7) de-energization with 25 µF LC filters reveals saturation

effect of the transformer’s magnetic core at lower frequencies,

8) based on conducted laboratory tests, PSCAD numerical

model was developed, achieved simulation results are

satisfactory in terms of overvoltage waveforms parameters,

namely maximum overvoltage peak value, steepness and

number of multiple arc re-ignitions, however, 100% perfect

convergence is hard to obtain due to lack of more precise data

regarding transformer capacitances and magnetization

characteristic.

This paper presents only a part of research that is under

consideration. Another issue is measurement of switching on

operations, which is out of scope of this article. Next directions

of research will cover analyses of other vacuum circuit breaker

related transient states in PV power plants. Main interest will

be put on investigation on de-energization under load

conditions during feedback generation as well as during short

circuit. Thanks to fitted models after laboratory experiments,

such analyses can be conducted in PSCAD software.

REFERENCES

[1] Jäger-Waldau A.: PV Status Report 2013, JRC Scientific And Policy Report, September 2013

[2] PV Power Plants, Industry Guide, 2011 [3] ABB Technical Application Papers, No. 10, Photovoltaic plants, 2010

[4] S. M. Yeo, H. I. Cho, C. H. Kim, Terzija V., Radojevic Z. M.: A Steady-state model of the Photovoltaic System in EMTP, International

Conference on Power Systems IPST 2009, Kyoto, Japan, June 3-6, 2009 [5] Mohan N., Undeland T., Robbins W.: Power electronics: converters,

application and design, John Wiley&Sons, 2003, ISBN 978-0-471-22693-2

[6] Teodorescu R., Liserre M., Rodríguez P.: Grid Converters for Photovoltaic and Wind Power Systems, 2011 John Wiley & Sons, Ltd.

ISBN: 978-0-470-05751-3 [7] Shipp D., Dionise T., Lorch V., MacFarlane B.: Transformer Failure due

to Circuit-Breaker-Induced Switching Transients, IEEE. Trans. on Industry Appl., vol. 47, no. 2, March/April 2011

[8] Piasecki W., Fulczyk M., Florkowski M., Werle P., Kouzmine O., Szczechowski J.: Mitigating VCB-induced very fast transients in

industrial installations. Case study: arc furnace transformer, CIGRE 2011 Join A2/D1 Colloquim Conf.,Kyoto, 11-16.09.2011

[9] CIGRE Joint Working Group A2/C4.39: Electrical Transient Interaction Between Transformers and the Power System, Part 1 and Part 2,

April 2014, ISBN: 978-2-85873272-2 [10] Kondala Rao B., Gajjar G.: Development and Application of Vacuum

Circuit Breaker Model in Electromagnetic Transient Simulation, 0-7803-9525-5/06/$20.00, 2006 IEEE

[11] Slade P.G.: The Vacuum Interrupter: Theory, Design and Application, CRC Press, 2008