iec standard high voltage circuit-breakers: practical

IEC STANDARD HIGH VOLTAGE CIRCUIT-BREAKERS: PRACTICAL GUIDELINES FOR OVERVOLTAGE PROTECTION IN GENERATOR APPLICATIONS D. Penkov Schneider Electric Grenoble France

C. Vollet Schneider Electric Grenoble France

C. Durand Schneider Electric Grenoble France

Abstract - The IEEE C37.013, [1] standard defines the overvoltage withstand requirements for circuit breakers (CBs) intended for use in generator applications. The constraints of these requirements may lead to larger sized CBs to be used. In Oil & Gas offshore applications, IEC 62271-100, [2], certified CBs are often preferred due to their more compact size. However, detailed validation of IEC CBs for generator applications has not been as well defined compared to IEEE CBs. Hence, validation of IEC CBs for overvoltage protection in generator applications is required, usually by performing computer transient simulation and analyses. Very often a simplified model of the generator is believed to be sufficient to provide reliable results. However, as this paper will demonstrate, correct modeling of the generator has a significant impact on the overvoltage results, especially with salient pole machines. The main purpose of this paper is to discuss the aspect of Transient Recovery Voltage (TRV) analysis that have to be conducted when an IEC breaker is intended for use in generator applications. Main guidelines to understand and perform such overvoltage analysis are also provided. The influence of generator saliency on the TRV peak and slope is demonstrated.

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INTRODUCTION

Previous paper [4], demonstrated how an IEC breaker can be verified for capability of interrupting current with high DC component for generator applications. Such analysis is mandatory; however it is not sufficient in order to validate the CB application. In fact, after current interruption there is high overvoltage that takes place between CB contacts. This voltage, called Transient Recovery Voltage (TRV) is characterized in IEC and IEEE standards by its steepness and peak value. In order that the current is successfully interrupted, the breaker shall withstand the TRV. Comparison of the two standards reveals a much higher overvoltage withstand capability in terms of peak and steepness in IEEE CBs specially designed for generator applications, as compared to IEC CBs. However, IEC CBs are often more preferred than IEEE CBs for generator applications due to the following advantages: IEC standard applications CBs are smaller in size than IEEE C37.013 generator applications circuit breakers. In an off-shore oil rig, space/weight is critical, and of major priority. IEC CBs are much less expensive.

K. C. Edey KBR UK Ltd Leatherhead UK

They allow IEC CB manufacturers to get into the market segment, allowing an alternative option from IEEE CB for generator applications. IEC CBs offer a technical challenge to the technology presented by existing IEEE generator CBs.

The purpose of this paper is not to show how IEEE certified generator breakers can be replaced by IEC ones. Instead, the paper aims to address the concerns of overvoltage protection in IEC CBs and validation steps needed to apply the IEC CBs in generator protection applications. The following chapters will be discussed:

Index Terms —Transient Recovery Voltage, switching over voltage, EMTP-ATP, transient simulation I.

A. M. Husin KBR UK Ltd. Leatherhead UK

Basics of Transient Recovery Voltage and factors that determine it Comparison of IEEE C37.013 / IEC62271-100 standards and scope of application Analysis of generator modeling for TRV simulations Analysis of required data/software/simulation in order to verify applicability of the IEC CB and recommend protection solutions Simulations of 2 typical applications where IEC breaker applicability has been proven, by means of additional overvoltage protection Conclusions II.

A.

BASICS OF TRV. MAIN CHARACTERISTICS

Origins of TRV Let us consider the simple circuit presented in Fig. 1: R

Lp

Cp

CB

Vm.cos(wt)

Fig. 1 Simplified equivalent circuit for TRV Initially the CB is closed. At current zero, the CB opens and the result is a transient voltage oscillation as shown in Fig. 2. This transient voltage oscillation has an initial maximum amplitude Vmax, before it stabilizes to the value of the source voltage, Vm.cos(wt).

2.0

3.0

Vmax TRV

1.5

2.5

Vmax with Isolated neutral

2.0 1.0 1.5

Vm Supply Voltage

0.5

Vmax with Earthed neutral

1.0 0.5

0.0 0.0 -0.5 -0.5 -1.0

-1.0 0

4

8

(f ile f irst_test.pl4; x-v ar t) v :SRC_1

12

*10 -3

16

20

0

v :TRV_1 -

Fig. 2 TRV waveform vs. steady state

2.

2.π . Lp.Cp

Amplitude Damping, D = exp(− t

2.R.Cp

)

The crest value of voltage is a function of the above parameters and moment of current interruption. It can not exceed 2 times the value of Vm. An amplitude factor is defined as:

k af =

8

12

v :CB_EA -CB_E_A

v :CB_IA -CB_I_A

16

*10 -3

20

Fig. 4 First pole to clear factor

The parameters of the transient voltage are: 1. Frequency, 1 F=

4

(f ile f irst_test.pl4; x-v ar t) v :SRC_1

Vmax , k af ≤ 2 Vm

This factor is close to but lower than 1.5 and is expressed as the following [5]: 3.( X 0 + 3.RN ) kpp = (2) X 1 + 2.( X 0 + 3.RN ) In case of RN->∞ kpp->1.5, if RN=0 and X1=X0, (Lm=0), then kpp=1. In practice X0>X1 (X0=3.X1) and kpp is typically around 1.3. The peak of the TRV across the breaker can be expressed as:

Vmax = d .Vm d = k af k pp

(1)

(3)

d − peak factor phase to earth Note that voltages (Vmax and Vm) are measured between phase and earth. Typical values for kaf lie in the range 1.4-1.7, depending on the moment of current interruption, i.e. the power factor of the system during the fault. This is also confirmed in IEC 62271-100, where kaf varies with the test duty. B.

First pole to clear factor Let us now consider a three phase system as per Fig.

3: R Vm.cos(wt)

CB

Lp Cp

R Vm.cos(wt-2/3.π)

Lp

Lm

Lm RN

Cp R

Vm.cos(wt-4/3.π)

Given typical values of kaf and kpp, one can estimate that TRV peak (or the peak factor d) across a breaker would lie in the range of 1.4-1.7 to 2-2.55 pu of steady state phase to earth voltage amplitude. The IEEE C37.013 defines the peak factor with respect to the rated maximum system voltage of the CB. The system voltage is the phase to phase rms voltage. Therefore the above value of d shall be multiplied by

2 / 3 in order to be compliant with that definition. For rated short-circuit current ANSI C37.04-1999 gives values as k af = 1.54 and k pp = 1.5 for systems below 100kV, which leads to the peak factor of d = 1.89 , defined in ANSI standards. The rate of rise of recovery voltage (RRRV) in the above case can be conservatively assumed as:

RRRV = d .Ur.π .F

Lm Lp

F − oscillating frequency

Cp

Ur − rated system voltage

Fig. 3 Three phase system for TRV analysis In this system, there are no mutual impedances (Lm=0). If the system has solidly earthed neutral (RN=0 Ohm), during a three phase to earth fault the amplitude factor is equivalent to a single phase system. However if the system is earthed through an impedance, or the fault is isolated from earth, the obtained overvoltage on the CB terminals would have a higher value. This is particularly true during the first pole to clear. The waveforms in Fig. 4 illustrate the earth fault voltage of these two cases. The ratio of the peak values of these waveforms is defined as the first pole to clear factor (kpp), which is linked to the earthing of the system and the ratio between the system’s positive and zero sequence impedances.

(4)

Note that the oscillating frequencies of the transient voltage also differ with earthing conditions. This will be explained in chapter IV. C. III. IEEE AND IEC STANDARDS COMPARISON This section summarises the main differences between the IEC 62271-100 standard for normal CBs and IEEE C37.013 standard for CBs designed for generator applications in determining the TRV parameters of a CB. A.

IEC TRV rating and parameters

The IEC HV circuit breakers (U −0.33 the TRV frequency of a solidly earthed generator will be higher than if it was isolated, also will the RRRV.

(8)

This frequency will vary with mutual inductances (see Appendix A. ); where Lm is related to Lp with a coefficient factor k as:

-0.3

Fig. 6 Ratio of TRV frequencies depending on generator earthing and factor k at the moment of current interruption

1

FTRV _ earthed _ Lm =0 =

k=

0.8 -0.4

Neutral Earthing

impedance

Additional data for TRV Attenuation and natural frequency (RC branch connected phase to earth) Capacitance primary/secondary to earth and primary to secondary winding Capacitance phase to earth

Capacitance to earth, traveling wave modeling, distributed parameters No particular

From Table IV, typical short-circuit data is easily accessible. However, parameters for additional data are often unknown and estimation is needed. IEEE standards C37.04-1999 (§5.9.2.2 b) and IEEE C37.011 [3], provides information about capacitances of several equipment, according to rated power and voltage. Also, information and typical capacitance values for generators and other equipment are also provided in [13]. For conservative approach, the circuit breaker is assumed as an ideal switch (with no arc voltage

introduced), to give worst case initial conditions. B.

Simulation software

There are two principal methods for TRV investigation: by analytical equations or by using dedicated transient simulation software. The first solution applies relatively easy and correctly when the analysis does not include generator applications, or with a single generator modeled as a round rotor machine. However, when there are multiple generators in the system, or when the generator is of salient pole type, and / or there are additional loads (parallel feeders, motors, reactors, etc…); it is necessary to conduct a computer simulation study instead. Let’s recall that salient pole generator can not be correctly represented by a source-impedance equivalent since this will eliminate the impact of second harmonic, (refer section IV-B. ). Therefore, for salient type machines, the usage of dedicated simulation software is mandatory. It is recommended that the simulation software is capable of verifying the proposed generator model as well as modelling traveling waves in conductors. Among the reputable software candidates includes EMTP-ATP, PSCAD, EMTP-RV and others. In this paper, the authors have used the EMTP-ATP software, [14], widely known for its capabilities for transient simulations, and the built-in synchronous machine model SM59. Although this machine model is not initially intended for TRV analysis, it can be easily “updated”. Authors observed that the SM59 model is capable of correctly representing the quadrature reactance for a terminal fault. Care must be taken to include generator tolerances during the TRV analysis, typically ±15% is retained. The high frequency of TRV voltage requires reconsideration of adjusting the machine values – mainly increasing the generator armature resistance and including self-damping parameters. C.

TRV values

Methodology of TRV analysis

The following details the recommended step-by-step procedure for successful TRV analysis: 1.) Model the power system according to A. and B.

6.) If necessary: conduct additional analyses with appropriate surge protections: a. Surge arresters b. Surge capacitors (0.25-1µF) c. R-C snubber (R=10-80 Ohm, C=0.251µF) Note: In case of generator with earthed neutral, when the fault is earthed, the TRV will be lower; there will not be line side contribution. However with the reduced voltage oscillation on the generator side, efficiency of surge arresters, if installed, may be reduced as well. 7.) Analyse tolerances on generator data Generator impedances are subject to tolerances. Typically it may be considered a range of ±15% for the subtransient values, which impact the most the TRV. 8.) Analyse the out of phase switching Note: Out-of-phase switching will induce a higher TRV, since there is no fault which could damp the impact from downstream network. However, since modern synchronising relays are very accurate, the probability of out-of-phase switching may not be required and requires verification, depending upon the network and network operating mode. When necessary this analysis shall be conducted for 90° phase difference/ shift as stated in IEEE C37.0 131997. 9.) Derive final conclusions for applicability of the IEC CB. VI. TRV ANALYSIS OF SUITABILITY OF IEC BREAKERS IN TYPICAL GENERATOR APPLICATIONS In order to demonstrate the applicability of IEC breakers in generator applications, two test cases will be considered- generators in industrial plant and utility generation plant. The prospective TRV of each case will be analyzed and results presented here. A.

Industrial plant generator application

2.) Establish the system fault locations, typically: a. Bus bar b. Generator side c. Downstream to bus bar side d. Special attention on fault locations when CB are separated from bus bar with cables; two fault locations shall be considered, one at each side of cable 3.) Run the short-circuit analyses – this is to determine the breaking current in order to select TRV envelope : T10, T30, T60 or T100 4.) Set up and perform analysis of TRV on CB opening : a. 50ms after fault appearance, b. One pole with zero DC component c. The above pole shall be the first pole to cut (clear) 5.) Compare the TRV envelope with the IEC standard

GT2

GT1

(A) CB n°1

CB n°2

(B) (C)

LOAD

LOAD

Fig. 7 Basic overview of the studied system Analysis will be presented in the order given in §V. -C. 1) Data Two identical gas turbine generators with following characteristics are connected to the main busbar:

TABLE V NETWORK DATA Data

I sym _ 20% = Value

Generator data Sr (MVA) 40 Ur (kV) 11 Pole Pairs 2 Xd” (pu) 0.23 Xq” (pu) 0.29 Cables to busbar Cross-section (mm²) 300 Positive resistance (Ohm/km) 0.06274 Zero sequence resistance (Ohm/km) 0.10196 Positive reactance (Ohm/km) 0.105 Zero-sequence reactance (Ohm/km) 0.220 Capacitance to earth (nF/km) 524 Length (km) 0.225 Cables per phase 5 Average Cables to transformers (loads) Positive resistance (Ohm/km) 0.1537 Zero sequence resistance (Ohm/km) 0.24313 Positive reactance (Ohm/km) 0.118 Zero-sequence reactance (Ohm/km) 0.353 Capacitance to earth (nF/km) 366 Length (km) 0.100

Generator reactance values are subject to a +/-15% tolerance. Generator capacitance is estimated as 500nF, [13]. Several loads are connected to the main busbar. The main substation is equipped with 50kA vacuum circuit breakers certified up to 15kV with IEC 62271 test duty. In the TRV analysis, the minimum clearing time, by [2], is equal to 50ms, which consists of 10ms for protection relay functioning and 40ms for circuit breaker opening. 2)

Fault locations

Three fault locations have to be analyzed: a. The fault takes place between the generator and the incomer circuit breaker (loc. A). CB n°1 sees the contribution of GT2 alone. b.

The fault takes place on the main busbar. CB n°1 sees current from GT1, (loc. B).

c.

The fault takes place on the outgoing feeder circuit breaker (loc. C). CB n.2 sees the contribution of GT1 and GT2.

3)

Calculation of short-circuit currents

=

I b × 1 + 2 × (%dc) 2 1 + 2 × (20%)2

8,5 × 1 + 2 × (95%) 2 1 + 2 × ( 20%) 2

(12)

= 13,7 kA

The test duty is chosen according to the ratio of the above calculated current to its rated short-circuit value. Test duty T60 will be chosen for a value in the range 30% - 60%, T30 for ration between 10-30%, etc... Results are summarized in TABLE VI: TABLE VI SHORT-CIRCUIT CURRENT ANALYSIS, (KA) Fault CB location to open A CB1 B CB1 C CB2

4)

Ib

%dc Isym_20% Test (Idc/Ib√2) duty

8.5 8.5 17

95 95 95

13.7 13.7 27.4

T30 T30 T60

IEC Peak TRV (kV) 29.4 29.4 27.6

IEC RRRV (kV/µs) 1.96 1.96 0.95

Set up analysis of TRV

In order to obtain the maximum TRV it is necessary to define the critical fault conditions: - Fault: Isolated three phase fault, allowing the voltages at both circuit breaker sides oscillating. An earthed fault will expose the breaker to the oscillation of the feeding side only -Fault Inception angle: defined to get symmetrical current on one phase, this angle is defined by the power factor before the fault. The phase with symmetrical current shall be the first to clear the current. - CB Opening time: The circuit breaker opens at least at 50ms after fault appearance. -Load: the presence of loads can reduce TRV by adding damping factor. But the presence of loads can also maximize the TRV by increasing the generator excitation that leads to a higher fault current that maximizes the TRV. For the fault location B and C, the presence of loads increases the TRV, but for the fault located in A, the presence of loads reduces the TRV. - Generator parameters variation: Generator datasheet gives reactance value with a +/-15% tolerance. The parameter variation has a major impact on TRV. The worst case is with +15% on X”q and -15% on X”d. 5)

Comparison of simulated TRV with standard defined (generator tolerances not applied)

Example curves will be given for fault at C, test duty T60: 50.0 kA 37.5 25.0 12.5

For each fault location the short circuit currents are calculated according to IEC 60909. The corresponding TRV test duty is determined for current with less than 20% of DC component. According to the IEC 62271-100, if the percentage of DC component at contact separation does not exceed 20%, the short circuit breaking current is characterised only by the r.m.s value of its alternating component. When the DC component is higher, which is often the case with generators; it is necessary to put in form the fault current with 20% of DC component. The ANSI C37.09 defines a formula to calculate the symmetrical current with a DC component wanted. The formula is also analysed in [4]:

0.0 -12.5 -25.0 -37.5 -50.0 0

10

20

30

(f ile QC_LNG_article_PS.pl4; x-v ar t) c:B_BARA-CB_2A

40

50

c:B_BARB-CB_2B

60

70

ms

c:B_BARC-CB_2C

Fig. 8 Fault currents through CB n°1, fault at A

80

40

50

kV

*103

[kV]

TRV envelope T60

40

30

30 20

20 10

10 0

0 64,0

64,3

64,6

(file QC_LNG_article.pl4; x-var t) v:B_BARA-CB_2A -10 64.2

64.4

64.6

(f ile QC_LNG_article_PS.pl4; x-v ar t) v :B_BARA-CB_2A

64.8

65.0

ms

65.2

m:TTR1

Fig. 9 TRV and T60 TRV envelope from IEC standard, fault at C In addition to the analysis for the fault locations, the impact of load has also been studied. It was found that in case of fault at A the maximum TRV was obtained without load. The presence of load at the same side with a generator did reduce the transient on that side. However, when the fault location has been moved to busbar, B, or on the outcoming feeder, C, the presence of load increases the TRV peak on CB2. The load in this case was on the other side of the circuit breaker; hence generator TRV has not been damped. TABLE VII summarizes the maximum TRV results for the different fault locations: TABLE VII TRV CALCULATIONS WITHOUT TOLERANCES Fault location

CB to open

Test duty

A B C

CB1 CB1 CB2

T30 T30 T60

IEC IEC Simulation Simulation Peak RRRV Peak TRV Rate of rise TRV (kV/µs) (kV) TRV (kV) (kV/µs) 29.4 1.96 0.16 30.5 29.4 1.96 0.17 34.6 27.6 0.95 0.19 32

Note the load damping impact to the TRV results, where the TRV peak on B is higher than that on A, even though it is almost the same current passing through the breaker. Finally, the most critical situation is with fault on C. In all the tests, the TRV peak values are over the withstand levels defined by standard. It is thus required to review the size of the overvoltage protection equipment. However this may not be necessary if the manufacturer of the circuit breaker can prove the capability of his CB to withstand the values without protection, i.e. if there are tests with similar values. It may also be possible that the required overvoltage protection gets “lightened” because of known capabilities of the CB. 6)

Analysis of tolerances on generator data

The calculation results presented in Table VII does not include the impact of generator impedance tolerance. In practice, there is always a variation to the quoted generator data and hence tolerances should always be included in the analysis. The calculation will be repeated for the fault is located at C; Test duty T60 (worst case), with tolerances applied. Fig. 10 illustrates the TRV analysis result:

64,9

65,2

[ms] 65,5

m:TTR1

Fig. 10 TRV in case of maximum tolerances on gen. data, fault at C It can be observed that the peak value is above any withstand rating of a 15kV circuit breaker, but in the range of 24kV CB. This means that even IEEE C37.013 certified breakers would need to be oversized in order to withstand that overvoltage. It also underlines the importance of consideration of the asymmetry of the rotor of the generator, which is actually often neglected. TABLE VIII below summarises the results with tolerances: TABLE VIII TRV CALCULATIONS WITH TOLERANCES Fault location

CB to open

Test duty

A B C

CB1 CB1 CB2

T30 T30 T60

7)

IEC IEC Simulation Simulation Peak RRRV Peak TRV Rate of rise TRV (kV/µs) (kV) TRV (kV) (kV/µs) 29.4 1.96 0.21 42.9 29.4 1.96 0.23 47.1 27.6 0.95 0.22 44.4

Recommendation of protection equipment

The preferred way to limit the TRV peak is by using surge arrester protection, connected phase-to-earth or phaseto-phase. The most appropriate connections are at the generator side of each generator CB and on the busbar side of CB2. For the fault cases at A and B, a 12kV rated phase-toearth surge arresters will limit the TRV peak under the envelope given in the standard IEC 62271-100. However for fault located at C, phase-to-earth surge arresters will not be sufficient to protect the circuit-breaker, therefore surge arresters connected phase to phase were used. Since the application involves generators, the surge arrester shall account for higher voltage fluctuations, hence a voltage rating of 15kV is recommended. Therefore, for this example, a 15kV rated phase-to-phase installed surge arrester will result in the TRV peak within the envelope of the IEC 62271-100 values, even if the maximum tolerance on generator reactances is taken into account. Fig. 11 and TABLE IX summarise the TRV analysis with surge protection considered: 30 *10 3

[kV]

25 20 15 10

5 0 64,0

64,3

64,6

(file QC_LNG_article.pl4; x-var t) v:B_BARA-CB_2A

64,9 m:TTR1

65,2

[ms] 65,5

m:PMAX1

Fig. 11 Example TRV with phase-to-phase installed surge arresters, fault at C

TABLE X UTILITY DATA

TABLE IX TRV CALCULATIONS WITH PROTECTIONS Fault location

CB to open

Test duty

A B C

CB1 CB1 CB2

T30 T30 T60

8)

IEC IEC Simulation Simulation Peak RRRV Peak TRV Rate of rise TRV (kV/µs) (kV) TRV (kV) (kV/µs) 29.4 1.96 25.8 0.19 29.4 1.96 27 0.2 27.6 0.95 26.2 0.2

Out of phase switching

A synchrophasor is used for coupling of the generators, therefore out of phase switching is not considered. 9)

Final conclusions on the TRV analysis

The following Fig. 12 summarizes the recommendations of overvoltage protection for the studied case: GT2

GT1

MOV

MOV

CB n°1

CB n°2 MOV

LOAD

LOAD

Fig. 12 Case study with location of the recommended protections (phase-to-phase surge arretesters) Note that the conclusion and recommendation derived remains valid for systems with rapid current interruption protection installed (i.e. fault current limiters). The configuration in Fig. 12 could also represent the one half of a section rapidly disconnected by such protection scheme. B.

Data Utility transmission grid Rated short-circuit power at 110 kV (MVA) Ur (kV) Earth Capacitance (at 110 kV) Series Resistance to the earth capacitance (Ohm)

Value 2000 110 5.8nF 2000

Step-down transformer Rated voltage on primary (kV) 110 Rated voltage on secondary (kV) 11 Rated power (MVA) 90 Coupling Ynd 11 Short-circuit voltage (% of rated) 10 Copper losses (kW) 201 Core losses (kW) 30 No-load current (% of rated) 0.2 Capacitances Primary–earth / Sec2.4/4.8/2.1 earth / Prim-Sec (nF) Short-circuit power at 11kV (MVA) 620 Cables to busbar Cross-section (mm²) 500 Positive resistance (Ohm/km) 0.0371 Zero sequence resistance (Ohm/km) 1.018 Positive reactance (Ohm/km) 0.0865 Zero-sequence reactance (Ohm/km) 1.2746 Capacitance to earth (nF/km) 438 Length (km) 0.02 Cables per phase 6

Note that utility short-circuit power is almost 350% higher than that of a single generator. The topology of the utility plant is presented on Fig. 13:

Utility generation plant

A utility generation plant has the same topology as the industrial plant, except that there are fewer loads connected to the busbar and that a connection to the transmission / sub-transmission grid exists. The presence of the high short-circuit power source will influence the overvoltage levels and TRV steepness and requires a different protection scheme, compared to the islanded industrial plant. For illustration purposes, the impact of the grid connection will be considered by providing the previously considered industrial case with additional connection to the transmission grid (110kV). This will limit the modifications to the already presented case and will facilitate the analysis and explanations. 1)

Data

The utility will be modeled according to the following data:

Fig. 13 Electrical diagram of the utility plant The utility is equipped with the same 50kA vacuum breaker as the generators. 2)

Fault locations

The fault locations considered is maintained as per in the previous case, with one additional location added, on the substation side (D). 3)

Calculation of short-circuit currents

TABLE XI summarises the results of the fault calculation of the utility plant configuration:

TABLE XI SHORT-CIRCUIT CURRENT CALCULATION Fault CB location to open A B B C D

CB1 CB1 CB3 CB2 CB3

Ib

%dc Isym_20% Test (Idc/Ib√2) duty

40.87 9.57 32.54 49 19.14

25 93 21 2 93

41.6 15.2 32.6 47.2 30

T100 T30 T100 T100 T60

IEC IEC Peak RRRV TRV (kV) (kV/µs ) 25.7 0.39 29.4 1.96 25.7 0.39 25.7 0.39 27.6 0.95

The current in case of fault at C is very high in the first 50ms and is above the breaking capability limit of the CB. Therefore, it is recommended that the minimum opening time on CB2 is increased to 200ms after the fault appearance.

TABLE XIII TRV CALCULATIONS WITH UTILITY AND TOLERANCES ON THE GENERATORS Fault location

CB to open

A B B C D

CB1 CB1 CB3 CB2 CB3

Recommendations for overvoltage protection

Set up analysis of TRV

The method of analysis adopted is the same as in the previous case. Note that the system initial conditions is set up by load flow analysis in which the substation is designated as a P, Q node in order to set up the system in generation mode. The generators are assumed to operate at full rated power. 5)

Comparison of simulated TRV curves with standard defined

TABLE XII summarizes the results of the TRV analysis of the utility plant configuration: TABLE XII TRV CALCULATIONS WITH UTILITY (WITHOUT TOLERANCES) Fault location

CB to open

A B B C D

CB1 CB1 CB3 CB2 CB3

Different protection solutions have been tested. The addition of a surge capacitor can reduce the slope of the TRV, however its peak value is increased above the withstand level. Hence it is recommended that a series resistance is added to reduce the peak value. Based on the results of the TRV calculation, the following recommended solutions are proposed: the most suitable protection equipment for the utility is an RC snubber, composed of 10 Ohm resistance and 0.75µF phase-to-earth capacitors phase-to-phase surge arresters are the most appropriate solution to reduce peak TRV values on the generators The diagram in Fig. 14 summarizes the protection installation with the utility:

Test duty

IEC IEC Simulation Simulation Peak RRRV Peak TRV Rate of rise TRV (kV/µs) (kV) TRV (kV) (kV/µs) T100 25.7 0.39 23.3 0.31 T30 29.4 1.96 0.21 36.4 T100 25.7 0.39 18.96 1.06 T100 25.7 0.39 22.95 0.27 T60 27.6 0.95 27.2 0.19

The results indicate a very important trend in the evolution of the overvoltage values: the peak values are reduced within the overvoltage withstand levels of the CB. This is due to the presence of the utility in parallel to the generators. The total short-circuit power is increased with the utility, which is equivalent to a reduction of the equivalent short-circuit impedance; and impacts the voltage variation on that impedance when the current is interrupted. It can be observed that the voltage on the “source” side remains close to its fundamental value. However, at the utility CB3 very steep overvoltages is observed in case of downstream faults. The CB1 also sees very high TRV peak. It is then necessary that additional protection equipment is used to mitigate the overvoltage constraints. 6)

IEC IEC Simulation Simulation Peak RRRV Peak TRV Rate of rise TRV (kV/µs) (kV) TRV (kV) (kV/µs) T100 25.7 0.39 24.15 0.32 T30 29.4 1.96 0.28 50.7 T100 25.7 0.39 18.96 1.07 T100 25.7 0.39 22.68 0.27 T60 27.6 0.95 0.28 42.22

As observed in the previous case, the additional generator tolerances give rise to an increased peak TRV results. 7)

4)

Test duty

Analysis of tolerances in generator data

The calculation in TABLE XII is repeated with generator reactance tolerances considered for worst case conditions. The results in TABLE XIII summarises the findings:

Fig. 14 Protective scheme for utility plant TABLE XIV summarizes results of TRV calculation with the proposed recommended solutions: TABLE XIV TRV CALCULATIONS WITH UTILITY AND TOLERANCES ON THE GENERATORS AND OVERVOLTAGE PROTECTION Fault location

CB to open

A B B C D

CB1 CB1 CB3 CB2 CB3

8)

Test duty

IEC IEC Simulation Simulation Peak RRRV Peak TRV Rate of rise TRV (kV/µs) (kV) TRV (kV) (kV/µs) T100 25.7 0.39 23.0 0.26 T30 29.4 1.96 26.87 0.23 T100 25.7 0.39 21.08 0.32 T100 25.7 0.39 21.87 0.23 T60 27.6 0.95 26.47 0.24

Out of phase switching

As previously out of phase switching is not considered because of the installed synchrophasor. 9) Final conclusions of TRV analysis with utility The addition of a new power source in the system played a significant role in the TRV evolution. Although the TRV peak value is reduced, the reduction is not sufficient to avoid installation of protection equipment. It was also necessary to add an RC snubber at the source side of the utility CB in order to reduce the slope of the overvoltage without increasing its peak value. However, the most important finding of the TRV analysis is that in both the islanded and utility connected generator plant cases, the possibility of applying an IEC certified circuit breaker can be validated. The applicability of IEC CB in generator applications can be made possible by adding appropriate protection solutions. Note that with tolerances in generator impedances, an IEEE C37.013 certified breaker would also experience difficulties to interrupt fault currents.

Xd, Xd’, Xd” Xq” Td’, Td” Ta w t Isym_20%

IX. ACKNOWLEDGEMENTS The authors would like to express their gratitude to M.Q. Nguyen (Schneider Electric France), A. Windhorn (Leroy Somer USA), and particularly P. Novak (Schneider Electric Germany) for their valuable contribution during the work progress of this paper. VI. REFERENCES

VII. CONCLUSIONS The presented paper deals with the overvoltage (TRV) analysis necessary to validate the application of an IEC circuit breaker in generator application. The main conclusions of the presented work are: • it is necessary to perform TRV analysis systematically in order to check the need of additional protection in generator applications • Special attention shall be taken with salient generators: saliency can substantially increase the TRV peak and render even IEEE C37.013 certified breaker out of its limits to withstand the overvoltage • Tolerances of ±15% in generator reactances may increase TRV peak to substantially higher values • The following mitigation methods are recommended to control the TRV within the CB withstand limits: − Surge arresters will reduce peak − Surge capacitors will reduce slope, but may increase peak of TRV − An RC snubber will act both on peak and slope of the overvoltage, however this is also the most expensive solution • A “natural” solution to reduce the overvoltage is to delay the opening time of the circuit breaker to ensure the TRV is within the CB capability. This is especially recommended if salient pole machines are the main generation system.

[1]

[2]

[3]

[4]

[5] [6]

[7] [8]

[9]

[10]

[11] Vmax Vm Ur

VIII. NOMENCLATURE Peak of TRV (V). Phase to earth voltage amplitude (V). Rated system voltage, Ur = 3 Vm (V). 2

e

Phase to earth voltage magnitude, (V). Ur e=

RRRV F Lp Lm k

Synchronous, transient and subtransient direct axis reactances of a generator (Ohm). Subtransient q-axis reactance (Ohm). Transient and subtransient time constants, d-axis (s). Armature time constant (s). Rated electrical angular speed (rad/s). Time (s). Symmetrical fault current with 20% DC component (A).

[12] [13]

3

Rate of Rise of Recovery Voltage. Frequency of TRV (Hz). Self inductance per phase (H). Mutual inductance between two phases (H). Ratio of mutual and self inductances.

[14]

ANSI/IEEE C37.013, IEEE Standard for AC High Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis IEC 62271-100:2008, High Voltage Switchgear and Controlgear-Part 100: Alternating Current CircuitBreakers IEEE std C37.011 -2005, IEEE Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers R. Cossé, T. Hazel, G. Thomasset, “IEC mediumvoltage circuit-breaker interrupting ratings-unstated short-circuit considerations “, IEEE Trans. On Industry Applications, Vol.36, No.3, May/June 2000 R. W. Alexander, D. Dufournet, “Transient Recovery Voltage (TRV) For High-Voltage Circuit Breakers” E. P. Dick, R. W. Cheung, J. W. Porter, “Generator Models for Overvoltage Simulations”, IEEE Trans. On Power Delvery, Vol.6, No.2, April 1991 Questionnaire 73(secretariat)29 E. L. White, “Surge-Transference characteristics of generator-transformer installations”, Proc. IEE, Vol.116, No.4, April 1969, pp.575-587 A. P. Hayward, J. K. Dillard, A. R. Hileman, “Lightning Protection of Unit-connected Turbine Generators – Field and Laboratory Studies”, AIEE Trans., Vol.75, Part III, pp.1370-1381,1956 T. Funabashi, N. Takeuchi, T. Sugimoto, T. Ueda, L. Dube, A. Ametani, “Generator Modeling for Transformer Transfer Voltage Study”, IEEE Trans on Energy Conversion, Vol.14, No.4, Dec. 1999 T. Funabashi, T. Ito, T. Sugimoto, K. Miyagi, T. Sano, T. Ueda, J. Martinez, A. Ametani, “Generalized Generator Model for Transformer Transfer Voltage Studies”, IEEE Trans on Energy Conversion, Vol. 19, No. 3, Sept. 2004 P. Barret, “Régimes Transitoires des Machines Tournantes Electriques”, Paris, Ed. Eyrolles, 1987 A. Greenwood, “Electrical Transients in Power Systems”, New York, Ed. Wiley-Interscience, 1971 EMTP-ATP Software RuleBook X.

A.

APPENDIX

Formula for frequency with mutual impedances, earthed neutral and earthed three phase fault:

Consider the system on Fig. 3, with the following indications: CB Va

Ia

Xc Vb

Ib Xm

Vc

Ic

Xc

Xp Xc

The voltage can be expressed as: Va = Ia.( Xp − Xc) + ( Ib + Ic). Xm

Vb = Ib. Xp + Ia. Xm + Ic. Xm Vb = Ic. Xp + Ia. Xm + Ib. Xm

(13)

Va + Vb + Vc = 0 After some operations one can obtain: − Va − 2 .Ia . Xm Ib + Ic = Xp + Xm Va = Ia .

( Xp 2 + Xm . Xp − 2 . Xm 2 − Xc ( Xp + Xm )) Xp + 2 . Xm

(14)

Xp 2 + Xm. Xp − 2. Xm 2 = Xc ( Xp + Xm) Xp 2 + Xm. Xp − 2. Xm 2 1 + k − 2.k 2 = Xp. ( Xp + Xm) 1+ k

Leq = Lp.

B.

(15)

1 + k − 2.k 2 1+ k

Explanation of the negative value of mutual inductance in generator at current interruption

In ref. [12], the mutual inductance between two phases spaced geometrically at 120° (i.e. b and a) of the stator of a generator is given as: Mba = M a 0 . cos(120°) + M a 2 . cos( 2.θ − 120°)

Christophe Durand received his MSc degree in electrical engineering from the Joseph Fourier University of Grenoble, in 2009. He is currently working for Schneider Electric as Power Systems Engineer. He is working on electrical network analyses such as stability, protection coordination and overvoltages. st

The equivalent inductance of the system at the natural frequency can be obtained by posing the right hand part of the equation to zero. It follows: Xp 2 + Xm. Xp − 2. Xm 2 − Xc ( Xp + Xm) = 0

Xeq =

Caroline Vollet received her Electrical Engineering degree from the National Polytechnic Institute of Grenoble in 1988. She joined Merlin Gerin (now Schneider Electric) in 1989. She is currently working on electrical network analyses such as stability, harmonic and overvoltages studies. She has been personally involved in several instances of equipment failure or malfunctioning in different kind of industrial plants.

(16)

In Eq.10 θ is the angle between phase a, which is the phase without DC component during the fault, and an arbitrary reference frame. In the case of rotating reference frame, the mutual inductance will oscillate at twice the electrical frequency. If the fault has been incepted at maximum of the voltage on phase a, i.e. when θ was equal to 0° or 180° then θ at the moment of current interruption will also take values of 0° or 180° (the current zero happens approximately at maximum of the voltage). As a consequence both terms of the mutual inductance will have negative values and the value of that mutual inductance will also be negative, corresponding to a negative k. XI. VITA Delcho Penkov was born in Haskovo, Bulgaria. He graduated from Technical University of Sofia in 2002. In 2006 he received his PhD degree in Electrical Engineering from the Institut National Polytechnique de Grenoble (INPG). He is currently working for Schneider Electric as Power Systems Engineer. Member of IEEE.

Adita Husin received a 1 class Honors in Electrical & Electronic Engineering from the University of Surrey, UK in 2000 and completed her MSc in Electrical Power System from the University of Manchester in 2007. She is now a Chartered Electrical Engineer with KBR and is responsible for power system studies for various oil & gas onshore plants and offshore platforms and FPSO. Kate Edey received her BEng (Hons) in Electrical & Electronics Engineering from University of Leicester, UK in 1990. She is a Chartered Engineer and has been heavily involved in generator excitation controls design and later in power distribution design with various companies. She is now with KBR and is responsible for power system studies for onshore, offshore and renewable projects. She is currently the Lead Electrical Engineer for an LNG project in Western Australia.