06125556

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Switching transients in long AC cable connections to

offshore wind farmsF. Moore

Cardiff [email protected]

A. HaddadCardiff [email protected]

H. GriffithsCardiff University

[email protected]

M. Osborne National Grid UK

[email protected]

Abstract- Overvoltages arising from energising a 45km132kV submarine cable connection to an offshore wind farmwere calculated using EMTP. Energisation of the submarinecable and remote 132/33kV transformer connected as atransformer-feeder was first simulated. The resultingovervoltages were compared with overvoltages caused byenergising the cable and transformer separately.

Index Terms-- ATP, ATPDraw, Cables, EMTP, Offshore,Overvoltage, Switching, Transients, Wind

I. I NTRODUCTION

In the UK, there are currently over 30GW of offshore

wind generation projects at various stages of development[1]. These result from government targets and incentives for

renewable energy [3][4]. The location of these large wind

farms means that long lengths of submarine cable are

required to connect the wind farms to the onshore

transmission network. It is important to investigate how these

long lengths of cable will influence the transient overvoltages

seen on the onshore transmission network.

Figure 1 shows a 33kV wind farm array connected to the

400kV onshore transmission system using 132kV submarine

cables.

Offshore

Substation

132/33kV

Onshore

Interface

Substation

(400/132kV)

Submarine

Cables (132kV)

Onshore Transmission Network (400kV)

To Wind Farm (33kV)

Offshore

Substation

132/33kV

Onshore

Interface

Substation

(400/132kV)

Submarine

Cables (132kV)

Onshore Transmission Network (400kV)

To Wind Farm (33kV)

Figure 1 Typical Offshore Network Configuration (Compensation plant

and harmonic filters at 400/132kV interface substation are omitted for

clarity)

The design of this offshore network requires a trade off

between cost and operability. Reducing plant on the offshore

platform substation has significant cost benefits. The

132/33kV offshore transformers could potentially be

connected directly to the 132kV submarine cables as

transformer-feeders, removing the need for 132kV circuit

breakers in the offshore substation.

This paper describes the development of a computer model

for the AC connection to an offshore wind farm. This paper

focuses on the overvoltages arising from energisation of wind

farm connection. Energisation of the submarine cable and

offshore 132/33kV transformer together as a transformer-

feeder is compared with energising the two items separately.

II. OFFSHORE CONNECTION MODEL

Figure 2 shows a model in ATPDraw, for the interface

between offshore and onshore transmission networks.

Network

Source

400/132/13kV

Transformer

400kV OHL

(Double Circuit)

400kV OHL

(Double Circuit)

Network

Source

Harmonic

Filter

13kV Shunt Reactor

Network Interface

Onshore 132kV Circuit Breaker

132kV Submarine Cable (45km)

Offshore 132kV Circuit Breaker

132/33kV

Transformer

Zy Earthing

Transformer

V V

V

V

V

V

V

V

Network

Source

400/132/13kV

Transformer

400kV OHL

(Double Circuit)

400kV OHL

(Double Circuit)

Network

Source

Harmonic

Filter

13kV Shunt Reactor

Network Interface

Onshore 132kV Circuit Breaker

132kV Submarine Cable (45km)

Offshore 132kV Circuit Breaker

132/33kV

Transformer

Zy Earthing

Transformer

V V

V

V

V

V

V

V

Figure 2 Offshore Connection Model in ATPDraw with a 400kV

Onshore Network. (Lettered circles mark where voltage measurements

were taken during simulations.)

A single 132kV submarine circuit, similar to those shown

in Figure 1, is modelled to allow energisation studies to be

carried out. The different aspects of the model are detailed in

the following section. No overvoltage protection was

included in the model.

UPEC 2011 ∙ 46th International Universities' Power Engineering Conference ∙ 5-8th September 2011 ∙ Soest ∙ Germany

ISBN 978-3-8007-3402-3 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach

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A. 400kV Onshore Network

The onshore transmission system is represented using two

network equivalents connected by a 400kV double circuit

overhead line. The offshore wind farm is connected by

turning in one of the 400kV overhead circuits.

Each network equivalent is a voltage source behind an

impedance comprising; an inductance, representing the

network fault level in parallel with a resistance representingthe network surge impedance. It was assumed that the fault

level contribution is shared equally between the two

equivalent sources. The network fault level is assumed to be

17.5GVA, or just over 25kA which is typical for the UK

Transmission system [4].

B. Transformers and Earthing

There are two power transformers represented in the

model; the 400/132/13kV (Yyd) autotransformer at the

onshore substation and the 132/33kV (Yd) transformer at the

offshore substation. Both are modelled using the BCTRAN

component. Saturation is represented using nonlinear

inductors connected to the lowest voltage winding in eachtransformer. Core losses are modelled as resistances within

each BCTRAN component. Winding capacitances are

connected externally.

Earthing transformers are installed on the 33kV side of the

132/33kV transformer to provide an earth to the delta

connected 33kV network. These are Zy transformers, which

are also used to supply power to the platform substation. The

earthing method used in this model is similar to that in

Lillegrund Wind Farm [5]; a 130kVA Zy transformer with

zero sequence impedance of 30ȍ is connected with a 67 ȍ

resistance between the neutral point and earth.

The 33/0.415kV Zy transformer is represented using the

Saturable Transformer component, with winding impedances

calculated from test certificate measurements according to

[6]. Saturation of the Zy transformer has been ignored, and

the flux-current relationship is linear; represented by a single

point based on the magnetising current.

C. Overhead Lines and Submarine CableThe 132kV submarine cable is a three core design, with

XLPE insulation and 630mm2

copper cores, and 45km in

length. Figure 3 shows the cross section of the cable.

Figure 3 Cross section of the 132kV submarine cable

The lead phase sheaths and the steel wire armour are

assumed to be continuously earthed. The frequency

dependent JMARTI model is used, with transformation

matrices calculated at 5 kHz. The frequency fitting is started

at a low frequency as suggested by [7].

The 400kV double circuit overhead lines are each 60km in

length, again represented using the JMARTI model. National

Grid’s L6 type overhead line is used; it consists of bundles of

four 400mm2 Zebra conductors per phase and a single

400mm

2

Zebra earth wire conductor. D. Filters and Compensation

A harmonic filter is connected at 132kV, on the

autotransformer side of the 132kV circuit breaker. The filter

used is a 20MVAR C-type filter, which is typical of similar

offshore wind farm connections. It is tuned to the 3rd

harmonic, which is most common [8], and a unity quality

factor has been assumed. The component values used were

selected using information from [9].

Reactive compensation is likely to take two forms; shunt

reactors to compensate the cable capacitance, and

compensation for voltage control (SVC or Statcom). The

13kV delta tertiary winding of the 400/132kVautotransformer provides an ideal low voltage point of

connection for reactive compensation. In this model, earthed

shunt reactors are connected to the 13kV transformer tertiary;

sized to compensate for the total capacitance of the cable and

filters. The shunt reactors are assumed to be air-cored so they

are represented as linear inductors. For the sake of simplicity,

no compensation for voltage control has been included.

III. SIMULATION

A. Circuit Breaker Operations Studied

The following scenarios were investigated usingsystematic switching:

1) Energisation of transformer-feeder circuit from

the onshore circuit breaker 2) Energisation of 132kV cable from the onshore

circuit breaker.3) Energisation of the 132/33kV transformer from

the offshore circuit breaker.4) Energisation of the 132kV cable with trapped

charge.

In the first scenario, with the offshore circuit configured as

a transformer-feeder, the submarine cable is connected

directly to the 132/33kV transformer at the offshoresubstation. The circuit is isolated by opening the 132kV

circuit breaker onshore, and the 33kV transformer breaker in

the offshore substation. This type of arrangement is very

common on UK distribution networks. This configuration

means that the cable and transformer have to be energised

together when the onshore circuit breaker is closed.

The other three scenarios assume that a 132kV circuit

breaker is located on the offshore substation between the

submarine cable and the 132/33kV transformer so that the

circuit can be energised in sections; first by energising the



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132kV cable from the onshore circuit breaker, then by closing

the offshore 132kV circuit breaker to energise the 132/33kV

transformer.

When the 132kV cable can be de-energised without the

transformer attached to the remote end, there is a likelihood

of leaving a trapped charge on the cable (provided there is no

circuit component to provide a discharge path to earth). A

trapped charge was introduced to the simulation by startingthe simulation with the onshore circuit breaker closed towards

the 132kV cable. After 1ms the circuit breaker is opened. The

circuit breaker is modelled as an ideal switch which opens

when the cable charging current is zero, leaving a voltage

remaining on the cable (in this case, phase B is left at 1pu,

phases A and C are left at -1pu). The magnitude of this

trapped charge is extreme, and represents the worst case when

re-closing the onshore circuit breaker onto the cable.

B. Statistical Switching Statistical switching was used to determine the range of

overvoltages from different circuit breaker pole closing times

across the 20ms/50Hz waveform. Switching across one thirdor half of the waveform would probably be acceptable if

symmetry is assumed; however, trapped charge is explored

by simulating a single breaker opening event prior to

statistically re-closing the circuit breaker. In this case,

switching across the whole waveform is required.

The approach taken for the simulations described in this

paper is similar to that used in [10] and the dependent model

described in [11]. A master switch determines the instant at

which circuit breaker closing is initiated. The closing time of

this master contact varies according to the uniform

distribution over a range of 0 to 1/ f . After the master switch

closes, each of the three associated phase contacts closes after

an average delay of 20ms. This delay is varied according to

the Gaussian distribution, with a standard deviation of

0.833ms corresponding to a maximum pole span of 5ms. The

end result is random switching events spread across one 20ms

waveform, centered on 30ms.

A total of 3500 operations were simulated for each circuit

breaker operation studied. In order to ensure the results were

directly comparable, the same statistical switching times were

used for each switching scenario studied.

IV. R ESULTS: STATISTICAL

Table 1 summarises the highest overvoltages found at each

network for the four different energisation events simulated.The overvoltages are calculated as per unit quantities; the unit

voltages used are the peak AC voltages shown in Table 2 . It

can be seen that the highest overvoltages of all were seen at

the 13kV tertiary winding of the 400/132kV autotransformer.

These overvoltages occurred whenever the cable was

energised.

Table 1 - The highest overvoltages found at the different network

voltages during statistical switching simulations.

Transformer-Feeder

(Cable & 132/33kV

Transformer)

132kV Cable 132/33kV Transformer 132kV Cable (with

Trapped Charge)

400kV 1.15 1.15 1.05 1.25

132kV 1.95 1.85 1.9 2.75

33kV 1.85 NA 2.6 NA

13kV 5.2 3.8 1.2 8.75

Network

Voltage

Plant Energised

The highest overvoltages seen on the 33kV network, 2.6pu,

occurred when the 132/33kV transformer was energised by

the closing of the offshore 132kV circuit breaker.

The maximum overvoltages seen by the 132kV network

were consistently around 1.9pu, for all operations except the

energisation with trapped charge. Energising the 132kV cable

with trapped charge produced greater overvoltages of up to

2.75pu on the 132kV system.

Under all energisation operations, little impact was seen

on the 400kV transmission network. The results of the

statistical simulations for each scenario are shown as

cumulative probability curves in Figure 4. The results showthe distribution of the overvoltages calculated using statistical

switching.

The minimum standard rated switching impulse withstand

voltage (SIWV) is 850kV for the insulation used on the

400kV transmission network [12]. This equates to 2.6pu,

whilst the highest overvoltage seen on the 400kV was only

1.25pu.

Standard SIWV voltages for 132kV, 33kV and 13kV are

not directly listed IEC 60071-1. The SIWV needs to be

calculated applying a test conversion factor to the standard

corresponding lightning impulse withstand voltages (LIWV)

[12]. This test conversion factor depends on the insulation

type. By dividing the LIWV by the test conversion factor of

1.25 for GIS, and then a safety factor of 1.15, we can

calculate the SIWV. Using the test conversion factor for GIS

will give more conservative answers than other insulation

media. A safety factor of 1.15 is used for enclosed insulation

systems. Table 2 summarizes the relevant insulation

strengths.

Table 2 Standard insulation strengths taken from IEC 60071 [12][13],

alongside corresponding pu values.

V o l t a g e ( L - L ,

k V )

U n i t V o l t a g e ( k V ,

p e a k )

S h o r t D u r a

t i o n P o w e r

F r e q u e n c y

W i t h s t a n d

( k V , r m s )

S I W V ( k V ,

p e a k )

L I W V ( k V , p e a k )

S h o r t D u r a

t i o n P o w e r

F r e q u e n c y

W i t h s t a n d

( P U ) *

S I W V ( p u ) *

*

400kV 326.6 NA 850-950 1050-1425 NA NA

132kV 107.8 275 NA 650 3.6 4.2

33kV 26.9 70 NA 170 3.7 4.4

13kV 10.6 38 NA 95 5.1 6.23

**Calculated by dividing LIWV by test conversion factor (1.25) and by safety factor

(1.15), then by refering to the base voltage.

*RMS values of short duration power frequency withstand voltages have been

given a pu value by assuming they are sinusiodal AC, and referring them to the

unit voltage.



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0

20

40

60

80

100

120

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Voltage (pu)

C u m u l a t i v e P r o b a b i l i t y ( % )

0

20

40

60

80

100

120

1 1.5 2 2.5 3 3.5 4

Voltage (pu)


0

20

40

60

80

100

120

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9

Voltage (pu)

C u m u l a t i v

e P r o b a b i l i t y ( % )

0

20

40

60

80

100

120

1 1.5 2 2.5 3

Voltage (pu)


400kV

132kV

33kV

13kV

Transformer-feeder

132kV Cable

132/33kV

Transformer

132kV Cable

with TrappedCharge

0

20

40

60

80

100

120

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Voltage (pu)


0

20

40

60

80

100

120

1 1.5 2 2.5 3 3.5 4

Voltage (pu)


0

20

40

60

80

100

120

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9

Voltage (pu)

C u m u l a t i v

e P r o b a b i l i t y ( % )

0

20

40

60

80

100

120

1 1.5 2 2.5 3

Voltage (pu)


400kV

132kV

33kV

13kV

400kV

132kV

33kV

13kV

Transformer-feeder

132kV Cable

132/33kV

Transformer

132kV Cable

with TrappedCharge

Figure 4 Cumulative probability curves of overvoltages calculated for

different voltages on the network using the statistical energisation

studies.

V. R ESULTS: WAVEFORMS

This section shows waveforms resulting from randomly

generated switching operation 431, which resulted in circuit

breaker poles closing at 33.6ms, 36ms, and 36.9ms for each

scenario studied.Figure 5 shows the voltages at various points during the

energisation of the circuit as a transformer-feeder. As the

circuit breaker poles close onto the uncharged 132kV cable,

there is instantaneous depression in voltage on the 132kV

side of the 400/132/13kV transformer, this can be seen in

Figure 5B. The sudden change in voltage on the 132kV

terminals, results in significant overvoltages being introduced

to the 13kV tertiary winding due to the capacitive coupling

between the windings. This can be seen in Figure 5C.

(A) Onshore Transformer 400kV Terminals

(B) Onshore Transformer 132kV Terminals

30 35 40 45 50 55 60 65 70-40

-30

-20

-10

0

10

20

30

V o l t a g e [ k V ]

Time [ms]

(C) Onshore Transformer 13kV Terminals

(b)

30 35 40 45 50 55 60 65 70-200

-100

0

100

200


V o l t

a g e [ k V ]

30 35 40 45 50 55 60 65 70-45

-30

-15

0

15

30

45

Time [ms] Time [ms]

(E) Offshore Transformer 33kV Terminals

(D) Offshore Transformer 132kV Terminals

(a)

30 35 40 45 50 55 60 65 70-400

-200

0

200

400


30 35 40 45 50 55 60 65 70-200

-100

0

100

200


Time [ms] Time [ms]



30 35 40 45 50 55 60 65 70-40

-30

-20

-10

0

10

20

30


Time [ms]30 35 40 45 50 55 60 65 70

-40

-30

-20

-10

0

10

20

30


Time [ms]


(b)

30 35 40 45 50 55 60 65 70-200

-100

0

100

200


V o l t

a g e [ k V ]

30 35 40 45 50 55 60 65 70-45

-30

-15

0

15

30

45

Time [ms] Time [ms]

(b)

30 35 40 45 50 55 60 65 70-200

-100

0

100

200


V o l t

a g e [ k V ]

30 35 40 45 50 55 60 65 70-45

-30

-15

0

15

30

45

(b)

30 35 40 45 50 55 60 65 70-200

-100

0

100

200


V o l t

a g e [ k V ]

30 35 40 45 50 55 60 65 70-45

-30

-15

0

15

30

45

Time [ms] Time [ms]


(D) Offshore Transformer 132kV Terminals

(a)

30 35 40 45 50 55 60 65 70-400

-200

0

200

400


30 35 40 45 50 55 60 65 70-200

-100

0

100

200


Time [ms] Time [ms]

(a)

30 35 40 45 50 55 60 65 70-400

-200

0

200

400


30 35 40 45 50 55 60 65 70-200

-100

0

100

200


(a)

30 35 40 45 50 55 60 65 70-400

-200

0

200

400


30 35 40 45 50 55 60 65 70-200

-100

0

100

200


Time [ms] Time [ms]

Figure 5 Voltage waveforms during transformer-feeder energisation

using onshore 132kV CB (Results shown for poles closing at 33.6ms,

36ms, and 36.9ms)

The high frequency oscillation in Figure 5C is a result of

oscillation between the coupling capacitance and the shunt

reactors. The oscillation is removed when the shunt reactors

were separated from the transformer tertiary winding by a

length of cable, although the initial voltage spike was still

apparent. Further work is probably needed to ensure that the

capacitive coupling between windings is accurately

represented.

Figure 6 shows the waveforms as the 132kV cable is

energised on its own. The behaviour is largely similar to the

scenario with the transformer-feeder energisation. The 132kV

waveforms from the transformer-feeder energisation in Figure5 show some additional distortion due to saturation of the

transformer during inrush.


Time [ms]


Time [ms] Time [ms]

(D) Offshore End of 132kV Cable

(C) Onshore Transformer 13kV TerminalsTime [ms]

30 35 40 45 50 55 60 65 70-400

-200

0

200

400

30 35 40 45 50 55 60 65 70-30

-20

-10

0

10

20

30


V o l t a g e

[ k V ]

30 35 40 45 50 55 60 65 70-200

-100

0

100

200

30 35 40 45 50 55 60 65 70-160

-80

0

80

160




Time [ms]


Time [ms] Time [ms]



30 35 40 45 50 55 60 65 70-400

-200

0

200

400

30 35 40 45 50 55 60 65 70-30

-20

-10

0

10

20

30


V o l t a g e

[ k V ]

30 35 40 45 50 55 60 65 70-400

-200

0

200

400

30 35 40 45 50 55 60 65 70-400

-200

0

200

400

30 35 40 45 50 55 60 65 70-30

-20

-10

0

10

20

30

30 35 40 45 50 55 60 65 70-30

-20

-10

0

10

20

30


V o l t a g e

[ k V ]

30 35 40 45 50 55 60 65 70-200

-100

0

100

200

30 35 40 45 50 55 60 65 70-160

-80

0

80

160



30 35 40 45 50 55 60 65 70-200

-100

0

100

200

30 35 40 45 50 55 60 65 70-200

-100

0

100

200

30 35 40 45 50 55 60 65 70-160

-80

0

80

160



Figure 6 Voltage waveforms during 132kV cable energisation using

onshore 132kV CB. (Results shown for poles closing at 33.6s, 36ms, and

36.9ms)

Figure 7 shows the voltage waveforms when 132/33kV

transformer is energised from the 132kV offshore circuit

breaker. Unlike Figure 5B, little voltage depression is

apparent on the 132kV terminal of the 400/132/13kV



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transformer in Figure 7B. The distortion visible here is

harmonic distortion due to transformer inrush. Figure 7D and

Figure 7E show high frequency oscillations as the 132/33kV

transformer being energised one pole at a time.

(A) Onshore Transformer 400kV TerminalsTime [ms]


Time [ms]

Time [ms]


Time [ms]


(D) Offshore Transformer 132kV TerminalsTime [ms]

30 35 40 45 50 55 60 65 70-400

-200

0

200

400

30 35 40 45 50 55 60 65 70-200

-150

-100

-50

0

50

100

150

V o

l t a g e [ k V ]

V o

l t a g e [ k V ]

30 35 40 45 50 55 60 65 70-150

-100

-50

0

50

100

150

30 35 40 45 50 55 60 65 70-60

-40

-20

0

20

40

60



30 35 40 45 50 55 60 65 70-12

-8

-4

0

4

8


(A) Onshore Transformer 400kV TerminalsTime [ms]


Time [ms]

Time [ms]


Time [ms]


(D) Offshore Transformer 132kV TerminalsTime [ms]

30 35 40 45 50 55 60 65 70-400

-200

0

200

400

30 35 40 45 50 55 60 65 70-400

-200

0

200

400

30 35 40 45 50 55 60 65 70-200

-150

-100

-50

0

50

100

150

30 35 40 45 50 55 60 65 70-200

-150

-100

-50

0

50

100

150

V o

l t a g e [ k V ]

V o

l t a g e [ k V ]

30 35 40 45 50 55 60 65 70-150

-100

-50

0

50

100

150

30 35 40 45 50 55 60 65 70-60

-40

-20

0

20

40

60



30 35 40 45 50 55 60 65 70-12

-8

-4

0

4

8


Figure 7 Voltage waveforms during 132/33kV transformer energisation

using offshore 132kV CB. (Results shown for poles closing at 33.6ms,

36ms, and 36.9ms)

Figure 8 shows the waveforms as the 132kV cable is

energised with trapped charge. The behaviour is largely

similar to the scenario with the 132kV cable energised

without trapped charge; however, the overvoltages are more

extreme.

30 35 40 45 50 55 60 65 70-300

-200

-100

0

100

200

300


30 35 40 45 50 55 60 65 70-300

-200

-100

0

100

200

300


30 35 40 45 50 55 60 65 70-400

-200

0

200


30 35 40 45 50 55 60 65 70-60

-40

-20

0

20

40

60

V o l t a g e [ k V

]


Time [ms]


Time [ms] Time [ms]



30 35 40 45 50 55 60 65 70-300

-200

-100

0

100

200

300

30 35 40 45 50 55 60 65 70-300

-200

-100

0

100

200

300


30 35 40 45 50 55 60 65 70-300

-200

-100

0

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200

300


30 35 40 45 50 55 60 65 70-400

-200

0

200


30 35 40 45 50 55 60 65 70-60

-40

-20

0

20

40

60

V o l t a g e [ k V

]


Time [ms]


Time [ms] Time [ms]



Figure 8 Voltage waveforms during energisation of 132kV cable with

trapped charge using onshore 132kV CB. (Locations shown are letteredas in Figure 2. Results shown for poles closing at 33.6mS, 36mS, and

36.9mS)

VI. CONCLUSIONS

Overvoltages on the 132kV and 400kV systems caused by

energising the submarine cable together with the 132/33kV

transformer were of similar magnitudes to those caused when

the cable and transformer were energised separately.

Energising the 132/33kV transformer separately using a

circuit breaker located offshore caused greater overvoltages

on the 33kV system.

Transformer-feeder circuits are common on UK

distribution networks because of the reduced number of

circuit breakers required. On the offshore platform, where

space and weight are at a premium, this is particularly

desirable. One crucial difference is that on the offshore

transformer, the 132kV cable connection to the transformer

would likely be enclosed due to space restrictions on the

platform, rather than terminated using open bushings. Thiscould potentially be difficult operationally, as flexible earths

cannot then be applied to earth the transformer HV locally.

Under all scenarios considered, little effect was seen on the

400kV system. Even in the extreme case of trapped charge,

only overvoltages of up to 1.25pu were introduced.

When the onshore circuit breaker is closed onto the

submarine cable, significant overvoltages can be introduced

to the 13kV tertiary winding of the onshore transformer. With

trapped charge, the overvoltage on the tertiary winding

reached 8.75pu in one simulation. Further refinement of the

model may be required to ensure the tertiary voltages areaccurately calculated. Despite the imperfections in the

existing model, overvoltages in the tertiary winding due to

capacitive coupling with the 132kV winding are a potential

concern. These overvoltages could perhaps be reduced using

surge arresters, but further work is required to confirm this.

Trapped charges on the 132kV cable cause the worst

overvoltages on the 400kV, 132kV, and 13kV systems. These

could be avoided by providing a path for the cable to

discharge to earth. This could be accomplished by

repositioning shunt reactors so that they are permanently

connected to the 132kV cable, or running the network as atransformer-feeder, as examined in this paper. Attaching

inductive VTs would also provide a discharge path. However

the discharge capability of inductive VTs may introduce some

restrictions on how frequently the circuit may be switched out

and re-energised.

VII.FUTURE WORK

The first step would be to check the accuracy of the

capacitive coupling on the 400/132/13kV transformer, and

improve the modelling of plant connected to the 13kV tertiary

winding. In the existing model, the earthed shunt reactors

provide the only path for earth fault current. A better

alternative may be to install a Zy earthing transformer and

leave the neutral point of the shunt reactors unearthed. The

capacitive coupling between windings used in this model is

potentially simplistic, and needs further examination.

The next logical step is to simulate faults on the network

and circuit breaker opening operations. Other network

topologies and alternative positioning of shunt reactors are to

be investigated.



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ACKNOWLEDGEMENTS

The authors would like to acknowledge the contribution

and support from National Grid.

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