Insulation coordination for wind power

plants

EMTP-RV Satelite meeting

Paris, FRANCE – August 30, 2018

Prof. Dr. Ivo Uglešić

Božidar Filipović-Grčić, PhD

Bruno Jurišić, PhD

Nina Stipetić, mag.ing.

Faculty of Electrical Engineering and Computing

University of Zagreb, Croatia

• Insulation coordination definition

• Wind Power Plant characteristics

• Challenges of insulation coordination in WPPs

• Examples in EMTP-RV

Temporary overvoltage due to SLGF

Vacuum circuit breaker switching

Direct lightning strikes

Overview

1

• The goal of insulation coordination is uninterrupted and reliablepower supply in all technical and atmospheric conditions.

• Definition by IEC 60071-1:

Insulation coordination is the selection of the dielectric strength

of equipment in relation to the voltage which can appear on the

system for which the equipment is intended and taking into

account the service environment and characteristics of the

available protective devices.

The insulation in a power system (with all its components) should

be designed on the way to minimize damage and interruption to

service as a consequence of steady state and transient

overvoltages and this should be done economically.

Insulation coordination

2

• Transmission system: MVAC, HVAC, HVDC

• Collector system: MVAC cable network, different topologies

• Reactive compensation, filters at POI

• Main components:

wind turbine generators (Type 1 – Type 5)

vacuum circuit breakers

transformers (step-up and power transformer)

cables

3

Wind power plant characteristics

WPP – typical layout

WTGs

LV/MV

step-up transformers

feeder

breakers

substation

MV/HV

transformer

4

Lightning transients Coupling through substation

transformer From direct tower strikes

Switching transients Coupling through substation

transformer Feeder energization Vacuum circuit breaker switching

(VFFT due to prestrikes and restrikes)

Temporary overvoltages Ground faults Feeder islanding, loss of ground

reference Can be higher than 1.73 pu

Continuous operating voltages Higher voltage at remote ends

Overvoltages in WPPs

Surge arrester selected

to protect equipment

from these

Surge arrester selected

to survive these

5

Surge arrester typical placement in MV network

6

• adjacent to the substation power transformer

• on the feeder side of each feeder breaker

• at each interface between overhead and underground

feeder sections

• at the end of each feeder and branch

+ LV surge arresters in the tower

7

Conventional process from

standards (IEEE C62.22, IEC 60071)Procedure for WPPs

1. Select the surge arrester to be used

(MCOV, TOV)

1. Select the available insulation level

(limited range)

2. Determine the protective level of

selected surge arrester

2. Select arresters needed to protect

that insulation level

3. Determine the locations for surge

arresters

3. Determine amount of TOV that can

be withstood

4. Determine the voltage at terminals

of the protected equipment

5. Select equipment insulation level

Evaluate voltage protection margins

Insulation coordination steps

if margins are inadequate, consider alternatives:

different arrester locations, higher insulation

level...

TOV mitigation methods

Temporary overvoltage due

to SLGF

8

9

Example 1

Temporary overvoltage: single-line-to-

ground fault, feeder disconnection and

loss of ground reference

1.

2.3.

R. Walling, WESC10

Example 1

• Overvoltage of healthy phases depends on grounding and WTG type

• TOV mitigating options: Transfer tripping (normaly used in new WPPs)

High-speed grounding switch

Grounding transfomer on each feeder

• The choice depends on WTG type

11

Example 1

voltage measurement

faultGT2

GT1

X

+

Vw

Z1

0.6

9kV

/_0

VM+m2

?v/?v/?v

LF

Phase:0

Slack: 132kVRMSLL/_0

LF1

+132kVRMSLL /_0

BUS:

VwZ10

FDQ+

FDQ2

FDQ+

FDQ12

CABLE DATA

model in: cable_sc_100m_33_6fd_rv.pun

cabledata4

FDQ+

FDQ13

CABLE DATA

model in: cable_33_800_6ph_fd_rv.pun

cabledata5

+ Vw

Z11

0.6

9kV

/_0

FDQ+

FDQ14

1 2

+30

DY_2

33/132+

1nFC1

+

1nFC2

+ Vw

Z12

0.6

9kV

/_0

+

-1|1

E15|0

SW

4

FDQ+

FDQ15

+ Vw

Z13

0.6

9kV

/_0

+

-1|1

E15|0

SW

11

FDQ+

FDQ16

+ Vw

Z14

0.6

9kV

/_0

+

-1|1

E15|0

SW

12

FDQ+

FDQ17

+ Vw

Z15

0.6

9kV

/_0

+

-1|1

E15|0

SW

19

FDQ+

FDQ18

+ Vw

Z16

0.6

9kV

/_0

+

-1|1

E15|0

SW

20

FDQ+

FDQ19

+ Vw

Z17

0.6

9kV

/_0

+

-1|1

E15|0

SW

21

FDQ+

FDQ20

12

+3

0

33/0

.69

+

-1|2

00m

s|0

SW

26

+

-1|1

E15|0

SW

27

+

-1|1

E15|0

SW

28

12

+3

0

33/0

.69

+

-1|2

10m

s|0

SW

29

CABLE DATA

cabledata6

VM+

?v

m8

Vacuum circuit

breaker model

DEV9

Vacuum circuit

breaker model

DEV10

Vacuum circuit

breaker model

DEV11

12

+3

0

33/0

.69

+

-1m

s|2

15m

s|0

SW

30

12

+3

0

33/0

.69

+

-1m

s|2

20m

s|0

SW

31

12

+3

0

33/0

.69

+

-1m

s|2

25m

s|0

SW

32

12

+3

0

33/0

.69

+

-1|2

30m

s|0

SW

33

12

+3

0

33/0

.69

+

-1m

s|2

15m

s|0

SW

34

12

+3

0

33/0

.69

+

-1m

s|5

00m

s|0

SW

35

VM+

?v

m10

VM+

?v

m11

+R

13

1m

+

R14

1m

+

R15

1m

+R16

100M

+

R17

100M

+R18

100M

+

RL7

?i

2,4

Ohm

+

RL8

?i

2,4

Ohm

+

RL9

?i

2,4

Ohm

+

12

Tr0

_7

1

+

12

Tr0

_8

1

+

12

Tr0

_9

1

+

20m

s|1

20m

s|0

?vi

SW

36

+R

19

1m

+

R20

1m

+

R21

1m

+

R22

100M+

R23

100M+

R24

100M

+

RL10?

i12,2

5O

hm

+

RL11?

i12,2

5O

hm

+

RL12?

i12,2

5O

hm

+

12

Tr0

_10

1

+

12

Tr0

_11

1

+

12

Tr0

_12

1

+

20m

s|1

E15|0

?vi

SW

37

+A ?

i

m12

+A?

im

13

+

20m

s|1

E15|0

?vi

SW

38

scope

?s

scp1

scope scp2

?sscope scp3

?s

+ A

?i

m1

+ A?i

m3

Data function

ZnO

arrester abb.dat

model in: arrester abb.pun

+Zn

O

ZNOc

?vi>e

66000+Z

nO

ZNOb

?vi>e

66000+Z

nO

ZNOa

?vi>e

66000

b

c

a

c

b

a

BUS_A2cc aa bb

BUS_A3

c cb ba a

BUS_A4

b ba ac c

BUS_A5

cc aa bb

BUS_A7

cc aa bb

BUS_A6

cc aa bb

BUS_A1

cc bb aa

c

a

b

a ab bc c

BUS_A8

a a

cc

bb

e_ZNOa

e_ZNOb

e_ZNOc

ynD 0.69/33 kV

dYn

33/132 kVGrounding transformer

zig-zag

33 kV cable network

VCB

WTG

1. SLGF occurs at 20 ms on phase C

2. Feeder breaker opens at 100 ms – loss of ground reference

3. WTGs continue to generate

12

Example 1 – GT 2 excluded

1. 2.

overvoltage limited

13

Example 1 – GT included

1. SLGF occurs at 20 ms on phase C

2. Feeder breaker opens at 100 ms – loss of ground reference

3. WTGs continue to generate

1. 2.

overvoltage remains limited

Vacuum circuit breaker

switching

14

15

Practical problems with VCB switching in wind farms

• The first off-shore wind farms were faced with substantialtransformer failures in a very early operation stage.

• In the first large offshore wind farms Horns Rev andMiddelgrunden (Denmark), almost all of the transformers had tobe replaced due to insulation failures.

• It is suspected that the switching of the vacuum circuit breakers(VCB) caused the transformer failures.

• In order to investigate this phenomenon, a laboratory setup wasbuilt, designed to give an insight into high frequency transientsgenerated during breaker switching in offshore wind farms andsimilar cable systems.

„ELECTRICAL TRANSIENT INTERACTION BETWEEN TRANSFORMERS AND THE POWER SYSTEM”, CIGRE WG A2/C4.39

16

Vacuum circuit breaker switching

17

Restrike phenomena during breaking of small

inductive current

18

Restrikes caused by VCB switching

„ELECTRICAL TRANSIENT INTERACTION BETWEEN TRANSFORMERS AND THE POWER SYSTEM”, CIGRE WG A2/C4.39

19

Example 2 – Failure of power transformer 110/25 kV

Failure of 25 kV winding

20

Example 2 – EMTP-RV model

• Detailed model of VCB

• Detailed model of power transformer (high-frequency behaviour)

VCB

MV cable network (capacitive load)

Power transformer

Trafo -A5

+ C30.100nF

VM+ m1?v

Data function

ZnO

model in: polimh29.pun

+L1

482mH

+

ZnO

72600

ZnO1

Vacuum circuit breaker model

DEV1

+

C2

3.5

uF

+

ZnO

72600

ZnO2

State

Space4

SS2

d__1

d__2

c

d__4

+

AC2

110kVRMSLL /_0

cb

a

d__2

GND

GND

GND

GND

VCB

Cable network

Power transformer

110 kV network

Simulation results

VCB voltage

21

Simulation results

VCB current

22

Simulation results

VCB current

23

Simulation results

Overvoltage at 25 kV winding of power transformer

24

Simulation results

Overvoltage at 25 kV winding of power transformer25

Direct lightning strike to wind

turbine

26

Damages caused by direct lightning strike

27

28

Lightning and surge protection for the wind turbine

29

Placement of the SPDs

SPD according to EN 61643-11/IEC 61643-11 type 2/class II

Nominal AC voltage (Un) 480 V (50/60 Hz)

Max. continuous operating voltage (Uc) 600 V (50/60 Hz)

Nominal discharge current (8/20 µs) (In) 15 kA

Max. discharge current (8/20 µs) (Imax) 25 kA

Voltage protection level (UP) ≤ 3 kV

Temporary overvoltage (TOV) 900 V / 5 sec.

Example:

protection for

the generator

30

Example 3 – Direct lightning strike to WT blade

SM

SM1

VM+w1B1a

?v

VM+w1B1c

?v

1 2

-30

YD_1

0.69/22

VM+w1B1b

?v

VM+m4

?v1 2

+30

DY_1

22/110

VM+m5

?v

+

VwZ1

?vi

123kVRMSLL /_0

VM+m1

?v

VM+m2

?v

+

100kA/3us?iIcigre1

FDQ+

CABLE DATA

model in: lv_cable_rv.pun

LV_cable

CP

+40

TLM

4

CP

+80

TLM

3

CABLE DATA

model in: cabledata1_rv.pun

cabledata1

FDQ+

FDQ2

+

I

YIY

0.04

?vi>i

Rn1

if(u) 1

Fm1

f(u) 1

Fm2

c0.02

C1

1

2

select

Sel1

+

1.155nFC2+

2.21nFC3

+

2.4

64nF

C4

Data function

ZnO

model in: arresterlv.pun+Zn

O

700

?vi

ZnO1

c

ba

BUS4

a

b

c

BUS3BUS3

c

b

a

BUS2

c

b

a

BUS7

GND

GND

GNDGND

SPD

22 kV cable

2 km

0,7 kV cable 80 m

TR1 0.69/22 kV

3 MVATR2 22/110 kV

15 MVA

110 kV

network

WTG

3 MVA

Tower

80 m

Blade

40 m

Grounding resistance 50 Ω

(ionization included)

Single WTG 3 MVA operating in the vicinity of substation 22/110 kV.

Lightning current 100 kA

31

Lightning current Current through grounding resistance

Potential rise at grounding resistanceGrounding resistance

Example 3 – lightning current distribution / ionization

32

Overvoltages at 0.69 kV side of TR1 Transferred overvoltages at 22 kV side of TR1

Overvoltages at WTG terminals

Example 3 – overvoltages at TR1 and WTG terminals

Example 3 – SPD connected in neutral point of TR1

and ideally grounded

33

Overvoltages at 0.69 kV side of TR1 Transferred overvoltages at 22 kV side of TR1

Overvoltages at WTG terminals

Conclusion

34

• Different approaches for reducing TOVs due to SLGF areinvestigated. Simulations have shown that application ofgrounding transformer efficintly reduces TOVs during SLGF.

• EMTP-RV simulations confimed the fact that VCB switchingproduces VFTOs which are not high in magnitude, but due tohigh freqency, the distribution across the transformer winding isnonlinear. This may cause dielectric breakdown of transformerinsulation system.

• The results of direct lightning strike simulation indicate that useof appropriate SPDs is crucial in order to protect the vulnerableelectronic equipment. However, the efficient operation of SPDsdepends on grounding resistance value which should be kept aslow as possible.

Insulation coordination for wind power

plants

EMTP-RV Satelite meeting

Paris, FRANCE – August 30, 2018

This work has been supported in part by the Croatian Science

Foundation under the project “Development of advanced high voltage

systems by application of new information and communication

technologies” (DAHVAT)