Ben Guth and Doug Jones - Wind Farm Grounding

8/10/2019 Ben Guth and Doug Jones - Wind Farm Grounding

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Wind Session

Effective Grounding of Wind Farm Collector Circuits

Byen u , . .

Doug Jones, P.E.


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What is transient overvoltage (TOV)?

What is the cause of TOV?

How is TOV mitigated?

Do all types of WTGs have problems with TOV?An o a mitigation tec niques wor or a types?

How is TOV mitigating equipment specified?


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Must design an economical grounding system that:

controls TOV to acceptable levels limits fault current while enabling secure ground fault

rotection

Transient Overvoltages (TOV) can lead to surge

arrester and other equipment failure roun ng rans ormers or g spee groun ng

switches usually provide adequate mitigation

Different wind turbine t es can mean differentgrounding methods


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69/92/115

MVA

230 kV

34.5 kV

34.5 kV Main Bus

52F1 52F2 52F3 52F452CAP

Collector

1

Collector

2

Collector

3

Collector

4DVAR


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Typically 34.5kV

Approximately 30MVA per circuit

capacitive

MOV t e sur e arrestors installed at each end of thecable system

Pad mount or nacelle mounted generator step up

trans ormers (35kV) solidly GndY primary (1kV-480V) solidly GndY

secondary -


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Type 1 squirrel-cage induction generators

Type 2 wound rotor induction generator withcon ro e ro or res s ance

doubly-fed induction generator

Type 4 generator interfaced to the grid totallythrough a variable speed power electronic drives stem.


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yp ca au t contr ut on range o to perunit


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Typical fault contribution range of 4 to 9 perunit


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Typical fault contribution less than 2 per unitfor remote faults before crowbar

er crow ar engages resem es a ype


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yp ca au t contr ut on s very ow andependant upon pre-fault loading

Usually Type 3 and 4 WTG short circuitcurrent contributions dont matter that much


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Ground fault

Inadvertent opening of circuit during generation

Capacitor bank switching

Collection cable ca acitance


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Ground reference primarily derived by Substationtrans ormer

ground fault

Overvoltage of non-fault phases depends ongrounding/generator type

A new ground reference must be derived fromcircuit to be effectively grounded


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Very small Neutral to Ground Impedance


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100

50

75

[kV]

-25

0

-100

-75

-50

(file Cable.pl4; x-var t) v:G5NCA -X0004A v:G5NCB -X0004B v:G5NCC -X0004C

. . . . . . . .


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Neutral potential & ground potential are coupledon y y t e ne-to-groun capac tances:

A

Gnd = 0 V

A-Gnd Fault on

Ungrounded System

N

VL-G = 1 p.u.

VL-G VL-L

A

C B = . p.u.

Un-faulted System

C B


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100

50

75

[kV]

-25

0

-100

-75

-50

(file Cable.pl4; x-var t) v:NGNCA -X0070A v:NGNCB -X0070B v:NGNCC -X0070C

. . . . . . . .


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Definition of Effectively Grounded

Effectively grounded when the coefficient ofgrounding (COG) is less than 80%

COG = VLG/VLL

Effectively grounded: VLG < 80% of VLL

nom na LL s . , e un- au e p asesmust be VLG < 27.6 kV


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An effectively grounded system can beapprox mate w en system:

X0 & X1 are both are inductive

R0/X1 < 1

To obtain effective ground either reduce R0 & X0, orincrease X1

Increase X1 = Increase Load Losses


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Install high speed ground switch

Requires very fast switching, < 1 cycle

Mechanical interlocks do the job well

Electric interlocks have not proven reliable, fast

enough

Install grounding transformer


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Wye-Delta or Zig-Zag

Equally effective for limiting TOV

Zig-Zag will be physically smaller but are specialty

built transformers available from a limited numberof manufacturers with generally long lead times andhigh pricing due to one-off design

Wye-Delta available pad-mount type are readilyavailable from multiple manufacturers


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Primary Voltage

Rated KVA (if needed)

Continuous Neutral Current Continuous non- .

Neutral Fault current and duration

Impedance Primary winding connection

Secondary connection


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Ohms per phase

Rated time 10 second, 1 minute, 10 minute, or extended (less then 90

Generally a 10 second rating for a WPP with an effectivelygrounded system due to fast and sensitive protectionre uirements

Continuous rated current Based on rated time (IEEE 32) = Example: 3500 amps for 10 sec = 3500 * 0.03 =105 amps

continuous


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Model the system

oose some yp ca parame ers , . - . ,

use 6.2%) Simulate ground fault

Determine VLG-unfaulted < 0.8 * VLL ess t an 27.6 V

Modify transformer parameters as needed, repeat

Once youve reached desired performance, calculate: ohms per phase

Maximum fault current from grounding transformer

One size does not fit all must consider the s stem

Check with transformer manufacturer to ensurebuildability/availability


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Its a good idea to limit TOV due to ground faults,an re uce trans ents ue to , resonance, etc.

asynchronous system

Must verify surge arresters, cables, and otherequipment will withstand the TOV during ground


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For Type 1 & 2 WTGs grounding transformers and

fast ground switch breakers are both effective in

mitigating TOV

TOV results whether the step-up transformers are

connected /Yg or Yg/Yg Generators are ungrounded, therefore do not provide zero

sequence fault current

Ensure the generator will trip off quickly if thesubstation breaker opens inadvertently


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Both roundin transformers and round switch

breakers can be effective in mitigating TOV Ground switch breaker ma be the referred o tion

as it forces the crowbar to engage and doesnt

require a full transient analysis Transient analysis may be needed

Transient analysis adds design time, costs and

complexity


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Type 4 machines should have low fault currentcontr ut ons to a au t

Verify fault current contribution from WTG

For SLG faults on the system the Type 4 power

electronic controller switches out ver uicklthereby not sourcing voltage to a SLG fault. As aresult no significant energy is provided that would


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Questions


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Chong Han, Don E. Martin, Member, IEEE, and Modesto R. lezama, Transient Over-Voltage (TOV) andIts Suppression For a Large Farm interconnection.

Eaton Application White Paper, Transient Over voltages on Ungrounded Systems from Intermittent

Ground Faults.

Pacific Crest Transformers Article, Grounding Transformers April, 02 2009.

even . ay ors, . ., arge n an o ec or es gn n arm o ec or ys em roun ng,PES Transmission and Distribution Conference 2008.

Mike Reichard, Fault Current Contributions From Variable Speed (Type 3 and 4) Wind Turbine

Generators Texas A&M Protective Relaying Conference Fault Current Contributions from Wind PlantsA ril 1 2009.

ANSI/IEEE Std 32-1972 IEEE Standard Requirements, Terminology, and Test Procedures for NeutralGrounding Devices.

Reigh A. Walling, Michael L. Reichard, Short Circuit Behavior of Wind Turbine Generators

EMA VDH/GSMI brochure (Combined 34.5 kV Vacuum Circuit Breaker & Mechanically Interlocked),available online at: http://www.ema-sa.com.ar

IEEE Standard C62.22-1997, Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems.

Documents

Ben Guth and Doug Jones - Wind Farm Grounding