Protection Requirements for a Large Scale Wind Park

7/30/2019 Protection Requirements for a Large Scale Wind Park

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U

Protection Requirements

Ufor a Large scale Wind ParkShyam MusunuriSiemens Energy

Abstract: In the past wind power plantstypically had a small power rating whencompared to the strength of the connectedelectrical network and the behavior of the windmills during faults in the network wasconsidered non critical and wind power plantswere simply pulled out of the system. Henceprotection requirement of a wind mill was justrestricted to simple current and voltage basedmeasurement.

This paper identifies the areas of concern whereproper protection has to be introduced apartfrom the basic wind park requirements.

Owing to the increase in demand for renewableenergy large wind parks are constructed in deepseas. Such wind parks are connected to theelectrical grid on the land by comprehensiveunderground cables. The underground cablesare a potential for a fault occurrence.

The next area of concern would be in themultiple mechanically switched capacitor banksor FACTS used for dynamic reactivecompensation. The voltage flicker produced bylarge wind parks owing to the varying speed of

the wind mills on the interconnected grid has adeteriorating effect on the other connectedequipment and also on the grid as well. Tomaintain voltage stability as per NERCguidelines and to mitigate voltage flickers,dynamic reactive compensation can be providedwith multiple mechanically switched capacitorbanks or FACTS.

Therefore the paper will discuss in detail theprotection possibilities for the undergroundcables and the reactors for large wind parkslocated offshore.

INTRODUCTION

The exorbitant and phenomenal rise in oil pricesin early 2008 and the drastic need to protect theenvironment from the climatic changes due toburning of fossil resources for power generationled to the increase in wind generation. Althoughwind generation is explored extensively on theshore, however the visually abhorrent pollutioncreated by large wind mills on the scenic beautyof the land , led to the fast emerging alternative -the location of wind parks offshore in deep seas.

The penetration of the wind mills in generatingclean energy is on the increase every day andthese wind mills when integrated into a largeexisting grid may require a redesign of theexisting power system and the operationapproaches. The challenge of integrating thewindmills into the grid has to address thefollowing pertinent questions.

a) How to maintain acceptable voltage level atall times to the consumers?

b) How can windmills co-generate with theother power plants according to thecustomer energy needs?

Therefore this paper will focus on the following

1. Introduction and analysis of the variouselectrical fault possibilities in an offshorewind park.

2. Detection and prevention against Ferroresonance due to inductive reactanceintroduced by wind generators, reactors andcapacitive reactance introduced by the longrunning cables and capacitor banks.

3. Automatic fault collection and analysis, andits benefits.

478978-1-4244-4183-9/09/$25.00 2009 IEEE


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Figure 1 indicates the typical single line diagram with the conventional protection elements in the wind park.

I) ANALYSIS OF THE VARIOUSELECTRICAL FAULT POSSIBILITIES IN A

WIND PARK

The wind park faults shall be split broadly into

(a)Network faults occurring in

1.1 Sea Submarine cable connectionbetween the Onshore and the Offshoresystems.

1.2 HV Transformer and cable connection tothe collector bus

1.3 Bus bar faults of 34.5kV system

(b) Wind park faults occurring in

1.4 Cable connection between the collectorbus and the Wind turbines

1.5 Cable connection between Windturbines

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(a) Network faults

1.1- Sea Submarine cable connection between the Onshore and the offshore systems

Figure 2

The conventional AC transmission via sea

cables, turns out to be technically andeconomically attractive, however the limitationbeing the distances over which the power canbe transmitted.

This limitation of the ac power transmissioncapacity can be suitably compensated by theFACTS and HVDC system. However in theHVDC system it will be difficult to determine thefault current fed from the network into the windpark during the design stage. The fault typicallyvaries from 0.0 to1.0 pu and the systembehavior without the actual fault current will not

be accurate enough to draw any conclusions.Alternate to the conventional current, voltageand frequency protection, the technically

superior and cost effective solution will be 2 in

1 protection , where there exists two mainprotections like Line Differential and Distanceprotection.

The sea submarine cable is protected by 3terminal differential protection connected in aring , with one relay connected to the feederconnecting the submarine cable at the on-shoreand the other two relays connected to thesecondary side of the step-up transformerlocated on the off-shore. For any faults in thesubmarine cable, the relays consider the fault asan in-zone fault, all the three 2 in 1 relays will

trip on differential protection and isolate all thefault feeding possibilities.

COMMUNICATIONCHANNEL(Dual)

On-shore

Off-shore

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In our case study, the earth fault current is limited by the grounding transformer to 400A.

Primary MVA of the Transformer = 220MVASecondary MVA of the transformer = 110MVACT ratio of the Transformer Secondary = 2500/1AEarth fault current = 400ASecondary voltage = 34.5kV

IBSec B= U220,000 U= 3682A

3*34.5

I BDiff B= U400 U= 0.1083682

3IB0 B= U400 U= 0.162500

The minimum setting for the differential relay isset at 0.2, to take care of the CT errors, errorsdue to the variation in the transformer taps etc.

When the relay is set at 0.2 and if the actualdifferential current works out to be 0.108 or 0.16,the relay will get desensitized and will fail to tripresulting in huge over voltage in the transformer,which can ultimately lead to a catastrophe.

The solution for such an application will be touse the 3IB0 Bsetting of the 51N function and setaccording to the utility standards.

TRANSFORMER FAULTS

Faults in the Step up Transformer can also beidentified, utilizing the distance protectionfunction available in all the 2 in 1 relays.

On the primary side , the 2 in 1 relay locatedin the on shore could be set to cover 80% of thetransformer primary side windings with suitableZone-1 / Zone-2 settings and the 2 in 1 relaysconnected in the secondary sides can be set tohave an offset reverse Zone-3 protection tocover 80% of the transformer secondarys.

A REF protection can be provided for thetransformer primary to locate sensitive earthfaults and any transformer winding faults

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Figure-4

Busbar protection of 34.5kV system

The bus bar fault is the most severe fault in anypower system and bus bar protection could berealized by the reverse interlocking functionfunctionally available in the over current relays.IEC61850 GOOSE helps in the intracommunication between relays exchanginginformation. .

The busbar faults can be realized by thefollowing logic and condition

Condition-1 : Relay in Circuit No2 should see thefaults in reverse direction

Condition-2 : Relay in Circuit No4 up to Circuit-8should see the faults in forward direction

2

1

4 5 876

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Reaction of a Wind park for network faults

For network faults, immediate disconnection ofthe large wind parks is not advisable as thedisconnection would put additional stress on thealready troubled system. As a rule the wind

parks are not disconnected as long as certainvoltage and frequency limits are not exceeded

as defined by the utility. Each utility has its owndefinition for the LVRT( Low Voltage RideThrough )and the commonly adoptedcharacteristics is as shown below

67(P) Reverse pickup

67(N) Reverse pickup

67(P) Forward pickup

67(N) Forward pickup

2

4

67(P) Forward pickup

67(N) Forward pickup

8

Trip all fault feedingCircuit Breakers

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(b) Wind Park Faults

1.4 -Cable connection between the collector bus and the Wind turbines

Figure-5

The wind mills connected radially ends up in thecollector bus. Therefore the collector bus carriesa huge power and removal of this feeder on faultis equivalent to the aggregate sum of the windgeneration connected to this feeder getting lost.

The collector bus is protected by main andbackup protections.

The main protection could be supplied by thedistance element of the 2 in 1 protection andthe backup provided by the Directional / non-directional relays.

The distance element can be suitably graded totake care of all the faults and the backupprotection as well.

The collector bus faults can be realized by the

following logic and conditionCondition-1: Relay in circuit no-2 should see thefaults in reverse direction

Condition-2: Relay in circuit no-4 should see thefaults in reverse direction

Condition-3: Relay Nos 5-8 should see the faultsin forward zone

2

1

4 5 876

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When the above conditions are met then therelay identifies the fault as a fault on thecollector bus feeder.

1.5- Wind mills and their interconnections

Let us understand what happens when a single phase to ground fault occurs in an isolated system.

A healthy isolated system is as shown in fig-(a). Whenthere is a solid A-phase to ground fault, the voltage atphase-A equals the neutral voltage. Because of this shiftin the neutral we observe that the phase to neutral

voltages of the other two healthy phases equals thephase to phase voltage. Hence, during ground faults thephase voltage equals the line voltage. If the systemcontinues operation and when the system gets stresseddue to over voltage, it can lead to a catastrophe.





2

4

67(P) Forard pickup

67(N) Forward pickup5-8

Trip the collector bus CB

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The individual wind mill after the step uptransformer is connected radially to the otherwind mills and the system is isolated without thegrounding transformer. There are only CTs inthe windmill switchgear and no VTs. When thereis an earth fault in the radial feeder, then thecollector bus feeder detects an earth fault from

the broken delta VT. However this does notexactly identify the position of the earth fault but

just an indication that there exists an earth faultsomewhere in this feeder. Each wind mill istaken out of the collector bus circuit and whenthere is an indication in the broken delta, andthen it is identified as a feeder with an earthfault. The minimum the time spent on theoffshore windmill, the better it is for themaintenance engineer, because the offshoreplants are wet and are even dangerous.

Let us assume a radial feeder with an earth faultconnected to a collector bus and to a bus bar asshown below. These CTs will not see this faultas the zero sequence currents will not flowthrough the step up transformer as the earthfault current will circulate between the circuitwhere the fault lies and the grounding

transformer.

So, if we place a core balance CT in the radialcircuit which senses the earth fault and if the CToutputs can be given as the fourth input of therelay in the switchgear then the relay senses theearth fault.

0.69/34.5kV0.69/34.5kV

150/34.5kV

34.5kV

NETWORK

Collectorbus

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Red dotted line shows the fault trajectory in case of an earth fault.

II) Ferro-resonance in Windmills- Is there a possibility?

Induction generators, reactors in a wind park area source of inductive reactance and cables,capacitor banks contribute capacitive reactance.This combination of inductive reactance and

capacitive reactance can lead to a complexelectrical phenomenon called Ferro resonance,characterized by the sudden onset of a very highsustained over voltage concurrent with highlevels of harmonic distortion.

The following are the conditions under whichFerro-resonance is likely to occur.

1. A sinusoidal voltage source A powersystem generator

2. Saturable Ferromagnetic inductances-Can be power transformers or

instrument transformers3. Capacitance- The large capacitance

from the cables, or the capacitance toground of an ungrounded system

4. Low Resistance- Unloaded Transformer,low short circuit power source

5. Existence of at least one point in thesystem whose potential is not fixed.

When the system capacitance is in parallel tothe inductance of the voltage transformer andif there is an initiating event such as atransient overvoltage due to switching or aphase to ground fault on the ungroundedsystem, this can drive the voltage transformerinto saturation.The ferro magnetic circuit during saturationcan lead to a condition when the inductive

reactance is exactly equal to the capacitivereactance and ferro resonance results. Thesaturation of the core is maintained by thecontinuous over voltage and the ferroresonant condition is stabilized.

G G G

Collector Bus

Busbar

GroundingTransformer

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Refer to figure-1 and assume that there is nogrounding transformer. Let us start analyzingbay no-H103 the collector feeder circuit ifFerro-resonance can be a possibility and seehow the above conditions are met.

1. A sinusoidal voltage source- The wind

generators feeding thro Kabel-C1-82. Saturable Ferromagnetic inductances-

The voltage transformer- ( T5 ) in thecollector circuit

3. Capacitance- The capacitance toground of the 34.5kV cables

4. Low Resistance- The voltagetransformer is probably very lightlyloaded.

5. The existence of at least one point in thesystem whose potential is not fixed.-Theinadequately grounded section of the34.5kV system.

The effect of Ferro resonance on a powersystem is excess overvoltage and harmonicdistortion. The over voltages can exceed thenormal phase to phase voltage and damage theinsulation of the connected equipment and theharmonics confuse the protection systems fromtaking the right decision

Prevention, Detection and Mitigation.There are relays in the market which detect theferro resonance and alarms the condition. Thefollowing conditions can be practiced forelimination of the ferro resonance effect.

1. To prevent the system from becomingungrounded at any point of time withsuitable grounding systems

2. Introduce Losses by means of loadResistances.

III) Automatic Fault Analysis

As the off-shore wind parks are wet, dangerousand many other inconveniences, minimum effortshould be exercised at the site for rectification offaults. The control engineer or the utilityengineer would be highly benefitted with the

following1. Automatic retrieval of fault records from

all devices installed.

2. Centralized Data archiving

3. Automated data analysis required forFault analysis including distance to faultlocation, monitoring facilities like devicemonitoring and also system monitoringlike communication monitoring.

4. One analysis tool instead of multitude ofsoftware packages-This reducessignificantly the analysis time and the

crew training times.

The automatic fault analysis systems must beable to process all kinds of data recorded bydigital devices like numerical protection relays,digital fault recorders and power qualityrecorders installed.

x Diagnostic results and PQ reports.

Now let us see how a best result is obtainedfrom an automatic event analysis

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The following tasks are being carried out for theautomatic fault analysis.

Grouping of fault records Transient records

are collected from different equipment capableof producing fault records. The transient recordsare grouped based on the trigger timeinformation. The goal of this action is to group allrecords related to the same network event inone folder and to facilitate by this the searchingof records.

Automatic diagnosis of faults in the networkThe automatic diagnosis is started immediatelyafter receipt of a new record file. The diagnosisconsiders all records pertaining to the samepower system event.

Principles of Fault Location:

Basically a single ended measurement of thereactance is used to determine the fault location.

This principle requires measured data from thethree phase to ground voltages and the threeline currents during the fault in one singlerecord. In the next step frequency and thephasor of all analogue signals are determined.Finally depending upon the fault type thedistance to the fault and the fault resistance arecomputed.

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Feature Explanation Benefit

Fast and reliable faultlocation after fault clearance

Identification of weak pointsin the network like

Identification of fault cause Minimizing outage times (down times)

Saves MoneySaves Time

Fast and reliable faultlocation after successfulauto reclosing

Identification of weak points in the network likedefective Insulator

Saves MoneySaves Time

Automatic data grouping andstorage

Grouping of all recorded data involved innetwork event. This allows

Availability of all concerned data sets

without searching in data bases Corrective action may be started

immediately after data analysis

Saves MoneySaves TimeGives peace of mind

Automatic analysis of faultrecorder and numericalprotection device recordsand messages

Identification of weak pointsin the electrical system like

Ferro resonance Breakers (switching time monitoring) Frequent transient faults caused by

trees

Saves Money

Conclusion

With todays Multifunction relays and standardopen communications protocol like IEC61850,the investment is secured. Due to highercapability with modern algorithms in thenumerical relays, a reduced number of relaysare required for the same protection requirementwhich otherwise will require many conventionalrelays. The 2 in 1 relay is one of such a typewhich can lead to a better asset managementbecause of usage of less number of relays.

Apart from the above mentioned advantages,the fault recording capabilities of the numericalrelays can be used as inputs to an automated

fault analysis system. The fault recordings fromdifferent relays for a same fault can be groupedand analyzed to find the exact fault location in awind park. The automated fault analysis systemalso helps in maintaining records and archiving itas per NERC or other regulators storageguideline.

The paper also points out to a must do in awind park that the system cannot be left

ungrounded at any point even if it is to foregosome benefits as this could lead to a complex

phenomenon called Ferro resonance.

References

1. Siemens Power Engineering Guide-2008

2. Siemens Line Differential protection withDistance protection-7SD52/3

3. Book: Wind Power in Power Systems byAckermann.T

4. Paper: Offshore Wind farm electricalconnection options byW.Grainger,N.Jenkins

The advantages of an automatic fault analysis system is as enumerated

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Protection Requirements for a Large Scale Wind Park