technical basics for grid connection

8/3/2019 Technical Basics for Grid Connection

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David Roberts

Dulas Ltd, UK

DULAS

Grid Connection ofEmbedded Generators


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This presentation will:

Outline the issues concerned with the gridconnection of embedded generators

Outline the work undertaken in Sri Lanka in theyear 2000 on the grid connection of embeddedgenerators

DULAS

Grid Connection of Embedded Generators


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What is Embedded Generation?

Embedded generation describes any generationsystem that is connected to the distributionnetwork. It derives from the generation beingembedded into the distribution network.

There is no formal definition of what is a distributionsystem and what is therefore an embeddedgenerator.

Commonly systems up to 33 kV are described as

distribution systems, where either consumer loadsor lines leading to consumer loads are present.

DULAS


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Why is this a subject requiring attention?

Distribution networks are primarily designedto distribute power from a centraltransmission system down to consumerloads connected to the distribution system,

the power flow is one wayWith embedded generation the power flow is

more complex and may be both ways

There are plans for the implementation ofincreasing amounts of smaller gridconnected generation systems.

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What are the key factors to be considered?

Grid stability and security Fault Level

Interconnection Protection

Islanded Operation Voltage levels Earthing Load flow

Connection application process Testing and maintenance of protection

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Maintenance of grid stability and security are primerequirements for the specification of interconnectionequipment and regulations.

The grid supply system may be subject to: increasing load demand

low availability of spinning reserve

load shedding to maintain system security

an increasing use of embedded generation

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Grid stability and security


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Connection and disconnection of embeddedgeneration may cause grid disturbances whichmay affect grid stability and reliability.

This is particularly the case if there is the lossof a large amount of embedded generation atone time and embedded generation may be asignificant contribution towards overall

generation capacity. There is usually no central control over

embedded generation operationDULAS

Grid Stability


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There will be increasing amounts of embeddedgeneration in Indonesia

This is likely to lead to an increasingproportion of the grid load being supplied by

embedded generation.

The security of supply of the embeddedgeneration will become more significant for the

security of the grid as a whole.

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Grid Security


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The connection of embedded generation willcontribute to the fault levels in the system

This may require changes to the protectionequipment on the distribution system

This is usually only a factor when there arelarge sizes or large numbers of embeddedgenerators in one area.

DULAS

Fault levels


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There is a requirement for some electricalprotection at the point of connection to ensurethat:

Operation of the generator will not compromise the

safety of the grid system Deviations in the grid system will not damage the

generator equipment

Safety on the distribution system and at the

generation station is maintained Islanded operation of sections of the distribution

system is prevented

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Interconnection Protection


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Islanded operation is where: a section of the distribution network is separated from

the rest of the network (islanded) and

the supply to the separated section is maintained by

embedded generationThis is potentially dangerous in that there is little

control on voltage or frequency and theprotection systems of the distribution network

may not operate due to the limited capacity ofthe embedded generation

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Islanded operation


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Embedded generation will cause changes to thevoltage levels in the distribution systemgenerally it will raise the voltage at points alongthe distribution network.

This voltage rise must be limited to ensure thatthe supplies to consumers remain withinstatutory and safe working limits

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Voltage Levels


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Embedded generators ususally genrate at lowvoltage (400V 3 phase) and require suitableearthing for their safe operation

The connection points will usually include switch

gear and transformers which require suitablehigh voltage earthing

These earthing systems must be separated to

ensure the safety of the plant in fault conditions

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Earthing


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Embedded generators will change the load flow inin the distribution system.

In some cases power will be fed from thedistribution system into the transmission system

These changes in load flow may causetransmission and distribution system voltagecontrol and protection devices to operate

incorrectly.

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Load Flow and Grid System operation


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A standard process is required for an embeddedgenerator to:

Apply for a connection

Agree the connection requirements,specifications and costs

Implement and test the connection protection

Operate and maintain the connection

protection

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Connection process


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The connection protection and earthing needs tobe maintained and tested.

The distribution system will rely on the operationof the embedded generation protection for safe

and reliable operation

Maintenance and testing requirements need to bedefined and implemented

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Testing and Maintenance


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DULAS

Typical connection arrangement


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DULAS

The Sri Lanka Guide to Interconnection

Of Embedded Generators

2000The work undertaken in Sri Lanka included:Meetings and discussions with many departments of theCeylon Electricity Board, existing and prospective

embedded generation companies and consultantsworking in the field of embedded generation.The man outputs were:

The production of a Guide

Training courses for CEB and private generationcompany personnel


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DULAS

The Sri Lanka Guide to Interconnection

Of Embedded Generators

2000

The work was undertaken by a consortium of companiesand consultants from Sri Lanka and the UK. The lead

partners were:Resource Management Associates in Sri LankaDulas Ltd in UK

The work was funded by the World Bank through the Pre

Electrification Unit of the CEB


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The CEB will need to undertake studies to ascertainthe acceptability and requirements for the connectionof a new embedded generator.

These studies will require information from the

proposed generating company.

The generating company will also require someinformation from the CEB to design suitable protectionarrangements and maybe to modify proposed designs

to suit particular grid requirements.

DULAS

Grid Interconnection of Embedded Generators - SriLanka

Studies and Information Requirements


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DULAS

Grid Interconnection of Embedded GeneratorsSri Lanka

Studies to be undertaken by the CEB at the

time of a connection applicationThe following studies may be conducted by theCEB:

Stability

Fault Level Grid protection Voltage levels Earthing Load flow


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The affect on local stability of new embedded generationcapacity should be analysed when the capacity of thenew plant exceeds 5MW, or when the total capacity ona single distribution line exceeds 5MW.

For small generators, typically less than 1MW, therequirement for stability information from theGenerating Company may be waived.

The Generating Company shall provide a model of the

AVR of the proposed generators where the capacityexceeds 5MW.

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Stability



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The cumulative effect of of the embedded generator(s)on the design fault level for the distribution systemshall be assessed by the CEB.

A study should be undertaken when the cumulative fault

level reaches 90% of the rating of the associatedswitchgear, or the design fault level.

The CEB may require more detailed information from thegenerator than that specified in Annex 3.

DULAS

Fault Level



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The effect on the distribution system protection ratingsand settings shall be studied if any of the followingapply:

the proposed generating site maximum short circuitcurrent is greater than 20% of the distribution systemshort circuit current

the cumulative short circuit current from all embeddedgenerators on a distribution line will exceed 30% of thedistribution system short circuit current

there will be a net export of power from the distributionsystem to the 132 kV transmission system.

DULAS

Protection



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The nominal voltage at the Point of Supply (POS) shallbe stated by the CEB in the LOI.

The voltage rise at the POS must be within operationallimits.

A two stage approach shall be made to studies 1) Exclude load connections

2) Include load connections

The stage 2 study is required when the stage 1 study

indicates a potential problem.

DULAS

Voltage Levels



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The Guide provides information on acceptable earthingpractices and earthing requirements for a variety ofsituations. An Annex on earthing is included to providebackground information on earthing.

The Generating Company shall provide informationabout the proposed earthing arrangement to the CEB.

It is the responsibility of the Generating Company toprovide adequate earthing at a generating site.

The interconnection of generating site and CEB earthsystems should be considered for each site situationwith reference to the Guide.

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Earthing



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To assess the load flow at the distribution transformer themaximum cumulative generation capacity and theminimum captive load on the distribution line shall becalculated.

This will indicate if there are conditions under which therewill be an export of power from the distribution line tothe 132 kV transmission system.

If export is likely to occur the protection at the sub stationwill need to be studied.

DULAS

Load Flow



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The initial information to be provided by the CEB andthe generating company are given in a pro forma inAnnex 3.

This information exchange is to follow the issue of anLOI.

The Generating Company shall later provide thefollowing information, prior to acceptance testing:

the proposed interconnection protectionimplementation

protection test procedures

drawings showing the protection arrangements

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Information Requirements



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Synchronisation

Synchronisation means the minimisation of thedifference in magnitude, frequency and phase anglebetween the corresponding phases of the generatoroutput and the grid supply prior to the connection ofthe two supplies.

Synchronisation can be achieved either manually orautomatically, the latter is preferable.

It is very unlikely that new installations will include onlymanual synchronisation. If manual synchronisation issuggested its safe and reliable operation should beseriously considered and implemented carefully.

DULAS

Grid Interconnection of Embedded Generators - Synchronisation of Generators


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Synchronisation (2)

Voltage Fluctuation

During Synchronisation of a single generator, theinduced voltage fluctuation on the grid should notnormally exceed 3% at the Point of Common

Coupling,

The requirements of voltage step and flicker givenin Section 11 should also be met.

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Synchronisation of Synchronous

Generators Generator output frequency must be the same asthe grid frequency.

The phase angle between the generator outputand the grid supply must be less than specifiedlimits

The rate of change in phase angle between thegrid and the generator must be within specifiedlimits

Then the generator may be connected to the grid

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Synchronisation Methods for Synchronous

Generators Control motive power to generator to achieve

synchronisation.

Usually indicated with Synchroscope lights or

indicator. Problems of controlling large rotatingmasses and motive power

Control of the load on the generator to achievesynchronisation. This is usually done with anElectronic Load Control (ELC) system

Requires a dump load, it provides very smooth anaccurate synchronisation.


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Synch Check relays

A synch check relay must be used to check thatthe synchronisation of the generator and the grid iswithin the specified limits

The relay must operate on at least two, andpreferably all three phases to ensure phaserotation is correct


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DULAS

Grid Interconnection of Embedded Generators- Synchronisation of Generators

DULAS

Synchronisation limits

The limits specified in the Guide for allowingsynchronisation are:

Phase angle +/- 20 degrees

Maximum voltage difference 7% Maximum slip frequency 0.44 %


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Synchronisation of Induction Generators

Two main methods of synchronisation:

Use of electronic soft start unit to motor the generatorup to synchronous speed

Mechanically drive the generator up to synchronous

speed.

Once at, or slightly above, synchronous speed thegenerator may be connected to the grid.

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Grid Interconnection of Embedded Generators - -Synchronisation of Generators


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Synchronisation of Induction Generators (2)

Inrush Current

There will be a large inrush current when the generatoris connected. This current is building up the field in the

generator.The inrush current may be reduced by adding seriesresistance for a short time after connection.

A large inrush current will cause problems to the local

grid, which must provide the current

DULAS

Grid Interconnection of Embedded Generators Synchronisation of Generators


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The addition of embedded generation may change therequirements for grid protection.

There will be a fault level contribution from theembedded generator, though this is usually small.

It is important that grid system protection will operatewhen required.

If a grid sub station becomes a net exporter of powerto the 132kV system the operation of voltage control

and distance protection systems will require study andmodification.

DULAS


Grid protection


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The hazards created during islanded operation are:

Unearthed operation of the distribution system

Lower fault levels

Out of synchronisation reclosure

Voltage levels outside statutory limits

Reduction in quality of supply

Risk to maintenance personnel

DULAS

Grid Interconnection of Embedded Generators - Sri Lanka

Hazards of Islanding


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Due to the hazards listed the operation of an islandedsituation is to be avoided.

There may be an advantage to consumers, andgenerating companies, to allow islanded operation in

the maintenance of supply when a line has beendisconnected. This advantage is small and thehazards outweigh the advantages.

DULAS


Islanding


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The onset of an islanded situation will be accompaniedby a disturbance in the grid.

The detection of islanded operation relies of thedetection of this disturbance.

The disturbance may take the form of:

a change in voltage or frequency

a single shift in voltage vector

a change in reactive power flow

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Detection of Islanded operation


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A prime consideration in the development of thedetection and protection requirements is to minimisethe potential for multiple tripping of embeddedgenerators due to grid disturbances or faults onadjacent lines which may not require a generator to be

disconnected.

This multiple tripping is known as Common Cause orCommon Mode tripping.

DULAS

Common Cause Tripping



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The protection types and settings specified in the Guide

are designed to avoid Common Cause tripping.

For larger generating sites (>5MW) inter tripping fromthe grid sub station, and possibly distribution breakers,should be used to ensure a generator is disconnected

when an islanded condition occurs. For sites < 5MW Loss of Mains (LoM) protection may

be used to detect an islanded condition anddisconnect the generator. For these smaller generating

sites the remaining small possibility of common causetripping is considered acceptable.

DULAS

Avoiding Common Cause TrippingGrid Interconnection of Embedded Generators - Sri Lanka


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All sites require under / over voltage and under / overfrequency protection

Other Loss of Mains protection may be provided by:

Rate of Change of Frequency (ROCOF)

Voltage Vector Shift

Reverse VAR

Other novel techniques

DULAS

Loss of Mains Protection



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Voltage

The voltage of all three phases shall be monitored.

The limits for operation are:

for HV connection +/- 10%

for LV connection +/- 14%

The relay should have a time delay to avoidspurious trips due to remote faults.

The maximum total tripping time shall be 0.5seconds

DULAS

Islanding Detection and Protection (1)



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Frequency

The frequency of the supply shall be monitored, thiscan be single phase.

The limits for operation are:

+4%, -6% (i.e. 52Hz to 47Hz)

There is no requirement for a time delay.


The low frequency limit may be reduced to 46Hz,this may require additional generator frequencyprotection.

DULAS

Islanding Detection and Protection (1)



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Rate of Change of Frequency (ROCOF)

True ROCOF detects the islanded condition

rather than the onset of islanding.

Some ROCOF relays may also be sensitive to an

initial change in voltage vector.

The limits of operation are:

2.5 Hz/second. This high level is specified toensure minimum spurious tripping.



DULAS

Loss of Mains Protection (1)



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Voltage Vector Shift

Voltage vector shift detects the onset on anislanded condition.

It is susceptible to spurious tripping due to grid

distrurbances

The limits of operation are:

6 degrees in a half cycle.

This may be de-sensitised to up to 12 degrees.



DULAS




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Reverse VAR

Reverse VAR relays detect the flow of reactivepower from the generator to the grid. This willoccur when during an islanded condition of a

single generator. Generators must have stable power factor control

to use reverse VAR protection.

The limits of operation are to be agreed between

the generating company and the CEB. Typically 1-5% of the magnitude of maximum kW export.

A time delay of up to 5 seconds may be required.

DULAS




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Intertripping

Intertripping is a direct means of islandingprotection. It can provide a reliable method oftripping isanded generators without any common

cause problems. A trip signal is sent from the circuit breaker or

recloer responsible for the islanding.

The reliability is dependant upon the security of the

trip signal communication method. Reliabilityshould be assessed for each particular site. A failsafe method of communication should be used.

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Fault Thrower

This is a special application of a fault thrower.

The fault thrower would be installed at the sourcesub station and would operate following the

opening of the source circuit breaker.

Operation would be delayed to allow generatorrelays to operate if sufficient load imbalance exists.

It is only effective for generators connected

between the source breaker and the first autorecloser.

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Other novel techniques

The Guide does not preclude the use of noveltechniques that may be developed to achieve adependable and reliable loss of mains function.

An example is the use of sensitive ROCOF blocked byvoltage vector shift to prevent operation during generalgrid instability.

DULAS




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Detection of islanding will not be possible in allsituations, for example a perfect load / generatorbalance may exist.

Secondary protection may be used such as:

dead line or synch check on auto reclosingdevices

Neutral Voltage Displacement (NVD)

NVD is a dependable means of satisfying safety

requirements and mitigating the risks of islandedoperation.

Details on NVD protection are given in the Guide.

DULAS

Islanding Network Protection



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The type and size of generator connection isclassified into five cases and the protectionrequirements defined for each case.

The factors that define which case applies are:

Generation site capacity

Generator type

Ratio of minimum captive load and maximumcapacity

Capacity and interconnection protection of othergenerators on the same distribution line and substation.

DULAS

Interconnection Protection Requirements



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DULAS

Interconnection Protection Requirements Summary

Case 1 Case 2 Case 3 Case 4 Case 5

Generator type All All See case

3descripti

on

All See case 5

description

Self commutated

static inverters

Minimum captive

loadL L L L

Maximum

cumulative export

capacity

0.8 x L

Max site export

capacity

< 5

MW

< 5 MW < 5

MW

> 5 Mw

Under and over

voltage protection

Under and over

frequency protection *

Vector shift

protection *

ROCOF protection

*True ROCOF

protection*

NVD protection *(1)

Intertripping * * * *Other requirements * * * * *



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All types of Generator The maximum cumulative export capacity is less

than half the minimum distribution line (orcaptive) load

the maximum export capacity is less than 5MW.Protection Required

Under and over voltage

Under and over frequency

Optional:

Three phase vector shift, subject to generatorpreference

DULAS

Interconnection Protection, Case 1



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All types of generator

The maximum cumulative export capacity is lessthan 0.8 times the minimum captive load, and

the maximum export capacity is less than 5MW.

Protection Required:



3 phase vector shift

Optional: True RoCoF may be used as well as vector shift

DULAS



id i f b dd d i k


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All types of generator except mains excited generatorsdefined in Case 5.

The maximum cumulative generation export capacityis greater than 0.8 times the minimum captive load,

such that load/generator balance is possible, and the maximum export capacity is less than 5 MW.

Protection Required:



3 phase vector shift, or true ROCOF

NVD

Dead line check

DULAS



G id I i f E b dd d G S i L k


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Alternative ProtectionAs a replacement for the combination of Vector shift

and NVD any one of the following may be used:

Intertripping

Fault thrower

Reverse VAR, where applicable

NVD is not required when the maximum exportcapacity is less than 1MW if the cumulative exportcapacity on a line is less than 0.8 times theminimum captive load.

DULAS

Interconnection Protection, Case 3 (2)


G id I t ti f E b dd d G t S i L k


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All types of generator The maximum export capacity of an Embedded

Generation site is greater than 5 MW.

It is preferred that the Embedded Generator is

connected directly to the primary bus rather thanteed into an HV distribution feeder.

Protection Required


Under and over frequency Intertripping from primary bus intake

Parallel earthing or NVD protection

DULAS



G id I t ti f E b dd d G t S i L k


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If the Embedded Generator is teed into a distributionfeeder, the following is also required:

Intertripping from the feeder breaker or

Fault throwing or

Reverse VAR protection where applicable.Generators larger than 5 MW will be encouraged to

obtain more secure connections. For largegenerators remote from the primary bus, adequate

security may only be achieved by double circuitconnection to the primary bus.

DULAS



Grid Interconnection of Embedded Generators Sri Lanka


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Mains excited asynchronous (induction) generator withlocal power factor correction less than the reactivepower demand, or a line commutated inverter.

The CEB network/circuit capacitance is not sufficient

to self excite the generator. The maximum cumulative connected generation

export capacity is greater than 0.8 times the minimumcaptive load. No synchronous generation or self-

excited generation are connected.

DULAS





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Protection Required Under and over voltage


3-phase vector shift

The total generation connected to a primary substationusing the vector shift method for loss of mainsprotection shall not exceed 20MW.

DULAS





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Some wind turbines and photovoltaic system invertersare examples of this type of generator.

The general requirements are covered with

synchronous machines in cases 1-4.

However inverters commonly include proprietaryprotection methods, including ROCOF.

It is the responsibility of the Generating Company to

demonstrate that the protection meets theacceptable levels of dependability and reliability.

DULAS

Interconnection Protection,Self Commutated Static Inverters




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It is the responsibility of the Generating Company to

organise, agree procedures with the CEB andundertake protection equipment testing.

Prior to testing, the Generating Company will certifythat:

the earthing system conforms to the provisions inthis Guide and other relevant standards.

the design and implementation of the protectionsystem complies with the requirements of the

Guide, and any protection specified in the PPA. the generating system is safe to operate and

complies with all the relevant requirements forelectrical installations.

DULAS

Test and Acceptance ProceduresGrid Interconnection of Embedded Generators - Sri Lanka



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It is the responsibility of the Generating Companyrepresentative to provide and complete the test forms.

Testing is to be witnessed by the CEB representative.The CEB representative is to certify by signature thatthe protection tests were witnessed as successful.

The CEB may provide staff and/or equipment to theGenerating Company to enable tests to be

undertaken.

A standard form to be completed during testing isincluded in Annex 5 of the Guide

DULAS

Test and Acceptance Procedures (2)Grid Interconnection of Embedded Generators - Sri Lanka



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The required grid connection protection is to be testedprior to acceptance of new generation plant forconnection to the grid. Short term connection may beallowed to set up and test the protection equipment.

Retesting at intervals of no greater than three years or:

Following any significant change in generation orprotection equipment.

Following any maintenance or repair, whichinvolved the disconnection or rearrangement of any

protection equipment.

DULAS

Test and Acceptance Procedures (3)Grid Interconnection of Embedded Generators Sri Lanka



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Thank you for attention

Any Comments or Questions?




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Information to be provided by the CEB

This information shall include the planned (or prospective) fault levels

expected by the CEB in 10 years from the time of connection

application.

Maximum fault levels (for equipment selection and earthing

design):

Network design symmetrical fault level (kA or MVA)Peak asymmetrical fault level at half cycle (kA)

3-phase symmetrical fault level at half cycle (MVA or kA)

X/R ratio for 3 phase symmetrical fault

1-phase to earth fault level (kA) (neglecting earth system

resistances)

X/R ratio for 1-phase to earth fault (neglecting earth system

resistances)




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Information to be provided by the CEB (2)

Minimum fault levels (for protection design):3-phase steady state symmetrical fault level (MVA or kA)

X/R ratio for 3 phase symmetrical fault

1-phase to earth fault level (kA) (Neglecting earth system

resistances)

X/R ratio for 1-phase to earth fault (neglecting earth system

resistances)




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Information from the Generating Company(1)Synchronous Generators with a capacity above 500 kW

Site Name

Location ...Site Reference Number ....

Generating Company Name.

Contact

Point of Supply (location) .

Maximum export capacity

Maximum import capacity ...

Power factor operating range ..

Generator (for each synchronous generator):

Terminal voltage (kV) ..

Machine rating (MVA) .

Stator resistance (pu) tolerance % .Sub-transient reactance (pu) . tolerance % .

Transient reactance (pu) . tolerance % .

Synchronous reactance (pu) . tolerance % .

Sub-transient time constant (ms) ... tolerance(ms) .

Transient time constant (ms) . tolerance (ms) .



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Information from the Generating Company(2)

Synchronous Generators with a capacity above 500 kW

Generator (for each synchronous generator):Terminal voltage (kV) ..

Machine rating (MVA) .

Stator resistance (pu) tolerance % .

Sub-transient reactance (pu) . tolerance % .

Transient reactance (pu) .. tolerance % .

Synchronous reactance (pu) .. tolerance % .

Sub-transient time constant (ms). tolerance(ms) .

Transient time constant (ms) . tolerance(ms) .



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Transformer (for each generator transformer);

Rating (MVA) ..

Reactance (pu) .. tolerance % .

Resistance (pu) .. tolerance % .

Voltage Ratio .. vector group .

Cable or Line between the Generator and Point of

Common Coupling where this cabling distance

exceeds 50 metres

Voltage (V) ..

Reactance (Ohm) Resistance (Ohm)



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Where a total generating capacity is less than 500 kW

there is a reduced requirement for information from the

Generating Company.

This information requirement is listed on page A3:4

Where Induction, or Asynchronous, generators are

proposed the same information should be provided

where relevant to the induction type generator.

technical basics for grid connection

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