David Roberts

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Grid Connection of Embedded Generators

• David Roberts
• Dulas Ltd, UK

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Grid Connection of Embedded Generators

• This presentation will
• Outline the issues concerned with the grid
  connection of embedded generators
• Outline the work undertaken in Sri Lanka in the
  year 2000 on the grid connection of embedded
  generators

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What is Embedded Generation? Embedded generation
describes any generation system that is connected
to the distribution network. It derives from the
generation being embedded into the distribution
network. There is no formal definition of what is
a distribution system and what is therefore an
embedded generator. Commonly systems up to 33 kV
are described as distribution systems, where
either consumer loads or lines leading to
consumer loads are present.
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Why is this a subject requiring
attention? Distribution networks are primarily
designed to distribute power from a central
transmission system down to consumer loads
connected to the distribution system, the power
flow is one way With embedded generation the
power flow is more complex and may be both
ways There are plans for the implementation of
increasing amounts of smaller grid connected
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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Grid stability and security

• Maintenance of grid stability and security are
  prime requirements for the specification of
  interconnection equipment 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

• Connection and disconnection of embedded
  generation may cause grid disturbances which may
  affect grid stability and reliability.
• This is particularly the case if there is the
  loss of a large amount of embedded generation at
  one time and embedded generation may be a
  significant contribution towards overall
  generation capacity.
• There is usually no central control over embedded
  generation operation

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Grid Security

• There will be increasing amounts of embedded
  generation in Indonesia
• This is likely to lead to an increasing
  proportion of the grid load being supplied by
  embedded generation.
• The security of supply of the embedded generation
  will become more significant for the security of
  the grid as a whole.

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Fault levels

• The connection of embedded generation will
  contribute to the fault levels in the system
• This may require changes to the protection
  equipment on the distribution system
• This is usually only a factor when there are
  large sizes or large numbers of embedded
  generators in one area.

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Interconnection Protection

• There is a requirement for some electrical
  protection at the point of connection to ensure
  that
• 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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Islanded operation

• 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 generation
• This is potentially dangerous in that there is
  little control on voltage or frequency and the
  protection systems of the distribution network
  may not operate due to the limited capacity of
  the embedded generation

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Voltage Levels
Embedded generation will cause changes to the
voltage levels in the distribution system
generally it will raise the voltage at points
along the distribution network. This voltage rise
must be limited to ensure that the supplies to
consumers remain within statutory and safe
working limits
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Earthing
Embedded generators ususally genrate at low
voltage (400V 3 phase) and require suitable
earthing for their safe operation The connection
points will usually include switch gear and
transformers which require suitable high voltage
earthing These earthing systems must be separated
to ensure the safety of the plant in fault
conditions
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Load Flow and Grid System operation
Embedded generators will change the load flow in
in the distribution system. In some cases power
will be fed from the distribution system into the
transmission system These changes in load flow
may cause transmission and distribution system
voltage control and protection devices to operate
incorrectly.
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Connection process

• A standard process is required for an embedded
  generator 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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Testing and Maintenance
The connection protection and earthing needs to
be maintained and tested. The distribution
system will rely on the operation of the embedded
generation protection for safe and reliable
operation Maintenance and testing requirements
need to be defined and implemented
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Typical connection arrangement

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• The Sri Lanka Guide to Interconnection
• Of Embedded Generators 2000
• The work undertaken in Sri Lanka included
• Meetings and discussions with many departments of
  the Ceylon Electricity Board, existing and
  prospective embedded generation companies and
  consultants working in the field of embedded
  generation.
• The man outputs were
• The production of a Guide
• Training courses for CEB and private generation
  company personnel

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The Sri Lanka Guide to Interconnection Of
Embedded Generators 2000 The work was
undertaken by a consortium of companies and
consultants from Sri Lanka and the UK. The lead
partners were Resource Management Associates in
Sri Lanka Dulas Ltd in UK The work was funded by
the World Bank through the Pre Electrification
Unit of the CEB
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Grid Interconnection of Embedded Generators -
SriLanka
Studies and Information Requirements

• The CEB will need to undertake studies to
  ascertain the acceptability and requirements for
  the connection of a new embedded generator.
• These studies will require information from the
  proposed generating company.
• The generating company will also require some
  information from the CEB to design suitable
  protection arrangements and maybe to modify
  proposed designs to suit particular grid
  requirements.

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Grid Interconnection of Embedded Generators Sri
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Studies to be undertaken by the CEB at the time
of a connection application

• The following studies may be conducted by the
  CEB
• Stability
• Fault Level
• Grid protection
• Voltage levels
• Earthing
• Load flow

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Grid Interconnection of Embedded Generators Sri
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Stability

• The affect on local stability of new embedded
  generation capacity should be analysed when the
  capacity of the new plant exceeds 5MW, or when
  the total capacity on a single distribution line
  exceeds 5MW.
• For small generators, typically less than 1MW,
  the requirement for stability information from
  the Generating Company may be waived.
• The Generating Company shall provide a model of
  the AVR of the proposed generators where the
  capacity exceeds 5MW.

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Grid Interconnection of Embedded Generators Sri
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Fault Level

• The cumulative effect of of the embedded
  generator(s) on the design fault level for the
  distribution system shall be assessed by the CEB.
• A study should be undertaken when the cumulative
  fault level reaches 90 of the rating of the
  associated switchgear, or the design fault level.
• The CEB may require more detailed information
  from the generator than that specified in Annex
  3.

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Grid Interconnection of Embedded Generators Sri
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Protection

• The effect on the distribution system protection
  ratings and settings shall be studied if any of
  the following apply
• the proposed generating site maximum short
  circuit current is greater than 20 of the
  distribution system short circuit current
• the cumulative short circuit current from all
  embedded generators on a distribution line will
  exceed 30 of the distribution system short
  circuit current
• there will be a net export of power from the
  distribution system to the 132 kV transmission
  system.

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Grid Interconnection of Embedded Generators Sri
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Voltage Levels

• The nominal voltage at the Point of Supply (POS)
  shall be stated by the CEB in the LOI.
• The voltage rise at the POS must be within
  operational limits.
• 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.

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Grid Interconnection of Embedded Generators Sri
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Earthing
The Guide provides information on acceptable
earthing practices and earthing requirements for
a variety of situations. An Annex on earthing is
included to provide background information on
earthing. The Generating Company shall provide
information about the proposed earthing
arrangement to the CEB. It is the responsibility
of the Generating Company to provide adequate
earthing at a generating site. The
interconnection of generating site and CEB earth
systems should be considered for each site
situation with reference to the Guide.
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Grid Interconnection of Embedded Generators Sri
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Load Flow
To assess the load flow at the distribution
transformer the maximum cumulative generation
capacity and the minimum captive load on the
distribution line shall be calculated. This will
indicate if there are conditions under which
there will be an export of power from the
distribution line to the 132 kV transmission
system. If export is likely to occur the
protection at the sub station will need to be
studied.
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Grid Interconnection of Embedded Generators Sri
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Information Requirements

• The initial information to be provided by the CEB
  and the generating company are given in a pro
  forma in Annex 3.
• This information exchange is to follow the issue
  of an LOI.
• The Generating Company shall later provide the
  following information, prior to acceptance
  testing
• the proposed interconnection protection
  implementation
• protection test procedures
• drawings showing the protection arrangements

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Grid Interconnection of Embedded Generators -
Synchronisation of Generators
Synchronisation
Synchronisation means the minimisation of the
difference in magnitude, frequency and phase
angle between the corresponding phases of the
generator output and the grid supply prior to the
connection of the two supplies. Synchronisation
can be achieved either manually or automatically,
the latter is preferable. It is very unlikely
that new installations will include only manual
synchronisation. If manual synchronisation is
suggested its safe and reliable operation should
be seriously considered and implemented carefully.
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Grid Interconnection of Embedded Generators -
Synchronisation of Generators
Synchronisation (2)

• Voltage Fluctuation
• During Synchronisation of a single generator, the
  induced voltage fluctuation on the grid should
  not normally exceed 3 at the Point of Common
  Coupling,
• The requirements of voltage step and flicker
  given in Section 11 should also be met.

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Grid Interconnection of Embedded Generators -
Synchronisation of Generators
Synchronisation of Synchronous Generators

• Generator output frequency must be the same as
  the grid frequency.
• The phase angle between the generator output and
  the grid supply must be less than specified
  limits
• The rate of change in phase angle between the
  grid and the generator must be within specified
  limits
• Then the generator may be connected to the grid

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Grid Interconnection of Embedded Generators -
Synchronisation of Generators
Synchronisation Methods for Synchronous Generators

• Control motive power to generator to achieve
  synchronisation.
• Usually indicated with Synchroscope lights or
  indicator. Problems of controlling large rotating
  masses and motive power
• Control of the load on the generator to achieve
  synchronisation. This is usually done with an
  Electronic Load Control (ELC) system
• Requires a dump load, it provides very smooth
  an accurate synchronisation.

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Grid Interconnection of Embedded Generators -
Synchronisation of Generators
Synch Check relays

• A synch check relay must be used to check that
  the synchronisation of the generator and the grid
  is within the specified limits
• The relay must operate on at least two, and
  preferably all three phases to ensure phase
  rotation is correct

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Grid Interconnection of Embedded Generators-
Synchronisation of Generators
Synchronisation limits

• The limits specified in the Guide for allowing
  synchronisation are
• Phase angle /- 20 degrees
• Maximum voltage difference 7
• Maximum slip frequency 0.44

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Grid Interconnection of Embedded Generators -
-Synchronisation of Generators
Synchronisation of Induction Generators

• Two main methods of synchronisation
• Use of electronic soft start unit to motor the
  generator up to synchronous speed
• Mechanically drive the generator up to
  synchronous speed.
• Once at, or slightly above, synchronous speed the
  generator may be connected to the grid.

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Grid Interconnection of Embedded Generators
Synchronisation of Generators
Synchronisation of Induction Generators (2)
Inrush Current There will be a large inrush
current when the generator is connected. This
current is building up the field in the
generator. The inrush current may be reduced
by adding series resistance for a short time
after connection. A large inrush current will
cause problems to the local grid, which must
provide the current
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Grid Interconnection of Embedded Generators Sri
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Grid protection

• The addition of embedded generation may change
  the requirements for grid protection.
• There will be a fault level contribution from the
  embedded generator, though this is usually small.
• It is important that grid system protection will
  operate when required.
• If a grid sub station becomes a net exporter of
  power to the 132kV system the operation of
  voltage control and distance protection systems
  will require study and modification.

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Grid Interconnection of Embedded Generators - Sri
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Hazards of Islanding

• 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

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Islanding

• Due to the hazards listed the operation of an
  islanded situation is to be avoided.
• There may be an advantage to consumers, and
  generating companies, to allow islanded operation
  in the maintenance of supply when a line has been
  disconnected. This advantage is small and the
  hazards outweigh the advantages.

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Detection of Islanded operation

• The onset of an islanded situation will be
  accompanied by a disturbance in the grid.
• The detection of islanded operation relies of the
  detection 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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Common Cause Tripping

• A prime consideration in the development of the
  detection and protection requirements is to
  minimise the potential for multiple tripping of
  embedded generators due to grid disturbances or
  faults on adjacent lines which may not require a
  generator to be disconnected.
• This multiple tripping is known as Common Cause
  or Common Mode tripping.

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Avoiding Common Cause Tripping

• The protection types and settings specified in
  the Guide are designed to avoid Common Cause
  tripping.
• For larger generating sites (gt5MW) inter tripping
  from the grid sub station, and possibly
  distribution breakers, should be used to ensure a
  generator is disconnected when an islanded
  condition occurs.
• For sites lt 5MW Loss of Mains (LoM) protection
  may be used to detect an islanded condition and
  disconnect the generator. For these smaller
  generating sites the remaining small possibility
  of common cause tripping is considered acceptable.

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Loss of Mains Protection

• All sites require under / over voltage and under
  / over frequency protection
• Other Loss of Mains protection may be provided
  by
• Rate of Change of Frequency (ROCOF)
• Voltage Vector Shift
• Reverse VAR
• Other novel techniques

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Islanding Detection and Protection (1)

• 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 avoid
  spurious trips due to remote faults.
• The maximum total tripping time shall be 0.5
  seconds

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Islanding Detection and Protection (1)

• Frequency
• The frequency of the supply shall be monitored,
  this can be single phase.
• The limits for operation are
• 4, -6 (i.e. 52Hz to 47Hz)
• There is no requirement for a time delay.
• The maximum total tripping time shall be 0.5
  seconds
• The low frequency limit may be reduced to 46Hz,
  this may require additional generator frequency
  protection.

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Loss of Mains Protection (1)

• 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 to
  ensure minimum spurious tripping.
• There is no requirement for a time delay.
• The maximum total tripping time shall be 0.5
  seconds

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Loss of Mains Protection (2)

• Voltage Vector Shift
• Voltage vector shift detects the onset on an
  islanded 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.
• There is no requirement for a time delay.
• The maximum total tripping time shall be 0.5
  seconds

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Loss of Mains Protection (3)

• Reverse VAR
• Reverse VAR relays detect the flow of reactive
  power from the generator to the grid. This will
  occur 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.

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Loss of Mains Protection (4)

• Intertripping
• Intertripping is a direct means of islanding
  protection. It can provide a reliable method of
  tripping 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. Reliability
  should be assessed for each particular site. A
  fail safe method of communication should be used.

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Loss of Mains Protection (5)

• Fault Thrower
• This is a special application of a fault thrower.
• The fault thrower would be installed at the
  source sub station and would operate following
  the opening of the source circuit breaker.
• Operation would be delayed to allow generator
  relays to operate if sufficient load imbalance
  exists.
• It is only effective for generators connected
  between the source breaker and the first auto
  recloser.

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Loss of Mains Protection (6)

• Other novel techniques
• The Guide does not preclude the use of novel
  techniques that may be developed to achieve a
  dependable and reliable loss of mains function.
• An example is the use of sensitive ROCOF blocked
  by voltage vector shift to prevent operation
  during general grid instability.

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Islanding Network Protection

• Detection of islanding will not be possible in
  all situations, for example a perfect load /
  generator balance may exist.
• Secondary protection may be used such as
• dead line or synch check on auto reclosing
  devices
• Neutral Voltage Displacement (NVD)
• NVD is a dependable means of satisfying safety
  requirements and mitigating the risks of islanded
  operation.
• Details on NVD protection are given in the Guide.

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Interconnection Protection Requirements

• The type and size of generator connection is
  classified into five cases and the protection
  requirements defined for each case.
• The factors that define which case applies are
• Generation site capacity
• Generator type
• Ratio of minimum captive load and maximum
  capacity
• Capacity and interconnection protection of other
  generators on the same distribution line and sub
  station.

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Interconnection Protection Requirements Summary
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Interconnection Protection, Case 1

• All types of Generator
• The maximum cumulative export capacity is less
  than half the minimum distribution line (or
  captive) 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 generator
  preference

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Interconnection Protection, Case 2

• All types of generator
• The maximum cumulative export capacity is less
  than 0.8 times the minimum captive load, and
• the maximum export capacity is less than 5MW.
• Protection Required
• Under and over voltage
• Under and over frequency
• 3 phase vector shift
• Optional
• True RoCoF may be used as well as vector shift

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Interconnection Protection, Case 3

• All types of generator except mains excited
  generators defined in Case 5.
• The maximum cumulative generation export capacity
  is 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
• Under and over voltage
• Under and over frequency
• 3 phase vector shift, or true ROCOF
• NVD
• Dead line check

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Interconnection Protection, Case 3 (2)

• Alternative Protection
• As 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 export
  capacity is less than 1MW if the cumulative
  export capacity on a line is less than 0.8 times
  the minimum captive load.

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Interconnection Protection, Case 4

• 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 than
  teed into an HV distribution feeder.
• Protection Required
• Under and over voltage
• Under and over frequency
• Intertripping from primary bus intake
• Parallel earthing or NVD protection

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Interconnection Protection, Case 4 (2)

• If the Embedded Generator is teed into a
  distribution feeder, 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 large
  generators remote from the primary bus, adequate
  security may only be achieved by double circuit
  connection to the primary bus.

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Interconnection Protection, Case 5

• Mains excited asynchronous (induction) generator
  with local power factor correction less than the
  reactive power 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
  minimum captive load. No synchronous generation
  or self-excited generation are connected.

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Interconnection Protection, Case 5 (2)

• Protection Required
• Under and over voltage
• Under and over frequency
• 3-phase vector shift
• The total generation connected to a primary
  substation using the vector shift method for loss
  of mains protection shall not exceed 20MW.

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Interconnection Protection, Self Commutated
Static Inverters

• Some wind turbines and photovoltaic system
  inverters are examples of this type of generator.
• The general requirements are covered with
  synchronous machines in cases 1-4.
• However inverters commonly include proprietary
  protection methods, including ROCOF.
• It is the responsibility of the Generating
  Company to demonstrate that the protection meets
  the acceptable levels of dependability and
  reliability.

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Test and Acceptance Procedures

• It is the responsibility of the Generating
  Company to organise, agree procedures with the
  CEB and undertake protection equipment testing.
• Prior to testing, the Generating Company will
  certify that
• the earthing system conforms to the provisions in
  this Guide and other relevant standards.
• the design and implementation of the protection
  system 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 for
  electrical installations.

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Test and Acceptance Procedures (2)

• It is the responsibility of the Generating
  Company representative to provide and complete
  the test forms.
• Testing is to be witnessed by the CEB
  representative. The CEB representative is to
  certify by signature that the protection tests
  were witnessed as successful.
• The CEB may provide staff and/or equipment to the
  Generating Company to enable tests to be
  undertaken.
• A standard form to be completed during testing is
  included in Annex 5 of the Guide

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Test and Acceptance Procedures (3)

• The required grid connection protection is to be
  tested prior to acceptance of new generation
  plant for connection to the grid. Short term
  connection may be allowed to set up and test the
  protection equipment.
• Retesting at intervals of no greater than three
  years or
• Following any significant change in generation or
  protection equipment.
• Following any maintenance or repair, which
  involved the disconnection or rearrangement of
  any protection equipment.

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Thank you for attention Any Comments or
Questions?
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Information to be provided by the CEB
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Information to be provided by the CEB (2)
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Information from the Generating Company(1)
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Information from the Generating Company(2)
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Information from the Generating Company(3)
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Information from the Generating Company(4)
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
A34 Where Induction, or Asynchronous,
generators are proposed the same information
should be provided where relevant to the
induction type generator.
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