EE 394J10 Distributed Technologies

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EE 394J10 Distributed Technologies
Grid-Microgrids Interconnection
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Motivation

• Reasons for connecting a microgrid to a main
  grid
• Availability Highly available power grids may
  act as an additional source for micro-grids.
• Operations/stability
• Direct connection of ac microgrids to a large
  power grid facilitates stable operation but only
  if the power grid acts as a stiff source to the
  microgrid.
• When using renewable energy sources, a grid
  connection may allow reducing the need for energy
  storage in the microgrid.
• If not all loads in a microgrid are critical, a
  grid connection may allow to reduce the
  investment in local generation.
• Economics
• Microgrids are typically planned with extra
  capacity with respect to the local load. This
  extra power capacity can be injected back into
  the grid in order to obtain some economic
  benefit.
• Grid interconnection allows to reduce fuel
  operational costs by using the grid at night when
  electricity costs are low.

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Definitions

• Point of common coupling (PCC) it is the point
  in the electric circuit where a microgrid is
  connected to a main grid.

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Standards

• There are several standards specifying various
  aspects grid interconnection of a local power
  generation source. Arguably the most important
  one is IEEE 1547.
• IEEE 1547 has several parts
• Main body
• IEEE Standard 1547.1 IEEE Standard Conformance
  Test Procedures for Equipment Interconnecting
  Distributed Resources with Electric Power
  Systems.
• EEE Standard 1547.2 IEEE Application Guide for
  IEEE Std 1547, IEEE Standard for Interconnecting
  Distributed Resources with Electric Power
  Systems.
• IEEE Standard 1547.3 IEEE Guide for Monitoring,
  Information Exchange, and Control of Distributed
  Resources Interconnected with Electric Power
  Systems.
• IEEE Standard 1547.4 IEEE Guide for Design,
  Operation, and Integration of Distributed
  Resource Island Systems with Electric Power
  Systems.
• IEEE Standard 1547.5 has not been issued, yet.
  Its intended scope is to address issues when
  interconnecting electric power sources of more
  than 10 MVA to the power grid.
• IEEE Standard 1547.6 IEEE Recommended Practice
  for Interconnecting Distributed Resources with
  Electric Power Systems Distribution Secondary
  Networks.
• IEEE Standard 1547.8 has not been issued, yet.
  Its intended scope is to provide supplemental
  support for implementation methods for expanded
  use of the previous standards, for example when
  addressing issues with high penetration of
  residential PV systems.

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Standards

• Main provisions from IEEE 1547
• The micro-grid must not actively regulate the
  voltage at the PCC.
• The grounding approach chosen for the local area
  power and energy system (LAPES) must not create
  overvoltages that exceed the ratings of the
  equipment connected to the main grid or must not
  affect ground fault protection coordination in
  the main grid.
• The distributed resources in the LAPES must be
  able to parallel with the main grid without
  causing voltage fluctuations at the PCC greater
  than 5 of the prevailing voltage level of the
  Area electric power system (EPS) at the PCC and
  flicker must be within acceptable ranges.
• The LAPES must not energized the main grid when
  the main grid is not energized.
• Each distributed resource (DR) unit of 250 kVA
  or more or DR aggregate of 250 kVA or more at a
  single PCC shall have provisions for monitoring
  its connection status, real power output,
  reactive power output, and voltage at the point
  of DR connection.
• A visible-break isolation device must be located
  between the main grid and a DR unit only when
  required by the main grid provider practices.
• The interconnection system must meet applicable
  surge and EMI standards.

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Standards

• Main provisions from IEEE 1547
• When a fault occurs in the main grid circuit to
  which a LAPES is connected, then the micro-grid
  local power generation units must stop to power
  this circuit before reclosure from the main grid
  happens.
• The interconnection system must be able to
  measure relevant indicated voltages and
  frequencies at the PCC or the point of connection
  of DR and disconnect within a given allowed time
  all local power generating units in the
  micro-grid when these measured voltages or
  frequencies fall within a range specified in a
  table in this standard. For example, when
  voltages fall below 50 of the base voltage, the
  LAPES must disconnect its DR within 0.16 seconds
  (one 60 Hz cycle). The time extends to 2 seconds
  for voltages between 50 and 88 of the base
  voltage. Disconnection must occur within 1 second
  if measured voltages are between 110 and 120 of
  the base voltage and within 0.16 seconds if the
  voltage exceeds 120 of the base voltage. For
  frequency measurements, any DR of 30 kW or less
  must disconnect 0.16 seconds if the measured
  frequency is above 60.5 Hz or below 59.3 Hz. The
  same disconnect time applies for DR of more than
  30 kW when the frequency exceeds 60.5 Hz, but for
  the lower range at these power levels disconnect
  within 0.16 seconds must occur if the frequency
  falls below 57 Hz, whereas disconnection is
  adjustable between 0.16 and 300 Hz if the
  frequency falls between 59.8 and 57 Hz.
• Reconnection of a LAPES to a main grid may occur
  at least 5 minutes after voltages and frequency
  fall within indicated required ranges.

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Standards

• Main provisions from IEEE 1547
• Reconnection of a LAPES to a main grid may occur
  at least 5 minutes after voltages and frequency
  fall within indicated required ranges.
• A microgrid must not inject dc current greater
  than 0.5 of the full rated output current at
  the PCC.
• Harmonic current injection by the LAPES into the
  main grid measured at the PCC must not exceed
  certain levels both in total and for given
  harmonic order ranges. The total demand
  distortion must not be more than 5 of the local
  main grid maximum load current integrated demand
  (15 or 30 minutes) without the DR unit, or the DR
  unit rated current capacity, whatever is
  greater. Base of this same base current, harmonic
  content for harmonics with an odd order below 11
  must not exceed 4 . If the odd harmonic order is
  between 11 and 17 the limit is 2 , whereas this
  limit falls to 1.5 for odd harmonics with an
  order between 17 and 23 and 0.6 for odd
  harmonics with an order between 23 and 35. For
  odd harmonics with an order above 35, the
  harmonic content with respect to the indicated
  current must not exceed 0.3 . For even harmonics
  their content limits are a quarter of those
  indicated for the odd harmonic orders.

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Standards

• Other important provisions from IEEE 1547.6
  about network protections (NP) on the grids
  side
• The presence of DR should not
• - cause any NP to exceed its fault-interrupting
  capability.
• - cause any NP to operate more frequently than
  prior to DR operation.
• - prevent or delay the NP from opening for
  faults on the network feeders.
• - delay or prevent NP closure.
• - require the NP settings to be adjusted except
  by consent of the area EPS operator.
• - cause an islanding condition within part of
  a grid network.

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Interconnection methods and technologies

• Interconnection methods
• Directly through switchgear
• Power electronic interfaces
• Static switches
• Directly through circuit breakers
• Relatively simple and inexpensive
• Slow (3 to 6 cycles to achieve a complete
  disconnection).
• Since electrical characteristics on both sides
  of the circuit breakers must be the same, then,
  electrical characteristics on the micro-grid side
  are dependent on the grid characteristics. For
  example, use of a circuit breaker implicitly
  limits the micro-grid to have, at least
  partially, an ac power distribution system in
  order to match the grids electrical
  characteristics.
• Power flow through the PCC cannot be controlled

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Interconnection methods and technologies

• Directly through circuit breakers
• Example of one of such systems
• Use of static switches
• Usually based on SCRs in antiparallel
  configuration to allow bidirectional power flow

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Interconnection methods and technologies

• Use of static switches
• They are costlier and more complex than using
  circuit breakers.
• Usually, conventional circuit breakers are still
  used to provide a way to achieve full galvanic
  isolation. A Bypass switch is also added for
  maintenance reasons.

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Interconnection methods and technologies

• Use of static switches
• They allow for many open/close operations
• They act much faster than conventional circuit
  breakers (in the order of half a cycle to a
  cycle). Sometimes IGBTs are used instead of SCR
  because IGBTs tend to be faster than SCRs and
  their current is inherently limited.
• Still power flow cannot be controlled.
• There are some conduction losses in the devices.

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Interconnection methods and technologies

• Power electronic interfaces
• It is the costlier option but it is also the
  most flexible one.
• Allow for power distribution architecture
  characteristics on both sides of the PCC to be
  completely different.
• Both real and reactive power flow can be
  controlled.
• Reaction times to connection or disconnection
  commands are similar to those provided by static
  switches, although in the case of a power
  electronic circuit, it response also depends on
  its dynamic performance, given by its controller,
  topology, and internal energy storage components
  characteristics.
• Still, in many cases, a circuit breaker will
  still be required at the grid-side terminal of
  the power electronic interface with a LAPES in
  order to provide a way to physically disconnect
  the micro-grid from the grid.
• Also, similarly to static switches, the presence
  of a power electronic circuit will lead to some
  power losses not found in the approach using
  mechanical interfaces.

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Grid-connected inverter control

• Consider the following configuration in which it
  is assumed that we can control both real and
  reactive power can be controlled at the inverter
  (this is not always the case).

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Grid-connected inverter control

• With a small voltage drop in the grid impedance
  (so the voltage at the PCC, va is fixed and
  cannot be regulated by the inverter
• The inverter impedance depends on the inverter
  output filter parameters and the inverter
  controller.
• In most typical applications, for low
  frequencies the inverter impedance is mostly
  resistive. Hence,

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Grid-connected inverter control

• If the inverter is controlled as in most PV
  grid-tied inverters so their power factor is
  close to 1, then
• Hence,
• If due to the output filter or the controller
  parameters the inverter presents an inductive
  equivalent series impedance, then

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Grid-connected inverter control

• Condition for unity power factor
• Consider the following triangle based on the
  above condition
• Then
• and

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Islanding

• In IEEE Standard 1547.4 an intentional island is
  said to be the result of intentional events for
  which the time and duration of the planned island
  are agreed upon by all parties involved.
• There are several reasons why intentional island
  operation of a micro-grid may occur, but a common
  one is a preemptive disconnection from the grid
  in anticipation of a power outage on the main
  grid side caused by an event that can be
  anticipated, such as an incoming hurricane or
  storm, or wildfires. The advantage of this
  intentional islanding operation instead of
  waiting for the outage in the main grid to occur
  in order to switch the LAPES to operate in
  islanding mode is that an intentional islanding
  allows for a controlled transition that prevents
  potential failures or quality issues in the
  micro-grid.
• Two phases can be distinguished in islanded
  operation
• transition from grid connected to island
  operation
• operation isolated from the grid.

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Islanding

• During the transition into island operation it
  is important that
• voltage disturbances are quickly dampened and
  that protection schemes both inside the LAPES and
  in the grid are not affected.
• When the transition is completed it is important
  that
• the micro-grid has sufficient local power
  generation and energy storage in order to ensure
  that loads are powered with the agreed quality
  level. For example, in ac micro-grids it is
  important that distributed resources are able to
  provide real and reactive power to the specified
  load range. This is particularly important in
  order to avoid loss of stability if there are
  large motors in the LAPES that require
  significant amounts of reactive power during
  startup
• Also for ac micro-grids, their control systems
  must be able to regulate both voltage and
  frequency within acceptable ranges. In dc
  micro-grids, neither frequency regulation nor
  reactive power generation are issues to consider.

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Islanding

• Eventually, it can be anticipated that the
  micro-grid would be connected to the main grid
  again. Grid connection of dc micro-grids or ac
  micro-grids with a power electronics interface
  with the main grid tends to be simpler than the
  case of ac micro-grids connected to the main grid
  through circuit breakers, contactors, or static
  switches because in the dc micro-grid and the ac
  micro-grid with a power electronics interface
  cases reconnection control resides only in this
  power electronic interface. That is, the
  controller in this power electronic interface
  would controlled in order to realize on its grid
  side some voltage waveform so its amplitude,
  frequency and phase angle are within specified
  limits to allow reconnection.
• In the other ac micro-grid casesthose directly
  connected to the main grid though mechanical
  switchgear or static switchesreconnection is
  more complicated because there is no possibility
  of directly controlling the voltage waveforms at
  the PCC. In this case, ensuring that the voltage,
  frequency and phase angle are within acceptable
  limits depend on how the LAPES distributed
  resources are controlled.

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Islanding

• According to IEEE Standard 1547.4 these
  approaches can be distinguished in this case of
  ac micro-grids in order to achieve a successful
  reconnection
• Active synchronization In this approach, the
  LAPES controller matches the voltage signal on
  the PCC micro-grid side to those of the PCC on
  the grid side immediately before closing the
  islanding devices, such as a circuit breaker.
  Implementation of this approach requires
  measuring these three voltage signal
  parametersamplitude, frequency and phase
  angleon both sides of the PCC. A communications
  channel in order to exchange information between
  the micro-grid and the main grid is also
  necessary. This need for sensing and
  communications may lead to a higher failure rate
  as the sensing and communications subsystems may
  become a single point of failure.
• Passive synchronization In this approach a
  device is used to monitor the voltage at both
  sides of the PCC and allows the LAPES to connect
  to the main grid only when the voltage signal on
  the LAPES side is within some given required
  range of the main grid analogous voltage
  parameters. Like the active synchronization
  approach, passive synchronization requires
  sensing and communications, leading to the same
  potential reliability concerns. In addition, this
  method may be slower than active synchronization.
• Open transition This approach is more basic
  than the other two, because the method involves
  connecting both ends of the PCC after
  interrupting disconnecting the LAPES load. Once
  the micro-grid is connected to the grid then this
  load is brought back online.

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Islanding

• According to IEEE Standard 1547.4,
  un-intentional islanding operations are
  inadvertent events that are typically initiated
  by loss of area EPS or equipment failure, and the
  DR island system may be automatically
  sectionalized from the area EPS by protective
  equipment.
• Once the island has been established, the same
  considerations that were considered for the
  intentional island condition applies to the
  un-intentional island.
• Contrary to the case of intentional islanding,
  during an un-intentional island it is not
  possible to prepare the LAPES for such
  transition, such as verifying that there is
  sufficient local generation to sustain a stable
  operation powering all loads. Hence, in case it
  is expected that local generation capacity may be
  insufficient to sustain the load during
  un-intentional islands, black start functions or
  standby generators with transfer switches have to
  be allocated within the LAPES.
• Once the issue in the main grid that led to loss
  of service to the micro-grid feeder is solved, it
  may be of interest to reconnect soon the main
  grid to the micro-grid. However, such connection
  cannot occur until the voltage and frequency of
  the grid are stable and within acceptable ranges.
  In order to ensure meeting such requirement, a
  delay of up to five minutes may be provided
  between the time power is restored at the PCC
  from the main grid and the time reconnection to
  the LAPES is established.