CIRED

1
Controllability of DG helps managing
Distribution Grids

• J. A. Peças Lopes
• (jpl_at_fe.up.pt)

2
Exploiting DG to improve system operation

• DG has been considered as non controllable and
  non dispatchable, since all the energy production
  has priority to be absorbed by the network
• The increase in DG foreseen for the next years
  will require a different approach regarding the
  way how DG units will be operated
• Concepts of controllability should be developed
  and exploited
• Participation in reactive power control
• Interruptability
• Delivery of ancillary services (primary and
  secondary reserves, according to the conversion
  technology and primary energy sources)
• Participation in system restoration strategies
• Development of concepts related with control of
  clusters of DG and virtual power stations

3
Main characteristics of DG units and
controlability concepts

• Three main types of energy conversion systems can
  be found among DG units
• Conventional synchronous machines (cogeneration,
  CHP, mini-hydro)
• Asynchronous generators (wind power, mini-hydro)
• AC/DC/AC electronic conversion systems used
  together with synchronous or induction machines
  (micro-turbines, fuel cells, wind generators).
• Classification (according to primary energy
  source and conversion system used)
• Non- controllable (Ex Wind park with
  asynchronous stall generators)
• Partially controllable (Ex Wind park with
  synchronous variable speed gen. and AC/DC/AC
  converters)
• Controllable (Ex Mini-hydro or Cogeneration
  plant with synchronous units).

4
DG units used to optimise the distribution system
operation

• DG can be used to optimise the operation strategy
  of distribution networks.
• The Problem can be formulated an optimisation
  problem
• Min (active power losses)
• Subj. to
• Vmax lt Vi lt Vmin
• Sij max lt Sij
• Qgmaxilt Qgi lt Qgmini taking into
  account the type of generator
• Qimpor max lt Qimpor
• Transformer tap limits are kept
• Control variables Qg, capacitor banks and
  transformer taps
• The need to use a motor of optimisation
• (Evolutionary Particle Swarm Optimisation
  EPSO)

5
Some results of the participation of DG in
Voltage VAR control

• Test System 60 kV distribution network with a
  large penetration of DG (mini-hydro and wind
  generation).

Activate control on reactive power generated in
the DG Units.
6
Some results of the participation of DG in
Voltage VAR control

• Changes in active Losses
• Peak load scenario
• A clear reduction on actives losses was obtained

7
Some results of the participation of DG in
Voltage VAR control

• Results concerning voltage in network busses

8
Dynamic Impacts

• Dynamic behaviour impacts need to be addressed
  using adequate DG modelling and DG equivalent
  representation
• Considering disturbances resulting from DG
  operation
• Considering disturbances in distribution
  networks
• Considering disturbances in the transmission
  system

9
Dynamic behavior analysis

• Scenario Week peak with maximum dispersed
  generation
• Disturbance Outage of Power Plant H 7,346MVA,
  production of 6,692j3,03 MVA, injection of
  2,678j1,081 MVA (tg j 0,404)
• Voltage profile
• 60kV bus at the substation

10
Dynamic behavior analysis
15 kV bus of the feeder where the power plant was
connected
15 kV bus of the feeder where the power plant was
not connected
11
Dynamic behavior analysis (Impact in the other
generators)
12
Relay coordination

• Under voltage relay coordination is needed
• Energy conversion systems need to able to
  withstand low voltages during short-circuits
  up-stream.

Frequency changes
Changes in contractual inter-area power flows
13
Impacts on Operation

• Load flows become bi-directional
• Voltage profiles have different patterns
• Losses change as a function of the production and
  load levels
• Congestion in system branches is a function of
  the production and load levels
• Short-circuit levels increase
• Power quality may be affected
• Voltage transients will appear as a result of
  connection disconnection of generators
• Risk of islanding operation
• Reliability may be reduced
• System dynamic behavior may be largely affected
• Protections coordination is needed

14
Conclusions

• The future
• DG units should be more actively used to help in
  the management of the distribution grid
• New DMS tools need to be developed
• Topology processor with capabilities of
  identification of energised areas
• Voltage and reactive power control
• Load and current forecasting
• Load flow including new generator models and load
  allocation algorithms to allow load flow to run
• Optimum network reconfiguration
• State estimation (considering that some DG units
  will not be monitored and new pseudo-measures
  need to be defined)
• Cluster control strategies should be implemented,
  involving the development of local dispatch
  centres
• Development of DMS training simulators for
  distribution grids with large amounts of DG
  (steady state and dynamic behaviour).