System Role of Generation

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System Role of Generation

• Leslie Bryans Alan Kennedy

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Generators contribute to

• System stability and power quality
• Inertia and fault level
• Reactive power balance
• SONI will discuss
• Active power balance
• SONI will discuss

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What is Inertia

• It is the energy stored in generators because
  they are rotating.

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Inertia

• Large spinning machines provide stored energy due
  to the rotating mass of their rotor, driving
  turbine shafts etc. Engineers model this as a
  number of rotating masses.
• When a disturbance occurs e.g. the unanticipated
  loss of a generator, the stored energy is
  released into the system and arrests the rate at
  which the system speed or frequency changes.

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System Frequency Issue
Solutions wind farms to contribute to
restorative power and system inertia?
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Frequency trace NI system only - low inertia

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Frequency trace for the combined NI RoI System
- higher inertia
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Why is the rate of change of system frequency
important?

• We need to understand this better as it likely to
  be a limiting factor in accommodating higher
  levels of wind.
• ROCOF protection
• Question could this become backup Loss of Mains
  protection? (DGSEE with Prof. Jim McDonald)
• System effects e.g. traditional generation
  auxiliary plant trips CCGT flame outs.
• Raised with DGSEE as requiring investigation

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Wind turbines

• Fixed speed turbines seem to deliver an inertia
  of around 4.3MWs/MVA which is similar to
  traditional plant so do not reduce system
  inertia.
• DFIGs and Fully converted wind farms are
  presently thought to deliver close to 0MWs/MVA
  so at present reduce system inertia.

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Work done

• In order to understand the problem, we have
  explored what happens to inertia and rate of
  change of frequency with very high levels of
  penetration of wind power on the island of
  Ireland.
• This is to understand what we need to tackle
  rather than alarm the present position.

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Inertia, Rate of change of frequency and
frequency nadir

• 80 wind operating
• (40 energy target)

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Inertia summer min ( 80 wind)
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Inertia - winter ( 80 wind)
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400MW trip in Winter 80 wind
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400MW trip in Summer 80 wind
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Enlarged Graph section 400MW trip in summer 80
wind
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Work by UMIST for DGSEE

• DFIGs could deliver inertia as required but
  require control modifications
• Hardware transient loading is not an obstacle. (
  Prof. Jenkins says that the transient loading on
  gearboxes, drive trains etc. falls well within
  that needed for dynamic braking of the turbines.)

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Our Proposal

• To seek a modification to the NI Grid Code to
  require wind turbines to provide inertia at 4
  6MWs/MVA.
• This would need to be harmonised with an
  equivalent provision in the EirGrid Grid Code and
  Distribution Codes.
• This will be a world first and waiting does not
  seem an option.

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Fault level

• With wind farms

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What is Fault level?

• Fault level is measured in current or MVA.
• When a short circuit occurs a large current flows
  this causes the operation of protective devices
  (fuses and circuit breakers open) to isolate the
  fault.
• Protection systems are designed to isolate only
  the faulty section of network.
• Fault level is also a measure of system stiffness
  which determines power quality issues.

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High and Low fault level

• Too high fault currents can be hard to interrupt.
• Too low and the protective devices may not
  operate.
• The problem with high penetration of wind farms
  is too low fault levels.
• (Work is on-going with DG and SEE for the UK
  Grid. They are going to study NI as well).

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Power quality

• Very low fault level results in excessive voltage
  dips when equipment is switched on.

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GB results

• With the expected penetration of wind farms in GB
  2010 the minimum fault level is reduced by a few
  percentage points on average.
• In some places because traditional generation is
  off-the bars the fault level is reduced by 70.
• This leads to problems with distance protection
  (the main 110kV system protection). In
  particular, resistance faults may result in
  mal-operation of distance protection.
• The NI situation is likely to be much worse
  because we only have 3 traditional generating
  units operating at periods of low fault level.

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Our proposal

• To ensure that the fault level tools developed by
  DGSEE (which assess fault level throughout the
  range of generation dispatches) are applied to
  the NI network and to consider what further
  action may be needed. At the extremewe may need
  to convert the distance protection schemes to
  unit schemes which is very expensive.
• Also, DGSEE have not considered the knock-on
  effect of low fault level on the distribution
  protection systems. This may be more serious and
  potentially intolerable. Further work is
  required.

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Reactive power
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Reactive Power Balance

• Static Balance
• To ensure an adequate voltage everywhere on the
  system and provide reactive power requirements to
  load
• More heavily loaded transmission system requires
  relatively more reactive power due to I²X losses
• Reactive power doesnt travel well on a
  transmission system due to the high X/R ratio,
  but NI transmission system is small.
• Studying the reactive power balance is more
  complex when embedded generation is included

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Static Reactive Power
Lines Cables capacitance Q V²/Xc
Gen AVR controls terminals to approx 1pu voltage

Capacitor banks Q V²/Xc
Transmission System
Gen transformer tapped to pump reactive power
into the transmission network.
Consumption of Reactive Power
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Problem with Embedded Gen
P
Q
G
R
X
Step-down Transformer
Gen Bus
Back-bone transmission network
P Q
R
X
load
Max limit at EG
Voltage
Min limit at load
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• Static Reactive Power Balance
• - Within reason this could be achieved with
  suitably sited capacitors but there are technical
  difficulties with many capacitors installed
• Dynamic Reactive Power Balance
• - Needed during and after system disturbances
• - Quantity depends on many factors but rule of
  thumb is dynamic static
• - Synchronous generators can supply high levels
  of dynamic reactive power
• - Reactive power output of DFIGs and
  fully-converted wind turbines is limited to
  rated MVA, but even this is of limited use on the
  transmission system
• - Need for alternative forms of dynamic reactive
  support on transmission system eg. SVC, STATCOM,
  synch-comp