Title: System Role of Generation
1System Role of Generation
- Leslie Bryans Alan Kennedy
2Generators contribute to
- System stability and power quality
- Inertia and fault level
- Reactive power balance
- SONI will discuss
- Active power balance
- SONI will discuss
3What is Inertia
- It is the energy stored in generators because
they are rotating.
4Inertia
- 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.
5System Frequency Issue
Solutions wind farms to contribute to
restorative power and system inertia?
6Frequency trace NI system only - low inertia
7Frequency trace for the combined NI RoI System
- higher inertia
8Why 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
9Wind 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.
10Work 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.
11Inertia, Rate of change of frequency and
frequency nadir
- 80 wind operating
- (40 energy target)
12Inertia summer min ( 80 wind)
13Inertia - winter ( 80 wind)
14400MW trip in Winter 80 wind
15400MW trip in Summer 80 wind
16Enlarged Graph section 400MW trip in summer 80
wind
17Work 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.)
18Our 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.
19Fault level
20What 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.
21High 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).
22Power quality
- Very low fault level results in excessive voltage
dips when equipment is switched on.
23GB 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.
24Our 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.
25Reactive power
26Reactive 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
27Static 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
28Problem 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
29- 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