Post on 29-Sep-2020
A.L. Hoskin, J.P. Rouse and S.D. Garvey
Grid Inertia: Current Perceptions and Directions of Travel
24 February 2020
Flywheels for Grid Inertia
Introduction
• Function of grid inertia
• How much inertia is needed?
• Possible ways of replacing inertia
• Serial Hybrid Kinetic Energy Storage System
(SHyKESS)
www.kfw.de2
Introduction
• Function of grid inertia
• How much inertia is needed?
• Possible ways of replacing inertia
• Serial Hybrid Kinetic Energy Storage System
(SHyKESS)
www.kfw.de3
Function of grid inertia
0
10
20
30
40
50
60
0 10 20 30 40 50
Dem
and (
GW
)
Time (hrs)
UK Grid Demand
Data from: www.gridwatch.templar.co.uk
• On the electrical grid supply must match demand quite closely
• Supply and demand must match over various time frames from seconds to
minutes or hours
• In the timeframe of seconds inertia helps balance supply and demand
• In the case of a generator or load disconnection inertia limits the RoCoF
4
• Inertia is a flywheel effect from the spinning mass of synchronous generators and turbines
• Synchronous generation rotational speed linked to grid frequency
• Demand > supply frequency decreases
• Supply > demand frequency increases
• Inertia limits Rate of Change of Frequency (RoCoF)
• Helps keep frequency within limits (49.5 Hz – 50.5 Hz for UK grid)
Function of grid inertia
5
• Renewable power sources such as wind turbines and solar energy have
less inertia
• This leads to a higher RoCoF for a given difference between supply and
demand
• Higher RoCoF can lead to larger frequency nadirs, islanding and blackouts
www.machinedesign.com
www.katuahearthfirst.org
Reduction of grid inertia
6
How much inertia is needed?
• National Grid max RoCoF is 0.125 Hz/s – above this generators will
start disconnecting causing further issues
• National Grid ensure that if the largest generator or demand comes
offline the RoCoF is less than 0.125 Hz/s
• Its does this by balancing ensuring the inertia (GJ) is 200 x the largest
generator or load power (GW) 7
How much inertia is needed?
• Three options of dealing with reduced inertia: reducing largest supply or demand,
increasing inertia or increasing max allowable RoCoF
• Some balance between all three options should be investigated
• Some level of inertia replacement will be needed8
Balance between inertia and
primary frequency control
• Large amount of inertia gives time
for primary frequency control to
respond and keep frequency above
limit
• As inertia decreases higher RoCoF
means less response time – more
and faster primary frequency
control needed to keep frequency
above limit
• Trade off between inertia and
primary frequency control
(Inertia requirement methodology – inertia requirements and
shortfalls - AMEO)
9
Current costs for National grid
Year RoCoF cost (£m)
2016 61.21
2017 59.00
2018 115.05
2019 173.77
• National Grid currently limit the largest generator and add other synchronous
generators on at low load to meet the required inertia
• This has significant and increasing costs
• This will increase in the future as National Grid aims to be able to run with
“100% zero carbon by 2025”. Inertia market starting in April.
(National grid balancing mechanism
monthly reports)
10
Inertia in the future
• Many grid operators have thought about paying for inertia
• National grid are starting an inertia market in April – inertia will have to
be cheaper than the other option of reducing largest generator and
adding other generators on low load
• Inertia price will decrease to the lowest method of creating inertia
(Economic value of inertia in low carbon power
systems. L.Badesa et al.)(Inertia requirement methodology – inertia requirements
and shortfalls - AMEO)
11
Methods of creating inertia
• Wind turbines • Synthetic inertia
• Synchronous compensator
12
Flywheels for inertia
Non-synchronous
Rotary stabiliser
Synchronous
• Non-synchronous - usually used for
primary frequency control
• Power in/out is externally controlled
• Synchronous with grid frequency
• Power is proportional to RoCoF
• Amount of energy limited to ±2% of
energy stored in flywheel
• Can be synchronous with grid frequency
• Can use DFIG to further discharge
flywheel below grid frequency13
Costs of creating inertia
[1] Lead Acid Li-ionSodium sulphur
Super-capacitor Flywheel
LCOIEur/kgm2year 275.22 205.64 247.48 192.12 167.64
[1] H. Thiesen, C. Jauch, A. Gloe, Design of a system substituting today’s inherent inertia in the European continental
synchronous area, Energies 914
SHyKESS
• Function of grid inertia
• How much inertia is needed?
• Possible ways of replacing inertia
• Serial Hybrid Kinetic Energy Storage System
(SHyKESS)
www.kfw.de15
SHyKESS
• Serial Hybrid Kinetic Energy Storage System
• Couples a synchronous machine to a flywheel via a differential drive
unit (DDU)
• Normal operation DDU locked adding inertia of flywheel to grid
• DDU can unlock and apply a torque to discharge or charge flywheel
Mode 0
• Grid frequency decreases
• Synchronous machine
rotational speed decreases
• Kinetic energy from
flywheel is given to grid
• Grid frequency increases
• Synchronous machine
rotational speed increases
• Energy from grid is taken
and stored as kinetic
energy in flywheel
Mode 1
• Grid frequency heads towards 49.5 Hz
• DDU discharges flywheel to grid powered by secondary energy
store
• Initially most of the power comes from flywheel
Mode 2
• Grid frequency recovers to above 49.8 Hz
• DDU charges flywheel and secondary energy store from grid
• Once flywheel is at the same speed as synchronous machine
DDU can lock and go back go mode 1
The Modes of SHyKESS
An extra dimension to Realness
Synchronous machines can supply fault currents that are many times
greater than rated currents for periods of many seconds (controlled by
the specific heat of copper and the highly-distributed nature of power
dissipation.)
Power electronic switches have very short time-constants - order of ms.
ILLUSTRATIVE CALCULATION:
Rated current density for machine windings typically ~2A/mm2
Resistivity of copper ~ 1.68e-8 Wm.
Specific heat of copper = 385 J/kgK. Density of copper = 8960 kg/m3.
5 Times Full Load Current: 0.47 °C/s
10 Times Full Load Current: 1.93 °C/s
25 Times Full Load Current: 12.1 °C/s 21
Sizing synchronous flywheels
• Choose pole-number first W is known (314.15 rad/s or 157.07 ..
• Use strong ductile steel for flywheel (e.g. EN24, sy = 500 MPa)
• Select the ratio g := Ri / Ro.
• Calculate
• Because flywheel will run in a vacuum and it is poor economics to
make a “pie-shaped” vacuum chamber …
set L 2RO.
• Diameter 5.4 m
• Length 5.4 m
• 12 MWh flywheel
• 50 MW synchronous machine
RI
Ro
22
Test demonstrator
Test demonstrator
Full Size SHyKESS
• Flywheel and DDU are
contained underground
• Magnetic bearings used
for most of flywheel
weight
• Contact bearings used
for stiffness
• Possibility of evacuating
chamber around
flywheel to reduce
losses
Further Reading
• Specs
Cost comparison
27
Flywheel System Battery System
• Synchronous machines are
cheap! 50MW machines can
be found for ~£800k
(Quartzelec).
• Steel costs (assume ~2/kg).
• 4 pole/12.412MWh variant
(984Tons, 5.4m diameter)
~£1.969m.
• 50MW hydraulic machine
~£1.6m
• Total costs ~£3m.
• PEC (MV) ~£100-200/kW.
• Battery cells ~£100/kWh
• For a 50MW system, PEC
alone accounts for £5m-10m.
• To realise 12MWh of energy
storage, battery cell costs
~£1.2m.
• Total costs are well in excess
of £10m.
• For comparison, Glassenbury
(Low Carbon) battery storage
plant, 40MW, 20MWh,
£12.6m.
Conclusions
• The “realness” of an inertia source sits on a
spectrum.
• “Real” inertia is not expensive and is a viable
solution in decarbonised grids.
• SHyKESS offers a secure, cheap, and high
efficiency method to replace inertia.
Questions?
29
Efficiency
30
Questions?
SHyKESS Efficiency