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