Technical limits of penetration of
asynchronous wind and PV
generation in an AC interconnection
Professor Janusz Bialek
School of Engineering and Computing Sciences
Durham University
©J. W. Bialek 2012, p1
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©J. W. Bialek 2012, p2
Contents

Overview of technical problems with high penetration
of wind and PV

– Balancing is an economic, not technical, problem
Results of All Island (Ireland) integration study

Policy implications for GB and other countries
©J. W. Bialek 2012, p3
Grid integration of renewable generation
©J. W. Bialek 2012, p4
Grid integration of renewable generation

Spain or Denmark have very high penetration of wind but can
lean on Europe

Continental Europe or USA (large interconnected AC networks)

GB, Ireland and Texas (?) are AC islands – what are the limits?
©J. W. Bialek 2012, p5
Comparison of wind penetration in AC
interconnections (2008)
Source: M. O’Malley.
©J. W. Bialek 2012, p6
Ireland and GB

Both synchronous islands (i.e.
connected by DC links to other
systems)

Ireland: high penetration of wind
(wind feed-in already may
exceed 50% of demand at night)

Indication of problems GB may
face in 10-20 years

Learn for the Irish - EirGrid, SONI: “All Island TSO Facilitation of
Renewables Studies”. Available at www.eirgrid.com

Operational strategy: spilling wind when above 50% of demand
©J. W. Bialek 2012, p7
Synchronous operation of a power system

Power system operation is based around traditional
synchronous generators running thermal and hydro
plants

Synchronous operation is vitally important for power
system operation:


– Frequency control
– Transient and dynamic stability
– Voltage stability
Replacing synchronous generators by asynchronous
ones (wind and PV) has a profound effect
Need to consider the whole AC interconnection, not just
one country or a control zone
©J. W. Bialek 2012, p8
Frequency control: how to
maintain a power balance
(generation=demand) in an AC
system?

In a time scale of minutes-to-minutes, balance of power is
maintained by power system operator instructing power plants
to follow demand

How is an instantaneous power balance maintained when e.g.
Sizewell B trips?
©J. W. Bialek 2012, p9
What happens when Sizewell B trips?

Immediately afterwards demand exceeds generation

But the lights stay on: where does the balance come from?

Kinetic energy stored in rotating masses
= inertia * (speed)2 /2

That kinetic energy stored provides a cushion for any
momentary imbalance of power
©J. W. Bialek 2012, p10
Frequency provides
the system-wide
signal about
generation/demand
balance
Load-frequency
control
Power system consists of many generators operating in
synchronism, i.e. with the same frequency (50/60 Hz), so there is
a lot
of kinetic
energy stored forces keeping the generators
The
strings
are electromagnetic
in synchronism
©J. W. Bialek 2012, p11
Primary frequency control: restoring
power balance

Following a trip of a large power plant, all synchronous
generators automatically slow down thus releasing additional
energy to cover the power deficit

Slowing down generators cause frequency to drop

Frequency drop activates automatic Turbine Governors that
open valves more hence releasing more steam (increasing
power output)

This continues until power
balance is restored:
frequency drop is halted but
it is less than 50/60 Hz
©J. W. Bialek 2012, p12
Primary frequency control

Primary frequency control is fully automatic – all generators
must be equipped with Turbine Governors with a droop
characteristic
speed
power

Power stations have to operate derated to provide a
headroom (frequency reserve)
©J. W. Bialek 2012, p13
Secondary frequency control: restoring the
nominal frequency

System Operator instructs power stations to increase
generation and restore 50/60 Hz

Manual in GB; automatic Europe, USA (Automatic Generation
Control – AGC)
©J. W. Bialek 2012, p14
Frequency trace following a large infeed loss
Restoring power
balance
Primary Freq.
Control
Fully automatic, decentralised
Restoring 50/60 Hz
Secondary Freq. Control
Centralised, manual or automatic
©J. W. Bialek 2012, p15
Frequency as a system-wide power balance
Frequency as an universal
power
balance
indicator
indicator
©J. W. Bialek 2012, p16
Frequency Limits in GB
52.0
Upper Operating Limit
50.5
Upper statutory limit
50.0
Normal operating frequency
49.5
48.8
47.8
47.5
Lower statutory limit
Demand disconnection starts
Demand disconnection complete
Lower Operating Limit
©J. W. Bialek 2012, p17
Importance of frequency limits

Design criteria for network and user equipment

Quality of supply

Risk of frequency collapse (positive fedback)
Italian blackout
2003
©J. W. Bialek 2012, p18
Impact of wind and PV on power system
operation

Less synchronous and more asynchronous

Wind farms use induction generators (double-fed or
classical) which react weakly to frequency changes –
smaller effective inertia
DFIG
©J. W. Bialek 2012, p19
Reduced inertia of DFIG and fixed-speed
induction generators
After M.O’Malley
©J. W. Bialek 2012, p20
Ireland: inertia expected to drop by 25% in
2020 compared to 2010
©J. W. Bialek 2012, p21
PV and DC
links

PV are connected by power electronics
DC/AC converters – no rotating masses, no
inertia

Similarly DC interconnectors do not respond to
frequency changes

Overall effect: lower inertia may cause a frequency drop
following the loss of a large infeed to be outside the
statutory limits

In extreme, a danger of frequency collapse

Preventive measure: restrict wind/PV feed-in
©J. W. Bialek 2012, p22
Overall effect of lower system inertia

Lower inertia may cause a frequency drop following
the loss of a large infeed to be outside the statutory
limits

In extreme, a danger of frequency collapse

Preventive measure: restrict wind/PV feed-in
©J. W. Bialek 2012, p23
Mitigation (Smart
Grids) measures:
synthetic inertia

“Synthetic inertia” may be provided by control abilities of
power electronics at DFIG

Artificial slowing down of the turbine after a frequency
drop was detected

Possible but not used yet

Make it a Grid Code requirement?
©J. W. Bialek 2012, p24
Mitigation measures: demand response
Temporary (a few mins) halting of a cooling cycle
Socially acceptable?
©J. W. Bialek 2012, p25
Mitigation measures: frequency response
from interconnectors

Europe has a very large inertia

It is possible to program AC/DC
converters to react to frequency
changes (behave similarly to AC lines)

Effectively sharing frequency response
with Europe

Sharing frequency response was the
main reason historically for connecting
countries

But sharing frequency reserve with
Europe would not be easy – significant
changes (harmonisation) required
©J. W. Bialek 2012, p26
Transient stability

Ability to restore synchronous
operation following a fault (e.g. shortcircuit)

Synchronous generators create a
spring-like synchronous torque that
restores synchronism (common
frequency)

DFIG has a different mechanism of
creating the torque than a
synchronous generator

Will the synchronism be restored
following a fault when penetration of
wind/PV is high?
©J. W. Bialek 2012, p27
Fault Ride Through

A significant problem at early stages of wind
expansion
 Synchronous generators
remained connected
following a severe fault but
wind generators (DFIG)
would drop out due to low
voltage

Remedy: Grid Code
requirements for Fault Ride
Through capability
Source: EON “2004 Wind Report”
©J. W. Bialek 2012, p28
Voltage stability

Voltage is connected with reactive power

Synchronous generators are the main sources of
reactive power

DFIG have lower reactive power capabilities than a
synchronous generator

DC/AC converter at PV may produce reactive power

Higher wind penetration may require additional
compensators (capacitors or solid-state)

It can be managed
©J. W. Bialek 2012, p29
©J. W. Bialek 2012, p30
Main issues emerging from All Island
EirGrid/SONI study

Frequency stability
following a loss of
infeed

Frequency as well as
transient stability
following a severe
network fault

Other issues can be
managed
©J. W. Bialek 2012, p31
Operational constraints at high wind
penetrations

Save operation for wind only up to to 50% of demand

Already spilling wind when more than 50% of demand

Operation up to 75% penetration possible but dangerous

PV would make it worse (not for Ireland!)
©J. W. Bialek 2012, p32
Conclusions

High wind and PV penetration poses significant technical
challenges due to replacing synchronous generators by
asynchronous DFIG or DC/AC converters

Not a problem in large AC interconnections but a problem
for GB or Ireland

Ireland an ideal lab for GB – already high wind
penetration

Main problems: managing system stability following a
large infeed loss or a network fault

Smart Grids fixes possible but untested
©J. W. Bialek 2012, p33