Impact of the Smart Grid on B lk S t R li bilitBulk System Reliability

What an operator can dop

K ith B llKeith BellDepartment of Electrical and Electronic Engineering

University of Strathclyde

How reliable is the system?y• Energy not supplied is due to

– (most frequently) faults on(most frequently) faults on distribution system

• lose small amounts of load

– (less frequently) faults near the interface of transmission and distributiondistribution

• lose bigger amounts of load

– (rarely) faults within the i t t d t i iinterconnected transmission system

• lose lots of load

Fig: Dobson, Carreras, Lynch, Newman, 2007Log-log plot of scaled pdf of energy unserved during North American blackouts 1984 to 1998

Managing riskg g• Risk = Σ (impact × probability) for all possible states

E– Easy:• Ask power system engineers for the probability of every state• Use models to quantify the impact of each one• Throw it all into an optimiser to decide a

dispatch of generation

– Actually not easy: system is largeActually, not easy: system is large, dynamic, non-linear, complex

• GB: >300 generating units (each modelled by at least(each modelled by at least 10 ODEs, 3 or 4 limiters), 2000 nodes, 3000 branches, hundreds of controllershundreds of controllers, thousands of circuit breakers, …

– Use sensible rules of thumb

Helping the operator: rules of thumbp g p• The power system facilitates

– reliable supply of electricity into an areareliable supply of electricity into an area• ‘reliability-driven’ network capacity

– competition/economic operation of a single market• ‘market driven’ network capacity• market-driven network capacity

Exports and importsp p• Obviously, an export of power must always be accompanied

by an import (and vice versa)y p ( )

• However, useful to think of – an ‘export constraint’: where an

increase in transfer would lead to a problem on the exporting side of a boundarya boundary

– an ‘import constraint’: where an increase in transfer would lead to a problem on the importing side of a boundary

• The consequences of export andThe consequences of export and import constraints are different

Risks and consequencesqSuccessful system operation depends on prediction of the effects of disturbances and accurate/reliable quantification of power transfer constraints

Disturbance withini i t t d

Risk assessment: it’s all relative…

Disturbance withinlocal system

main interconnectedsystem

‘Easy’ to quantify Very difficult to quantifyImport ofpower

Easy to quantify failure modes &

loss of supply effects

Very difficult to quantify failure modes &

loss of supply effects

Export of‘Easy’ to quantify failure modes &

A bit tricky to quantify failure modes &

power economic effectsof curtailment

economic effectsof curtailment

Security standardsy• A security standard defines

– the set of secured events (‘contingencies’) andthe set of secured events ( contingencies ), and– the consequences to be avoided

• e.g. overloads, unacceptable voltages, instability, unacceptable frequencyEffect of secured events studied in ‘security assessment’• Effect of secured events studied in ‘security assessment’ – Take or identify action to avoid unacceptable consequences

• Examples of secured events:– single circuit outage– double circuit outage– loss of infeed, e.g. generator or an interconnector such as French linkoss o eed, e g ge e ato o a te co ecto suc as e c– loss of reactive compensation– outage of a bus section or mesh corner

• NETS ‘Security and Quality of Supply Standard’ (NETS SQSS) is a• NETS Security and Quality of Supply Standard (NETS SQSS) is a licence condition of the 3 GB transmission companies: https://www.nationalgrid.com/uk/Electricity/Codes/

System securityy yLoss of

load (MW)Zone 2 – Unacceptable consequencesWhere necessary, preventive measures taken to avoid this zone, even if costlyLimit of

unacceptableconsequences

Zone 1Extremeevents

Zone 3 – Unacceptable risksWhere necessary, preventive measures taken to avoid this zone, even if costly

Isorisk

(Maximu

Zone 4 Acceptable risks

sk curve

um accepted risk)

Zone 4 – Acceptable risksAny preventive measures must be result of technical and economic analysis

Probability of event

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Figure: RTE

The power transfer Generation

Demand

challenge B4

B6

SCOTLAND (SHETL)

SCOTLAND (SPT)UPPER NORTH

4561 1726

7061 3899

400

400

NORTHERN 2484 3303

2834

5997

5177

Figure 8.B6 - Boundary Transfers and Capability(B d B6 SPT NGET)

B7

B8

B9

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MIDLANDS

NORTHERN

400 400

REPUBLIC OF IRELAND

14998 12319

76928153

2668

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B13

B15

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4990

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7396

28

3000

4000

5000

6000

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1000

2000

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2011/12 2012/13 2013/14 2014/15 2015/16 2016/17 2017/18

Boun

dar

2011/12 2012/13 2013/14 2014/15 2015/16 2016/17 2017/18

Years90% Range of Transfers 50% Range of Transfers SYS Transfer

SYS Required Capability SYS Capability Figure: National Grid

Smarter transmission…

• The planner might seek support for investment in:d i f d t– new designs of conductor

• higher thermal ratings– real-time monitors for ‘dynamic’ thermal ratings– new series devices and facilities for their coordination

• quadrature boosters, thyristor controlled series compensation– wide area monitoring and control– ‘supplementary’ controls for damping of power oscillations– decision support for operational planning of corrective actions

• When will demand response realistically be available?When will demand response realistically be available? • How much is available? How will it be accessed?

– bypassing of the AC grid with HVDCfacilities to ‘fail gracefully’– facilities to fail gracefully

• The planner should deliver a set of facilities that the operator can operate– What kind of ‘security’?

Facilitating higher transfersg g

Figure: National Grid, from ELSI simulations

GB ‘Vision for 2020’HVDC circuit

Re-conductor or re-insulateRe conductor or re insulate existing double circuit overhead line route

Full re-build or new build double circuit overhead line route

S i tiSeries compensation• equipment located at terminal substations

250kmunderseacable

Combination of ‘embedded HVDC’

cable

Combination of embedded HVDCand series compensation on ameshed network:• nothing like this done before

270kmundersea cable

Source: Electricity Networks Strategy Group “Vision 2020”

nothing like this done beforeanywhere in the world

Examples of ‘embedded’ HVDCp

• Existing– Caprivi link (Namibia)– Kii channel (Japan)

F Sk (Fi l d S d )– Fenno-Skan (Finland-Sweden)• Planned

– France-Spain– France-Spain – ALEGRO (Belgium-Germany)– CobraCable (Netherlands-Denmark)( )– France-Italy– Switzerland-Italy– West and East coast ‘bootstraps’ (GB)

CIGRE JWG B4-C4-C1.604

Hazards• How much can apparent system limits be squeezed?

– Does the operator always know what the limits really are?– Need both for accurate measurements and robust analysis

• Usual possibility of local AC network constraintsMi h f fl ibl l i d l ?• Might fast, flexible controls interact adversely?– Risk of creation of unstable modes

• Interactions of HVDC SVC and power electronics on DFIG andInteractions of HVDC, SVC and power electronics on DFIG and FRC wind farms

• (Experience from Swiss railways of small signal instability)• How should supplemental controls be tuned on a large system?• How should supplemental controls be tuned on a large system?• (Anecdotal suggestion of increased SSR risk when DFIGs are close

to series compensation)

• Effect on distance protection of widespread VSC?• Loss of infeed risk with loss of commutation of multiple LCC

Hazards - 2

• What are the ideal locations for measurements for damping f ill ti ?of oscillations?– What if measurements are delayed or lost?

• In coordination of QB tap positions:• In coordination of QB tap positions:– Are post-fault tap changes required? Can they be relied

upon?– Can a single set of ‘optimal’ pre-fault tap positions be

found wrt contingency constraints?How best to reach a compromise?• How best to reach a compromise?

• How to explain settings to operators?

– What if conditions have changed in the meantime?g• Increased need for coordination of 132kV voltages with

transmission voltages?

Hazards - 3

• Can inter-trips be implemented– sufficiently quickly?– in a coordinated manner?

ith t d l bilit t j li bilit t ?– without undue vulnerability to major unreliability events?• Might it sometimes better not to arm them? How to identify such

cases?

• What dependency remains on dirty, conventional generation?

Can inertia be synthesised on wind farms?– Can inertia be synthesised on wind farms? • Risk of extracting too much energy from wind turbines leading to

stall?

– How much is needed for frequency regulation?

Undesired behaviours

• Western US, August 1996: l f ill ti t i ti b f lt li– low frequency oscillations set in motion by a fault on a line

– disturbance exacerbated by AGC increasing flows from north to try to re-establish schedule

– then more faults and erroneous tripping of generators by excitation protection

• Brazil, Dec 1994– Testing at a HVDC Converter Station– Human error caused the operation of the forced isolation scheme. – The two HVDC bipoles were blocked, resulting in a shortage of 5,800The two HVDC bipoles were blocked, resulting in a shortage of 5,800

MW in the interconnected system• Lots of examples of protection being triggered, even when correctly set

but under circumstances not anticipatedbut under circumstances not anticipated

Is action needed and justified?j• The huge set of possible cascade mechanisms and paths

makes quantification of risk very difficultmakes quantification of risk very difficult– Analysis inevitably requires approximations, e.g.

• Filtering of search space using heuristics, e.g.– Study only certain initial conditions– Study only certain consequential events

• Simplified modelling of system behaviour, e.g.p g y g– Assume electro-mechanical equilibrium is achieved– Assume adequate voltage support

• Are some risks acceptable? Can we spot them?

risk = impact × probability

Are some risks acceptable? Can we spot them?What is the value of this anyway?• Probabilities of consequential events are

conditional on initial conditionsVery large Very small conditional on initial conditions

• Evidence for quantification of probabilities?• Heuristic methods often either approximate or

ignore probabilitiesWhat is VOLL?

“Security standards limit economic transfer of po er”transfer of power”• Would Dobson et al’s plot be reproduced in Britain?

– Perhaps not: the Brits (currently) secure to ‘N-D’ (double circuit outage)In the US in the past the system– In the US in the past, the systemwas not always secured at all

• Should we relax operational security?– Perhaps: secure only to N-1 when it seems safe to do so

• South London event of 2003 was initially N-1…trip revealed a ‘hidden failure’ (‘really’ N 2?)• … trip revealed a ‘hidden failure’ (‘really’ N-2?)

– Interruptions would inevitably occur more often• Still so rare as to be ‘acceptable’?p• Containing interruptions becomes even more important

– Defence measures…

Change to the security doctrine?g y

• What is already done in respect of post-fault actions on t t i t ?export constraints?

– Is there consistency?Can the requirements be better articulated?– Can the requirements be better articulated?

– Can decisions be better supported?– Should more frequency excursions be accepted?S ou d o e eque cy e cu s o s be accepted– Should greater risks of instability be accepted?– What does ‘operational complexity’ mean?

• Should demand security in importing areas be relaxed?– What should be the criteria?– (To what extent do import constraints hinder the

achievement of renewables targets?)

What makes an operator unhappy?p ppy• “Complexity”• Possible failure of corrective measures

– Corrective measures do not take place or prove to be insufficient• Operators might err on side of caution: schedule more action than needed

• Rapidly changing initial conditionsWhat are the weather conditions in different parts of the system?– What are the weather conditions in different parts of the system? How quickly are they changing?

• Possibility of multiple trips– Near simultaneous loss of many generators– Cascading of transmission outages

• System instability– Oscillatory, voltage or transient

L k f li bl ft d/ l k f li bl d t• Lack of reliable software and/or lack of reliable data– e.g. ratings, the actual stability limit, probabilities of faults, prevailing weather

• Computer analyses that are difficult to set up or interpret• Lack of clarity on requirementsLack of clarity on requirements• Scope for variation in interpretation• Possibility of post-event litigation

What makes a generator unhappy?g ppy

• Possibility of pole slipping• Frequent unexpected tripping• Undamped oscillations

L f t th k t• Loss of access to the market

What makes a consumer unhappy?ppy

Loss of supply• Unhappiness depends on the nature of loss of supply

– Short duration loss of supplyF t h t d ti l f l– Frequent short duration loss of supply

– Long duration loss of supply

What makes a politician unhappy?

Standard reliability indicesy

• How to take account demand side management?– ‘Authorised’ interruptions of partial interruptions

• Weighting of long interruptions versus short onesA hid th d l i t– Averages can hide the underlying story

– (Who is responsible for recording restoration time?)• Weighting of widespread interruptions versus local ones• Weighting of widespread interruptions versus local ones

– Averages can hide the underlying story• Roger’s point: voltage dips can be as important as g p g p p

disconnections– What is the voltage dependency of load anyway?

• What does 99.9999% really mean?• (New CIGRE C1 WG on reliability indices: contact Keith)

What makes an operator unhappy?• “Complexity”• Possible failure of corrective measures

p ppy

– Corrective measures do not take place or prove to be insufficient• Operators might err on side of caution: schedule more action than needed

• Rapidly changing initial conditionsWhat are the weather conditions in different parts of the system?– What are the weather conditions in different parts of the system? How quickly are they changing?

• Possibility of multiple trips– Near simultaneous loss of many generators– Cascading of transmission outages

• System instability– Oscillatory, voltage or transient

L k f li bl ft d/ l k f li bl d t• Lack of reliable software and/or lack of reliable data– e.g. ratings, the actual stability limit, probabilities of faults, prevailing weather

• Computer analyses that are difficult to set up or interpret• Lack of clarity on requirementsLack of clarity on requirements• Scope for variation in interpretation• Possibility of post-event litigation

Standards and measureable performanceagainst those standards are important

Risks and consequencesq

K t ki d t di f ‘ it ’

Successful system operation depends on prediction of the effects of disturbances and accurate/reliable quantification of power transfer constraints

Disturbance withini i t t d

Key to making progress on new understandings of ‘security’Risk assessment: it’s all relative…

Disturbance withinlocal system

main interconnectedsystem

‘Easy’ to quantify Very difficult to quantifyImport ofpower

Easy to quantify failure modes &

loss of supply effects

Very difficult to quantify failure modes &

loss of supply effects

Export of‘Easy’ to quantify failure modes &

A bit tricky to quantify failure modes &

power economic effectsof curtailment

economic effectsof curtailment

Trying to understand the risksy g

Generate random sample covering all situations of interest and credible initial disturbances

unacceptable

and credible initial disturbances

For each situation and disturbance,determine the final system state

ram

eter

p2 unacceptable

Determine system limits by distinguishing between acceptable and unacceptable situations

pera

ting

par and unacceptable situations

Explain the distinctionbetween acceptable

Op

acceptable

pand unacceptable

Determine control requirementsOperating parameter p1

and operational limits fromexplanation of distinction

Software requirementsq• Provide easy definition of data variations• Reproduce operators’ despatch decisions

Basic data

St ti ti l l i• Reproduce operators despatch decisions– accurate determination of initial state

• Compute system responses accuratelymodel equipment adequately

Random sampling• availability• load level• events, ...

Initial point determination• MW despatch

Scenarios

Statistical analysis& data mining toolbox

Descriptive• distributions• correlations• ...

Decisional• decision trees

i– model equipment adequately– access to quasi steady state

simulation• Manage large quantity of data

MW despatch• voltage targets• shunts, ...

Power system simulation‘ Astre’‘Eurostag’

DatabaseInitialsystemstate

• regressions • ...

‘Knowledge’• security rules• risk assessment• Manage large quantity of data

– input variations (variants and events)– results

• Provide access to powerful tools for

Systemresponse

• ...

• Provide access to powerful tools for interpretation of results

Can we guarantee continuity of supply?Can we guarantee continuity of supply?No, so get defence measures in place to limit impact• See, for example, CIGRE WG C1.17

Summing upg p• ‘Adaptive’ security standards (more risk based) can make more network

capacity available at different timescapacity available at different times– We need to be sure

• that standards are relaxed only at the ‘right’ times• that we have a fall-back position to limit adverse impactsp p

• New technologies promise significant enhancement of grid power transfer capability…

• … but they are not without their issues and complexitiesy p• To be (reasonably) confident of not messing up system behaviour in a

big way, analysis and testing are required• Steady state analyses are necessary but not sufficient

– e.g. ELSI gives a flavour of an initial cost-benefit analysis• We need dynamic simulations of lots of different operational scenarios

– ‘adequate’ models of wind farms and power electronic converters are essential

‘Supersmart’ gridsp g

• Not just MENSA level but IMMENSA…• ‘Supersmart’ grids depend not just on local ‘intelligence’ but

‘coordinated intelligence’…• and that depends on comms• …and that depends on comms

100 single circuit faults2 double circuit faults

6 concurrent single circuit faults

5 cable faults

4 busbar faults

10 transformer faults

Should we beworried about this?

2000 protection or2000 protection orcommunication failures 20 circuit breaker faults

Graphic: National Grid, 2003

Action• Some clever power system risk analysis software already exists – see,

for example, IEEE TF on Cascading Outages– Much of it depends on heuristics: are they proven?p y p– Are tools based on steady state analyses adequate?

• What can you really do with the results?– Good data available? How to turn data into information?

• We could develop cleverer software that is really useful in operational timescales– Great for researchers to research

• But, can we deliver?• Competition (among vendors or among research groups) v collaboration

• In the meantime, better to concentrate on the things a competent utility h ld b d i ?should be doing?– Construct decent models of existing and planned plant– Make state-of-the-art software available to engineers

• Teach engineers to use it– Ensure that the system is visible– Develop better rules of thumb