All Island TSO Facilitation of Renewable Studies- Ireland II

Ravi SundariaJay Panchal

Contents

1. Background of study2. Methodology and key assumption3. Key results4. Impacts of operational strategies.5. Recommendation to Future work6. Conclusion7. Summary 8. Observations

Background

• Future system Government in Ireland has set a binding electricity target of 40% from renewable resources by 2020

•Base study- implications for the real time operation of the power system studied in All Island Grid Study [DCENR (2008)]

• Objective -technical and operational implications associated with high shares of wind power

Methodology and Assumptions

EIRgrid and SONI (2009 ) provide requirement for voltage frequency stability

Wind Input data Years 2007-2009 (15 min interval)

63 dispatch cases are considered

Peak wind generation 7550 MWs 90 % of installed capacity

2020 generation portfolio from AIGS

Installed – 2210 new OCGT for high flexibility however committed is used for technical studiesBase Renewable – biomass and tidalOther Renewable- run of river hydro power plantsStorage- pumped hydro power plants

Modeling for Stability studies

Tools

1. Siemens power system simulation package-• fault and stability analysis

2. System frequency model • frequency response and regulation

Rate of change of frequency relay (ROCOFs)

Different wind turbine models• Doubly Fed Induction generators• Full converter wind turbine generators

Key ResultsClassification of issues

Operational metrics

1. Issues that imposes fundamental operational limit

Loss of largest infeed

Issue- Frequency instability

•Change in active power Less load- high f, more load – low f

•Load shedding -0.7 hz Curtailment of wind power - +0.25 hz

•Kinetic energy in rotating mass= inertia

• inertia opposes the frequency change

Result 1 Cont.

Result 1 Cont.

Result 1 Cont.

Technical mitigation measures

•Synchronous compensator mode

• Emulated inertia from wind turbines

Result 1 Cont.

2. Issues may impose operational limits but need further analysis

Issue-

• Frequency excursion due to network faults

• Wind turbine generators need more time to restore active power than conventional generators

• Grid code – 90% of active power within 1 sec of voltage recovery

Result 2 Cont.

Result 2 Cont.

• On ROCOF 0.6 hz /s operational metric 1> 40..50 % may be unstable

Technical mitigation measures• greater curtailment of wind•Frequency protection trip setting

Further study required in•Inertial response of wing turbine generators•Modern wind turbine can recover active power faster than grid code

Result 2 Cont.

3. Issues that impose operational limits but can be mitigated

Issue- • Voltage stability • Due to change in load & supply (wind power supply)• Due to reactive power injection ( a local phenomena)

Result 3 Cont.

Technical mitigation

• Transmission connected windfarm should comply gridcodes

•Placement of reactive power sources

•Must run conventional units and there location

Result 3 Cont.

Transient Stability

• Issues:

‘Ability of grid to maintain synchronism when subjected to a severe transient disturbance’

40KV Generator Subjected to fault for two different duration:- 100ms (Clearing time)- 150ms

Result 3 Cont.

Issues: Cont…

• Issues dealing with Dynamic & transient stability analysis- loss of generation, loss of load- transmission system faults- Calculation of critical clearance times (CCTs)

• Results indicate:- high levels of wind lead to smaller CCT (i.e. less transiently stable)- Provision of reactive power during faults improves transient stability- recovery of active power post-fault impacts on transient stability

Result 3 Cont.

Disturbances with critical clearance times as indicated in the legend as a function of “operational metric 1”.

Result :The results suggest that the transient stability of the 2020 All Island Power System decreases significantly if the value of “operational metric 1” exceeds 70%...80%.

Issues: Cont…Result 3 Cont.

• Technical Measures :Technical Mitigation Measure was done by evaluating transient analysis of system for one dispatch.Mitigation measures on the number of faults that requiring a certain critical clearance time.

Result 3 Cont.

• Issues

- Behaviour of the load is regular and well understood.

- Generation follows this pattern balancing the load in all time domains.

- But wind pattern is not correlated with the load and not as cyclic.

Power balance fluctuations and frequency regulation

Result 3 Cont.

Load changes and wind ramps during the week. On Monday the load increase in the morning coincides with a decrease of wind generation

Issues: Cont…Result 3 Cont.

• Results:

- The statistical analysis derived distributions and confidence intervals for the change of 2020 wind generation and net load for various time frames. (2 years of data was used for analysing wind generation and net load variation for 2020.

Upper and lower control requirement :

Result 3 Cont.

Technical mitigation measures :

• In case of extreme positive ramps of generation, balancing of the system can be supported by curtailing wind farms. To a certain extent demand response may help to reduce power gradients

Result 3 Cont.

Network Loading Analysis

• Network loading analysis - Investigated various wind patterns in five regions.Results show: - >110kV transmission system not overloaded - 110kV transmission system may overload - 110kV “bottlenecks” occur in same areas - ‘optimal use’ of power system

(with lowest network loadings) were seen for 25% wind

Issues:- Network congestions in rural area- Wind generation implemented at

remote locations changes power flows

Result 3 Cont.

W 0% W 25%

W 50% W 100%

Loading of transmission network for the base case (35% of wind farms connected to the transmission system) at winter maximum load and 0%, 25%, 50% and 100% instantaneous wind power.

Network at voltage level 110kV

Results : Cont…

The results suggest that even at high wind levels the network above 110kV is only lightly loaded.

Networks at voltageslevels >110kV

Results : Cont…

• Technical mitigation measures :

- Reinforcement of the 110kV network- Development of dedicated 220kV transmission- Temporary curtailment of wind farms connected to the 110 kV

network during high wind periods.

Result 3 Cont.

4. Issues that imposed no problem to the model

• Issues:- Fault Levels- Small Signal Stability

Small-Signal Stability

Issues:

• Ability of the All Island Power System to maintain synchronism when subjected to small disturbances

• Potential instability phenomena as a steady increase in generator rotor angle due to lack of synchronizing torque have not been considered.

• Lead to increase system losses and may lead to tripping of units.

• Dynamic system behaviour may change

Result 4 Cont…

Results:

- Qualitative conclusions only- Increase in wind improves the system damping

Model results do not suggest any problem with small signal stability with increasing wind power in the All Island Power System.

Because addition of wind doesn’t add to system impedance (As its power flow is controlled by power electronics hence, analysis of mitigation measures was obsolete.

Result 4 Cont…

Fault Levels

• Fault levels quantify the maximum currents equipment may be exposed to in case of a short circuit.

• Effects on system:- Equipment Rating- Changes in network parameter.- Protection Schemes- Wind power may affect short circuit levels

Issues :

Result 4 Cont…

Results:

• Short-circuit calculations were carried out according to UK Engineering Recommendation G74 [National Grid(2008)]

• Assumed break times were about 50-80ms depending on voltage level.

Result 4 Cont…

• For three-phase or single-phase-to-earth faults, the planned maximum short circuit fault levels shall not be greater than the values indicated in Table

Planned maximum short circuit fault levels [ESB National Grid (1998)]

Observed minimum short circuit fault levels (Exception)

Results: Cont…Result 4 Cont…

- For high wind penetration, short circuit levels can decrease down to 50%.- Short circuit levels are unlikely to drop below the minimum levels that are experienced without wind power (During Summers).- The short circuit level of some buses close to large wind farms but remote from conventional generation increases with wind generation.

Observation from Modelled results :

Results: Cont…Result 4 Cont…

Results from short circuit studies for three-phase faults at selected bus bars for the base case:

Results: Cont…Result 4 Cont…

Results from short circuit studies for single-phase faults at selected busbars for the base case

Result 4 Cont…

Technical mitigation measures:

Measures Targeting problem of critically low short circuit levels:- Define “must run units”. They shall be kept running at there low active power limit.- Presence of Static Var compensator (SVC) and Flexible AC Transmission Device (FACTs). This study already envisaged their presence in network.- Upgrading line between low short level regions and high short circuit level regions.- Increasing short circuit capabilities of Wind turbine, can enhance system SC level but it is not feasible over other alternative, As it involve lot of money.

Result 4 Cont…

5. Impact of the operational strategy

• In an unconstraint situation, the current scenario facilitates a renewable

energy share of nearly 50% in the annual load. ˃ 40% target set by IRISH government in 2008

• Proposed limiting operational metric 1 for instantaneous inertia less generation 60…80%.

• Above this value, wind generation units need curtailment.

Estimated total wind energy share in annual load coverage due to curtailment for different maximum allowable instantaneous wind penetrations

Estimated lost wind yield due to curtailment for different maximum allowable instantaneous wind penetrations

6.Recommendations for Future Work

System Planning and Operation:• Improved system & equipment modelling by evolving Dynamic system

operational metrics, possibly more sophisticated than “operational metric 1.• Allowing more quantitative & refined results and reduced uncertainties.

Further Extension to Knowledge base:• Cost effective methods to verify and monitor grid code compliance to

completely deploy the operational range of the 2020.• with particular focus on voltage & reactive power issues

Anticipating future Developments:• Monitoring of actual system performance & feedback to modelling

7. Conclusion Increasing wind power penetration

results in:Dominating issues:- Frequency response to disturbances- Transient stability *Significantly worse system frequency response & stability.

• Less transiently stable system, reduced CCT’s• Potential 110kV overloads• Issues with voltage control & reactive power provision• Mainly adequate system reserves & flexibility• Improved small-signal stability/damping• Reduced short-circuit levels

“But main issues can be mitigated by optimised operational metrics-1values”

8. Summary•2020 policy targets aiming at 40% electricity from renewables is achievable (6,000MW peak wind generation)

•Two dominating issues frequency stability after loss of generationfrequency as well as transient stability after severe network faults

• Maximum limit of wind generation and import is 60-80% for fundamental issues that need further analysis Additional adaptation needed (ROCOF relay, reactive power sources)

•50% if ROCOF relays at distribution connected wind farms and other generators were not disabled

9. Observations

-Different wind turbine models are considered

-Study helps the future policy formation for renewables

-Grid codes are not followed in case of rocofs where allowed 0.5 hz whereas 0.6 Hz has considered

-Other renewable sources are not considered

-FACTS / SVC devices inclusion