NPTI CONFERENCEChallenges & Issues in Grid Integration & Stability

11-01-2019

Greening the Grid -Solar & Wind Integration, Issues, Challenges and

Case Studies

ERLDC, POSOCO

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Wind Potential ~ 100 GW(at 80 m hub- height)

Solar Potential ~ 750 GW(4-7 kWh per sq. m per day)

National Offshore Wind Energy Policy

Solar Cities~ 60 Nos.Green Energy Corridors

Renewables CAGR~ 18 %

Installed RE Capacity – 72013 MWas on 31st Oct’18

34 Solar Parks – 20 GW

Solar RPO ~ 8 % of total generation by 2022 (Excluding Hydro)

Solar PV Reverse Bidding ~ Rs. 2.44/kWhr

Wind Atlas, 2015

Renewables on the Rise…MW to GW

Wind Reverse Auction ~ Rs 2.43 / kWhr

(21%)

(7%)

(13%)

(57%)

(2%)

Renewable in Indian Power System

Region Installed Capacity (MW)Northern Region 13634Southern Region 35971Western Region 21024Eastern Region 1084

North Eastern region 287Islands 13

All India 72013Source : CEA Oct’18

Eastern Region Renewable Target 2022: 12646 MW (12237 MW Solar; 135 Small Hydro & 244 MW Biomass )

Source : Niti Ayog report

Challenges in integrating 175 GW RE by 2022

Technical Standards for

Grid Connectivity

Forecasting Scheduling Metering

Accounting Settlement

Frequency & Voltage Control,

Congestion

Flexibility

Market Design

Visibility at control centres

Intermittency & Variability

Uncertainty

Location Specificity

Characteristics

Renewable : Dependency on Various Factors

Variability of Renewable PowerNon variable renewable energy generation refers to sources ofelectricity that can be generated at the request of power gridoperators or of the plant owner. Since wind power and solarpower cannot be controlled by operators, so these are termed asVariable Renewable Energy (VRE) sources.

Lesser Variability

High Variability

Renewable : Variability

RES injection coinciding with

Morning Demand ramp

RES injectionopposite of

Morning Demand ramp

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Renewable : Ramp Variability

RE Impact on Net Load

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Peak load = 7 MWSolar generation = 10 MW

(A)Solar increasing

Import decreasing

A

(B)Solar decreasing

Import increasing

B

Diu Island

Effect of aggregation on variability

One Wind Turbine

100 Wind Turbines

One PV plant

18 PV plants

Forecasting RE generation

• A prime necessity to minimize deviations betweenschedule and actual dispatch

• Different approaches are preferable for differingtime frames to produce the best forecast

• Most accurate forecasts can be obtained by usingmany local and global scale models and combiningthem to form a single multi model ensemble.

• Indian Meteorological Department (IMD) hassignificant resources and experience in traditionalweather forecasting

Two main approaches• Statistical approaches working on single Numerical

Weather Prediction (NWP) forecasts• Uncertainties derived from ensembles of predictions

Physical wind power forecasting• derive wind speeds at turbine hub height from the NWP

model and use explicit descriptions of relevant physicalprocesses to calculate the electric power output of theturbine

Statistical wind power forecasting• describe the relationship between various model

parameter of the weather model and the measuredpower output

• Based on past data analysis, following clustersare formed for integrated scheduling andforecasting of wind generation for high windpenetration states

State Area/zone wise Wind generation passes

Tamil Nadu Muppandal, Udumalpete, Theni, Thirunelveli

Karnataka Chitradurga, Chikkodi, Gadag

Rajasthan Udaipur, Jaisalmer, Jodhpur

Gujarat Kutch, Saurashtra

Maharashtra Sangli, Ahmednagar, Satara

• Time series models using on-site measurementsare adequate for the very short term time scalefrom minutes up to a few hours

• Forecasts based on cloud motion vectors fromsatellite images show a good performance for atemporal range of 30 minutes to 6 hours

• Grid connected bulk solar power generationrequires forecasts up to two days ahead or evenbeyond

• Numerical weather prediction (NWP) modelsare considered to be most appropriate

Challenges in RE Forecast

• Dependency on Accuracy of Weather Forecast.• Continuous Improvement in Forecast and Confidence level.• Number of Forecasting Revision is fixed : Sufficient Time for

taking action in Short Term Forecast.• Ramp Forecast for meeting the Ramping requirement :

Reducing Deviation, Absorbing RE and Optimizing OtherEnergy Sources.

• Turbulent & chaotic processes : Localized Wind/Rainfalls• Including the impact of local topography : Not captured in

standard weather model• Change in Method with Change in Time of Forecast• Continuous Assessment : Which one is Best Forecast

Forecast Error Impact on Reliability and Economy

Handling Renewable Generation

- Provisions in IEGC & CEA Connectivity Standards

Special provisions for renewable sources

MUST RUN STATIONS– Run-of-river power stations - Controlling may lead to

spillage– All renewable energy power plants, whose tariff is

determined by the CERC– Shall not be subjected to ‘merit order despatch’

principles.EXCLUSION

– Biomass power plants, , and non-fossil fuel basedcogeneration plants (dispatchable)

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Scheduling of solar generation• Scheduling compulsory with effect from 1.7.2011, for

new solar generating plants with capacity of 5 MW andabove connected at connection point of 33 KV level andabove and , where pooling stations commissionedafter 03.05.2010

• Schedule to be given by the generator based onavailability of the generator, weather forecasting, solarinsolation, season and normal solar generation curve

• To be vetted by the RLDC in which the generator islocated and incorporated in the inter-state schedule.

• If RLDC is of the opinion that the schedule is notrealistic , it may ask the solar generator to modify theschedule.

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Wind generation Scheduling

With effect from 15.07.2013 Scheduling of Wind Gen.necessary:

i) where the sum of generation capacity of such plantsconnected at the connection point to the transmission ordistribution system is 10 MW and aboveii) and connection point is 33 KV and above,iii) and where pooling stations commissioned after03.05.2010

For capacity and voltage level below this, as well as for oldwind farms ( A wind farm is collection of wind turbinegenerators that are connected to a common connectionpoint) it could be mutually decided between the WindGenerator and the transmission or distribution utility, as thecase may be, if there is no existing contractual agreement tothe contrary .

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There may be one revision for each time slot of one and halfhours starting from 00:00 hours of a particular day subject tomaximum of 16 revisions during the day.

Such revisions shall be effective from 4th time-block, the firstbeing the time-block in which notice was given.

Deviations from schedule by solar and wind generators to bedelinked from frequency and shall be accounted for and settled inaccordance with the provisions of CERC DSM Regulations

Schedule Revision, treatment of deviation

General connectivity standards• B1. Requirements with respect to Harmonics, Direct

Current (DC) Injection and Flicker• (1) Harmonic current injections from a generating station

shall not exceed the limits specified in IEEE Standard 519.• (2) The Generating station shall not inject DC current

greater than 0.5 % of the full rated output at theinterconnection point.

• (3) The generating station shall not introduce flickerbeyond the limits specified in IEC 61000. Provided that thestandards for flicker will come into effect from 1st April2014.

• (4) Measurement of harmonic content, DC injection andflicker shall be done at least once in a year in presence ofthe parties concerned and the indicative date for the sameshall be mentioned in the connection agreement

General connectivity standards• (1) The generating station shall be capable of

supplying dynamically varying reactive powersupport so as to maintain power factor within thelimits of 0.95 lagging to 0.95 leading.

• (2) The generating units shall be capable ofoperating in the frequency range of 47.5 Hz to 52 Hzand shall be able to deliver rated output in thefrequency range of 49.5 Hz to 50.5 Hz. Provided thatabove performance shall be achieved with voltagevariation of up to ± 5% subject to availability ofcommensurate wind speed in case of windgenerating stations and solar insolation in case ofsolar generating stations.

Fault Ride Through (FRT) capability

• VT/Vn is the ratio of the actual voltage to the nominal system voltage atthe interconnection point during the voltage dip

• the individual wind generating units in the generating station shallgenerate active power in proportion to the retained voltage;

• during the voltage dip, the generating station shall maximize supply ofreactive current till the time voltage starts recovering or for 300 ms, whichever time is lower.

(3) All wind farms connected at 66 kV and above shall remain connected to the grid when voltage at the interconnection point on any or all phases dips upto 15% of the rated (nominal) voltage:

Instances of > 800 MW wind generationloss due to absence of LVRT capabiltiy

General connectivity standards(4) Wind generating station connected at voltage levelof 66 kV and above shall have facility to control activepower injection in accordance with a set point, whichshall be capable of being revised based on thedirections of the appropriate Load Despatch Centre.As far as possible, reduction in active power shall bedone without shutting down an operational generatingunit and with reduction being shared by all theoperational generating units pro-rata to their capacity.(5) The standards in respect of the switchyardassociated with the generating stations shall be inaccordance with the provisions specified in respect of'Sub-stations' under Part III of connectivity regulations.

Flexibility

Flexibility for the 21st century power system

Symptoms of power system inflexibility– Balancing generation and load difficult leading to

frequency excursions or dropped load– Significant renewable energy curtailments; mostly

due to excess supply and/or transmissionconstraints

– Area balance violations– High price volatility

Need for Flexibility

• Large 660 MW, 800 MW and 1000 MW sets being introduced• Changing load profile, Flexible conventional generation needed to facilitate

integration of renewables• CEA Standard Technical Features for 660MW/800 MW Sets

• Technical minimum recommended – 40%• Two shifting of machines envisaged

Need of More Ramping (Flexibility) from Hydro, Thermal and Gas to Absorb 170 GW Renewable.

Renewable : Ramping Support Requirement

Net load curves for Low, Intermediate and High RE penetration scenarios

G-t-G Study : 2021-22 conditions

Ramp up and Ramp down requirements in each of the three RE penetration scenarios

Scenario Ramp-down requirement (GW) (8AM-1PM)

Ramp Up Requirement (GW) (2PM-9PM)

Low RE Scenario 63 101

Intermediate RE Scenario

77 115

High RE Scenario 96 134

G-t-G Study : 2021-22 conditions

More Spinning Reserve to Meet Variability of Demand and Renewable Energy

Renewable : Spinning Reserve Requirement

Impact on System Inertia

Reduction of power system inertia• The increasing penetration of wind energy in the power

grid will effectively reduce the overall system inertia andincrease the equivalent system droop

• The change in these two vital system parameters willadversely affect the frequency regulation capability of thenetwork, leading to increased peaks and dips in frequencyafter every load events

• If the WTGs provide no inertial and droop capabilities intimes of low system frequency, the robustness of thesystem will be in danger

• If variable speed WTGs are to be used for integrating windenergy into the power grid, the hidden inertia and droopcapabilities from these machines can be extracted

Synthetic Inertia• Synthetic inertia is achieved by reprogramming power

inverters attached to wind turbines so that they emulate thebehavior of synchronized spinning masses

• intelligent, distributed response of an entire wind plant to afrequency disturbance could potentially minimize overall gridfrequency deviation and improve recovery speed.

• The rotating mass of variable-speed wind turbines is decoupledfrom the grid frequency and does not inherently exhibit aninertial response unless controlled for that specific purpose.

• the kinetical energy of the rotating blades could be used for asynthetic inertia control

• The controlling methods could be implemented as anadditional feedback loop for the converter

• If the frequency is lower than the nominal, the controllingextracts additional energy with torque increase, which slowsthe rotation of the blades

Control Concept• Use controls to extract stored inertial energy• Provide incremental arresting energy (meaning kW-s)

(5%-10% of rated power) during the first 10 seconds ofgrid events.

• Allow time for governors and other controls to act• Target incremental energy similar to that provided by a

synchronous turbine-generator with inertia (Hconstant) of 3.5 pu-sec.

• Focus on functional behavior and grid response: doNOT try to exactly replicate synchronous machinebehavior

• The technical constraints and limits like the rotationspeed and the converters scale must not be exceeded.

• Demand for additional torque should reduce as theWTG approaches its minimum allowable speed

Balancing Capability Enhancement

• Primary objective is to avoid frequency deviations and unscheduled power flows arising out of RE integration

• Avenues• Short Term - improvement in load forecasting• Improving accuracy of RE generation forecasting• Enhancing system operation and plant flexibility• Medium Term - intraday balancing with storage

type hydro plants

Balancing Capability Enhancement

• Enhancing geographical size of control areas thatare to be balanced

• Development of large scale pumped storage typehydro-electric plants

• Introduction of demand side management• Long Term – Increasing geographical dispersion

of wind generators• Power storage options, technology improvement

Suggestions

Primary•All India - 4000 MW

•Outage of Ultra Mega Power Plant (UMPP) or any similar event

• Automatic, Decentralized, mandated as per Grid Code

Secondary• All India - 3623 MW

•NR – 800 MW•WR – 800 MW•SR – 1000 MW•ER – 660 MW•NER – 363 MW

• Automatic, regional level with suitable compensation

Tertiary

• All India - 5218 MW•NR – 1658 MW•WR – 1353 MW•SR – 1343 MW•ER – 857 MW•NER – 65 MW

• Manual, state level• UnRequisitioned Surplus

Adequate Reserves and Ancillary Services Market

Higher number of units should on Bar….for handling large and fast ramps

Need of Adequate PrimaryResponse from generators(Presently under RGMOand inadequate responseobserved)

Need of Pan IndiaOperation of AGC toprovide Secondary Controlfor balancing (Pilot Projectin Northern RegionCompleted)

Requirement of adoptionof RRAS at State level tohave adequate TertiaryControl.

Faster markets and shorter settlement period

Airlines Banking

May 08, 2017National Electronic Funds Transfer

(NEFT) system – Settlement at half-hourly intervals

Railways

Data Time-Interval Annual Cost*

1 Minute Rs. 13,20,000

2 Minutes Rs. 7,50,000

5 minutes Rs. 2,75,000

15 Minutes Rs. 60,000

Stock Exchanges

*NSE Annual Data Charges, Capital Market Segment

Petroleum

5-minute Scheduling vs. 15-minute Scheduling

Strong Interconnections, SAMAST and enabling framework would enhance RES absorption by

taking advantage of diversity

Need of More Number of PSP for Flexibility Support

A Few Significant Steps

Renewable Energy Management Centres

NLDC, Delhi

NRLDC, Delhi

WRLDC, Mumbai

SRLDC, Bengaluru

Andhra Pradesh

Tamil Nadu

Karnataka

GujaratMadhya Pradesh

Maharashtra

Rajasthan

• Co-located with Load Despatch Centres (LDCs) –State/Regional/National (11 Nos.)

• Forecasting , Real time tracking of generation from renewable sources

• Advanced decision-making and control systems

• Single source information repository and coordination point

• Different skill sets for personnel manning REMCs and LDCs

• Implementation under progress

Dedicated IMD Portal for Power Sector

Intra-State Settlement System and Imbalance

Handling

Forecasting (Load/ RE generation/ Net Load) Spinning Reserves

Frequency Control (Primary/Secondary/

Tertiary)

Technical Standards for RE Generation Ancillary Services

Balanced Portfolio Flexibility - Harnessing and Incentivizing

Market Design Enhancements

Communication in Power Sector

Capacity Building of all stakeholders

Web-Link: http://powermin.nic.in/sites/default/files/uploads/Final_Consolidated_Report_RE_Technical_Committee.pdf

Technical Committee on Integration of RE

Greening the Grid National Study – Key Points

https://posoco.in/reports/india-renewable-integration-study-report/

Collaboration of Ministry of Power, USAID, CEA, CTU,

POSOCO and state utilities

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https://posoco.in/download/fold-posoco-report-on-operational-analysis-for-optimization-of-hydro-resources/?wpdmdl=14168

Operational Analysis for Optimization of Hydro Resources

Key Recommendations• Optimization & Incentives for Flexibility• Ancillary Services from hydropower

(Regulated tariff ISGSs already within the ambit of FRAS)

• Coordinated Scheduling & Despatch Transmission Planning impacting Hydro Flexibility

• Multi Part Tariff• Silt Forecasting & Coordinated Flushing • Inflow Forecasting• Review of Standards• Revisiting Hydrological Constraints• Renovation & Modernization

Summarizing…..• Integration of wind and solar plants in large scale is the only

way to achieve sustainability and energy security• Accuracy of forecasting VRE sources and state load forecasting

has to improve• Proper scheduling, accounting and settlement practices at

intra-state level need urgent implementation• Country-wide operationalization of primary and secondary

control as early as possible. (Tertiary control in the form ofRRAS is already in place)

• Rejuvenation of existing pump-storage capabilities andjudicious utilization of hydro flexibility

• Incentivizing flexible operation of existing coal fired powerstations. (CERC regulated generators are already compensatedfor backing down generation, as per the provisions of IEGC)

• Improvement of real-time data availability from RE stations

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“Engineers like to operate sophisticated power systems,economists like to think about optimal incentives, andlawyers like to write rules and agreements. Power sectorreform brings all of them into close contact. But none ofthem can succeed at their chosen tasks unless they worktogether in designing sustainable institutions.”

- World Bank Report: Transmission System Operators – Lessons From The Frontlines- Beatriz Arizu, William H. Dunn Jr. and Bernard Tenenbaum