Grid Integration

Renewable Energy Sources (RES) Grid Integration: Key Issues
and Challenges
Bharat Singh
Indian Institute of Technology Mandi
Introduction
• The world needs energy innovation
• Problem : Security of supply, climate change and
sustainability
• Solution : Transitions to clean, reliable and affordable energy
• Methods : Renewable energy, energy savings and clean use of
fossil fuels
• Wind power/Solar power
• Proved as a potential source for generation of electricity
• Minimal environmental impact
• Fastest-growing source and expected to remain in future
• Modern wind/solar farms
• Can capture several MWs of power (7 MW largest wind
turbine)
• Can supplement the base power
Key Drivers
Towards Sustainability in Electric Energy
Renewable Purchase Obligation (RPO)
-Solar & non-solar
-Issued by States (SERC & CERC)
-Obligated by State power utilities
*RPO is the minimum amount of renewable energy to be purchased by the States
in order to meet the mandatory electric energy requirement.
Conventional power plants
Large synchronous machines
Provides primary &secondary control
Meet the specific grid connection requirements (GCR)
With increased penetration of wind/solar power
Mostly converter interfaced small asynchronous generators/ generations,
Poses new challenges in maintaining reliability and stability of electricity supply,
Advanced GCR are required to be developed for efficient, stable and secure
operation of grid ,
Loss of generation cannot be tolerable,
Need to perform suitable studies to analyze the interaction of wind/solar power
with existing grid,
Need to develop several solutions to improve and mitigate negative consequences
on the existing grid, if any.
Intermittency (reliability, stability etc.) and High Cost
With high penetration of RES
Outline
Grid connection issues
Economic issues
System Operational Aspects
Reasons for Renewable Energy in India
why and why now??
DecliningFossilFuelSupplies
Environmental*Concerns(Kyoto Protocol,1997)
IncreasingCostofFossilFuels
BusinessOpportunities
EnergySecurity
Energyindependence
* 1 MW of wind plant in one year can displace 1500 tons of CO2, 6.5 tons
of SO2 and 3.2 tons of NOx. (REPP report, Washington July 2003)
Introduction
Huge power shortages in the range of 3% to 21% in
States with national average energy shortage of about
10.3%and peak demand shortage of 15.4%.
Renewable energy technologies have a good potential
in India.
Substantial efforts have been made to harness the
non-conventional and renewable sources of energy
during the last three decades.
The vision of Indian renewable energy programis
“to develop new and renewable energy technologies, processes,
materials, components, sub-systems, products and services at par with
international specifications, standards and performance parameters in
order to make the country a net foreign exchange earner in the sector
and deploy such indigenously developed and/or manufactured products
and services in furtherance of the national goal of energy security”.
Introduction
In 1981, the Government of India (GOI) established
a Commission for Additional Sources of Energy
(CASE) under the Department of Science and
Technology (DST).
In 1982, the commission was given full status of a
department called as Department of Non-conventional
Energy Sources (DNES) and put at par with other
energy departments, such as coal and power.
Finally, in 1992, the GOI upgraded the department
to a full-fledged ministry known as Ministry of Non-
conventional Energy Sources (MNES) exclusively
devoted to the renewable energy promotion.
MNES is renamed as Ministry of New and
Renewable Energy (MNRE) in 2006.
8
The MNRE efforts have been identified in following four areas:
Power generation fromrenewable
Grid-interactive renewable power (Wind Power, Small
Hydro Power (SHP), Biomass Power, Urban & Industrial
Waste-to-energy and Solar Power),
Captive/CHP/Distributed Renewable Power (such as
Biomass/Cogeneration, Biomass Gasifier, Waste-to-
energy, Aero-generator/Hybrid Systems)
Rural & Decentralized Energy System (such as
Family-type Biogas Plants, Home Lighting System, Solar
Photovoltaic (SPV)/Thermal Program, Wind Pumps)
Remote Village Electrification
Other programs (such as Energy Parks, Akshay Urja
Shops and Hybrid Vehicles, Geothermal and Tidal)
Introduction
9
Government of India has created conducive
environment for speedy development of RES and to
attract private investors by providing
Subsidy and fiscal incentives,
policies support,
regulatory & legislative framework,
finances, consultancy services,
research design & development,
up-gradation of existing technologies, and,
planning & resource assessment.
Government is a catalyst and facilitator, however, the
implementation is being carried out by the States or by
the private sector.
Several States have so far announced promotional
policies for RES.
Renewable Energy Development in India
10
11
All India Installed Capacity (on 29.02.12)
S.
No.
Fuel Share
(in MW)
Percentage
(%)
1 Total Thermal
Coal
Gas
Oil
124730
105437
18039
2000
65.45
2 Hydro 38848 20.38
3 Renewable* 22233 11.65
4 Nuclear 4780 2.50
Total 190592 100
25%
9%
63%
3%
HYDRO RENEWABLES
THERMAL NUCLEAR
12
Indian Power System - Present
NER
ER
NR
WR
SR A
N
D
A
M
A
N

&
N
I
C
O
B
A
R

L
A
K
S
H
A
D
W
E
E
P
Trans. Grid Comprises:
–765/400kV Lines: 77500 ckt-km
–220/132kV Lines:114600 ckt-km
–HVDC bi-poles : 3 nos.
–HVDC back-to-back : 7 nos.
–FSC – 18 nos.; TCSC – 6 nos.
NER, ER, NR & WR operating as
single grid of 90,000MW
Inter-regional capacity: 14600 MW
New & Renewable Energy Cumulative deployment
of various Renewable Energy Systems/ Devices as on
31/01/2012
Jawaharlal Nehru National Solar Mission (JNNSM): To create an enabling
policy framework for the deployment of 20,000 MW of solar power by 2022.
Centre of Wind Energy Technology (CWET), Chennai, established
in 1982, to serves as the technical focal point for wind power
development.
SolarEnergyCentre(SEC), Gurgaon, establishedin1982,isa
dedicatedunitoftheMNREfordevelopmentofsolarenergy
technologiesandservesasaneffectiveinterfacebetweenthe
Governmentanduserorganizations.
AlternateHydroEnergyCentre(AHEC),Roorkee,was establishedin
1982,topromoteSHPprojects&otherrenewableenergysourcese.g.
biomass,solar,wind,etc.
SardarSwaranSingh- NationalInstituteofRenewableEnergy,
Kaputhala, isbeingestablishedfordevelopmentofbio-energy,
includingbio-fuels,andsyntheticfuels.
IndianRenewableEnergyDevelopmentAgencyLimited(IREDA),
NewDelhi,establishedin1987,promotesandfinancesrenewable
energyandenergyefficiency/conservation.
Specialized Centers/Institutes
14
Jawaharlal Nehru National Solar
Mission
To create an enabling policy framework for the deployment of 20,000
MW of solar power by 2022.
To ramp up capacity of grid-connected solar power generation to
1000 MW within three years – by 2013;
an additional 3000 MW by 2017 through the mandatory use of the
renewable purchase obligation by utilities backed with a preferential
tariff.
This capacity can be more than doubled – reaching 10,000MW
installed power by 2017 or more, based on the enhanced and enabled
international finance and technology transfer.
The ambitious target for 2022 of 20,000 MW or more, will be
dependent on the ‘learning’ of the first two phases, which if
successful, could lead to conditions of grid-competitive solar power.
Jawaharlal Nehru National Solar
Mission
To create favourable conditions for solar manufacturing capability,
particularly solar thermal for indigenous production and market
leadership.
To promote programs for off grid applications, reaching 1000 MW
by 2017 and 2000 MW by 2022.
To achieve 15 million square meters solar thermal collector area by
2017 and 20 million square meters solar thermal collector area by
2022.
To deploy 20 million solar lighting systems for rural areas by 2022.
CERC has announced preferential tariff of Rs. 18.44 per unit for
solar PV power and Rs. 13.45 per unit for solar thermal power for 25
years;
The Electricity Act 2003 has paved the way for setting up of
State Electricity Regulatory Commissions (SERCs) for providing
conducive atmosphere for the rapid development of power
generation including renewable energy.
The National Tariff Policy that was notified by the Ministry of
Power in January 2006, in continuation with the Electricity Act
2003.
The National Electricity Policy 2005 stipulates the progressive
of electricity generation from non-conventional sources. Setting
renewable energy targets and preferential feed-in tariffs for
renewable energy procurement by the respective SERC.
Several SERCs in turn, provided concessional feed-in tariffs
(mostly decided by cost-based approach), wheeling, banking of
energy for future use, third party sale and power evacuation
facilities.
Tariff Policy
17
Preferential Tariffs/Policy Announced by the
SERC’s for Wind/Biomass/SHP
Sources Wind Biomass/Cogeneration SHP
Items
States
Wheeling
Charges
Banking Buy-back (INR/kWh) Wheeling
Charges
Banking Buy-back
(INR/kWh)
Buy-back
(INR/kWh)
Andhra
Pradesh
2% of energy 12 Months 3.37 28.4 % +
INR 0.5 kWh
Allowed at 2%
for 8-12
2.63 2.69
Chhattisgarh -- -- -- 6 % of energy Not Allowed 2.67 --
Gujrat 4% of energy -- 3.37 fixed for 20 yrs. 4 % of energy Allowed 3.0 --
Haryana 2% of energy Allowed 4.08 + Escalation 1.5% 2 % of energy Allowed 4.0 2.25
HP -- -- -- -- -- -- 2.50
Karnataka 2% of energy 2% /Month
for 12
Month
3.40 fixed for 10 years 5% surcharge Allowed at 2%
INR 1.13/kWh
2.27 2.90
Kerala 5% of energy 9 Months
(Jun.–Feb.)
3.14 fixed for 20 years 5% of energy Allowed 4 Months 2.80 --
Maharastra 2% of energy
+5%trans.loss
12 Months 3.50 + Escalation of 0.15 for 13
years from documentation of the
project
7% of energy Allowed 3.05 2.25
Madhya
Pradesh
2% of energy Not allowed 3.97 (with decrease of 0.7 upto
4
th
yr ) then fixed at 3.30 from 5
th
yr onwards uniformly for 20
years
-- Allowed 3.33 2.25
Punjab -- -- -- 2% of energy Allowed12
Months
3.59 2.73
Rajasthan 10% of energy 3 Months 3.59 for Jaisalmer, Jodhpur etc.
and 3.67 for other districts
10% of energy Allowed 12
Months
3.60 2.75
Tamil Nadu 5% of energy 5%
12 months
2.90 (Levelised) 2-10% Allowed at 2%
charge
2.73 --
Uttar Pradesh -- -- -- 12.5% of energy Allowed
24 Months
2.67 2.25
West-Bengal INR 0.3/kWh 6 Months To be decided on case to case
with a cap of 4
-- -- -- --
Open access transaction is allowed in all States. Third party sale is allowed in case of wind in all States. RPS is announced in each State
The MNRE has been providing capital subsidy as Central
Financial Assistance (CFA), depending on type and the size of
the plants, and category of institutions and areas to promote
RES.
Incentives/Promotion Policies
S.
No.
Sources/Systems Central Financial Assistance (in INR)
Special Category States Other States
(NE region, , J&K, HP and Uttaranchal)
1 Wind Power 3 × MF 2.5 × MF
2 Biomass Power (Agro- residues) 0.25 × MF 0.20 × MF
3(i) Small Hydro Power (up to 25 MW) 2.2 × MF 1.5 × MF
3(ii) Renovation and Modernization of
SHP
1.125 × MF 0.75 × MF
4 Biomass Power using Advanced
Technologies
1.2 × MF 1.0 × MF
5 Cogeneration Bagasse (Private)
40 bar & above 0.18 × MF 15 × MF
6 Cogeneration Bagasse (Public/joint)
40 bar & above
60 bar & above
80 bar & above
0.40 × MF
0.50 × MF
0.60 × MF
0.40 × MF
0.50× MF
0.60 × MF
7 Waste-to-energy 20% higher than other States (0.50-1.0)×10
7
MF = 10
7
× C
0.646
, C = Capacity of the project in MW
Incentives/Promotion Policies
Item Description
Accelerated
Depreciation
80%depreciation in the first year can be claimed for the following
equipment:
for Wind: Extra 20% after March 2005 for new plant &
machinery
for Cogeneration Systems:
1. Back pressure, pass-out, controlled extraction, extraction–cum-
condensing turbine for co-generation with pressure boilers
2. Vapour absorption refrigeration systems
3. Organic Rankine cycle power systems
4. Low inlet pressures small steam turbines
Tax Ten years tax holidays.
Customs Duty Concessional customs and excise duty exemption for machinery
and components for initial setting up of projects.
Sales Tax Exemption is available in certain States
Besides the CFA, fiscal incentives such as 80% accelerated
depreciation, concessional import duty, excise duty, tax
holiday for several years, etc., are available for RES.
Grid Connection Issues
TechnicalguidelinesandrequirementsforRESarevaryingwith
oneStatetootherStatesandnotgoodenoughforthelargeRES
integrationintothegrid.
InordertopromoteRESandtomaintaincommongriddiscipline
thereisanurgentneedofaspecificandcommongridcodefor
RESinIndia
Wind Power
What is Wind Power
Wind turbines extract the kinetic
energy from the wind and converts
into generator torque.
Generator converts this torque into
electricity and feeds into the grid
Power in the Wind
KineticEnergy=Work=½mV
2
where:m=massofmovingobject
V=velocityofwind
Whatisthemassofmovingair?
=density(ρ)xvolume(Areaxdistance)
=ρxAxd
Power=KineticEnergy/t
=½ρAV
3
V
A
d
Wind Power Status (Jan. 2011)
The total installed wind power capacity is 195 GW
China (45 GW)
USA (40 GW)
Germany (27 GW)
Spain (20 GW)
India (14 GW)
Estimated Wind Power Potential in India
State Gross Potential (MW)
Andhra Pradesh 9063
Gujarat 7362
Karnataka 7161
Kerala 1026
Madhya Pradesh 4978
Maharashtra 4519
Orissa 1520
Rajasthan 6672
Tamil Nadu 4159
West Bengal 32
TOTAL 46092
The wind power potential on macro level based data collected from 10 states
considering only 1% of land availability is around 46092 MW.
Government Support for RES
With different policies such as tax holiday etc
Regulatory & legislative framework
Subsidy and fiscal incentives
Finances and consultancy services
Research design & development
Planning & resource assessment
Indian Scenario for Wind power
Wind potential 46 GW, WT manufacturers claim double
Concentration in few regions only
Installed capacity has exceeded 10% in many states and increasing with high
rate
Specific nationally harmonised GCR for wind power are in urgent need
Existing Situation
Each State has different requirements for grid integration
Mostly power factor is specified & maintained at PCC
PCC is highly disputed among states and wind farm developers
No active grid support requirement is specified
Indian Perspective plan 2022
REPORT OF THE WORKING GROUP on, “NEW AND RENEWABLE ENERGY” for XI
th
FIVE YEAR PLAN (2007-12)
Global Growth of Wind Power
Annual Installed Capacity
Cumulative Installed Capacity
Conventional power plants
Large synchronous machines
Provides primary & secondary control
Meet the specific grid connection requirements (GCR)
With increased penetration of wind power
Mostly converter interfaced small asynchronous generators,
Poses new challenges in maintaining reliability and stability of electricity
supply,
Advanced GCR are required to be developed for efficient, stable and
secure operation of grid ,
Loss of generation cannot be tolerable, wind turbines should remain
connected and actively support the grid,
Need to perform suitable studies to analyze the interaction of wind power
with existing grid,
Need to develop several solutions to improve and mitigate negative
consequences on the existing grid, if any.
INTRODUCTION
WIND ENERGY CONVERSION SCHEME
GENERATOR SYSTEMS FOR WIND TURBINES
Fixed speed
IG
Gear Box
Transformer
Wind Turbine
Soft
Starter
Capacitor Bank
A reactive power compensator is needed to control the voltage
Poor power quality
Gearbox torque is of concern
Robust and maintainece free
Reduced cost
Variable speed converter interfaced generators
Contorlabiltiy over active and reactive
power
High energy efficiecny
Improved Power Quality
More expensive
More complex system with more
sensitive electronic parts
DFIG is most popular scheme
Reduced Power Converter Rating
Reduced Losses
Reduced EMI filters
Reduced cost
GENERATOR SYSTEMS FOR WIND TURBINES
Powerfromawindturbinerotor=C
p
½ρAV
3
• C
p
is called the power coefficient
• C
p
is the percentage of power in the wind that is converted into mechanical
energy
• C
p, max
= 0.5926 (Betz limit)
w
r
v
ω
λ =
Power coefficient varies with tip speed ratio
GENERATOR SYSTEMS FOR WIND TURBINES
Wind turbine power
Maximum power extraction can be realized
with a speed variable system
By measuring the wind velocity and adjust the
turbine rotating speed to keep the power
coefficient C
p
at its maximum value
Grid Code Requirement’s Issued by TSO’s
Country TSO Author Title Issue Year
Denmark Eltra/Elkraft Eltra/Elkraft Regulation TF 3.2.5, Wind turbines connected to grids with voltages above 100 kV 2004
Denmark Eltra/Elkraft Eltra/Elkraft Regulation TF 3.2.5, Wind turbines connected to grids with voltages below 100 kV 2004
Germany E.ON. E.ON. Grid Code High and Extra High Voltage 2006
U.K. NGET NGET Grid Code 2008
Sweden Svenska Kraftnät
(SvK)
Svenska Kraftnät
(SvK)
Affärsverket svenska kraftnäts föreskrifter och allmänna radom driftsäkerhetsteknisk
utformning avproduktionsanläggningar
2005
Ireland EirGrid EirGrid EirGrid Grid Code: WFPS1- Wind Farm Power Station Grid Code Provisions (Ver. 3) 2007
Scotland Scottish Hydro
Electric
Scottish Hydro
Electric
Guidance note for the connection of wind farm 2002
China ALL CEPRI Technical Rules for Connecting Wind Farm to Power System 2005
U.S.A. FERC FERC Order No. 661-A, Interconnection for Wind Energy 2005
Poland PSE PSE INSTRUKCJA RUCHU I EKSPLOATACJI SIECI PRZESYŁOWEJ 2006
Grid Connection Issues/ Grid Codes
Grid connection requirements: Intermittency
• Active power/ Frequency Regulation
• Reactive power/Voltage Control
• Fault-ride through capability and protection
• Power quality (unbalance, harmonics, flicker)
• Scheduling/Dispatch/Forecasting
Grid connectivity or evacuation of energy:
• Remote areas, augmentation of existing power
transmission lines or new lines
• Interconnection point
Point of Common Coupling
Add on
Equipment
PCC
P
INDIAN ELECTRICITY GRID CODE FOR WIND
FARMS (IEGCWF)
IEGCWF must be read in conjunction with
Indian electricity grid code (IEGC),
Technical standards for connectivity to the grid, regulations 2007, and
State electricity grid codes issued by respective states of India.
It must also be emphasized, that all capabilities will not be
exploited in all wind turbines at all times. Connection codes
shall provide the capabilities and characteristics of system
components are available when ever needed for safe and
reliable system operation.
All requirements are to be met at the connection point.
Connection point can be defined as a point in the
transmission network, to which the wind turbine or wind
farmis to be connected.
INDIAN ELECTRICITY GRID CODE FOR WIND
FARMS (IEGCWF)
It is assumed that wind power is fed into the grid when and
where available on priority basis
It is assumed that wind power is remunerated for active
power by fixed price in respective States and for reactive
power according to Section 6.6 of IEGC
The Beneficiary pays for VAr drawl when voltage at the metering
point is below 97%
The Beneficiary gets paid for VAr return when voltage is below 97%
The Beneficiary gets paid for VAr drawl when voltage is above 103%
The Beneficiary pays for VAr return when voltage is above 103%
ACTIVE POWER CONTROL
Active Power Control is a requirement for generating units to be able to
deliver power and remain connected to the network even if the system
frequency deviates from specified one
An adjustable upper limit to the active power production from the wind
farm shall be available whenever the wind farm is in operation
Ramping control
Fast down regulation.
Automatic frequency control
ACTIVE POWER CONTROL
Active Power Control is a requirement for generating units to be able to
deliver power and remain connected to the network even if the system
frequency deviates from specified one
An adjustable upper limit to the active power production from the wind
farm shall be available whenever the wind farm is in operation
The upper limit shall control that the active power production, measured as a 15
minute average value, does not exceed a specified level and the limit shall be
adjustable by remote signals.
It must be possible to set the limit to any value with an accuracy of ±5%, in the
range from 20% to 100% of the wind farm rated power.
Ramping control of active power production must be possible.
Limit on ramping speed of active power production from the wind turbine in
upwards direction to 10 % of rated power per minute.
No requirement for down ramping due to fast wind speed decays, but to limit the
down ramping speed to 10 % of rated power per minute, when the maximum power
output limit is reduced by a control action.
ACTIVE POWER CONTROL
Fast down regulation. It must be possible to regulate the active power
from the wind turbine down from 100% to 20% of rated power in less
than 5 seconds.
This functionality is required for system protection schemes.
Some system protection schemes implemented for stability purposes
require the active power to be restored within short time after down
regulation.
For that reason disconnection of a number of wind turbines within a wind
farm cannot be used to fulfill this requirement.
Frequency control. Automatic control of the wind turbine active
production as a function of the system frequency must be possible.
The control function must be proportional to frequency deviations and
must be provided with a dead-band as shown earlier.
The detailed settings will be provided by the respective State utilities.
Voltage control is a requirement for generating units to supply
lagging/leading reactive power at the grid connection point.
Wind turbine should be capable of supplying a proportion of the
systems reactive capacity, including the dynamic capability and
should contribute to maintain the reactive power balance.
Requirements of the grid codes for reactive power capability
demand that the power factor be maintained in the specified
range.
The wind farm must have adequate reactive capacity to be able to
be operated with zero reactive exchange with the network
measured at the connection point, when the voltage and the
frequency are within normal operation limits.
REACTIVE POWER (OR VOLTAGE)CONTROL
Wind farms are required to balance voltage deviations at the
connection point by adjusting their reactive power exchange and
moreover by setting up predetermined power factors.
Wind Farms shall be capable of operating at rated output for power
factor varying between 0.9 lagging (over-excited) to 0.95 leading (under-
excited).
The above performance shall also be achieved with voltage variation of
± 5% of nominal, frequency variation of ± 3% and combined voltage and
frequency variation of ±5%, as given for conventional generators.
The reactive output of the wind farm must be controllable in one of the
two following control modes according to SU specifications:
The control shall operate automatically and on a continuous basis. The
wind farm shall be able to maintain acceptable small exchange of
reactive power at all active power production levels.
The wind farm must be able to automatically control its reactive power
output as a function of the voltage in the connection point with the
purpose of controlling the voltage.
REACTIVE POWER (OR VOLTAGE)CONTROL
Voltage control requirements
According to the German grid code, the wind turbines must provide, being a
mandatory requirement, voltage support during voltage dips.
Wind turbines have to supply at least 1.0 p.u. reactive current when the voltage falls
below 50%.
A dead band of 10% is introduced to avoid undesirable control actions. However, for
the wind farms connected to the high voltage grid, the continuous voltage control
without dead band is also under consideration.
REACTIVE POWER (OR VOLTAGE)CONTROL
Older WPG based on IG require a reactive power support from the
power systems.
Modern WPG can provide dynamic reactive power capability directly
fromthe power electronics converters
Several TSO required that a wind farm owner supplies a PQ diagram
showing the regulation capability for reactive power of the installation at
the connection point.
REACTIVE POWER (OR VOLTAGE)CONTROL
Capability curve of DFIG
FRT: is the requirement for generating units to revert to
normal operation when fault on power system is cleared.
FRT requirement is imposed on a wind power generator so
that it remains stable and connected to the network during
the network faults.
Disconnection from grid may worsen a critical grid situation
and can threaten the security standards when wind
penetration is high.
FAULT-RIDE THROUGH
Definition of FRT requirements
In Germany, the wind generating plants are
expected to acquit themselves during a low-
voltage disturbance as summarized in a voltage
vs. time curve.
Wind turbines are required to stay on the grid
within areas 1 and 2. If a wind turbine faces
overloads, stability or other kinds of technical
problems in area 2, it can be disconnected itself
from the grid provided a resynchronization can
take place after 2s.
Moreover, it must be able to increase the active
power output following the resynchronization by
gradients of at least 10% of the nominal power
per second.
FAULT-RIDE THROUGH IN GERMANY
The wind farm must be able to continue operation during
and after disturbances in the transmission network. This
requirement applies under the following conditions:
The wind farm and the wind turbines in the wind farm must be able to
stay connected to the system and to maintain operation during and after
dimensioning faults in the transmission system.
The wind farm may disconnect from the system, if the voltage in the
connection point during or after a system disturbance do fall below the
certain levels.
The fault duration, where the voltage in the connection
point may be zero, is 100 milliseconds for 800 kV & 400 kV
and 160 milliseconds for 220 kV & 132 kV.
FAULT-RIDE THROUGH
Inferences
GCR are set up to specify the relevant requirements for
efficient and secure operation of power system for all
network users and these specifications have to be met in
order to integrate wind turbines into the grid.
The GCR for Indian scenario has been laid down in order
to provide for adequate safe operation and reliability of
the interconnected Indian power system.
Crowbar
Chopper
Series connection
Extra series converter
DOUBLY-FED INDUCTION GENERATOR
Fault-ride through (FRT) : Objectives & Solutions
Protection of converters
Grid codes fulfillment
Reducing mechanical stress
UNIFIED ARCHITECTURE (UA) OF DFIG
An extra series grid side converter (SGSC) sharing common
DC bus with conventional DFIG system
According to advanced GCR wind farms should have
To supply/consume active/ reactive power to/fromthe grid
To supplies a P-Q diagram showing the regulation capability for
reactive power
These requirements are defined with respect to the power factor as a
function of the voltage at the PCC with the main grid
Selection of partial rated converters rating is crucial in terms
of cost, efficiency, operating speed range and reactive power
capability
A complete capability curve for DFIG system is developed
considering optimum RSC converter rating and GSC
capability.
Reactive Power Capability of WT
DFIG WITH BACK-TO-BACK PWM VOLTAGE
SOURCE CONVERTERS
GRID DFIG
3
3
3
GB
ROTOR SIDE
CONVERTER
CONTROL
GRID SIDE
CONVERTER
CONTROL
Capability curve of SG
End heating limit
REACTIVE POWER CAPABILITY OF DFIG
First, 3 steady state models of DFIG are derived in terms
stator and rotor voltage (V
S
,V
R
),
stator voltage and rotor current (V
S
,I
R
), and
stator voltage and stator current (V
S
,I
S
),
to derive the limitations in the reactive power production /consumption,
caused by the rotor voltage, rotor current and stator current limits,
respectively
Secondly, reactive power capability of the Grid Side
Converter (GSC) is included.
Finally, a complete capability curve of conventional &
modified DFIG for stator voltages is developed by
optimization of rotor speed employing maximum power point
tracking (MPPT) algorithm.
GRID DFIG
3
3
3
GB
ROTOR SIDE
CONVERTER
CONTROL
GRID SIDE
CONVERTER
CONTROL
The fundamental steady state equations for the DFIG are given by:
Voltage equations: &
Flux equations: &
Eliminating flux linkages, we have &
S
S
s
j V ψ ω = ( )
R
R S R R
r
j I R V ψ ω ω − + =
R M S S
S
I L I L + = ψ
S
M
R
R
R
I L I L + = ψ
( )
R M S S S
S
I L I L j V + = ω
( )
S M R R S R R
R
I L I L js I R V + + = ω
REACTIVE POWER CAPABILITY OF DFIG
ROTOR VOLTAGE LIMITATION
( )
( )
( )
2
2
2
2
2
2
sin
3 3
K
M R M
S S S R
S S
R S SC
R S SC
L s R L
P V V V
L L
R s L
R s L
δ δ
ω
ω
⎛ ⎞
+ ⎛ ⎞ ⎛ ⎞ ⋅
⎜ ⎟
= −
⎜ ⎟ ⎜ ⎟
⎜ ⎟
+ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠
+
⎝ ⎠
( )
( )
( )
( )
2
2
2
2
2
2
c o s
3
1 3
S M S C K
S m
S S r
S S
R S S C
r S C
s L L
V L
Q V V
L L s
R s L
R s w L
ω δ δ
ω
ω
⎛ ⎞
⎡ ⎤
+
⎜ ⎟
⎢ ⎥
= + −
⎜ ⎟
⎢ ⎥
+ ⎜ ⎟
+
⎣ ⎦
⎝ ⎠
θ cos 3
R S
S
M
S
I V
L
L
P − =
θ
ω
sin 3
.
3
2
R S
S
M
S M
S
S
I V
L
L
L
V
Q + =
ROTOR CURRENT LIMITATION
cos 3 φ
R S S
I V P − =
φ sin 3
R S S
I V Q =
STATOR CURRENT LIMITATION
CAPABILITY CURVE OF DFIG
The locus of capability curve is obtained by the minimum absolute value of
three limiting curves governed by stator current, rotor current and rotor
voltage during entire operation of optimized slip
Rotor voltage limitation curves for
Variation in RSC rating
Percentage increase in RSC rating w. r. t.
|±s
max
| × P
s
required for variation in grid voltage
to exclude rotor voltage limits from capability curve
SELECTION OF OPTIMAL RSC RATING
For RSC rating = 0.3789 p.u. i.e. (1.263 × |s
max
|)
CAPABILITY CURVE WITH OPTIMAL RSC RATING
COMPLETE CAPABILITY CURVE OF DFIG
( )
2 2
.
R GSC GSC GSC
P I V Q − ± =
GSC reactive power capability
For RSC rating = 0.3789 p.u. For RSC rating = 0.3 p.u.
CAPABILITY CURVE OF UNIFIED ARCHITECTURE
Inference from RPC curve
It is established that the total reactive power generation is limited
by rotor voltage at low speeds and by rotor current at higher
speed.
However, the total reactive power consumption is limited by stator
current, for entire operating region.
Rotor voltage limitation can be avoided by increase of rotor side
converter rating.
During operating region of optimized slip, rotor power remains
considerably small and hence RPC of GSC can be effectively
utilized.
In the recent years, the controllability of wind turbines has been
improved by the introduction of power-electronic technologies.
DFIG schemes have utilized PI controllers for power electronic
converters.
However, the main problem with the PI controllers is that these will
work effectively under the conditions in which these have been
designed. If there is any deviation in the system conditions, such as
variation in load, variation in system parameters etc., the
conventional PI controllers may not give the satisfactory results.
Computational intelligence (CI) techniques, such as fuzzy logic (FL),
neural network (NN), genetic algorithm (GA), etc., are showing
promising results in the several engineering applications.
CI based controllers can lead to improved performance, enhanced
tuning and adaptive capabilities.
NONLINEAR ADAPTIVE NEURO-FUZZY CONTROLLERS FOR WT
67
Mostly the FL and ANN are used, where an existing PI or PID
controller is simply replaced by a CI based controller
FL lack of a formal design methodology, the difficulty in
predicting stability and robustness of FL controlled systems
difficult to define the crisp rules for control difficult to define
and tune fuzzy rules and the fuzzy system parameters due to
large numbers of fuzzy rules
ANN require least computational time after training to select
optimal structure, parameter values and to minimize training set
are some of the issues to be addressed in the neural network
applications.
An ANFIS based hybrid intelligent controller is proposed for fast,
accurate, and efficient control of power electronic systems used
for wind turbines
s
r
L
r
r
rr
r
+ 1
1
s
K
K
i
p
+
*
) (
s
dr
i
Ψ
) (
s
dr
i
Ψ
'
dr
V
) (
s
dr
i
Ψ
s
r
L
r
r
rr
r
+ 1
1
s
K
K
i
p
+
*
) (
s
qr
i
Ψ
) (
s
qr
i
Ψ
'
qr
V
) (
s
qr
i
Ψ
*
) (
s
P
) (
s
P
s
K
K
i
p
+
*
) (
s
Q
) (
s
Q
s
K
K
i
p
+
ROTOR SIDE CONVERTER CONTROL
Aims to control the stator active and reactive powers
ANFIS
ANFIS
ADAPTIVE NEURO-FUZZY INFERENCE SYSTEM
In an ANFIS technique, using a given input/output data set,
a fuzzy inference system (FIS) is constructed from an
extremely limited mathematical representation of the
system, whose membership function parameters are tuned
(adjusted) using either a back-propagation algorithm alone,
or in combination with a least squares type of method.
This allows fuzzy systems to learn from the data they are
modeling.
Hence ANFIS architecture can identify the near-optimal
MFs of FLC for achieving desired input-output mappings
BASIC STRUCTURE OF THE ANFIS
The adjustment of modifiable parameters is a two step process.
First, information is propagated forward in the network until Layer-4,
where the parameters are identified by a least-squares estimator.
Then the parameters in Layer-2 are modified using gradient descent.
BASIC STRUCTURE OF THE ANFIS
Layer 1
Every node i, in this layer, is a square node with a node function where, x is the
input to node i, and A
i
is the linguistic label (small, large, etc.,) associated with this node
function. Parameters in this layer are referred to as premise parameters.
Layer 2
Every node in this layer is a circle node, labeled Π, which multiplies the incoming signals
and sends the product out.
Layer 3
Every node in this layer is a circle node, labeled N. The I node calculates the ratio of the
I rule’s firing strength to the sum of all rule’s firing strengths, as given below. Outputs of
this layer are known as normalized firing strengths.
Layer 4
Every node i in this layer is a square node with a node function
where, is the output of layer 3, and { p
i
, q
i
, r
i
} is the parameter set. Parameters in
this layer will be referred to as consequent parameters.
Layer 5
The single node in this layer is a circle node labeled Σ that computes overall output as
the summation of all incoming signals.
4
( )
i i
i i i i i
O w f w p x q y r = = + +
i
w
1
( )
i
i A
O x μ =
PI
Error
ANFIS
Error
Actual Signal
Output
Actual Signal
Reference Reference Output
DESIGN OF ANFIS CONTROLLERS
Training and testing the ANFIS are generated by designing and testing
PI controllers for a set of operating conditions
Operation at different wind speed conditions.
Operation at different ramp increase in wind speed.
Operation at different step increases in wind speed.
Operation at 1-phase fault.
Operation at 3-phase fault.
Operation at different voltage sag conditions
Training was performed using Hybrid Algorithm
q-axis stator current
d-axis stator current
d-axis rotor current
q-axis rotor current
Stator flux
DC link voltage
SIMULATION RESULTS FOR DFIG DURING THREE-PHASE
FAULT
SIMULATION RESULTS FOR UA DURING THREE-PHASE
FAULT
d-axis stator current q-axis stator current
d-axis rotor current q-axis rotor current
Stator flux
DC link voltage
Inferences
The dynamic performance shows that, with ANFIS, the settling
time is reduced, peak overshoot of values are limited and
oscillations have been damped out quickly.
ANFIS based controller performs better than PI controller.
ANFIS based controllers can perform more better by rigorous
training using large data sets.
Economic Issues
High capital/energy Cost
Promotion of Renewable Energy by Policy Design
and Regulatory Initiatives
oElectricity Act 2003 (Jun 2003)
oNational Electricity Policy (Feb 2005)
oNational Tariff Policy (Jan 2006)
oNational Action Plan on Climate Change (Jun 2008)
Central Government
o Regulations for Preferential Tariff for RE (Sep 2009)
o Renewable Energy Certificate Mechanism (Jan 2010)
o Implementation Framework (2010 – ongoing)
Central Electricity
Regulatory
Commission
o Preferential RE Tariff Orders by SERCs (2002–2010)
o Over 19 States have mandated Renewable Purchase
Obligations (2004 – 2010)
o Modification to RPO and adoption of REC
framework
State Electricity
Regulatory
Commission
Renewable Purchase Obligations : Potential Problems
• SERCs of those states, in which RE potential is not significant
are constrained fromspecifying higher RPOs.
o DERC has fixed RPO of 1%.
o MPERC has fixed RPO of 10%, actual achievement < 1%.
• SERCs of those states, in which the RE potential is very high
(Rajasthan, Tamil Nadu etc.) specified higher RPOs. Though
significant headroom is available in these states for enhancing
the RPOs, expensive renewal electricity becomes a constraint.
CERC REC Regulation, 2010 – Conceptual Framework
Electricity
Component
* Self consumption by CPPs based upon renewable generation are eligible for RECs
RE Generation*
Sale at
Preferential
Tariff
REC Component
Obligated Entities /
Voluntary Buyers
Distribution Company
/ Third Party Sale/
Power Exchange
Obligated Entities
Economic Issues
Preferential Tariff & Incentives
CERC/SERChasannouncedpreferentialtariffforRES;
Zeroorconcessionaldutyapplicableonimportofcertainspecificitems;
ZeroExcisedutyondomesticmanufactureofmanysolarenergydevicesand
systems;
CERC/SERCwillreviewthecostsperiodicallyandfixtariffaccordinglyfornew
projects.
Preferential Tariffs/Policy Announced by the
SERC’s for Wind/Biomass/SHP
Sources Wind Biomass/Cogeneration SHP
Items
States
Wheeling
Charges
Banking Buy-back (INR/kWh) Wheeling
Charges
Banking Buy-back
(INR/kWh)
Buy-back
(INR/kWh)
Andhra
Pradesh
2% of energy 12 Months 3.37 28.4 % +
INR 0.5 kWh
Allowed at 2%
for 8-12
2.63 2.69
Chhattisgarh -- -- -- 6 % of energy Not Allowed 2.67 --
Gujrat 4% of energy -- 3.37 fixed for 20 yrs. 4 % of energy Allowed 3.0 --
Haryana 2% of energy Allowed 4.08 + Escalation 1.5% 2 % of energy Allowed 4.0 2.25
HP -- -- -- -- -- -- 2.50
Karnataka 2% of energy 2% /Month
for 12
Month
3.40 fixed for 10 years 5% surcharge Allowed at 2%
INR 1.13/kWh
2.27 2.90
Kerala 5% of energy 9 Months
(Jun.–Feb.)
3.14 fixed for 20 years 5% of energy Allowed 4 Months 2.80 --
Maharastra 2% of energy
+5%trans.loss
12 Months 3.50 + Escalation of 0.15 for 13
years from documentation of the
project
7% of energy Allowed 3.05 2.25
Madhya
Pradesh
2% of energy Not allowed 3.97 (with decrease of 0.7 upto
4
th
yr ) then fixed at 3.30 from 5
th
yr onwards uniformly for 20
years
-- Allowed 3.33 2.25
Punjab -- -- -- 2% of energy Allowed12
Months
3.59 2.73
Rajasthan 10% of energy 3 Months 3.59 for Jaisalmer, Jodhpur etc.
and 3.67 for other districts
10% of energy Allowed 12
Months
3.60 2.75
Tamil Nadu 5% of energy 5%
12 months
2.90 (Levelised) 2-10% Allowed at 2%
charge
2.73 --
Uttar Pradesh -- -- -- 12.5% of energy Allowed
24 Months
2.67 2.25
West-Bengal INR 0.3/kWh 6 Months To be decided on case to case
with a cap of 4
-- -- -- --
Open access transaction is allowed in all States. Third party sale is allowed in case of wind in all States. RPS is announced in each State
Renewable Energy Certificate
(Regulations 2010)
CERC makes the following regulations for the development of
market in power from Non Conventional Energy Sources by
issuance of transferable and saleable credit certificates.
The concept of renewable energy certificate seeks to address the
mismatch between availability of renewable energy sources and
the requirement of obligated entities to meet their renewable
purchase obligations.
The REC mechanism mainly aims at promoting investment in the
renewable energy projects and to provide an alternative mode to
the RE generators for recovery of their costs.
Key Objectives for Introduction of
REC Mechanism
Effective implementation of RPO
Increased flexibility for participants
Overcome geographical constraints
Reduce transaction costs for RE transactions
Enforcement of penalty mechanism
Create competition among different RE technologies
Development of all encompassing incentive mechanism
Reduce risks for local distributor by limiting its liability to energy purchase
In Indian Context, following aspects had to be considered for REC design
Electricity Market is regulated to great extent
(> 90% of electricity volumes continue to be transacted at regulated rate)
Preferential RE Tariff regime to continue (Feed-in tariff & REC to co-exist)
Concept of REC Mechanism in India
Avg. PP Cost
of Host Utility
(regulated)
Market Rate
as per
Power Exchange
At Tariff
Determined by
Regulatory
Commission
Renewable
Energy
Electricity
REC
Distribution
Company
Renewable
Energy
Electricity
REC
Distribution
Company
Obligated
Entity
(Buyer)
Existing
Mechanism
REC
Mechanism
OA / Trader
Bilateral
agreement
(de-regulated)
REC Pricing Framework
Renewable Energy
Electricity Component
REC Component
(Environmental
Attribute)
Market Discovered Price
(Obligated Entity/Voluntary
Buyer)
Average
Power Purchase Cost
(Distribution Utility)
Andhra Pradesh - Rs 1.78/kWh
Maharashtra - Rs 2.43/kWh
Karnataka- Rs 1.85/kWh
Kerala - Rs 1.46/kWh
Tamil Nadu - Rs 2.62/kWh
Himachal Pradesh - Rs 1.48/kWh
Rajasthan - Rs 2.48/kWh
Parameters
Non Solar
REC
Solar
REC
Forbearance
Price (Rs/MWh)
3900 17000
Floor Price
(Rs/MWh)
1500 12000
Bilateral Agreement
(de-regulated)
(OA User/Trader)
Renewable Energy Certificate
(Regulations 2010)
EligibilityandRegistrationforCertificates:
Dealinginthecertificates:
PricingofCertificate:
ValidityandextinctionofCertificates:
Complementary Commercial Mechanism for
Scheduling of RES
ABT & Unschedule Interchange
‘Unscheduled Interchange’ in a time-block for a generating
station or a seller means actual generation minus its scheduled
generation and for a beneficiary or buyer means its total actual
drawal minus its total scheduled drawal.
Special dispensation for scheduling of solar
generation
The schedule of solar generation shall be given by the generator
based on availability of the generator, weather forecasting, solar
insolation, season and normal solar generation curve and shall be
vetted by the RLDC in which the generator is located and
incorporated in the inter-state schedule.
In case of solar generation no UI shall be payable/receivable by
Generator. The host state , shall bear the UI charges for any
deviation in actual generation fromthe schedule.
Purchasing state to pay to solar generator at contracted rate for
whatever power is generated by the solar generation.
Remaining under drawal / over-drawal to be settled in UI
mechanismand RRF.
Illustrative examples for commercial settlement for
Solar Generation
Case1:Actualaspergenerationschedule
Schedule
( MW)
Actual
generation
(MW)
Implication
on
purchaser
Unscheduled Interchange (UI)
Implication on
host state
Implication on
solar Generator
5 5 Purchaser to pay
Solar Generator
for 5 MW at
contracted rate.
No implication
on host state.
No implication
on solar
generator.
Illustrative examples for commercial settlement for
Solar Generation
Case2:UnderInjectionbytheSolarGenerator
Schedule
( MW)
Actual
generation
(MW)
Implication
on
purchaser
Unscheduled Interchange (UI)
Implication on
host state
Implication on
solar Generator
5 4 Payment to be
made by purchaser
for 4 MW (as per
actual) at
Contracted rate and
for 1 MW
to RRF at UI rate
For 1 MW UI
liability on the
host state, as a
result of under
generation by
the Solar
Generator
embedded in the
State system,
the same shall
be received by
the host state
from RRF at UI
rate.
No implication on
Solar generator.
Illustrative examples for commercial settlement for
Solar Generation
Case3:OverInjectionbytheSolarGenerator
Schedule
( MW)
Actual
generation
(MW)
Implication on
Purchaser
Unscheduled Interchange (UI)
Implication on
host state
Implication on
wind Generator
5 6 To pay for 6 MW to
Solar generator
at contracted rate
Purchaser shall
receive payment for
1 MW from RRF at
contracted rate.
For 1 MW, UI
benefit for the
host State on
account of
overgeneration
by
solar generator to
be passed on to
the RRF at UI
rate
No implication on
Solar generator.
Conclusions
High penetration of RES can have a significant influence on the
operation of power systems.
Grid codes are set up to specify the relevant requirements for
efficient and secure operation of power system for all network
users and these specifications have to be met in order to
integrate RES into the grid.
Economic issues and practices in India.
Development of suitable market mechanism for emerging
electric power systems.
Modeling Issues
the model complexity depends on the target of the
investigations or intended application,
the models shall be in agreement with the interface of
simulation tools,
the level of detail representation of each component
should truly represent its physical phenomenon,
appropriate simulation time-scale is crucial for
phenomenon under study
94
Modeling Issues
aggregated wind park models, which represent a whole
wind park instead of only a single wind turbine, must be
suitably addressed to study the impact of high penetration
levels of wind power on the dynamic behavior of large
power systems, by considering all parameters.
this reduces the size of the power system model, the data
requirements and the computation time.
95
Modeling Issues
Development of models must reflects the technological
development (or next generation WT) in the area of wind
power generation especially in meeting existing Grid
Code Requirements (GCR) as well as GCR’s required in
future
Reactive power capability of WT & inclusion of FACTS
devices
96
Conclusions
Wind farms have a significant influence on the operation of
power systems.
Grid codes are set up to specify the relevant requirements for
efficient and secure operation of power system for all network
users and these specifications have to be met in order to
integrate wind turbines into the grid.
In this paper, existing GCR of several countries, which are
proactively meeting the challenge of considerable wind power
penetration, are analyzed.
Conclusions
GCR’s are discussed, like, are active power control, frequency
control, voltage control, wind farm protection (fault ride
through control) etc.
These interconnection regulations for wind turbines or wind
farms tend to add the following requirements:
• to maintain operation of WTduring a fault in the grid,
known as ‘fault ride-through’ capability;
• to operate the wind turbine in the predefined frequency range;
• to control the active power during frequency variations (active
power control);
Conclusions
• to limit the power increase to a certain rate (power ramp rate
control);
• to supply or consume reactive power depending on power
system requirements (reactive power control);
• to support voltage control by adjusting the reactive power,
based on grid measurements (voltage control).
These interconnection requirements can increase the total cost
of a wind turbine or wind farm. Hence, interconnection
regulations should be enforced for secure & economic
operation of power system.
Outlook (1)
Further increase of wind turbine size – limitation only by
transportation
DFIG or full size converter based wind turbines? Still not
clear!
Voltage control by WT becomes common feature
WT will be provide STATCOM function
WT stay connected to the grid during voltage dips
WT will participate in grid frequency support
Outlook (2)
Wind farm operators have to provide certification about grid
compliance
Standards for certification are necessary
Manufacturer have to provide WT simulation models for grid studies
Further improvement of grid codes is necessary
Financial compensation for Var generation?
Outlook (3)
Wind Turbine modeling (DFIG)
• Model for stability studies
• Aggregated Model for Wind Farms/Parks
Impact of Large Wind Power on Transient Stability and
development of Controllers to improve Transient Stability
Impact of Large Wind Power on Small Signal Stability and
development of Controllers to improve Small Signal Stability
Coordination of Wind Power with Conventional Power Plants
Trading Issues and Development of Suitable Market Mechanism
for Emerging Electric Power System.
Concluding Remarks
India has huge potential for producing power from Renewable Energy
Sources (RES).
Government of India has endeavored to lay the foundation for a broad-based
renewable energy program and designed it specially to meet the growing
energy needs, and to fulfill energy shortage and security concerns of the
country.
Considerable experience and capabilities exist in the country on renewable
technologies. Although at present the contribution of renewable energy is
small, but future developments might make RES technology more
competitive to displace conventional energy sources.
Prospects for RES are steadily improving in India towards a great future. It
is destined to take a leading role in the global renewable energy movement
aiming towards sustainable development.
The strategy for achieving these enhanced goals will mainly depend on the
active participation of all players i.e. from government agencies to NGO’s,
from manufactures to R&D institutions, from financial institution to
developers and of course a new breed of energy entrepreneurs.