Integration of Renewable Energy
P.R.Raghuram, General Manager, SRLDC
Structured of the this write-up:
1. Introduction
2. RE Technologies
2.1 Various types of RE :
2.2 Technology of Wind
2.3 Requirement of wind farms
2.4 Technology of Solar PV
3.
Scenario of Renewable Energy generation in India :
4.
Issues involved in Grid Integration of RE:
4.1 Planning Transmission system for RE
4.2 Variability and Intermittancy
4.3 Forecasting and Scheduling
4.4 SCADA / telemetry
4.5 Network related Problems and Congestion
4.6 Protection
4.7 Commercial mechanism implementation
5. International Experience
6. The Road ahead
1. Introduction: The electric power production in the world is predominantly fossil fuel based.
Fossil fuels are non-renewable that is, they draw on finite resources. In addition, they contribute to
the production of greenhouse gases and particulates. In contrast, renewable energy resources, such as
wind, solar, ocean, biomass, hydro, etc., can be replenished at a generally predictable rate and have
no direct greenhouse gas or particulate emissions.
Due to depleting nature of these reserves, efforts are on worldwide to ensure energy security through
alternate technologies for electric power generation. Parallely, there has been a growing concern
about the ever increasing pollution levels contributed by conventional electricity generation.
International conventions on action plan for mitigating the climatic changes mandated phasing out
of fossil fuel generating technologies and adoption of Clean Development mechanisms for
encouraging Renewable and green energy technologies.
Global demand for energy is increasing at a breathtaking pace, which will require significant
investment in new power generation capacity and grid infrastructure. Just as energy d emand
continues to soar, supplies of fossil fuels are dwindling and prices are at their most volatile.
Wind energy, however, is a massive indigenous power source which is available virtually
everywhere in the world. There are no fuel costs, no geo-political risk and no supply import
dependency.
Variable generation technologies generally refer to generating technologies whose primary energy
source varies over time and cannot reasonably be stored to address such variation. Variable
resources differ from conventional and fossil-fired resources in a fundamental way: their fuel source
(wind, sunlight, and moving water) cannot presently be controlled or stored. Unlike coal or natural
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gas, which can be extracted from the earth, delivered to plants thousands of miles away, and
stockpiled for use when needed, variable fuels must be used when and where they are available. Fuel
availability for variable resources often does not positively correlate with electricity demand, either
in terms of time of use/availability or geographic location. Additionally, peak availability of wind
power, the most abundant variable resource in terms of megawatt value today, can often occur
during periods of relatively low customer demand for electricity. Additionally, peak availability of
wind power, the most abundant variable resource in terms of megawatt value today, can often occur
during periods of relatively low customer demand for electricity.
2. RE Technologies :
Various types of RE :
1. Wind (On-shore, Off shore)
2. Solar (Solar PhotoVoltaic, Solar Thermal)
3. Micro Hydro (with Pondage , without Pondage )
4. Biomass (Bagasse, other bio-mass material like rice husk, cotton stalk, mustard stalk,
groundnut shell, coconut fronds, waste cotton stalks, bark, roots of trees, cane trash, arecanut
shells, Prosopis juliflora, poultry litter)
5. Non‐fossil fuel‐based co‐generation
Wind is most predominantly used technology world-wise, hence we focus more on it in this
discussion.
2.1 Technology of Wind Generators
Wind power systems convert the movement of air into electricity by means of a rotating turbine
and a generator. There are two types of Wind generators viz On- and off-shore. In the beginning,
generators of a few kW typically 250kW were manufactures.. Nowadays with advancement in
technology very large wind turbines (up to 5 MW) are in operation. Offshore windfarms are
expected to have higher load factors.
2.1.1 Types of Wind Generators :
1. Induction (Type-1)
a squirrel cage induction generator that is driven through a gearbox. It operates within a very
narrow speed range and is now obsolete. Generally fixed speed is achieved through a gear
box.
2. Variable-slip Induction Generator (Type-2)
It includes a wound rotor and a mechanism to quickly control the current in the rotor and
results in better response to fast dynamic events.
3. Doubly Fed Induction Generator (DFIG) (Type-3)
The turbine-generator power output passes through two components 1) about 70% through a
mechanism that produces a variable-frequency current in the rotor circuit 2) AC-DC-AC
power converters
The first mechanism enables the wind turbine generator to operate at a variable speed
typically about 2:1 range from max to min speed). This improves the power conversion
efficiency and controllability of the wind turbine generator.
The AC-DC-AC power converters need only be rated to carry a fraction, typically 30%, of
the total wind turbine-generator power output.
4. Full conversion Wind Turbine-Generator (Type-4)
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The entire turbine power output through an AC-DC-AC power electronic converter system.
the output current of a Type 4 wind turbine generators can be electronically modulated to
zero; thereby limiting its short-circuiting current contribution and reducing the short-circuit
duty of standard protection equipment. It has a comparable inertial response/ performance to
a conventional generator.
Out of the above, type 1 and 2 are now obsolete and type 3 & 4 are being more popular.
Wind Farm : A group of wind generators located in an area connected to a common pooling station
is called a Wind Farm. Its aggregated capacity ranges from 25-50MW
Penetration level : The percentage extent of participation of RE output in the total grid generation is
called penetration levels. As low wind penetration level RE is generally treated in terms of its
energy component. As the penetration levels started increasing, its participation is treated in terms of
MW and more stringent requirements are imposed on the RE.
Sizes of Wind Turbine : As the technology advanced, larger sizes of WTG are being deployed with
higher tower height and larger diameter of blades. Thus smaller machines of 50kW of earlier era
gave way to larger machines of MW capacity. As of now in India WTG of 1.25MW capacity with
tower heights of 80m and blade diameter of 52m are in service.
Year
MW
Hub
height
1985
50kW
18m
1988
300kW
38m
1990
500kW
48m
1995
600kW
60m
1998
1.5 MW
80m
2000
2.5 MW
90m
2005
6MW
120m
2.1.2 Main components of a wind electric generator:
1. Tower
2. Nacelle
3. Rotor
4. Gearbox
6. Braking System
7. Yaw System
8. Controllers 9. Sensors
2.1.3 Various forms of control :
Mechanical Controls :
1. Pitch control
Electrical controls :
1. Voltage Control
2. Yaw control
2010
10MW
150m
5. Generator
3. Stall control (Active/ Passive )
Pitch control :
Pitch control, which allows their output to be curtailed in real-time by adjusting the pitch of
the turbine blades (i.e., “spilling wind” ). By throttling back their output, wind plants are able
to limit or regulate their power output to a set level or to set rates of change.
2.1.4 A wind turbine’s pitch controller uses advanced computer-based schemes to ensure the rotor
blades pitch exactly the amount required. This control scheme will normally pitch the blades a few
degrees every time the wind changes to keep the rotor blades at the optimum angle and maximize
output for all wind speeds. The same control mechanism could be used, in aggregate, by the operator
to dispatch variable generation between minimum and maximum available power output.
2.1.5 Yaw control :
The WTG is rotated as per wind direction so that the rotor is subjected to forces of the wind either to
increase or decrease power. This technique of yaw control is achieved through a motor controlled
mechanism which senses direction and velocity of wind and rotates the WTG. If full rotation (upto
360 degrees) is completed, the mechanism will again rotate the WTG so as to unwind the power
cables of WTG.
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2.1.6 Stall Control :
In Active Stall control, the machines are programmed to pitch their blades much like a pitch
controlled machine at low wind speeds. When the machine reaches crosses scheduled (normally)
rated power, however, the machine will pitch its blades in the opposite direction and will increase
the angle of attack of the rotor blades in order to make the blades go into a deeper stall and control
the over-speeding.
In Passive Stall control, the rotor blades of the WTG are bolted at a fixed angle, but the blades are
designed aerodynamically to maximise the wind force.
2.2 Electrical charactorestics of Wind generators:
a) Is Governor operation available in WTG?
Governor action for the WTG (controlling the input as a response to frequency changes) in the
conventional sense per se is not available because wind can not be controlled to regulate the
frequency and WTG is expected to utilise the entire wind energy available. However, reduction or
curtailment of the output is possible to some extent by varying the effective force of wind on the
turbine blades in the event of storms, over speed, over generation etc . Similarly through a
command from the control centre also output reduction is possible.
b) What is the Inertia contribution to Grid?
Because of low rotational mass, there is negligible contribution of inertia to the grid from these
generators.
c) What is the short circuit contribution of the WTG?
Type 1 and Type2 (Induction/ fixed speed type of generator) can contribute to short circuit level to
some extent. But the variable speed (type3 and 4) generators can not contribute due to asynchronous
connection through AC-DC-AC convertors
d) Can WTG be Black Started?
All types of WTG require Voltage from Grid to start generation for either excitation or Reference
volateg for AC-DC-AC convertor.
2.2.1 Reactive requirement:
The type-1 and Type-2 machines being induction generators can not participate in voltage
regulation and require switched shunt capacitor banks for reactive compensation.
Due to variable speed, WTG depend on AC-DC-AC convertors which will provide an
asynchronous link between the WTG and Grid. These power electronic devices generally have
inherent control of reactive power and can participate in voltage regulation. Due to restriction on
drawl of reactive power from Grid, a combination of switched capacitor banks and/or power
electronic transmission technologies such as SVC/STATCOM are provided for Reactive support and
power factor control.
2.2.2 Fault Ride Through (FRT) /Low Voltage ride-through (LVRT) : It is ability of WTG to stay
connected to the grid during voltage dips caused by short-circuit one or all phase of its terminal
current upto a specified voltage level. It is achieved through modifications of the turbine generator
controls . This capability is essential as large scale trippings of Wind Turbines in large Wind farms
result in disturbance in load flows. This should be achieved without damaging the WTG due to
unbalance torque, Electronic and mechanical components.
WTG act like a current source.
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2.2.3 Load Following capability : Ability to reduce power output when there is no matching load.
2.2.4 Load Regulation : Control of Load to match the generation.
2.2.4 General Requirements of wind farms:
i) Wind farms shall have the ability to limit the active power output at grid connection point as per
system operator’s request.
ii) The grid connected wind farms shall have the ramp up/ramp down capability
iii) The reactive compensation system of wind farms shall be such that Wind farms shall maintain
power factor between 0.95 lagging and 0.95 leading at the connection point.
iv) The wind generating machines shall be equipped with fault ride through capability. During a
Fault Ride through, the Reactive power drawl fro Grid shall be minimum and active power
generation shall be in proportion to the retained grid voltage. They should have the capability to
withstand repetitive faults.
The
Wind
generating machines shall have the operating region as shown in Figure given below during system
faults. Wind farms can be disconnected if the operating point falls below the line in the Figure .
Wind farms connected to high voltage transmission system must stay connected when a voltage dip
occurs in the grid, otherwise, the sudden disconnection of a large amount of wind power may
contribute to the voltage dip, with adverse consequences. Wind farms must remain connected when
the voltage dip profile is above the line shown in the figure. The per unit voltage at the point of
connection to the grid is shown in the vertical axis and the duration (seconds) of the fault in the
horizontal axis. This code requires Fault Ride-Through (FRT) capability during voltage drops in
Transmission System to 15% of nominal voltage during 300 ms with recovery up to 80% of nominal
voltage after 3 sec, with the slope shown in figure given above
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2.3 Technology of Solar PV
Solar PV technology converts the electromagnetic energy in sunlight directly into direct current
(DC) through use of modified silicon (Crystalline Silicon) or other semi conductor (thin film)
material. It can use both Diffuse Solar Radiation (DSR) and Direct Normal Irradiance (DNI). It is
the most popular technology as it does not require larger plant sizes to achieve economies of scale
and is often deployed as distributed generation. It uses DC-AC convertor for grid interface. The
energy from the sun is less variable than wind energy but is also affected by changing atmospheric
conditions. The power outputs during the year are fairly consistent but there are differences in the
number of hours (according to the latitude) which determine the total production in each installation.
As it does not have any rotating mass, it has no inertia meaning the it has large ramp downs in
partially cloudy days.
2.4 Technology of Solar Thermal
Concentrating solar power plants convert sun’s energy in to high temperature heat using various
mirror configurations. The heat is then channelled through a tubular path containing heat transfer
fluids like water, oil molten salt etc. the heat so captured is used to produce steam which in turn acts
as prime mover to a conventional turbine-generator. These plants consist of four parts; a
concentrator, a receiver, heat transfer fluid, storage and power conversion equipment. For the
purposes of system operation, the main difference is that some of these plants are able to store
thermal energy to produce electrical energy during a period of 7 hours which reduces fluctuations of
power output.
3. Scenario of Renewable Energy generation in India :
Government of India is giving thrust to increase Renewable Energy generation with a view to
increase alternate energy resources. The Ministry of New and Renewable Energy (MNRE) has
embarked on ambitious plan of installing 45GW of wind generation by 2020 and 20GW of Solar
generation. Already over 18 GW of RE capacity has been operation as per following breakup:
Wind Power
Small Hydro Power
Biomass Power
Bagasse Cogeneration
Waste to Power (Urban & Industrial )
Solar Power (SPV)
Total
Capacities in MW as on 31-12-10
Installed Capacity
13066
2939
997
1562
72
18
18654
Potential
48756
14292
8680
5000
7000
200000
283728
Source : MNRE
Winds created by three types of motions a) General Circulation caused by uneven heating of earth
and sea , Earth’s rotation and seasons b) Secondary Circulation caused by pressure difference due to
cooling and heating of lower atmosphere c) Tertiary Circulations which are purely due to local
effects such as mountains, valley passes, land masses and terrain effects.
Land masses are heated by sun more quickly than sea in day time. The air from land rises and flows
out to sea and creates a low pressure at ground level which attracts cool breeze from sea. At night
fall there would be a time when no breeze flows due to temperatures are equal between sea and land
and wind becomes standstill. Gradually wind blows in reverse direction albeit at lower speeds as
temperature gradient between land and sea also reverses.
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It can be seen from the map
that Wind distribution is
more in the Western side of
India due to Westerly winds
and for reasons mentioned
above paragraph
Distribution of Wind potential in India, Source : CWET, Chennai
The mapping of Wind
and Solar potentials in
India shows that they are
concentrated in the same
geographical locations.
Distribution of Solar potential in India, Source : MNRE,
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The National Action Plan for Climatic Change (NAPCC), a high power council under The
leadership of the Prime Minister has embarked on action plan to reduce its per capita greenhouse
gas emissions through deployment of RE technologies and envisages to expand the scope of other
renewable and non-fossil options such as nuclear energy, wind energy and biomass.
For a multi-pronged, long-term and integrated strategies for achieving key goals in the context of climate
change, it has announced eight national missions. One of them is JNNSM (Jawaharlal Nehru
National Solar Mission) to significantly increase the share of solar energy in the total energy mix.
Diurnal and seasonal pattern of Wind in India :
The wind pattern (both diurnal and seasonal) varies from region to region. In South peak season of
Wind is from April to October. In a typical day, normally wind picks up from around 10 hrs to
18.00hrs and gradually reduces in the late evening hours. It touches least in the morning between
0600 hrs to 0900 hrs.
S-W monsoon
N-E monsoon
KARNATAKA WIND GENERATION ON MAXIMUM GENERATION DAY
29.277 MU on Date - 31-07-2010
1400
1300
1200
1100
0:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
9:00
10:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
0:00
1000
Diurnal patterns
Seasonal patterns
4. Issues involved in Grid Integration of RE:
1. Planning Transmission system for RE
2. Variability and Intermittancy
3. Forecasting
4. Scheduling
5. Network related Problems and Congestion
6. SCADA / telemetry
7. Protection
8. Commercial mechanism implementation
Variable generation plants are often located in remote areas of the network where the short circuit
level is weak and, as a result, problems such as under-/over-voltages, harmonics or
voltage unbalances may be observed.
4.1 Planning Transmission system for RE
The traditional tools and techniques for the planning and operation of transmission system so far
have been based on the conventional generations like thermal type (coal, gas and oil) and nuclear.
With increasing levels of renewable generation, these tools and techniques for the planning and
operation of the transmission system have to be reviewed. Large-scale penetration of variable
generation should be considered in terms of timeframes: seconds-to-minutes, minutes-to-hours,
hours-to-days, days-to-one week and beyond.
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Time frame
Planning criterion
1 -30 years
transmission and resource adequacy assessments.
1 year- 1 month
New capacity addition, Tr. Adequacy assessment
1 day- 1 week
Wind forecast, Demand forecast, Congestion
monitoring, Market operations,
Minutes - hour
Wind forecast, Demand forecast, Unit Commitment
and L-G balance
Seconds-to-minutes
Protection, AGC, Governor, Excitations systems, PSS,
AVRs, SPS, FRT capability
Source : NERC’ report on “Accommodating High Levels of Variable Generation” April 2009
Wind generator terminal voltages are typically 415V and are stepped up to 11kV upto the pooling
station which are further stepped upto either 33kV or 66kV/ 110kV depending on the level of
distribution/ sub-transmission system. Wind being spread across a large geo-graphical area calls for
huge investment in distribution and transmission levels. Usually the network upto the pooling station
is constructed and maintained by the wind developer. The network beyond the pooling station is
owned maintained and operated by the Distribution licensee for which wheeling charges and other
incidental charges would be born by the wind farm developer.
As the penetration levels are generally very low, there is a need to strike a balance between optimal
loading of the network and ensuring network redundancy.
To perform transmission planning, the planner needs to study power flow, time-domain and smallsignal stability along with short-circuit duty analyses tools.
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Wind Turbine Generator modelling issues: The WTG is to be modelled depending on its type viz.
Induction type, DFIG, Fully conversion etc. The modelling can be done either individual plant wise
or entire wind farm wise. The steady state model is to consider the wind farm wake in atmospheric
turbulence through widely used models like K-epsilon model (for turbulent kinetic energy), Eddy
viscosity etc. The problems faced by System Study engineers through out the world is that the
steady state and dynamic models provided by the manufacturers are highly detailed, specific,
proprietary in nature, non-standard and confidential. There is a unanimous requirement of
generalised and standardised models which can perform load flow, short circuit, and stability
studies necessary to ensure system reliability. NERC specified Modelling data and Analysis
standards as given below :
MOD-011 and MOD-013 : for steady state and dynamic performance
MOD-012
: for reliability studies and
MOD-014 and MOD-015 : for planning purposes of interconnected systems,
MOD-023
: for verification of reactive power limits
IEEE, WECC and CIGRE (technical brochure 328) also provide valuable information on modelling
of variable generation plants.
Dynamic Line Rating : During high wind periods the network can be loaded to a higher levels of
thermal ratings of the network because of higher heat dissipation due to wind. This concept is
called dynamic Line rating.
Diversity Factor :
Diversity factor: Wind farms are usually located across large geographical spreads, hence wind
pattern for all the wind mills and wind farms is not the same, thus the wind production at a given
point of time is not same for all the wind generators and farms. This is called Diversity factor and is
to be duly factored while designing evacuation systems.
Outage planning: Outage of wind generator should be planned during lean wind season, outage of
solar, if required during the rainy season and outage of run-of-the-via hydro power plant in the lean
water season.
4.2 Variability and Intermittancy
There are two major attributes of variable generation that notably impact the bulk power system
planning and operations:
Variability: The output of variable generation changes according to the availability of the primary
fuel (wind, sunlight and moving water) resulting in fluctuations in the plant output on all time scales.
• Uncertainty: The magnitude and timing of variable generation output is less predictable than for
conventional generation.
In a power system 3 levels of control are provided for maintaining Load-Generation Balance :
1. Free Governor Mode of operation (FGMO) – Primary Control
2. Automatic Generation Control (AGC) – Secondary Control
3. Load Shedding and Unit re-despatch – Tertiary Control
However in Indian context, FGMO is operational only to a limited extent. AGC is not available as
the machines operate at almost full load and there are no margins to change. Spinning reserves are
very scarce in a deficit ridden Indian power system. Even the limited reserves in the form of liquid
based generation is not being fully harnessed due to high costs and poor financial health of the
utilities. The overdrawls from pool have become costly due to regulatory measures and tightening of
operating frequency. As a result utilities are having to resort to large scale Load shedding is resorted
inspite of a huge Value Of Lost Load (VOLL).
Wind generation by its very nature is exogenously variable. Its variability, largely, is beyond
control of the wind farm operator. By its very nature the generation of a wind farm varies over the
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day. In Tamil Nadu, the wind generation typically peaks in the late afternoon hours and tapers off
during night. On a daily basis one can find that the wind generation typically varies from 800 to
2000 MWs. This unpredictable shift would upset the day ahead economics and affect the power
system operation in the grid viz. the balancing market mechanism (UI) gets disturbed and a loss of
around 1000 to 1200 MW would dip the system frequency thereby increasing the spot market cost of
power. As the deficit due to fall in wind generation rises in its own control area of Tamil Nadu State,
the drawal over schedule by the State entity would increase considerably. To bring back the system
frequency to normalcy, TamilNadu would have the option of either resorting to massive load
shedding or running the depleted/storage hydro and costly gas based power plants. During such a
contingency, any committed export, bilateral open access contracts, would still have to be honored
which may cause additional stress on the power system in the state. A typical graph of hour wisemonth wise average wind generation in Tamil Nadu depicting the variability is shown at Figure .
1400
1200
August
Wind Generation in MW --->
1000
July
800
May
June
600
October
Jan
November
Sep
400
April
Figure :
A
December
200
March
hour
23:30
22:30
wise-month wise average of wind
for past 5 years in Tamil Nadu
Time generation
in HH:MM ---->
21:30
20:30
19:30
18:30
17:30
16:30
15:30
14:30
13:30
12:30
11:30
10:30
09:30
08:30
07:30
06:30
05:30
04:30
03:30
02:30
01:30
0
00:30
of
typical
To offset the variability and ensure L-G balance, more balancing power is needed through spinning
reserve, back-up generation, power storage facilities like fuel cells, etc.
Availability of good hydro generation within the control area gives a very good flexibility to
avoid UI charges consequent to the variability / uncertainty. For example Karnataka has good hydro
generation which can to a large extent meet the balancing needs. Luckily the variability in Karnataka
is to a lesser extent than Tamilnadu. Karnataka wind sites have lower wind density than that of TN
but they are spread across a larger geo-graphical area.
Whereas TN has sites with higher wind density in smaller geo-graphical area. This could be one of
the reasons why wind generation in Karnataka is less variable than that of TN.
As Tamil Nadu has high wind penetration but low hydro resources, the wind farms in
Tamilnadu have to schedule synergistically with gas based power generating units as balancing
sources. For example, if wind generation is above 2500 MW with a variability of 1000 MW and the
control area can handle the variability of only 500 MW (with its reserves), the System Operator of
Tamil Nadu has to arrange on contract gas units to supply the balance 500 MW reserve as and when
required. However, such additional costs of ancillary services would have to be borne by the wind
generators.
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TAMILNADU WIND GENERATION ON MAXIMUM GENERATION DAY
59.61 MU on 16-07-2010 (28% of Energy)
3000
2500
SR Maximum Wind -17.08.10 AT 18:59 HRS
% of Wind in SR I/C
– 17%
% of Wind Gen in SR Demand Met – 14.9 %
% of Wind in TN I/C
– 39.4 %
% of Wind Gen in TN Demand - 31.3 %
0:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
0:00
2000
TIME ?
It can be seen from the figure below that the penetration levels are as high as 31% of the
total demand met by the Tamilnadu and 28% in terms of energy and about 15% of the SR demand
on a typical day.
In terms of installed capacity wind constitutes about 17% in SR as a whole and 39% in Tamilnadu.
4.3 Forecasting :
Due to the reasons of Variability and intermittency explained in section 4.2, Wind generation needs
to be predicted with reasonable accuracy for proper scheduling and dispatching of power from these
sources in the interconnected system. As the penetration levels increase, wind generation
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forecasting becomes essential. Wind generation forecasting can be done on an individual developer
basis or joint basis for an aggregated generation capacity of 10 MW and above connected at a
connection point of 33 kV and above. If done jointly, the wind forecasting facility shall be built and
operated by wind developers in the area and sharing of the cost shall be mutually discussed and
agreed.
Form the following figure it can be seen that the uncertainity between hourly generation is within
+/-200MW for levels even for levels of 2000MW. Therefore it is possible to predict the wind
generation with reasonable accuracy.
SCATTERED PLOT OF UNCERTAINTY Vs ACTUAL GENERATION OF TNEB WIND
GENERATION (HOUR WISE) (Aug-10 to Mar-11)
UNCERTAINTY BETWEEN HOURLY GENERATION->
1000
800
600
400
200
0
-200
-400
-600
-800
0
500
1000
1500
2000
2500
3000
3500
ACTUAL WIND GENERATION IN MW --->
Forecasting of wind is a complex issue and is geo-locational and terrain specific. Weather related
inputs with high confidence level are a pre-requisite for forecasting wind. The forecast errors are
high as the forecast horizon increases and forecast of more than 24 hours ahead are generally
representative and cannot be used for dispatch.
As per IEGC, the wind energy forecasting system shall forecast power based on wind flow data at
the following time intervals:
i) Day ahead forecast: Wind/ power forecast with an interval of 15 minutes for the next 24 hours
for the aggregate Generation capacity of 10 MW and above.
The wind generators shall be responsible for forecasting their generation upto an accuracy of 70%.
Therefore, if the actual generation is beyond +/- 30% of the schedule, wind generator would have to
bear the UI charges.
There are different technologies for forecasting the Wind. Based on one or more techniques, world
over wind forecasts are being successfully incorporated by the utilities in operational planning. The
earth surface is divided into a grid of 35km x 70km and divided earth’s atmosphere into 64 layers.
The meteorological data from satellites is used as main input of Weather data. The meso scale
models further divide the data into 2.5 sq km or 0.5 sq.km grid.
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The forecast methods are based on statistical techniques involving Numerical Weather Prediction
(NWP), Site topography, SCADA/ current data of weather, power, and historical data of weather
and power as inputs. These inputs are fed to different Suite of models which are distinctly based on
Adaptive techniques, Time Series Anlaysis, Climatology. In intelligent model will combine the
results of these individual models and gives a best fit of results. This is achieved by incorporating
the mesoscale models from different NWP providers. The results are continuously fine tuned by
taking real time data inputs from wind farms on live updates of wind speed, live SCADA and site
geography. The wind forecast is based on a digital model of Flow modelling, wake effect, and
turbine output.
Forecast process is done upto 16 days ahead, but first 7 days are more accurate. In the first 7 days it
NWP
Foreca
NWP
stForecaNWP
st Forecast
Historic
SCADA
Live
SCADA
Site
geography
Suite of Models
Adaptive statistics
Model
adaptation
•
•
•
Time Series
Climatology
Intelligent Model Combination
Live
SCADA
Wind speed
forecast
Optimised combination of NWP suppliers
•
Incorporation of mesoscale models
Regular live feedback from the wind farm
Power
model
“Learning” Algorithms for:
•
•
Model
adaptation
Meteorology
Power models
Power
forecast
is run for 3 hour intervals while for the remaining period it run for 8 hour intervals. On day ahead
basis, it is run for 4 times for a window of 6 hours with a granularity of 10 minutes, which has to be
ultimately aligned with 15 min average values. The errors will be lesser and uncertainty band will
be tighter for shorter forecast horizons.
There are 22 group of different organisations who are members in an ensemble of Global Ensemble
Forecast System.
Integration of Renewable Energy – Study material for System Operator’s training program, NPTI
14
Hourly data 24 hours in advance
Certainty information particularly
useful for ramp events
Source : Enercon
Sudden rise or fall of wind generation is called ramp events which are required to be predicted
accurately to prepare for operational planning of reserves. For the ramp events the forecast model
would be put to acid test. Hence one of the attributes to evaluate the efficacy of the forecast model
is the correctness of the ramp events. The accuracy of the forecast is measured by either Mean
Average Error or Mean Absolute Error. The latter is a better measure of accuracy.
4.4 Scheduling
To enable RE generators to participate in trading across inter regional boundaries, CERC has
permitted Scheduling of Wind Generators of 10 MW (Solar 5MW)and above capacity connected at a
connection point of 33kV and above who do not have PPAs with host state as on 03-05-10.
The schedule by such wind power generating stations supplying interstate power under long–term
access and medium-term and short-term open access may be revised by giving advance notice to
RLDC. Such revisions by wind power generating stations shall be effective from 6th time-block ,the
first being the time –block in which notice was given. There may be maximum of 8 revisions for
each 3 hour time slot starting from 00:00 hours during the day.
A common control centre for co-ordinating the scheduling activities on behalf of all the Renewable
energy generators is very useful in smooth implementation of the CERC provisions.
4.7 Network related Problems:
Usually in the beginning of development of wind farms, existing transmission system will be
sufficient to evacuate the wind generation. However, as the wind installed capacity increases system
augmentation will become necessary and dedicated lines may have to be laid. Wind generation
Integration of Renewable Energy – Study material for System Operator’s training program, NPTI
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capacities are often added in small increments at usually slow time frames. With low penetration
levels and uncertainity in project execution, generally there is an inherent tendency for the network
service provider to delay transmission augmentation commensurate to the capacity addition. This
sometimes results in sudden congestion in the network. The problem becomes more severe if the
telemetering is either insufficient or inaccurate.
Power system planning with requisite transmission corridor for a generating station with low PLF
would be uneconomical and hence existing network resources are normally utilized by the wind
firms thereby sacrificing the reliability criteria. Lines are added by the wind generators/utility as the
bottleneck become more severe and alarming. However, in case of Tamil Nadu and Karnataka,
having largest wind penetration, the service providers have taken system augmentation at regular
intervals as and when congestion was experienced. POWERGRID as CTU has provided sufficient
network infrastructure thereby the evacuation of wind upto 2500MW became hassle free as on date,
seen at Figure 3 from the Power System Study Report for Tamil Nadu wind generation network.
EVACUATION STUDY FOR TN WIND:
NETWORK SUFFICIENCY
PRESENTLY THERE ARE NO
NETWORK CONSTRAINTS FOR
EVACUATION WIND UPTO 2500
MW.
NEW ELEMENTS ADDED FOR STRENGTHENING ARE HIGHLIGHTED
However in future when large amounts of conventional generation comes up in Kudankulam,
Coastal Tamilnadu and Andhra Pradesh area, congestion may again occur. To cater to the needs of
RE generators and to provide access to inter-regional markets, Government is contemplating high
power corridor to from South to Western regions.
4.8 SCADA and telemetry :
For real time monitoring of the actual generation and network flows, telemetry from wind farms is
very essential. The telemetry assumes more significance due to the variable and intermittent nature
of wind generation and consequent requirement of planning demand side and generation side
management . Due to dispersed and distributed nature of the wind generation across a large geographical area, telemetering the data is a challenge and costly affair.
Generally all real time data from wind turbines is metered and transmitted to the local control centre
of each wind farm. The net injection of the wind farm is also measured at the pooling station and
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16
transmitted to the Area control centre (SubLDC), which in turn is re-transmitted to SLDC and
RLDC.
As per proposed Wind grid code :
- Wind farms shall have communication channel which is continuously available to system operator.
- Data Acquisition System facility shall be provided for transfer of information to concerned SLDC
and RLDC
Telemetry issues with grid integrated wind farms are of major concern for the present day operation
of the grid and there is a challenge due to distributed nature and remoteness of the wind mill units.
Most of the wind mills report to the manufacturers’ control centre all the data and operates online
through aggregation. This information has to be shared with the system operators of the control area.
Telemetry is a must for scheduling and monitoring and as matter of fact, in India there is a lot of
scope for improvement in providing telemetry for these wind farms. Presently an approximate value
is available with the Load Despatch Centres which partly contains assumed manual data and partly
telemeter data at a maximum of 110 KV level.
WIND MILLS TELEMETRY
TYPICAL LAYOUT
110 kV Bus
33kV or
11 kV Bus
WIND MILLS
LOADS
X MW injected
to 110kV Bus
(Telemetered)
Untelemetered
However for a good telemetry application, it is desirable by the grid operator to receive telemeter
data (as seen from Figure ) at least from wind aggregation point at 11 KV level Wind Mill side.
4.7 Wind turbine generator protection issues
4.7.1 Protection requirements for RE :
4.7.2. Under-Voltage/ Over Voltage protection
4.7.3 Under frequency / Over frequency protection
4.7.4 Over current and earth fault protection
4.7.5 Load unbalance (negative sequence ) protection
4.7.6 Differential protection for WTG and grid connecting Transformer.
4.7.7 Capacitor bank protection
Low Voltage Ride-Through (LVRT) or Fault Ride-Through capability :
For faults in transmission systems, the generator may experience very low voltages at its terminals
for a few cycles until fault is cleared by relaying of Transmission system. Till that time, the
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protection of the Generator should be designed to withstand this voltage and see that generator does
not trip so as to avoid large scale cascade tripping of the wind farms.
a) All the grid connected wind farms must have protection systems to protect the wind farm
equipment as well as the grid, such that no part system shall remain unprotected during faults.
b) The protection co-ordination for the wind farms shall be done by the SEB/STU and RPC.
c) The following are the minimum protection schemes that shall be installed for wind farm
protection:
- Lightning protection of WTG system shall be according to IEC TR 61400-24 “Wind turbine
generator systems – Part 24: Lightning protection.”
- Wind turbine grounding systems shall follow the recommendations of IEC TR 61400-24 (sec. 9).
- The grid connecting transformer configuration shall be designed to provide:
i) A circuit to block the transmission of harmonic currents.
ii) Isolation of transmission system side and wind farm side ground fault current
contributions
The preferred configuration of the grid connecting transformer is delta connection on the
wind farm side and grounded wye connection on the transmission system (grid) side. Delta
connection on the high voltage side of the grid connecting transformer is not permitted.
Alternate transformer configuration including wye-wye or wye-wye with a delta connected
tertiary is also acceptable for the grid connecting transformer. If the wind farm is directly
getting connected to the existing utility substation, the standard practice of utility shall be
followed.
The purpose of prohibiting delta connection on the high voltage side of the grid connecting
transformer is to block the harmonics current and to detect the earth faults on the grid side.
4.8 Commercial mechanism to encourage RE :
Traditionally RE generators have long term PPAs for around 25 years with the host state for
supplying at a preferential tariff of around Rs. 3.5/kwh(for Wind). With a view to enable RE to
enter in to market so as to bring in a sense of commercial viability, Hon’ble Commission envisaged
market across the regions to third party buyers. For implementing this mechanism, scheduling,
computation of actual energy and accounting of deviations (UI) are important activities. The relevant
extract from IEGC are reproduced below:
5. The wind generators shall be responsible for forecasting their generation upto an accuracy of
70%. Therefore, if the actual generation is beyond +/- 30% of the schedule, wind generator
would have to bear the UI charges. For actual generation within +/- 30% of the schedule, no UI
would be payable/receivable by Generator, The host state , shall bear the UI charges for this
variation, i.e within +/- 30%. However, the UI charges borne by the host State due to the wind
generation, shall be shared among all the States of the country in the ratio of their peak demands
in the previous month based on the data published by CEA, in the form of a regulatory charge
known as the Renewable Regulatory Charge operated through the Renewable Regulatory Fund
(RRF). This provision shall be applicable with effect from 1.1.2011,for new wind farms with
collective capacity of 10 MW and above connected at connection point of 33 KV level and above ,
and who have not signed any PPA with states or others as on the date of coming into force of this
IEGC. Illustrative calculations in respect of above mechanism are given in Appendix.
6. A maximum generation of 150% of the schedule only, would be allowed in a time block, for
injection by wind, from the grid security point of view. For any generation above 150% of
schedule, if grid security is not affected by the generation above 150%,, the only charge payable
to the wind energy generator would be the UI charge applicable corresponding to 50- 50.02 HZ .
7. 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 from the schedule. However, the net UI
charges borne by the host State due to the solar generation, shall be shared among all the States of
the country in the ratio of their peak demands in the previous month based on the data published by
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CEA , in the form of regulatory charge known as the Renewable Regulatory Charge operated
through the Renewable Regulatory Fund as referred to in clause 5 above.. This provision shall be
applicable ,with effect from 1.1.2012, for new solar generating plants with capacity of 5 MW and
above connected at connection point of 33 KV level and above and , who have not signed any PPA
with states or others as on the date of coming into force of this IEGC.
Illustrative calculations in respect of above mechanism are given in Appendix of IEGC.
Renewable Energy Certificates :
Further to bring in more investment in RE sector and to make RE business more attractive,
Commission introduced the concept of REC (Renewable Energy Certificate). According to the
scheme each SERC has to prescribe a certain amount of RE generation in the their energy portfolio.
This obligation is called Renewable Purchase Obligation which can be fulfilled by either purchasing
power from RE sources or purchasing equivalent amount of Renewable Energy Certificates. The
certificates are classified as Solar and non-Solar and one certificate is issued for 1 MWh of energy
produced which has a validity period of 365 days.
As per this mechanism, the RE generator has to obtain accreditation form State Nodal Agency (SNA)
and Register with Central Nodal Agency (CNA) as a one time measure. The RE has to apply to CNA
for issue of REC. The actual energy injected to grid would be verified by SNA and CNA would
issue REC. The RE is eligible to get a one REC for each MWh of energy produced. The REC can be
traded in a Power Exchange platform and price discovery is through a double sided anonymous
bidding.
A. Accreditation and Registration (One-time activities) :
1. Apply for Accreditation
2. State Nodal Agency (SNA) to give accreditation
3. Apply for Registration
4. Central Nodal Agency to register
B. Issue of REC:
1. Apply for REC to CNA
2. SLDC to issue Energy Injection report.
3. CNA to issue REC with unique number
C. Trading in Power Exchange platform :
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The RECs are issued in an electronic (‘demat’) form and can be traded in a Power Exchange. The
maximum and minimum ceiling prices for trading in the Px are specified by CERC and they are Rs.
3900/- and Rs. 1500/- for non-Solar REC and Rs. 17000/- and 12000/- for Solar REC respectively.
5. International Experience
VLPGO WG#3 on Integration of Renewables conducted a study among the member countries which
include India. According to the study, many of the countries have already added significant levels
of Renewable generations and have ambitious plans to enhance the RE portfolio to levels of up to
40%-50% of the total generation. Spain which has got a capacity of over 16GW of RE generation,
has a dedicated Control centre called Control Centre for Renewable Energies (CECRE) (for
managing Renewables in the grid in real time operation.
A study was conducted among the international community to study the practices followed with
regard to transmission planning for RE, the variability of wind intermittency, Demand side
management, Short term forecasting, Grid Code requirements for renewables, Feed-in tariffs and
incentives for renewable like RECs, Black Start etc. following are the Conclusions:
 Integration of renewables face significant challenges with a 400% increase between 20082020 among the member states resulting in approx 300 GW of energy coming from
renewable sources
 Wind contribution to renewables is most significant in large parts of Europe and India
 Advancements of wind turbines and in particular wind related transmission technologies are
essential to meet climate change targets in the majority of the member states for example
(wind turbines are currently manufactured to circa 3-5 MW with developments in this area
there could be the possibility for sizes up to 10 MW per turbine output
 Wave technology is not seen as a major renewable contributor with the majority of its
developments focused on near shore devices
 The significance of wave technology can only be realised in the off shore (water depth >40m)
where large developments are conceivable
 At the moment, tidal power is not widely used due to the lack of suitable sites, its high cost
and the huge environmental impact it has across a wide area. Tidal power also only provides
energy periodically for around 10 hours in a day, when the tides are moving in and out.
 Thermal solar and Photovoltaic generation have power outputs that are fairly constant but
differs due to latitude and hence are more important for member states located nearer to the
equator. In addition, thermo solar have the possibility of storage of energy
 Hydro plants can be seen to be the most flexible form of renewable energy in the form of
either micro or large hydro plants, they play an important role in feeding local demand and
balancing energy sources on the power system
 In order to integrate renewables, greater interconnection between different power systems
should be planned.
 Short term forecasting and the variability of wind intermittency were sighted as the most
challenging issues. In particular the subject of short term forecasting of wind output to
balance the overall system is essential in ensuring proper use of the renewable sources to
make good any shortfalls in generation from conventional plants
 Battery storage technology is still in early stages with only small scale implementations of
1MW batteries (totalling 34MW) on the PJM system in 2010
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CIGRE 293 report of Working Group C1.3 on Electric Power System Planning with the Uncertainty
of Wind generation studied the following :
Plant tolerance to voltage and frequency variation
plant capability regards




tolerance to system faults and performance during and after faults
reactive power
active power management
power quality
control capability to manage
 power ramping up
 response
 voltage and power factor control
Real time information and control including the provision of modelling information.
There is a requirement for accurate standardised dynamic modelling and for compliance testing.
suggests the need to review standards for connection and suggests that wind farm curtailment could
be used to reduce backbone network development.
It is recommended, with respect to voltage tolerance:
Embedded Generation and renewables should be capable of tolerating voltage variations to the
same extent as other plant in that area of network.
It is recommended that, with respect to frequency withstand capability:
Since all parts of a network are subjected to similar frequency fluctuations, embedded and
renewable generation should be required to tolerate the fluctuations to the same extent as other
generation.
System Operators periodically review their requirements. Increased system size, differing plant mix
and the advent of interconnection may affect the possibility of large frequency movements or rates of
change. The smaller the frequency change requirements the easier it will be to detect wide area
islanding using traditional protective approaches.
“It is recommended that System Operators consider the degree of penetration of wind farms likely in
the long run and make provision in Grid Codes for future control of frequency using wind turbines,
if the penetration is likely to warrant the need inside the life of the wind turbine control systems.
System Operators should consider to what extent any energy regulation element of their Code should
apply retrospectively. System Operators should consider the extent to which control of ramp rate is
required to avoid serious frequency fluctuations from the variability of wind farm output. Various
automatic mechanisms are available but perhaps the simplest prevents ramping upwards when the
system frequency is already high.”
6. The Road ahead
To manage the challenges faced by the variable and intermittant nature of the RE, there is need for
Energy storage technologies and deployment of Smart Grid technologies.
Energy storage technologies :
Pumped hydro storage
High Energy Battery storage
Storage Capacitors
Superconducting Magnetic Energy Storage (SMES)
Compressed Air Energy Storage (CAES)
Flywheel energy storage
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Thermal Energy Storage
Smart Grid applications
Plug-in Hybrid Electric Vehicles (PHEV)
Pumped hydro storage :
Pumped hydro is the only energy storage technology deployed on a gigawatt scale in the United States
and worldwide. In the United States, about 20 GW is deployed at 39 sites, and installations range in
capacity from less than 50 MW to 2,100 MW. Many of the sites store 10 hours or more, making the
technology useful for load leveling. PHS is also used for ancillary services. PHS uses conventional
pumps and turbines and requires a significant amount of land and water for the upper and lower
reservoirs. PHS plants can achieve round-trip efficiencies that exceed 75% and may have capacities that
exceed 20 hours of discharge capacity. Environmental regulations may limit large-scale above-ground
PHS development. However, given the high round-trip efficiencies, proven technology, and low cost
compared to most alternatives, conventional PHS is still being pursued in a number of
locations.Alternative lower-impact configurations have been studied, including using a natural or mined
underground formation for the lower reservoir, but this configuration has yet to be commercialized.
High-Energy Batteries can generally provide rapid response hence can be “designed” for energy
management. Several battery technologies have been demonstrated or deployed for energy management
applications. The commercially available batteries targeted to energy management include two general
types: high-temperature batteries and liquid electrolyte flow batteries.
The most mature high-temperature battery as of 2009 is the sodium-sulfur battery, which has worldwide
installations that exceed 270 MW. Alternative high-temperature chemistries have been proposed and are
in various stages of development and commercialization. One example is the sodium-nickel chloride
(ZEBRA) battery.
The second class of high-energy batteries is the liquid electrolyte “flow” battery. This battery uses a
liquid electrolyte that flows across a membrane. The advantage of this technology is that the power
component and energy component can be sized independently. As of 2009, there has been limited
deployment of two types of flow batteries – vanadium redox and zinc-bromine. Other combinations such
as polysulfide-bromine have been pursed, and new chemistries are under development
Capacitors Capacitors store electricity in an electric charge. Capacitors have among the fastest response
time of any energy storage device, and are typically used in power quality applications such as providing
transient voltage stability. However, their low energy capacity has restricted their use in longer timeduration applications. A major research goal is to increase their energy density and increase their
usefulness in the grid
Superconducting Magnetic Energy Storage (SMES) SMES stores energy in a magnetic field in a coil
of superconducting material. SMES is similar to capacitors in its ability to respond extremely fast, but it
is limited by the total energy capacity. Work is on for increase their energy storage capacity.
Compressed Air Energy Storage (CAES) CAES technology is based on conventional gas turbine
technology and uses the elastic potential energy of compressed air. Energy is stored by compressing air
in an airtight underground storage cavern. To extract the stored energy, compressed air is drawn from the
storage vessel, heated, and then expanded through a high-pressure turbine that captures some of the
energy in the compressed air. The air is then mixed with fuel and combusted, with the exhaust expanded
through a low-pressure gas turbine. The turbines are connected to an electrical generator.
CAES effectively is a combined cycle gas turbine (CCGT) with an electrical compressor acting as
storage load. Air is compressed into an underground cavern by operating the compressor during
periods of low marginal cost and the compressed air is expanded and used for power generation in a
CCGT during peak hours. CAES has been modeled in a similar way as the UPAC, but with a roundtrip electrical efficiency of 181% and a consumption of natural gas of 4.1 GJ/MWh. This comes
down to a total efficiency – (natural gas + compressor energy) / electricity generation – of 60%,
which is the best practice for a modern CCGT.
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Flywheel energy storage (FESS),
Flywheels store energy in a rotating mass. Flywheels feature rapid response and high efficiency, making
them well-suited for frequency regulation to take advantage of high prices in frequency regulation
markets
Thermal Energy Storage is most often associated with concentrating solar power storing thermal
energy from the sun that is later converted into electricity in a conventional thermal generator. In this
application, thermal energy from the solar field is stored in molten salt or another medium. This energy
can be recovered later and used to generate electricity, which turns this technology into a dispatchable
source of energy.
Plug-in Hybrid Electric Vehicles (PHEV)
Electric vehicles (EVs – used here to represent both “pure” electric vehicles or plug-in hybrid electric
vehicles) are a potential source of flexibility for VG applications. The charging of EVs can potentially be
controlled, and provide a source of dispatchable demand and demand response. Controlled charging can
be timed to periods of greatest VG output, while charging rates can be controlled to provide contingency
reserves or frequency regulation reserves. Vehicle to grid (V2G) (where EVs can partially discharge
stored energy to the grid) may provide additional value by acting as a distributed source of storage. EVs
could potentially provide all three grid services discussed previously. Most proposals for both controlled
charging and V2G focus on short-term response services such as frequency regulation and contingency.
Their ability to provide energy services is more limited by both the storage capacity of the battery, as
well as the high cost of battery cycling. This could restrict their ability to provide time-shifting (energy
arbitrage) beyond their ability to perform controlled charging.80 The role of V2G is an active area of
research, and because electrified vehicles in any form have yet to achieve significant market penetration,
it is difficult to assess their potential as a source of grid flexibility. However, analysis has demonstrated
potential system benefits of both controlled charging and V2G. The role of EVs as an enabling
technology requires additional analysis of their unique temporal characteristics of availability, unknown
battery costs and lifetimes, and the availability of smart charging stations to maximize their usefulness
while parked.
Smart Grid applications:
Many of the Smart Grid technologies can be successfully deployed in integrating variable generation
sources.
With the impetus provided by the MNRE and CERC through a slew of incentives given to the
RE generators, the huge potential of RE will be tapped and eventually the Grid operators
should gear up for Large Scale Integration of RE to Grid.
Acknowledgements :
Sincere thanks are due to authorities of TANTRANSCO, Enercon for sharing their experience and
information in understanding the Renewable energy generation which helped in making this
document.
Bibliography:
1. NERC’ report on “Accommodating High Levels of Variable Generation” April 2009
2. European Wind Integration Study (EWIS)- Towards a Successful Integration of Wind Power
into European Electricity Grids
3. CIGRE 293 report of Working Group C1.3 on Electric Power System Planning with the
Uncertainty of Wind generation April 2006
4. CIGRE SC C6 report on Distribution systems and Dispersed generation and renewables.
5. CAISO report on Transmission and operating issues and recommendations for integrating
renewable resources on the California ISO-controlled Grid November 2007
6. Draft Indian Wind Grid Code by PRDC , Bangalore
7. IEC 61400 –Standards for Wind Generators
Integration of Renewable Energy – Study material for System Operator’s training program, NPTI
23
8. CERC (Terms and Conditions for Tariff determination from Renewable Energy Sources)
Regulations, 2010.
9. CERC (Terms and Conditions for Tariff determination from Renewable Energy Sources)
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intermittent generation on the British electricity network
Websites :
21. http://www.mnre.gov.in/
22. http://www.cwet.tn.nic.in
23. http://www.gwec.net/
24. http://www.wind-integration.eu
25. http://www.windpower.org/en/
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