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

  1. Scenario of Renewable Energy generation in India :
  2. 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

  1. International Experience
  2. 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 demand 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 conventionaland 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 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 components1) 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, typically30%, of the total wind turbine-generator power output.

4. Full conversion Wind Turbine-Generator (Type-4)

The entire turbine power output through an AC-DC-AC power electronic converter system. the output current of a Type 4 wind turbinegenerators 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 conventionalgenerator.

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 / 1985 / 1988 / 1990 / 1995 / 1998 / 2000 / 2005 / 2010
MW / 50kW / 300kW / 500kW / 600kW / 1.5 MW / 2.5 MW / 6MW / 10MW
Hub height / 18m / 38m / 48m / 60m / 80m / 90m / 120m / 150m

2.1.2 Main components of a wind electric generator:

1. Tower 2. Nacelle 3. Rotor 4. Gearbox 5. Generator

6. Braking System7. Yaw System8. Controllers9. Sensors

2.1.3 Various forms of control :

Mechanical Controls :

  1. Pitch control 2. Yaw control 3. Stall control (Active/ Passive )

Electrical controls :

  1. Voltage Control

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 rotorblades pitch exactly the amount required. This control scheme will normally pitch the bladesa few degrees every time the wind changes to keep the rotor blades at the optimum angle andmaximize output for all wind speeds. The same control mechanism could be used, inaggregate, by the operator to dispatch variable generation between minimum and maximumavailable 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.

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 responseto frequency changes) in the conventional sense per se is not available because wind can not be controlledto 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.

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 inFigure given below during system faults. Wind farms can be disconnected ifthe 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

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:

Installed Capacity / Potential
Wind Power / 13066 / 48756
Small Hydro Power / 2939 / 14292
Biomass Power / 997 / 8680
Bagasse Cogeneration / 1562 / 5000
Waste to Power(UrbanIndustrial ) / 72 / 7000
Solar Power (SPV) / 18 / 200000
Total / 18654 / 283728
Capacities in MW as on 31-12-10 / 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.

Distribution of Wind potential in India, Source : CWET, Chennai

Distribution of Solar potential in India, Source : MNRE,

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

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 shortcircuit level is weak and, as a result, problems such as under-/over-voltages, harmonics or