Tag: Wigton Wind Farm

  • Electrical Integration of Wind Farms

    Electrical Integration of Wind Farms

    In this article, we will look at the integration of modern wind turbine generators (and by extension wind farms) into the electric power grids.

    Modern wind turbine generator

    Modern wind turbine generators (WTG) can be broadly categorized as either vertical axis or horizontal axis. The horizontal axis wind turbine design is much more efficient at extracting power from the wind than a vertical axis wind turbine and is widely accepted as the industry standard design for large-scale applications. It generally consists of a rotor with three blades that are connected to the nacelle, which contains the electric generator and other auxiliary parts, via its hub. The nacelle houses the gearbox (where applicable), electric generator and other auxiliary parts at the top of the tower. An electric transformer, normally located at the base of the tower, is used to step up the voltage at the terminal of the generator (usually less than 1 kV) to a voltage level suitable for integration (usually medium voltage, i.e. up to 36 kV).

    Parts of a wind turbine

    Wind energy conversion

    A WTG, like all other forms of power generators, is an energy conversion system. The wind turbine itself converts the kinetic energy in the wind to mechanical (or rotational) energy. The mechanical energy is then converted to electrical energy using one of several types of electric generators (alternator). Overall the WTG converts the energy in the wind into electrical energy that can be fed directly into modern power grids (micro, mini, island or interconnected) or used in standalone installations. The former is the focus of this article.

    Wind energy conversion process

    The mechanical power produced by a turbine is dependent on the air density, rotor swept area and wind speed as per the following expression.

    where:
    ρ – air density (typically 1.225 kg/m3 at sea level with standard conditions, i.e. at a temperature of 15 °C and an atmospheric pressure of 101.325 kPa)
    A – area swept by the rotor blades
    v – wind speed
    Cp – so-called power coefficient of the wind turbine. The power coefficient is a nonlinear function of the blade pitch angle (β) and the tip-speed ratio (λ). The electrical power (Pe) is usually given by the WTG power curve, as shown below.

    The electrical power (Pe) is usually given by the WTG power curve, as shown below.

    WTG power curve (pitch regulated vs stall regulated)

    The electrical power curve also shows the impact of the various control techniques applied to the WTG. For example, stall regulated and pitch regulated. Stall and pitch regulation refers to the aerodynamic/mechanical control techniques that are applied to the turbine itself to ensure that the mechanical power produced by the turbine does not exceed the power rating of the electrical generator.

    Stall regulation was primarily used on Type 1 WTG given its fixed speed nature. It involved the natural/aerodynamic stalling/slowing of the turbine blades at high wind speeds (generally above the rated wind speed) until the turbine stalls or shutdown occurs at the cut-out wind speed.

    Pitch regulated, on the other hand, is the more modern of the two techniques and it involves the pitching or axial rotation of the turbine’s blades to control the rotational speed of the turbine’s shaft.  This allows for optimization of the electrical output of the generator over the entire operating range of the WTG, hence it also helps to smooth the electrical output of the WTG.

    Wind turbine integration concepts

    A wind turbine integration concept primarily refers to the method used to convert the mechanical energy generated by the turbine to useful electrical energy. There are four industry-standard wind turbine integration concepts, namely, Type 1 WTG, Type 2 WTG, Type 3 WTG and Type 4 WTG as described below.

    Type 1 WTG:

    The Type 1 WTG is the simplest wind turbine integration concept. It is implemented with a squirrel-cage induction generator (SCIG) and is connected directly to the grid via its step-up (coupling) transformer, as shown below. The WTG speed is fixed (or nearly fixed) to the grid frequency, and it generates real power (P) when the WTG shaft rotates faster than the grid frequency (i.e under a negative slip condition). Slip is the difference between the grid speed (frequency) and the generator shaft speed. Positive slip occurs when the SCIG is operated as an electric motor.

    Type 1 – Fixed-speed, squirrel cage induction generator (Molina & Mercado, 2011)

    Type 1 WTG are simple, robust and economical. However, one major drawback of the induction machine used herein is that it consumes reactive power for its excitation. The solution to this problem is usually to include capacitors within the nacelle or at the collector bus. Another drawback is the large currents the machine can draw when started “across-the-line.” To ameliorate this effect a thyristor-based (AC/AC controller) soft starter is generally used to manage the connection of the WTG to the grid on starting. See our previous article titled “Wind turbines – I’m a Big Fan!” for additional information on how a Type 1 WTG starts up.

    Type 1 WTGs are installed at the following wind farms in the Caribbean:

    1. Wigton I wind farm in Jamaica. It consists of twenty-three (23) NM52/900kW WTGs. The NM52/900kW is a stall regulated turbine with a two speed (pole switching) SCIG.
    2. Maddens wind farm in St. Kitts. It consists of eight (8) GEV/275kW WTGs. The GEV/255kW is a pitch regulated turbine with a two speed (pole switching) SCIG.

    Type 2 WTG:

    Another problem with the Type 1 WTG generator is its near fixed speed operation (at approximately 1-2% slip), which cause large variation in electrical output during gusty wind conditions. This is because the aerodynamic control techniques described above are not fast enough to limit or smooth power output during these fast-changing wind conditions. The Type 2 WTG was developed to solve this problem in a cost-efficient manner.

    In Type 2 WTG, the squirrel cage induction generator is replaced by a wound rotor induction generator with the stator circuit connected directly to the grid via the step-up transformer (as in the case of the Type 1 WTG) and a variable resistance wired into the rotor circuit, as shown below.

    Type 2 – Variable Speed, wound rotor induction generators with variable rotor resistance (Molina & Mercado, 2011)

    The variable resistance is generally accomplished with a resistor and power electronics external to the rotor with currents flowing between the resistors and rotor via slip rings. Alternately, the resistor and electronics can be mounted on the rotor, eliminating the slip rings.

    The variable resistors control the rotor current so as to keep constant power even during gusting conditions. It allowed for a speed variation of 10% compared to the typical 1% slip of the Type 1 WTG. The corresponding drawback with this solution is that excess energy due to overspeeding from gusty conditions is dissipated in the resistors as waste heat energy. That is not a problem in itself, however, since the only alternative is to waste the excess wind energy by pitching the rotor blades out of the wind.

    This concept is not used in any of the wind farms in the Caribbean. It was first introduced by Vestas in the early 1990s termed OptiSlip and formed part of the V39/600kV to V47/660kW WTGs. This was further enhanced and renamed OptiSpeed in Vestas Type 3 WTG design.

    Type 3 WTG:

    The Type 3 WTG, known commonly as the doubly-fed induction generator (DFIG) or doubly-fed asynchronous generator (DFAG), takes the Type 2 design to the next level, by adding variable frequency ac excitation (instead of simply variable resistance) to the wound rotor circuit, as shown below.

    The additional rotor excitation is supplied via slip rings generally by a current regulated, voltage-source converter, which can adjust the magnitude and phase of the rotor currents nearly instantaneously. This rotor-side converter is connected back-to-back with a grid side converter, which exchanges power directly with the grid via the coupling transformer.

    Type 3 – Variable speed, wound induction generators with a rotor-side converter (Molina & Mercado, 2011)

    In the event of over‐speed conditions, the back-to-back convert absorbs this extra energy and provides additional output energy to the grid (this mode of operation is called super-synchronous mode). This energy would have been wasted as heat in a Type 2 WTG. On the other hand, when under‐speed conditions persist, the back-to-back converter extracts energy from the grid and supply this to the rotor (this mode of operation is called sub-synchronous mode). The converter is therefore bidirectional.

    These two modes allow a much wider speed range, both above and below synchronous (grid) speed by up to 50%. The greatest advantage of the DFIG is that it offers the benefits of separate real and reactive power control, much like a traditional synchronous generator, while being able to run asynchronously.

    The Type 3 WTG is the most widely used, on wind farms in the Caribbean, of the four concepts discussed here. For example, it is used on wind farms in the following islands:

    1. Jamaica – Wigton II & III and BMR wind farms (Vestas and Gamesa, respectively),
    2. Dominican Republic – All wind farms (Vestas and Gamesa), 
    3. Puerto Rico –  Punta Lima wind farm (Vestas)
    4. Aruba and Curacao – All wind farms (Vestas)
    5. Cuba – Gibara wind farms (Vestas and Goldwin)

    All of which are pitch regulated.

    Type 4 WTG:

    The Type 4 WTG offers a great deal of flexibility in design and operation as the output of the rotating machine is sent to the grid through a full-scale back-to-back frequency converter, as shown below. The turbine is therefore allowed to rotate at its optimal aerodynamic speed, resulting in a “wild” AC output from the electrical generator.

    In addition, the gearbox may be eliminated, such that the machine spins at the slow turbine speed and generates an electrical frequency well below that of the grid. This is no problem for a Type 4 WTG, as the back-to-back converter manages the difference in frequency, and also offers the possibility of reactive power support to the grid.

    Type 4 – Variable speed, conventional generator with a full-scale converter (Molina & Mercado, 2011)

    One advantage of a Type 4 WTG is that it can use any type of electrical generator (alternator), but is primarily fitted with wound rotor synchronous machines (WRSG), similar to those in hydroelectric plants, permanent magnet synchronous machines (PMSG), or SCIGs.

    One drawback, however, is that the power electronic converters must be sized to pass the full rating of the rotating machine, plus any capacity to be used for reactive compensation. This increases the overall cost of ownership of the WTG.

    Type 4 WTG can be found at the following wind farms in the Caribbean:

    1. Munro wind farm in Jamaican. It consists of four (4) U50/750 kW. The U50/750kW is a pitch regulated turbine with a PMSG.
    2. Morotin wind farm in Bonaire. It consists of twelve (12) E44/900kW. The E44/900kW is a pitch regulated turbine with a PMSG.
    3. Santa Isabela wind farms in Puerto Rico. It consists of forty-four (44) SWT108/2300kW. The SWT108/2300kW is a pitch regulated WTG with a SCIG.

    Wind farm electrical connectivity

    The following diagram illustrates the interconnectivity of the WTGs within the Wigton I wind farm in Jamaica.

    Electrical oneline diagram of a wind farm

    In this plant, twenty (23) Type 1 WTGs are connected directly to the grid (i.e no power electronic converters) via step-up transformers that convert from the 690 V at the terminal of the WTGs to the collector feeder voltage of 24 kV. The top thirteen (13) WTGs are connected in a daisy-chain fashion (parallel) to create one collector circuit and the bottom ten (10) WTGs are similarly connected to form the other. The two feeders terminate on a 24 kV collector bus at the wind plant substation that is used to parallel connect the two collector circuits to the grid interconnection (station) transformer. The grid interconnection transformer steps up the voltage from the collector bus to 69 kV. In this case, the wind farm is interconnected to the utility grid via a single 69 kV transmission line.

    In the next article, we will look at some of the technical requirements when integrating WTGs/wind farms into regional grids. These technical requirements are generally outlined in the national grid codes or interconnection agreements where no grid code exists.

  • Electricity from the Wind – Part 1

    Electricity from the Wind – Part 1

    Wind as a source of electric energy in the Caribbean is now becoming commonplace, with utility-scale wind power plants in operation on Aruba, Bonaire, Curacao, Cuba, Dominican Republic, Guadeloupe, Jamaica, Nevis, Puerto Rico, and Martinique. Barbados, Guyana, and St. Lucia are next in line to add utility-scale wind energy to their energy mix.

    Utility-scale wind power plants consist of several wind turbines, most of which are usually connected to each other in a daisy-chained fashion. The turbine, which is the heart of the plant, converts the kinetic energy of wind into electricity. A modern wind turbine consists of a three-blade rotor that captures the energy from the wind and drives a generator to produce electricity. The rotor and the nacelle, which contained the electric generator and the other necessary parts, are installed at the top of a tower, as shown below. The nacelle and the blades are controlled based on measurements of the wind speed and direction.

    parts of a wind turbine

    The amount of power that a wind turbine can extract from the wind is primarily dependent on the rotor swept area (A) and the wind speed (U). Therefore, to extract maximum energy from the wind, turbine manufacturers have been increasing the rotor diameter of their wind turbines over the decades, as shown below. Likewise, wind farm developers are always scouting for areas across the globe with high and stable wind speed all year round to develop economically competitive wind projects.

    wind turbine growth over the decades

    The actual power output of a wind turbine is limited by physical restrictions and is best illustrated by its power curve. The power curve of a wind turbine shows the electrical power output of the wind turbine versus the wind speed. An example of a power curve is shown below. It represents a Vestas V112-3.3 wind turbine as used in the case of the BMR wind farm in Jamaica. It has a rotor diameter of 112 meters and a rated/nominal power of 3.3 MW.

    V112-3.3 Power Curve

    The operating range of the wind turbine is defined by the cut-in and cut-out wind speeds. At the cut-in wind speed, typically around 3 m/s, the turbine starts to operate and produce electric energy. The cut-out wind speed, 25 m/s in the case of the V112-3.3 turbine, demarcates the upper safe operating wind speed at which point the turbine will stop producing electric energy and shut itself down. The rated wind speed is the wind speed at which the turbine produces its rated power output. The rated power of the V112-3.3 turbine is reached at 13 m/s.

    If this wind turbine was to operate at rated power for one hour it would produce 3.3 MWh (3,300 kWh). This is approximately 150% of the annual energy consumption of the average family home in Jamaica. However, wind turbines don’t always operate at their rated power output, due to the variability of the wind speed. Therefore, a measure known as capacity factor, is typically used to assess the efficiency of a turbine or wind farm. It is defined as the average power output of a wind turbine/farm as a percentage of the rated power of the turbine/wind farm.

    For most wind turbines erected on land, the capacity factor is between 20-40% or expressed in full-load hours it is around 1,800-3,500 hours per annum. The capacity factor for the Wigton and BMR wind farms in Jamaica are shown in the following table along with their rated power and estimated annual energy production based on their capacity factors.

    Wind Farm Capacity (MW)Capacity Factor (%)Annual Energy (MWh)
    Wigton I20.73563,466.20
    Wigton II183352,034.40
    Wigton III243063,072.00
    BMR36.334108,115.92
    Munro34010,512.00
    Total 10232286,688.52
    Capacity Factor for wind farms in Jamaica

    From the total install capacity of 102 MW and the total estimated annual energy of 286,688,52 MWh, an overall capacity factor of 32% is estimated.

    In part 2, we will look at turbine design parameters for specific wind sites.

  • Companies go Head to Head to Supply RE in Jamaica

    Companies go Head to Head to Supply RE in Jamaica

    Jamaica is currently leading the English speaking Caribbean in the use of renewable energy (RE) at the commercial level and will probably continue to do so for some time to come. This statement is based in one part on its (more…)

  • The Economics of Wind Power in Jamaica

    The Economics of Wind Power in Jamaica

    In late 2013, the Office of Utilities Regulation (OUR) named three preferred bidders for the supply of up to 115 MW (megawatts) of electricity generation capacity from renewable energy. The three preferred bids amounted to a total 78 MW of energy only renewable energy capacity, including two projects offering energy from wind amounting to 58 MW, and one offering solar amounting to 20 MW. The proposed delivery price to the grid ranged from US$0.1290 to US$0.1880.

    The preferred bidders were:

    1. Blue Mountain Renewables LLC, to supply 34 MW of capacity from wind power at Munro, St. Elizabeth;

    2. Wigton Windfarm Limited, to supply 24 MW of capacity from wind power at Rose Hill, Manchester; and

    3. WRB Enterprises Inc., to supply 20 MW of capacity from Solar PV from facilities in Content Village, Clarendon.

    The 20 MW solar farm will be the first of its kind in the Island, however Jamaica’s first grid-connected wind-powered generator was commissioned in February 1996 at Munro College. This wind turbine-generator, a Vestas V27 – 225 kW, was also the first grid-connected wind-energy source in the English-speaking Caribbean. The project was funded primarily by the Environmental Foundation of Jamaica (EFJ), but also included a long list of local companies and individuals. The total installation cost of the facility was US$300,000. However, much of the local services, such as JPSCo’s services and Alpart’s crane services, were donated free of cost.

    The overwhelming success of the the Munro College wind turbine encouraged the Petroleum Corporation and the Government of Jamaica to commission Jamaica’s first large scale wind farm at Wigton (in the parish of Manchester) in 2004. The initial 20.7 MW wind farm, which came to be known as Wigton I, comprises of twenty three (23) NEG Micon NM52 – 900 kW wind turbines. The project was financed at a total cost of US$26.2 million with equity injection of US$ 3.2 million from the Petroleum Corporation of Jamaica (PCJ), a US$ 16 million loan from the National Commercial Bank of Jamaica (NCB) and a grant of US$ 7.0 million from the Netherlands Government.

    A midst several changes, including $150 million in lost revenues due to unfavorable energy rates and $120 million due to penalties imposed by JPS for reactive power demand and a fail divestment attempt in early 2007, the Wigton wind farm was expanded during the period 2009 to 2010 to include nine (9) Vestas V80 -2.0 MW wind turbines. The 18 MW project, now called Wigton II, was financed from the PetroCaribe Development Fund at total cost of US$49.9 million.

    In late 2010, JPS (the owner and operator) commissioned its first wind project – a 3 MW wind farm at Munro, St. Elizabeth. This project comprises of four (4) UNISON U50 – 750 kW wind turbines and was completed at a total cost of US$9.3 million.  The Munro wind farm interconnects to JPS 24kV distribution system unlike the Wigton wind farms, which interconnects to JPS 69kV system via a 11km long tie-line. It is worthwhile noting that the grid interconnection cost can account for as much as 8-9% of the total project cost. In the case of the Wigton wind farms the 11kM 69kV line was included in the capital cost of the initial project.

    The two new wind farms coming out of the OUR latest request for renewable energy in addition to the national grid are projected to cost US$40 million for the WWF’s (Wigton Windfarm) 24 MW wind farm and US$90 million for the BMR’s (Blue Mountain Renewables) 34 MW wind farm. The cost of these two project forces me to ask one key question “how does public vs private investor wind power projects costs compare?”. I thought that a good way to get a fair comparison was to look at the projects that had/have  the same/similar time horizon. So, I decided to firstly compare the Wigton II and JPS Munro wind farm projects (which were both commissioned in 2010) and secondly the proposed Wigton III and BMR Munro wind farm projects (both scheduled to be commission in 2016), as shown below.

    privatevspublic

    This comparison revealed two important facts:

    1. Private investor wind projects in Jamaica cost more than public wind projects. In the first case, the JPS Munro wind farm cost approximately 1.1 times the cost of the Wigton II wind farm on a per megawatt basis. Similarly, the proposed BMR Munro wind farm will cost approximately 1.6 times the proposed Wigton III wind farm on a per megawatt basis. It would be good to see a breakdown of the project cost to see exactly where the projects varied in term of cost.

    2. The cost of wind power has come down by 40% for public projects and 15% for private projects since 2009.

    Wind Capital Cost StructureThe cost of a wind project has a lot to do with its total size (economics of scale) however the most common way to compare wind project cost is on a per megawatt basis, as was done here. It is also worthwhile to add that the basic cost components of  wind projects typically include: turbine cost, grid interconnection, foundation, electrical installation, consultancy, financial cost, road construction, control systems, etc. The inserted table gives a break down of the % share of the total cost for each component.

    Public projects, in most cases, could have a competitive advantage in terms of the land rental, financial cost and road construction components which could possibly explain to some extent why public projects have been carrying lower project cost compared to the few private projects that we have seen in Jamaica’s recent renewable energy history.

  • Wigton makes move to increase its Capacity by 62%

    Wigton makes move to increase its Capacity by 62%

    Wigton Windfarm Limited’s (WWFL) plans to start expansion of its 38.7 megawatts (MW) wind farm by an additional 24 MW this year . Wigton phase III, which will be located in Rose Hill, Manchester on lands adjoining the (more…)

  • Is RE high on JPS’s agenda?

    Is RE high on JPS’s agenda?

    JPS posted on Linkedin that they are “the #1 investor in renewable energy in Jamaica.” Is there any truth to this? No there isn’t! See my comment and their follow-up comment below. The Petroleum Corporation (more…)

  • Jamaica Leads with 41.93 (MW)

    Jamaica Leads with 41.93 (MW)

    Jamaica currently leads the Caribbean in wind energy integration, boasting an installed capacity of 41.93MW (Megawatts). Its latest addition to the national grid being the Munro Find Farm, completed in September 2010. (more…)