Tag: BMR 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.

  • Weighing in on T&T’s 10% RE Target

    Weighing in on T&T’s 10% RE Target

    Trinidad and Tobago (T&T), has set an ambitious renewable energy (RE) target of 10% of installed capacity by 2021. This equates to approximately 200 MW given the combined installed capacity of the two islands is over 2000 MW of natural gas based power generation.

    T&T is the only nation in the western hemisphere, and the second in the world, that generates 100% of its electricity needs from natural gas. Therefore, unlike the other islands in the Caribbean T&T is already energy independent, since all the natural gas used is sourced locally through its sophisticated network of pipelines. As a consequence, T&T have seen the lowest and most stable electricity rates in the region over the last decade.

    Given that T&T is  already energy independent, the integration of renewables will have the effect of reducing the natural gas demand for electricity production and thereby increasing the levels available for export and/or for use in the well developed local petrochemical industry. This is now being championed by the energy sector as a means to increasing government revenues in a time when the nation is witnessing a significant decline in revenues and consecutive budget deficits.

    We decided to weigh in on the potential savings to be derived from this level of renewable energy integration. In order to do this we first had to assume a mix of renewable energy technologies. Since the objective is to use renewables as a means to reduce the consumption of a natural gas and thus increase government revenues, it thus implies that the 200 MW will come from utility scale renewable energy projects only.

    We therefore opted to break up the 200 MW into 120 MW of onshore wind, 60 MW of solar pv and 20 MW of waste to energy. No consideration is given to the technical feasibility of this RE mix. There are, however, ongoing discussions on the subject of undertaking solar and wind resource assessments and there are currently no known technical barrier limiting grid connection.

    As the based case, we looked at Jamaica, which has over 150 MW of utility scaled renewables connected to the grid, to formulate a case for wind and solar in T&T. In 2016, Jamaica commissioned 60 MW of wind and 20 MW of solar capacity at a cost of approximately US $200 million.

    If we use the 36 MW BMR Wind Farm in Jamaica, commissioned in 2016 at a cost of US $90 million, as an example then, 120 MW of utility scale onshore wind capacity should not cost T&T more than US $300 million in 2018, given that the capital cost of onshore wind fell by 20% between 2010 and 2017. Conservatively, 120 MW of wind can generate 285,000 MWh annually, thus avoiding the use approximately 2,850,000 MMBTU of natural gas annually for the production of electricity.

    Similarly, if we use the 20 MW Content Solar Farm in Jamaica, also commissioned in 2016 at a cost of US $63 million, then 60 MW of utility scale Solar PV should not cost T&T more than US $190 million in 2018, since the capital cost of solar PV fell by 68% between 2010 and 2017. 60 MW of solar can conservatively generate 95,000 MWh annually, thus avoiding the use of approximately 950,000 MMBTU of natural gas annually.

    There has been some discussion around the potential of a waste to energy (WtE) facility at the country’s largest landfill, located on the outskirts of the capital city. The Solid Waste Management Company (SWMCOL) estimates that the landfill receives approximately 1000 tonnes of uncharacterized waste daily. We estimate that a 20 MW WtE facility can be developed at the proposed site to produce energy for the national grid.

    Using the information on the Solid Waste Authority of Palm Beach County Renewable Energy Facility 2 (REF2), a 100 MW mass burn WtE facility commissioned in 2015 at a cost of US $672 million, we assume, therefore, that a similar facility rated at 20 MW should not cost T&T more than US $150 million. Given that a mass burn WtE facility is a steam power plant at its core, then a 20 MW plant should generate approximately 150,000 MWh annually and thus avoiding the use of approximately 1,500,000 MMBTU  of natural gas annually.

    natural gas price projections

    Therefore, from our selected portfolio of renewables we see that the potential exist to avoid approximately 5,300,000 MMBTU of natural gas annually. However, this does not come cheap as total investment cost estimates to US $640 million. The chart to the left shows the projected price of natural gas up to 2040.

    If we therefore look at the pessimistic case, we see that the price of natural gas in the US is projected to vary between US $3.00 to $4.00 over the remaining period and averages about US $3.50. Using this price we estimate a potential earning of US $18.6 million annually. The optimistic outlook, on the other hand, shows an average price of approximately US $6.80 resulting in a potential earning of US $36 million per annum.

    Both the pessimistic and the optimistic outlooks gave very large negative net present values using a 10% discount rate over a 20 year period. The optimistic case only gave a positive net present value for a discount rate of about 1%.  The analysis assumes that the projects would be government owned and did not take into consideration the operation and maintenance cost over the life of the project. Overall, it shows that the projected revenues to be derived from the sale of the avoided natural gas on the open market will not return the capital invested over a 20 year horizon.