WIND POWER Utilization

Technology Growth in Wind Turbine Generators

• Wind turbine generators (WTGs) started as fixed-speed wind turbines with conventional induction generators and capacitor banks as static reactive compensators. Capacitors supplied reactive power for the air gap magnetic flux, which the induction generators could not produce.

• Denmark initially standardized on this model, terming it the Danish Concept. These turbines contributed 71.6% of the total WTGs there by 2006.

• Later, squirrel-cage rotors in induction generators were replaced with wound rotors. Variable rotor resistance, variable speed compatibility with gears, and capacitor banks became standard features.

• Doubly fed induction generators (DFIGs) followed with partially rated power electronic convertors. The converter helped to provide independent control of active and reactive power outputs of the WTGs. The PE converter rating was generally at 30% of the WTG rating.

• Finally, TWGs with added functions in the PE convertors arrived. The PE portion increased the costs but gave better control and helped in the fault-ride-through facility.

• This category constitutes just 0.2% of the total WTG population.

Nature of Wind

• Wind may blow steadily during certain periods, varying by day, season, location, and so on. Let us say the velocities fall within some zones. The wind may die down, falling almost to nil. Then it may rise from a very low speed.

• There may be a wind lull, when the wind dies out and then rises in short bursts. A wind gust is the opposite phenomenon to a wind lull. A very strong wind is a storm.

• This nature of wind makes it an unreliable source of power due to its variability and uncertainty.

Components of A Wind Turbine Generator

• The rotor blades, whose pitch is adjustable as per wind velocity so as to catch maximum wind energy.

• The gear box which adjusts the rpm of the rotor of the generator as closely as possible to the grid synchronous frequency.

• The generator, which converts mechanical input into an electrical output. Wind Turbine

Operating Conditions

1. For a given wind condition it should produce maximum possible power. This is possible when λ stands at λopt.2. There is a minimum wind condition below which the WTG becomes unstable.λis represents the crossover point for the stable condition limit. At λstall, the WTG will stall.two operating conditions for a WTG:

• Note that the ratio of WTG blade tip speed to wind speed, λ, plays an important part.

• The WTG control should perform in such a way that it is at λopt under different conditions of wind load.

• The rate of change in λ is given by another quotient x:

• A WTG is in an unstable condition when becomes positive. The past figure shows power versus speed curves of a wind turbine with wind velocity as a parameter. The dashed line is a boundary between high- and low-speed regions.

WIND POWER

• The figure relates the power coefficient to the tip speed ratio, λ, defined as the relationship between the rotor blade tip speed and the free speed of the wind for several wind power turbines. As stated earlier, in quantitative terms, the tip speed ratio is defined as λ=v/V=wR/V,

• where w is the angular speed of the turbine shaft, v the blade tangent tip speed, and R the length of each blade.

• It emphasizes the importance of knowing the purpose for which the energy will be used, to allow determination of the best selection for wind power extraction.

Mechanical Power

• The turbine mechanical power can be given by

• The air density ρ can be corrected by the gas law (ρ = P/RT) for every pressure and temperature with the following expression:

Wind generator technologies

Converter

DFIG

Gearbox

Grid

Converter

Converter

EESGGrid

Converter

PMSGGrid

Advantages Disadvantages

Smooth grid-connection Multi-stage gear box

Reactive power compensation Need carful protection

Rotor energy can be fed to grid Control complexity

Advantages Disadvantages

Flux control for minimizing loss External field excitation

PMs is not required (less cost) Heavy weight

Voltage is controllable Expensive solution

Converter

SEIGGrid

Gearbox

Advantages Disadvantages

flexible control More expensive conv.

Absence of brushes Higher conv. losses

Less cost and maintenance Multi-stage gear box

Advantages Disadvantages

Direct drive operation High cost of PMs

Higher efficiency and energy PM demag. at high T

Higher power to weight ratio Manuf. Difficulties

No additional power supply

Double Fed Induction Generator Electrical Exc. Synchronous Generator

Self Exc. Induction Generator Permanent Magnet Synchronous Generator

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SEIG Dynamic Modeling

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DFIG• A three-phase wound-rotor induction machine can be set up as a doubly-

fed induction motor. In this case, the machine operates like a synchronous motor whose synchronous speed (i.e., the speed at which the motor shaft rotates) can be varied by adjusting the frequency fRotor of the ac currents fed into the rotor windings.

• The same wound-rotor induction machine setup can also serve as a doubly-fed induction generator. In this case, mechanical power at the machine shaft is converted into electrical power supplied to the ac power network via both the stator and rotor windings.

• Furthermore, the machine operates like a synchronous generator whose synchronous speed (i.e., the speed at which the generator shaft must rotate to generate power at the ac power network frequency fNetwork can be varied by adjusting the frequency of the ac currents fed into the rotor windings.

DFIG

• The ac currents produced by the generator are converted into dc current by an AC/DC converter, then converted by another AC/DC converter back to ac currents that are synchronous with the ac power network. It is therefore necessary for the power electronics devices used in such a circuit to have the size and capacity to process 100% of the generator output power.

• The power electronics devices used in doubly-fed induction generators, on the other hand, need only to process a fraction of the generator output power, i.e., the power that is supplied to or from the generator rotor windings, which is typically about 30% of the generator rated power.

• Consequently, the power electronics devices in variable-speed wind turbines using doubly-fed induction generators typically need only to be about 30% of the size of the power electronics devices used for comparatively sized three-phase synchronous generators.

AC/DC power Conversion topologies:

• There are several ways for converting the AC alternate (from wind) power to DC,as a first stage of interfacing with the grid.

Advantages:• Simple designDisadvantages:• Uncontrolled output• Large harmonics

Advantages:• Controlled outputDisadvantages:• Poor power factor• Large harmonics

Advantages:• Controlled output• Large power (MW)Disadvantages:• Large harmonics• Large size & weight

Advantages:• Controlled output• Less harmonicsDisadvantages:• Large switching loss• Complex control

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DC/AC Inverter interface topologies for ac-loads and utility grid connectivity:

Voltage source inverter

Z-source inverter

Small and medium power applications such as:• Medium voltage industrial appl.• Wind farms• VAR compensators• active filters• FACTS

Small power applications such as:• Fuel cell Vehicles• UPSs• ASDs

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Current source inverters (CSIs) can be utilized for new ideas:

Large power applications such as:•plug-in HEVs•PM motor control•PVs grid tie inverters•Propulsion drive systems

Advantages:• Boost design • Large power (MWs)Disadvantages:• Large inductor size

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APPROPRIATE LOCATION

• To select the ideal site for placement of wind power turbines, it is necessary to study and observe the existence of enough wind to make extraction of energy possible at a desired rate.

• Although flat plains may have steady strong winds, for small-scale wind power the best choice is usually along dividing lines of waters (i.e., the crests of mountains and hills). In those geographical locations, there is good wind flow perpendicular to the crest direction. Some basic characteristics to be observed for defining a site are:• Wind intensities in the area• Distance of transmission and distribution networks• Topography• Purpose of the energy generated• Means of access

Performance Improvement through Blade Pitch Control

• At low speeds, the pitch angle is almost zero. Maximum possible energy is scooped up (maximum power strategy).

• At high speeds, the pitch angle increases. Beyond a certain wind speed, automatic mechanical brakes apply and electrical dumping resistances are used as loads.

Efficiency of a WTG• Average efficiency of a WTG is defined as the ratio of energy

delivered to grid to the energy at the turbine rotor shaft. As the energy is transmitted from one member to the next in the transmission system of a WTG, losses are incurred.

Losses in a WTG

• Average and rated efficiencies for the three different types of WTGs are 82–86% at low wind speeds and 89.7–89.9% at high wind speeds.

• Thus, weather forecast and past statistical data form important requirements for efficiency and reliability when integrating wind farm energies into today’s mega grids.

Flickers in the Output of a WTG

• There are two main causes of flicker in the supply from a WTG: Mechanically Related Causes• Motor turbine imbalance• Rotor blades passing in front of the wind structure• Structural modes due to mechanical eigenfrequencies

(frequencies at which there is mechanical resonance) Rotational sampling

• The flickers caused by these mechanical causes have a regular pattern, low amplitude, and a low-frequency range of 0.65 to 0.71 Hz.

Wind Velocity Related Causes. Wind flow has regular bursts that can cause flicker. This flicker has a high amplitude and a range of 0.01 Hz–10 Hz. This flicker is objectionable and has been investigated deeply.

CONNECTION OF WIND ENERGY PLANTS TO THE GRID—THE GRID CODE

• In the early days of wind electricity generation, the plant sizes were small. With an induction generator, there was no problem of synchronizing with grid frequency.

• External capacitors took care of voltages; when there was a disturbance in the grid leading to low voltages at the point of connection, the wind plants were disconnected and stayed disconnected until the grid disturbance was cleared.

• Today, wind plant sizes have increased. Should a wind plant get disconnected due to a grid disturbance, it could aggravate the situation. A grid code for interconnection has evolved.

Grid Code

• The main features of the grid codes are:

• A low-voltage ride-through (LVRT) is essential for getting into a grid.

• Accurate power control at a PF of ±0.95 has to be maintained at the point of connection.

• Accurate plant models must be submitted.

• SCADA data must be supplied as agreed with the system operator.

Low-Voltage Ride-through

• Wind energy farms are now a sizeable constituent of the power supply. If a fault develops in the grid, with voltages at the point of common coupling between the grid and the wind farm, falling to low levels during the fault clearance time, the wind farm should not disconnect. In other words, it must have a low-voltage ride-through capacity.

• If a fault occurs in the grid system to which a wind energy farm is connected, the voltage at PCC dips to a low percentage, depending on the severity and location of the fault.

• The control system clears the fault within the time frame specified under grid discipline.

A RESISTIVE BRAKING OF A WTG

• The figure below shows power-versus-speed characteristics of a WTG. The top curve represents conditions with a series damping break resistor (SDBR). The bottom curve shows the same without SDBR. A grid fault occurs at point 1 and the SBDR is switched in. The characteristics change over to the bottom one at point 2. It travels along 2–3 within the duration of the fault. At point 3, the SDBR is shorted and the characteristics jump to point 4.

• However, actual WTG at this changeover is at a low speed, corresponding to point 3, and also has a low power output. So the characteristics travel pack to point 5, which coincides with the original point 1. The SDBR has prevented a runaway increase in WTG speed.

• The ride-through periods for a WTG must be naturally much longer than those allowed under fault-clearance codes. Danish grid codes specify testing each WTG as per the specified voltage profile

MODELING OF A WIND TURBINE GENERATOR

• It is desirable first to understand how a vastly spread electricity power system operates physically. The following gives a brief sketch.

• A transmission system operator (TSO) looks after load following and power quality on a very small scale time scale, say on a minute or 10 minute basis.

• For this, he has a schedule of power offers from various generators. The TSO has also a schedule of time-bound requirements from the customers. He matches these and balances the load.

• The TSO also has an updated chart of transmission facilities with all their characteristics in his computer. He selects an optimum route for a load dispatch. • This route has minimum operating losses and costs. Modern

fast-operating computers and accurate data are essential for his work. Supplying of accurate models of WTGs is compulsory for this reason.

Method

• Electrical and mechanical parameters of a WTG are converted into algebraic quantities. These algebraic notations are used to develop algorithms to arrive at characteristic functions.

• Most of the grids in the world require a WTG dynamic model to be submitted to the transmission system operator for permission to join the grid.

• Typical Irish grid requirements are listed below. Any WTG greater than 5 kW must submit a model incorporating the following features:

1. Generator general characteristics

2. Turbine generator and drive train mechanical characteristics

3. Variation of power coefficients and pitch angle to tip speed ratio

4. Blade pitch control

5. Converter controls

6. Reactive components

7. Protection relays

• Time per step for simulation should not to exceed 5 microseconds. Although models for simulations for thermal and hydro generators have long been used and standardized, those for WTGs are still evolving and there are no standards.

Present Problem Areas in Modeling

1. Secrecy by WTG manufacturers who are developing the newest machines.

2. Non-standardization, beginning with numbering and naming components.

3. Numerical instability arising out of rounding off and truncating practices, among

others.

4. A large number of differing models.

Unit Commitment and Scheduling

• With power marketing growing in scale, load balancing becomes important. For this, the system operator must know accurately how much unit commitment he can have from the wind farm and how much capacity he should hold in reserve.

• This is a demanding task, considering the erratic nature of wind and the magnitude of loads to be handled. But the system operator has many tools in his hand, including dynamic scheduling and accurate hourly weather forecasts.

Dynamic Scheduling

• Dynamic Scheduling. With accurate modeling of the system and computerized software he can find out what can happen to the system when an electric component is added or subtracted or controlled in power output. All system components must be put in models for accurate simulation. Modeling right to the last details becomes important.

• Since the WTG technology is fast developing, standards for modeling these are not yet in place. Take the case of a 4.5 MW TWG weighing 450 t developed by Enercons in Germany. The very size contributes to LVRT on small duration faults. This large piece of apparatus is not the last step in WTG development.

Weather Forecasts

• Accurate Hourly Weather Forecasts. Weather is not all that erratic. Weather behavior falls into a pattern—daily, seasonal, periodical, and geography specific. Excursions out of this pattern might be in a band that can be anticipated.

• This, along with daily weather forecast by meteorology departments, can help the system operator on unit commitments from the wind farm on the day-ahead schedule as well as on the daily schedule, balancing them fairly closely.

• The system operator need not commit too much capacity to reserves. In fact, although wind energy costs are next to nil, their operational costs go largely toward unit commitments.

CAPACITY FACTOR OF A WTG

• Effective load-carrying capacity (ELCC) is defined as the amount of additional load that can be reliably served. In determining this capacity, two factors enter into the picture.

• A load served during peak hours has the highest going market rate. If there is a LOLP during this period, cost of power is taken into for determining the capacity factor of a WTG. This period will not necessarily always coincide with the period of delivery at the rated output of a WTG.

• Thus, WTG capacity during the period of LOLP counts against its rated capacity for planning purposes, unit commitments, and so on. Capacity value of a WTG has been shown to range approximately from 10% to 40% of the wind plant rated capacity.

CAPACITY CREDIT CONSIDERATIONS• How do the capacity considerations affect wind energy installations? In many

countries today, major utilities, mostly based on thermal generation, have to buy “green” energy such as wind energy at prices stipulated by the energy regulator. Typically, the basis for these prices is fixed on the following factors:

• Capacity factor

• Operations and maintenance cost reductions for the utilities

• Electrical loss reduction for the utilities

• Environmental benefit

• Hourly capacity factors are determined. The average of top 50% of the load hours (on both sides of the peak hours) is close to the estimation of the capacity factor for wind energy converters (WECs). In the deterministic method, historical data on wind is collected and the capacity factor is calculated. In the probabilistic method, this is calculated from the wind data statistics and forecasts. This gives too high a capacity.

• Thus, the capacity factor is important for WEC’s since it directly affects their earnings. The capacity factor is also important from the grid operational point of view, for daily planning as well as for hourly operations. The capacity factor is important for future planners of the grid system. Probabilistic methods might be more useful here.

WIND PENETRATION LIMIT

• The wind penetration is defined as the extent to which wind power can be added to a power system without compromising its operational reliability.

• Operational reliability in turn is defined in terms of a system’s dynamic response, requiring the system frequency to stay within the limits of protection for a set of wind-related events. This makes acceptable system dynamics the sole criteria for increasing wind penetration.

• Wind penetration simply means the extent to which wind power can be added to a power system without affecting its operational reliability. This reliability depends on the extent of reserves, external to the wind capacity, which are available to the system, internally or externally.

• The grid rule for Irish Electric Systems requires that the primary reserve in a system should be 75% of the single largest feed of the system. This should be available within 5 to 15 seconds subsequent to the event of the feed going out.

• Again, if the load factor of a system is 60% and the average wind plant capacity is 40%, then the penetration has been worked out by Smith as 30%. Average wind penetrations has been between 20% to 30%.

WIND ENERGY FARMS

• With rising contribution by wind power to electricity systems, individual WTGs or clusters thereof are giving up their place in the energy scenario to wind energy farms.

• These farms have a few noteworthy features. They are generally located away from load centers. Connecting them into the grid system requires almost dedicated transmission arms. Whereas WTGs can be planned, installed, and put in operation within a short time span, connecting transmission links require long-time planning and execution.

• Existing transmission lines might not have the capacity to carry the wind farm power, as happened in the case of an East European customer who contracted for wind energy from a Dutch supplier.

• An irregular flow on the direct line could also affect load flows and voltage regulation on the neighboring lines. This necessitates central control from the system operator over the operation and contribution by the wind energy farm.

MAINTENANCE OF WTG

• Problems applying to modeling also apply to standardizing maintenance. Manufacturers treat failure data as proprietary and are not very cooperative in disclosing this. Loads are high and of a fluctuating nature. So far, maintenance is of a corrective type.

• Downtime, loss of production, and repair costs are high. Problems are aggravated in offshore installations. Preventive maintenance is desirable. Geared WTGs operate through a weak shaft with torsion stresses.

• Quite often, there is mechanical resonance in the system, similar to electrical resonance. At this mechanical resonance frequency (about 1–2 Hz), there is shaft relaxation at shaft faults, leading to failures. Including mechanical characteristic in modeling is essential.

Home Assignment

• Given the MATLAB/SIMULINK model of the wind energy conversion system example explained during the lecture, apply the following changes to the model,

1. Replace the simple step change wind input with a properly-scaled actual wind pattern and comment on the effect of the wind speed variations on the turbine output power and voltage.

2. Change the rating of the 500 kW load to any another rating value in the range of (1-3 MW), and change the rating of the 2 MVA plant to any value in the range of (4-6 MVA), then apply appropriate design changes to the rating of wind turbine in order to successfully supply the local loads without violating the voltage or frequency limits on the grid connection point.