ECE 566: Grid Integration of Wind Energy Systems S. Suryanarayanan - - PowerPoint PPT Presentation

Power electronics for wind turbines References

ECE 566: Grid Integration of Wind Energy Systems

• S. Suryanarayanan

Associate Professor ECE Dept.

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References

Reminders and notifications

1

Mid-term exam scheduled for October 14, 2014 Next Tuesday

75-minutes take-home exam 645–8 PM (Mtn time) Closed notes, closed book, etc. 1 page cheat sheet allowed: letter-sized 2-sided; to be scanned and submitted along with answers Access and submission via RamCT Blackboard

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Capacitor bank controller [1]

Capacitor bank Cap banks are used with induction generators to supply reactive power (VARs) Traditionally, mechanically-switched cap banks were used with Type-I and Type-II wind turbines to minimize electrical power drawn by the induction machine from the grid Advantages of mechanically-switched banks: easy to control and cheap

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Capacitor bank controller [1]

Capacitor bank Induction generators in wind turbines can have a full load dynamic compensation

A certain number of caps may be connected or disconnected continuously based on average reactive power demand over a fixed window of time Since this reactive power drawn is dependent on wind speeds, it may lead to an excessive number of switching leading to:

Transient over-voltages Mechanical and electrical stress of capacitors

Number of switching operations will affect the capacitor lifetime, and also the wind turbine and gearbox reliability Over-voltage on the grid-side may cause heavy loading and damages to the wind turbine leading to increased maintenance costs

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Capacitor bank controller

Capacitor bank control figure taken from [2]

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Capacitor bank controller

Reactive power control in wind farm [3] Local capacitor bank control at terminals of WTG

1

For Type-I WTG, 2 sets of cap bank configurations, each corresponding to one set of poles, is included

2

Based on the number of poles in the operation, the banks are switched appropriately

Cable capacitance provides reactive compensation Substation power factor capacitors STATCOM or SVC

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Capacitor bank controller

Capacitor bank: Hybrid reactive compensator While mechanical switching is easy when the demand is known, it is difficult to do so when the demand for reactive power is variable such as in the wind turbine Local capacitor banks can be used with power electronic solutions such as a static compensator (STATCOM) to provide a hybrid compensator [4] Advantages

1

Dynamic control of power factor of wind farm

2

Independent control of reactive power at the turbine terminals, thus reducing losses in the plant

3

Available resources can be used for optimal reactive power management

4

Smooth electronic control instead of multiple mechanical switching events

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [1]

What is it?

1

A cheap and efficient power electronic component used for the interconnecting Types I and II WTG to the grid

2

At start-up, induction generators produce high currents (inrush), approximately 7–8 times the rated value which decays with time as the rotor picks up speed

3

If left unaddressed, this momentary condition may damage grid components and/or cause severe voltage disturbances

4

Protection devices must not trip for this momentary situation

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [1]

What is it?

1

A soft starter has two thyristors connected in anti-parallel for each phase

2

By controlling the firing angle (α) of the thyristors, a smooth interconnection of the Types I and II WTG to the grid can be achieved

3

Once the inrush currents subside and steady rated current flows, the thyristors are by-passed to allow direct-coupling to the grid

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [6], [5]

Firing a thyristor

1

A thyristor is a 3-junction 4-layered semiconductor device with alternating P and N types material

2

Also known as silicon controlled rectifiers (SCR)

3

Three terminals known as anode, cathode, and gate

4

“A thyristor can be switched off if the external circuit causes the anode to become negatively biased (a method known as natural—or line—commutation). In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.” [5]

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter

Firing a thyristor. Figure taken from [5]

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [6], [5]

Firing a thyristor

1

Unlike diodes, the SCR does not automatically start conducting when a positive voltage across the anode-cathode is applied

2

Rather, the conducting state is achieved by supplying the gate terminal with an impulse current; this is knows as ‘firing’ a thyristor

3

Once fired, the thyristor behaves like a diode until an

• ff-state is reached

4

Off-state is reached by letting the conducting current value fall below a minimum value (holding current)

5

Transition from off-state to conducting state is achieved by firing new pulse current sequence to the gate

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [6], [5]

Firing a thyristor

1

Unlike diodes, the SCR does not automatically start conducting when a positive voltage across the anode-cathode is applied

2

Rather, the conducting state is achieved by supplying the gate terminal with an impulse current; this is knows as ‘firing’ a thyristor

3

The point on the waveform at which the thyristor is triggered into conduction is called the firing angle (α)

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [6], [5]

Firing a thyristor. Figure from [7]

1

Once fired, the thyristor behaves like a diode until an

• ff-state is reached

2

Off-state is reached by letting the conducting current value fall below a minimum value (holding current)

3

Transition from off-state to conducting state is achieved by firing new pulse current sequence to the gate

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [6], [5]

Firing a thyristor. Figure from [7]

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [7]

Firing a thyristor Types of firing

1

Zero-voltage cross-over: The thyristor is turned on only when the instantaneous sinusoidal voltage at the gate reaches zero value

2

Phase-angle control: By varying the phase angle, the timing

• f the triggering of gate pulses is controlled (via a delay)

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [8]

Soft starter A soft starter has 6 thyristors—2 per phase—connected in anti-parallel or back to back configuration To limit the rate of change of voltage across the thyristors, a snubber (RC) circuit is used

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [8]

Soft starters in motors Advantages By varying the votlage across the terminals of an induction motor gradually, the following advantages can be obtained:

1

Smooth acceleration reducing heating and stress on drive system due to high starting torques

2

Such high torques may affect reliability of belts, gears, chains and conveyors, bearings and shafts

3

Reduction in starting current avoids large voltage dips

4

Fully adjustable acceleration provides adequate torque to accelerate loads while keeping electrical and mechanical shocks under check

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [8]

Soft starters in motors Control of current or torque is based on the assumption that terminal voltage is controlled (regulated) by adjusting the firing angles of the thyristors Relationship between α and voltage is highly nonlinear and depends on power factor, which depends on the rotor slip Makes it difficult to obtain an exact α for a required motor speed or torque

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [8]

Soft starters in wind turbines In induction generators, the slip varies within a narrow range around 0 The soft starter provides the induction machine with a variable voltage which is generated using a predefined firing angle sequence The gates are triggered by pulse trains that begin at a fixed (desired) firing angle α

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter [8]

Soft starters in wind turbines Forward phase Firing angle Reverse phase Firing angle (1) A+ (reference) 0 deg (4) A- 180 deg (3) B+ (reference) 120 deg (6) B- 300 deg (5) C+ (reference) 240 deg (2) C- 60 deg

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter firing pulse controller. Figure taken from [9]

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter. Figure taken from [9]

Firing pulse controller [9] The phase locked loop (PLL) which converts phase quantities to αβ frame Phase detector which generates an error function φerr = VαKvsin(φ) + VβKvsin(φ) PI controller which yields φ = Kpφerr + Ki t

0 φerr + ω0

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter. Figure taken from [9]

Firing pulse controller [9] Vα = 2

3(Va − .5Vb − .5Vc) and Vβ = 2 3(

√

(3) 2

(Vb − Vc)) are the Clarke‘s components Kv is the voltage proportional constant Kp and Ki are the PI controller‘s gains ω0 is the PLL tracked frequency φ is the phase output signal synchronized with phase-A

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter. Figure taken from [9]

Firing pulse controller [9]

Using φ as reference, a set of 6 sawtooth signals is obtained, each separated by 60 deg The firing pulse generator receives the phase identifiers (i.e., sawtooths) as input ‘H’ An appropriate firing delay (firing characteristic) is provided to yield the firing pulse trains Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Soft starter. Figure taken from [9]

Firing pulse controller [9]

The firing characteristic is given as a down ramp signal over time, ‘L ’ The sawtooth inputs in ‘H’ are constantly compared against the firing characteristic ‘L ’ The pulse sequencer is used to block the pass the firing pulses Typically, the firing pulses are blocked and the soft starter is by-passed in about 10 cycles Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1]

Adjustable speed drives An adjustable speed drive, which is a traditional frequency converter, consists of

1

rectifier: a component for converting AC to DC, while energy flows into the DC system

2

capacitors: which store energy

3

inverter: a component for converting DC to AC, while energy flows into the AC system

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1]

Rectifier Common rectifier solution is the diode with advantages of easy control, low costs, and low losses It produces harmonics due to the switching and non-linear

• peration

Also conducts in only one direction, unless more diodes are used in a special arrangement It cannot control voltage or current Diodes are only used for rectification while electronic switches can be employed for inverter functions as well

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1]

Inverter Common inverter solution is the thyristor with advantages

• f easy control, low costs, and low losses

It produces harmonics due to the switching and non-linear

• peration; as well as high consumption of reactive power

GTO thyristor and IGBT are better solutions for inverters; but, costlier and complicated controls

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1]

Rectifier & inverter The rectifier and generator should be chosen/designed as a combination The inverter can be designed as an independent solution/selection Diode rectifier or thyristor rectifier can be used together

• nly with a synchronous generator (because there is no

need for a reactive magnatizing current) When GTO and IGBT rectifiers are used, they must accompany variable-speed induction generators so that they are able to control the reactive power

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1]

VSC and CSC (or VSI and CSI) Self-commutated converter/inverters can be classified as: voltage or current sourced Implemented using many techniques such as pulse amplitude modulation (PAM) or pulse width modulation (PWM) PWM: low-frequency harmonics are removed; freq of the first higher order harmonic occurs about the switching freq

• f the converter

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1]

VSC and CSC (or VSI and CSI) VSC supplies a relatively well-defined switched voltage waveform at the generator terminals and the grid VSC stores energy in the DC bus by keeping the voltage constant across a capacitor CSC supplies a relatively well-defined switched current waveform at the generator terminals and the grid CSC stores energy in the DC bus by keeping the current constant through an inductor r

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1]

Rectifier & inverter Rectifiers and inverters are combined into a frequency converter in 5 different topologies:

1

Back-to-back converter

2

Multilevel converter

3

Tandem converter

4

Matrix converter

5

Resonant converter

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1], [10]

Back-to-back converters. Figure from [10] Bi-directional converter topology with two conventional PWM-VSI converters Full control of current on grid side is achieved by boosting the DC bus voltage higher than the grid line-line voltage

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Rectifiers & inverters [1], [10]

Capacitor on DC bus decouples the two converters and enables individual controls on each side without affecting the other side Power flow on the grid side is controlled by keeping the DC bus voltage constant Generator-side converter is controlled to match the magnetization demand and rotor speed of the machine Disadvantages: high switching losses; harmonics; reduced lifetime and efficiency due to DC link capacitor

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Future of power electronics in wind farms Wind farm installations—at large capacities—are becoming more and more prevalent in electricity grids Bold initiatives such as ‘20% wind by 2030’ [11] are in effect in the US and other countries As wind power penetrations grow to significant values, problems/challenges may arise:

1

Regulation of active and reactive power

2

High technical demand

3

Frequency and voltage regulation needs

4

Fast response during transient and dynamic conditions

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Standard controls such blade pitching, yaw control, dumping excess electricity into resistor or inductor banks and disconnections may not contribute to system stability Advances in power electronics must be adapted to wind farms for extending the reach and reliability Storage technologies may also become viable in the future, pending economies of scale reduction in costs Using power electronics may make the wind farm appear more like a fully controllable traditional power plant

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Depending on the application of power electronics, topologies can vary

1

Completely decentralized

2

Partially decentralized

3

Completely centralized

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Completely decentralized. Figure from [1] Completely decentralized with internal AC network connected to main grid Each turbine has its own freq. converter and control system Adv: Optimal control for individual turbines, reduced stress and noise Disadv: Costs Popularly used with Type III DFIG

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Partially decentralized. Figure from [1] Each turbine having its own converter for optimal operation Rectified output of each turbine collected into a central inverter for grid connection Originally suggested for multipole high-voltage sync. gens Has all advs of variable-speed concept Can be used with SCIG too, if rectifier is VSC

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Completely centralized Centralized converter connects the collected (unrectified)

• utput of turbines to grid

Can be used with SCIG or WRIG Adv: Isolated grid operation from internal wind farm issues Disadv: Non optimal performance as all machines are spinning with same angular speed (average)

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Completely centralized. Figures from [1] Centralized converter type: HVDC transmission for off-shore installations Low/medium voltage wind farm is connected to grid via HVDC systems E.g.: Gotland, Sweden uses ABB technology (HVDC Light)

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References Capacitor bank controller Soft starter Converters Future of power electronics in wind farms

Future of power electronics in wind farms [1]

Completely centralized. Figures from [1] Uses SVC or STATCOM units for improving voltage stability by centralized reactive compensation Can also use Advanced SVC (ASVC) that are self-commutated and offer full continuous ‘Q’ control (e.g., Rejsby HEde, Denmark) When voltage varies during faults, the available reactive power can be better managed compared to SVCs

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References

• T. Ackermann. Wind Power in Power Systems. Wiley,
• 2012. ISBN: 9781119941835. URL: http:

//books.google.com/books?id=QM60LmgaeeQC. F . van Roemburg J. Reindl T. van de Steeg. Power Capacitors For Wind Turbines. in Passive Component

• Industry. URL: http://goo.gl/C3ts9c.
• M. Steurer et al. “Voltage Sensitivity to Capacitor

Switching on an Existing Fixed Speed Induction Generator Wind Farm”. In: Power Engineering Society General Meeting, 2007. IEEE. 2007, pp. 1–4. DOI: 10.1109/PES.2007.386107.

• A. Jones. Grid Connection of Renewable Generation.

SandC Electric. URL: http://goo.gl/uyHDq7.

• Thyristor. Wikipedia. URL:

http://en.wikipedia.org/wiki/Thyristor.

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References

• S. Heier. Grid Integration of Wind Energy: Onshore and

Offshore Conversion Systems. Wiley, 2014. ISBN: 9781118703304.

• T. Agarwal. Power control Using SCR. URL:

http://www.elprocus.com/power-control- using-scr/. Angel Rodrgiuez. “Improvement of a fixed-speed wind turbine soft-starter based on a sliding-mode controller”. Chapter 5. PhD thesis. University of Seville, 2006.

• G. Quinonez-Varela and A Cruden. “Modelling and

validation of a squirrel cage induction generator wind turbine during connection to the local grid”. In: Generation, Transmission Distribution, IET 2.2 (2008),

• pp. 301–309. ISSN: 1751-8687. DOI:

10.1049/iet-gtd:20060180.

Suryanarayanan ECE 566 Lecture/Week 7

Power electronics for wind turbines References

• L. H. Hansen et al. Conceptual survey of generators and

power electronics for wind turbines. Tech. rep. Risø National Laboratory, Roskilde, Denmark, December 2001. 20Contribution to U.S. Electricity Supply. Tech. rep. National Renewable Energy Laboratory, Golden, CO, July 2008.

Suryanarayanan ECE 566 Lecture/Week 7