Matrix Converters Applied to Wind Energy Conversion ...

This is the author's version of an article that has been published in this proceedings record. Changes were made to this version by the publisher prior to publication. The final version of record is available at http://dx.doi.org/10.1109/CERMA.2009.38

Matrix Converters Applied to Wind Energy Conversion Systems, Technologies and Investigation Trends J. L. Elizondo, M. E. Macías, and O. M. Micheloud Tecnológico de Monterrey, Campus Monterrey Cátedra de Energía Eólica [email protected], [email protected], [email protected]

Abstract The matrix converter (MC) has been subject of investigation since 1980 and with the constant improvement of power electronics semiconductors along with the microelectronics it is now possible to efficiently and reliably implement such a topology. The MC finds its application wherever bidirectional power flow and controlled voltage and current in AC systems is needed and proves to be superior to its competitors when applied in certain specific environments and circumstances. Wind energy conversion systems (WECS) is a recent growing research topic where MC is found to be promisingly more efficient as power electronic controller. This paper discusses what a MC is, what are the different WECS configurations and the latest published advances and trends in this area of knowledge.

1. Introduction Matrix converters applied to wind energy conversion systems is a topic recently approached in dissertation thesis [11], [19], [27], indexed and high impact publications [1], [2], [9], with most of the literature presented in conferences [4]-[8], [10], [12], [13], [15]-[18], [20]-[22], [24], [28] and highly unlikely to be present in text books [23]. This topic is nowadays subject of investigation around the world by renewable energy systems enthusiasts as well as MCs investigators to develop a system as efficient as possible taking into account the control strategy implemented (as almost two thirds of the references cited), the electromagnetic interference present [5], using modified MCs topologies [24], [27] as well as different WECS topologies (five among the references cited), with all efforts having one sole goal in mind: To

build a WECS capable of capturing the maximum wind power possible, as efficiently as it can be done. The use of MC in WECS is feasible due to the advantages that a direct AC-AC converter as the MC have [26]. There are publications such as [14], where a comprehensive WECS technologies comparison has taken place, which can be used to decide where is more likely to take the most advantage of the MC. The following sections show an overview of each different aspect of this topic towards clarifying the actual situation of the research about MCs in WECS.

2. The Matrix Converter Matrix converter is the name given by scientific literature for a direct AC-AC converter built as an m x n solid state switches matrix with bi-directional power flow capability. The MC can directly connect a fixed voltage and frequency source to a load requiring variable voltage and frequency, as with induction motors where it's needed to control torque and velocity in its shaft independently of the mechanical load applied. The MC is also capable to convert a variable voltage and frequency source to a fixed voltage and frequency AC as in the case of grid-connected WECS where the generator shaft velocity depends on the wind velocity and where steady voltage and frequency at its output is required. Conventionally, the m x n matrix is a 3 x 3 matrix because three phase AC environments are the most common in industry. A MC generic schematic diagram, along with input filters and a clamp circuit is shown in figure 1. Matrix converters require the switches matrix to have harmonic components suppressors filters implemented in its input (to keep the source free from harmonic components generated by the switches commutations) and also in its output if necessary (as in the case of determined harmonic components sensitive

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synchronous generator (SG), the induction generator (IG) and the doubly-fed induction generator (DFIG). Figures 2, 3 and 4 denote simplified diagrams of each basic WECS along with the usual power electronics controller blocks used.

Figure 2. WECS for PMSG or IG with rectifier inverter technology as the interface with the grid Figure 1. Conventional matrix converter loads), along with a clamp circuit for switches protection (when abrupt failures occur, as a source or a load line disconnection). The input and output three phase voltages, vi and vo respectively, can be represented by vectors and the switches s11, s12, …, s33 can be represented by a matrix that requires a modulation with a determined duty cycle defined as m11, m12, …, m33. So the equations for voltages and currents are given by:  v1o t   m11 t  m12 t  m13 t   v1i t  v t   m t  m t  m t   v t  22 23  2o   21   2i  v3o t  m31 t  m32 t  m33 t  v3i t 

 i1i t   m11 t  m12 t  m13 t   i1o t  i t   m t  m t  m t   i t  22 23  2i   21   2o  i3i t  m31 t  m32 t  m33 t  i3o t 

Figure 3. WECS for SG with rectifier inverter technology as the interface with the grid

(1)

T

(2)

The solution for these equations leads to the desired output waveforms to achieve the implemented application control. This explanation serves as a brief introduction to the MC technology. More details about the MC state of the art can be found in [3], and a comprehensive analysis of modulation and commutation techniques as well as different issues related to MCs can be read in [26].

3. Wind Energy Conversion Systems (WECS) There are four common WECS configurations, each of them depending on which kind of generator is going to be used. The common generator types are the permanent magnet synchronous generator (PMSG), the

Figure 4. WECS for DFIG with rectifier inverter technology to control the rotor current and stator directly connected with the grid It can be seen from the figures 2 and 3 that the generators aren’t directly connected to the grid, there is an interface in between. This power electronics interface has the capability to convert variable voltage and variable frequency to fixed ones and because of that these WECS can function with variable speeds [25]. In figure 4 the stator is directly connected with the grid, but the rotor current is controlled in order to obtain fixed voltage and frequency in spite of variable generator shaft velocities as well. There is a wider variety of WECS than the presented in this document [23] but these are the most representative where the MC can be applied.



4. Advantages of MC applied to WECS The MC advantages when applied to WECS are the very same advantages of MC against other power electronics topologies. Besides of supporting bidirectional power flow, the MC is known as a "pure-silicon" converter. This adjective has been gained because the MC offers the possibility of implementing small input and output filters when the correct modulation and commutation technique is used, and because it omits the presence of a dc-link capacitor as the one implemented in the rectifier inverter technology that usually has a big capacity and consequently a big size. The main reason motivating this topic research is the possibility of constructing a small sized power electronics controller, free of bulky reactive elements, that promises a more efficient use of energy as a direct AC-AC converter without any DC stages, taking advantage of stored energy in mass inertias, and having a longer useful life due to its lack of significant reactive elements and consequently being capable to operate at low and high temperatures and adverse atmospheric conditions [3].

Figure 5. WECS for DFIG with matrix converter technology to control the rotor current and stator directly connected with the grid The figure 5 can be compared with the figure 4 to visualize the opportunity of better energy conversion efficiency as in figure 5 there is only one stage, the AC-AC one, and in figure 4 there is a stage of AC-DC conversion, then a dc-link (with the possible need for a DC-DC converter or Chopper to guarantee a continuous DC voltage) and finally a DC-AC conversion. The reason why figure 5 was chosen to depict the use of the MC in a DFIG based WECS, and not within another WECS technology, is discussed at the end of the next section as a reflection after the

citation of the different WECS topologies advantages and disadvantages.

5. Comparison of WECS Technologies [14] There are comprehensive reviews of WECS and its power electronics controllers in the scientific literature as in [14]. The advantages and disadvantages of each of the types of generators implemented in WECS have been analyzed and are presented within the subsections below.

5.1. Permanent Generator (PMSG)

Magnet

Synchronous

Advantages  Flexibility in design allows for smaller and lighter designs  Higher output level may be achieved without the need to increase generator size  Lower maintenance and operating costs; bearings last longer  No significant losses generated in the rotor  Generator speed can be regulated without the need for gears or gearbox  Very high torque can be achieved at low speeds  Eliminates the need for separate excitation or cooling systems Disadvantages  Higher initial cost due to high price of magnets used  Permanent magnet costs restricts production of such generators for large scale grid connected turbine designs  High temperatures and sever overloading and short circuit conditions can demagnetize permanent magnets  Use of diode rectifier in initial stage of power conversion reduces the controllability of overall system

5.2. Induction Generator (IG) Advantages  Lower capital cost for construction of the generator  Known as a rugged machine that have a very simple design  Higher availability especially for large scale grid connected designs  Excellent damping of torque pulsation caused by sudden wind gusts  Relatively low contribution to system fault levels



Disadvantages  Increased converter cost since converter must be rated at the full system power  Results in increased losses through converter due to large converter size needed  Generator requires reactive power and therefore increases cost of initial AC–DC conversion stage of converter  May experience a large in-rush current when first connected to the grid  Increased control complexity due to increased number of switches in converter

Disadvantages  Increased control complexity due to increased number of switches in converter  Stator winding is directly connected to the grid and susceptible to grid disturbances  Increased capital cost and need for periodic slip ring maintenance  Increased slip ring sensitivity and maintenance in offshore installations  Is not direct drive and therefore requires a maintenance intensive gearbox for connection to wind turbine

5.3. Synchronous Generator (SG)

As it can be seen from the above data, there are a lot of possible constraints to take into account when a cost-efficiency system analysis is going to be made to decide which generator type to use in a WECS. So, it is not surprising that the IG is the most investigated generator type (based on the references presented in this document) because of its almost lack of maintenance and lower construction cost. PMSG and SG don’t have such advantages and thus MCs implemented with these generator types are minimal. The DFIG configuration for WECS, proves to be better than the IG because of its capability to output more energy than rated and because of its lower power rated electronic controller located at the rotor side. These DFIG advantages overcome its so called disadvantage of having slip rings and higher maintenance cost and hence the MC finds a promising application as the DFIG based WECS power electronics controller as it is the second generator type with more research publications. Figure 5 shows such a WECS controlled by a MC because it is the one which represent the most cost-efficient system, combining the advantages of both, the MC as a controller and the DFIG as the generator for a WECS.

Advantages  Minimum mechanical wear due to slow machine rotation  Direct drive applicable further reducing cost since gearbox not needed  Allow for reactive power control as they are self excited machines that do not require reactive power injection  Readily accepted by electrically isolated systems for grid connection  Allow for independent control of both real and reactive power Disadvantages  Typically have higher maintenance costs in comparison to that of an IG  Magnet used which is necessary for synchronization is expensive  Magnet tends to become demagnetized while working in the powerful magnetic fields inside the generator  Requires synchronizing relay in order to properly synchronize with the grid

5.4. Doubly-Fed Induction Generator (DFIG) Advantages  Reduced converter cost, converter rating is typically 25% of total system power  Improved efficiency due to reduced losses in the power electronic converter  Suitable for high power applications including recent advances in offshore installation  Allows converter to generate or absorb reactive power due to DFIG used  Control may be applied at a lower cost due to reduced converter power rating

6. Actual Investigation Trends In accordance with the information presented in the introduction, it can be easily shown that the main research topic about MCs applied in WECS is, and will remain to be for quite a long time, the control one. Control algorithms is the most common research topic involving MC in WECS due to the minimum hardware investment it requires, taking advantage of already installed generators, converters and development platforms. Also, the generator type which costs the less and has the best efficiency will remain to be the one under focus. The following subsections are meant to review the main investigation topics of MCs in WECS and the main generator types involved.



6.1. Overall investigation topic

6.2. Generator type

The publications about control aspects referenced in this document are [1], [2], [4], [6], [8], [10]-[13], [15], [17]-[22], [25], [27] and [28]; all of them ranging from 1998 to 2009, proving this trend to be ongoing and the most investigated. A variety of control topics can be depicted from the publications related to this trend. There has been investigation of fuzzy logic control principles in [1] and [12]; implementation of input current observers and current modulation schemes in [2] and [4] along with nonlinear control loops and the decoupling of the active and reactive power of the generation system respectively; speed sensor-less control algorithms are proposed and developed in [6], [8], [10], [20] and [25]; and state-feedback controllers based on the LQ optimization method to achieve zero steady-state errors are proposed in [11] and [15]. All of the previous documents worked on IG based WECS. The publications which investigated control algorithms in DFIG based WECS [17], [18], [22] and [28], all of them worked with stator flux oriented schemes to control the generator rotor current. The one which worked with the uncommon generator type “brushless doubly-fed induction generator” (BDFIG) used the gap flux oriented vector transformation control technology. And finally there were also publications that presented control schemes for a multilevel MC [27], simplified carrier-based modulation algorithms for MCs [19] and another which worked with the concept of direct-drive wind generator based on converter-driven SG [13]. All of these control strategies with the same goal, to capture the maximum amount of wind power possible doing it as efficiently as the implemented technologies permit to do. There are other kinds of publications, not as frequent as control ones, like this paper, which make a technology review to give the reader an overview of the whole themes of MC as in [3] and [26], of WECS as in [23] and of MC applied to WECS as in [14] and [16]. These publications serve as illustrating references for new enthusiastic investigators. There are also publications related to control but focused in resolving specific issues like electromagnetic interference [5] to investigate common-mode effects in MCs, power quality enhancement in WECS [7] by the means of MC use, the study of converter reactive power capability [9] and different approaches of modified converter topologies like the ones in [1], [24] and [27].

It can also be seen from the references that the generator type research trend gives the IG as leader with [1], [2], [6], [8]-[12], [15], [19], [20] and [24] working on it, then the DFIG with [4], [5], [7], [17], [18], [22], [28] investigating about it, and PMSG, SG and the un-common generator type BDFIG in third place having just one publication within the presented references, [27], [13] and [21] respectively. So, there is another main investigation trend besides the control algorithms one and that is the investigation of MC on WECS based on IGs and DFIGs. The interest in IG based WECS is understandable due to the low price of an IG as well as its low maintenance cost and the interest in DFIG is also comprehensible because it overcomes its disadvantage of needing periodic maintenance with the better than IG efficiency advantage.

7. Conclusions The MC applied to WECS is a fresh on-going investigation topic as the majority of publications which contribute to the state of the art are conference proceedings. Just a few are specialized magazine articles and doctoral dissertation thesis. So, it can be inferred that its investigation will continue to create more high impact documentation. The actual trends of investigation are control algorithms to achieve better efficiency and maximum wind power capture as well as the investigation of MC in IG and DFIG based WECS. The main reasons for this is to take advantage of already installed equipment and minimize initial construction and maintenance cost. Based on a cost-efficiency analysis of MC applied to WECS, it can be told that the DFIG based high power rated WECS is the one with the most advantages over other topologies for the MC to be implemented and successfully used as an efficient direct AC-AC converter with bi-directional power flow capability.

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