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Abstract—As the power of wind energy system increases, the control of their active and reactive power becomes increasingly more important from a system standpoint given that these are typical frequency and voltage control parameters. In this paper a family of wind energy systems with integrated functions of active power transfer, reactive power compensation and voltage-conversion is proposed. The proposed wind energy systems using solid state transformer (SST) can effectively suppress the voltage fluctuation caused by the transient nature of wind energy without additional reactive power compensator and as such may enable the large penetration of wind farm (WF) into the power grid. To this end, a simulation study for WF driven by squirrel-cage induction generators is presented to verify the effectiveness of the proposed system. In addition, a modular type high voltage and high power three-phase SST topology is presented for the proposed system, and its basic building block, which is a single-phase SST, is analyzed. The functions of SST in the presented wind energy system are verified in a single-phase laboratory prototype with scaled down experiments. Lastly, cost issue of the proposed technology is analyzed with comparison to the traditional one.
Index Terms—Wind generation, Solid State Transformer,
SST has been a hot research direction recently thanks to
the rapid development of power device [12]-[19]. On one hand, efforts are focused on topology investigation in this new technology for high voltage and high power application [12]-[15]. On the other hand, SST is considered as the promising candidate for the applications where low frequency transformers are dominating, such as traction/locomotives, solar/wind farm, charge station, and smart grid, for reducing the volume and weight of the system [16]-[20]. In all the previous applications, only the benefit of reducing volume and weight of SST is considered. Although it is known that SST may provide the reactive power compensation capability, to best of the authors’ knowledge, no literature has explored the SST in these applications, especially in the wind energy systems with reactive power compensation in addition to the solely voltage-conversion functionality. In addition, some of the previous SST topologies can not fulfill the reactive power compensation due to lack of DC links [18]. Considering the wind generation system architecture and function, that of a STATCOM and power transformer, may be inherently embodied by the SST, thus makes it an attractive alternative to interface wind energy system into the grid, which is the key concept explored in this work. Specifically, this paper contributes to looking at the advantages and possibilities offered by SST-interfaced wind energy systems, focusing on their integrated active power transfer, reactive power compensation capability, and voltage-conversion functions. To this end, a wind energy system with the squirrel-cage induction generator is studied as an example with comparison to the conventional wind energy system architecture. Further, this paper addresses the challenges for such a system: how to design a high voltage and high power SST for wind energy system. Correspondingly, a modular high voltage and high power three-phase SST topology is presented for this application with its basic single-phase building block analyzed in detail. Scaled down experimental results with single-phase SST prototype are presented for validation purposes of the integrated functions of active power transfer, reactive power compensation, and voltage-conversion. Lastly, Cost issue of the SST is also covered in the paper.
II. SYSTEM DESCRIPTION
A. Wind generation systems overview Several techniques are used to convert wind energy into
electric energy, but the most popular and widely used is based on the induction generator. Presently, there are three main wind farm architectures, namely squirrel-cage induction generator (SCIG) based wind energy system [21], doubly-fed induction generator (DFIG) based wind energy system [22],
Wind Energy System with Integrated Functions of Active Power Transfer, Reactive Power compensation, and Voltage conversion
Xu She, Student Member, IEEE, Alex Q.Huang, Fellow, IEEE, Fei Wang, Student Member, IEEE, and Rolando Burgos, Member, IEEE
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and directly driven synchronous generator (DDSG) based wind energy system [23], shown in Fig.1. Fig. 1(a) describes the wind farms (WF) with SCIG, which is directly coupled with the grid and thus the most economical solution. The slip, and the resultant rotor speed of the generator, varies with the amount of power generated, thus a capacitor bank is generally placed in the terminal of the wind generator for the local reactive power compensation, which is necessary for the operation of this system. Fig.1 (b) shows the WF with DFIG, which employs partial power back to back converters to decouple the mechanical and electrical rotor frequencies. Fig. 1 (c) lastly shows the WF with DDSG, which uses full power back to back converters that totally decouple the generator from the grid. The new WFs are tending to use DFIG or DDSG connected to grid with back to back converter; nevertheless, more than half of the existent WFs are still based on SCIG.
The nature of WFs is that their operation is highly dependent on the active and reactive power transferred to the grid, a condition normally reflected by the fluctuation of voltage magnitude at the point of common coupling (PCC). This sensitivity is exacerbated in the case of SCIG, since this type of generator is an inherent consumer of reactive power [2]. Additionally, in the case of faults causing a voltage drop at PCC, the induction generator speeds up consequently due to unbalance between the mechanical shaft torque and the generator’s electromagnetic torque, in which case it draws more reactive power acting as a positive feedback contributing to the grid destabilization and PCC voltage collapse [21].
(a)
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Fig. 1 WFs with induction generator interfaced by normal transformer: (a) WF with squirrel-cage induction generator; (b) WF with doubly-fed
induction generator; (c) WF with directly-driven synchronous generator. As seen, the modern power system has to confront some
major operating problems such as voltage regulation, power flow control, transient stability, and damping of power oscillations, etc. Reactive power compensators, such as static compensator (STATCOM), are hence good solutions for regulating the PCC voltage [3]-[8], and it is also shown in Fig.1. The downside of these WFs however is the use of bulky power transformers for both STATCOM and WF generators, although a low cost solution. The SST on the other hand has been regarded as a promising technology integrating active power transfer, reactive power compensation, and voltage-conversion, while no literature has explored the application of SST with full utilization of all the functions [12]. Accordingly, the major contribution of this paper is to propose a new family of SST-interfaced WF architectures effectively replacing the conventional transformer and reactive power compensator.
B. Solid state transformer (SST) Conventional copper-and-iron based transformers have
been challenged by solid state technologies. Specifically, a conventional transformer in ideal terms represents a simple input-output voltage and current transformation, thus disturbances on one side, which are typical active and reactive power, are fully reflected on the opposite side. Overcoming this seeming drawback, the SST has been a promising technology in recent years [13]-[20]. Potential advantages of SST over conventional transformers include low volume and weight ( due to its high frequency operation compared with 60Hz transformer), fault isolation, voltage regulation, unsusceptible to harmonics, easy integration of renewable energy resources and energy storage, and etc [12]. As functionally shown in Fig. 2, the SST is typically composed of a high voltage AC/DC rectifier that regulates a high voltage DC bus (and AC voltage when for reactive power compensation), an isolated high frequency operated DC/DC converter to regulate the secondary DC bus, and a DC/AC inverter to regulate the output terminal AC voltage. With this structure, the active power transfer, reactive power compensation, and voltage transformation may be inherent if appropriate topologies are chosen.
Admittedly, this technology has its penalties, such as reliability, which is addressed by the possible solution of this paper. Besides, a high voltage and high power SST that can be interfaced with the distribution system is also not easy with state-of-art technology. Numerous technologies are being investigated and may be feasible for this high voltage and high power application, such as advanced power device, multilevel converter, converter series/parallel connection, and etc [12]. A 2.75 MVA, 13.8 KV to 465V three-phase SST has already been designed and 1MVA single-phase SST has been developed, which verifies the feasibility of high voltage and high power field application of the technology [24]. In this paper, a three-phase modular type high voltage and high power solid state transformer suits for the presented application is also proposed together with its single-phase building block implementation illustrated. The following sections will introduce the proposed novel WFs interfaced by
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SST in detail.
C. A family of SST interfaced wind farm systems A family of wind energy system has been proposed as
shown in Fig .3. Fig. 3(a) shows the case of a WF with SCIG, where the SST acts as the grid interface. The local capacitor bank, two conventional transformers, and the STATCOM as shown in Fig.1 (a) are all functionally integrated into a single SST, which will be illustrated in the later case study. Fig.3 (b) shows the case with DFIG, where the AC/AC back to back converter is retained for fully utilization of advantages of SCIG based WF, while two transformers and STATCOM are replaced by a SST. Lastly, Fig. 3(c), featuring DDSG with full power converters, shows how the SST can be used to replace the AC/AC converter, both step-up transformers and STATCOM. From these diagrams it is easily seen how the proposed SST-interfaced WFs represent a possible more compact and cable solution, and as such can be deemed to be a promising technology. This will be investigated in what follows.
Fig. 2 Functional representation of SST
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(b)
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Fig. 3 Proposed WFs with induction generators interfaced by SST: (a) WF with squirrel-cage induction generator; (b) WF with doubly-fed induction generator; (c) WF with directly-driven synchronous generator
III. SYSTEM CASE STUDY
A. Three-phase SST for system evaluation Although different topologies can be adopted to implement
the SST for the proposed application, the overall control objectives intrinsic to its operation are the same regardless of its circuit topology; namely AC voltages for reactive power compensation and the DC voltages for active power transfer. For the sake of better illustration of control objectives of the SST in the proposed system, which are active power transfer, reactive power compensation, and voltage-conversion, a simple three-phase SST topology is initially adopted and analyzed neglecting the physical limitation of the power device and magnetic materials.
Fig. 4 shows a cascaded type three-phase SST. Its first stage is a three-phase bidirectional AC/DC PWM rectifier, which can also be used in the DC/AC power conversion stage as depicted. Its DC/DC stage is embodied by a dual active bridge (DAB) converter, which represents the most attractive candidate for high power applications requiring isolation, as it can perform zero-voltage-switching (ZVS) in a wide operation range[25]-[26]. In the above SST configuration,
habcv is the PCC voltage, habci is the PCC current flowing into the SST, hdcv is the high DC bus voltage, ldcv is the low DC bus voltage, labcv is the output voltage of inverter, and labci is the output current of inverter.
Fig. 4 Three-phase SST for control illustration
Fig. 5 shows the control system adopted with the integrated active power transfer, reactive power compensation, and voltage-conversion functions, where single L filter ( L ) is adopted for the input side and LC filter ( ,s sL C ) is adopted for the low voltage side. The controllers for all three conversion stages are conventional ones and thus only brief description is included as follows.
A d-q axes vector controller is used to regulate the input currents of the three-phase AC/DC rectifier, where the d-axis loop is used to regulate the DC bus voltage, and the q-axis loop to regulate the SST reactive power generated, which in turn is governed by the PCC voltage magnitude loop. This control structure is exactly the same as that used in the well-known STATCOM, evincing how the SST integrates this compensator functionality. To regulate the active power flow, a phase regulation scheme is adopted for the DAB using a simple PI controller, which adjusts the phase shift between high- and low-side H-bridge converters. Lastly, the inverter stage is controlled using a conventional dual loop strategy in d-q coordinate, as shown in Fig. 5. It is shown in the figure that the outer voltage loop is used to compensate for the reactive power consumption at the induction generator terminals, thus implementing the function carried out by the capacitor banks in the case of WFs with SCIG. In addition, the inductor current loop is cascaded as the inner loop such that fast dynamic responding can be guaranteed. It is also shown in the Fig .5 that the couplings between d-axis and q-axis control loop have also been taken into the consideration as this is also necessary for a high performance system. Due to the bidirectional power transfer characteristics
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Fig. 5 Control system for the three-phase SST in wind energy systems
(a) Conventional transformer interfaced wind farm with SCIG
(b) SST interfaced wind farm with SCIG
Fig. 6 Power system plus WFs used in the case study of the system, this controller can also transfer the power from the low voltage side to the high voltage side, which is the case in the presented system. Obviously, the presented control logic can enable the reactive power compensation function without the wind power input, thus the fully reactive power compensation function can always be maintained.
B. System study in SCIG driven WFs A system study is carried out in a typical WF system, as
shown in Fig. 6 (This system is a demo from MATLAB 2008b). The SCIG driven WF was adopted as the example since it presents the largest demand of the reactive power, and hence compensation needed, among the three types of generators under consideration. Nonetheless, the conclusion drawn from the case study can also be applied to WFs driven by DFIG and DDSG since the control objectives are exactly the same for all three systems.
In this system, two 3.3MVA, 575V WFs with SCIGs are connected to a 25KV distribution system. Fig.7 shows the power characteristics of the wind turbine as a function of turbine speed under varying wind speeds. The base wind speed for the wind turbine is 9m/s(the cut-in speed of wind turbine is 4m/s and the cut-off speed of wind turbine is 25m/s), and the base rotational speed at the base wind speed is 1pu with the base of generator speed. The active power
generated by the wind turbine at base wind speed is 3MW, which corresponds to a power factor of 0.9. The pitch angle is set to zero and no pitch control is implemented in this study for simplification. The parameters of the SCIG are set as shown in Table.I. The parameters of the 120KV generator impedance and the transmission line are also listed in Table.II and Table.III, respectively, which give the detailed description of studied power system. For the evaluation, Fig. 6(a) represents a WF interfaced by a conventional 10MVA power transformer (25KV/575V) without any additional compensator at the PCC. A 0.8MVar capacitor bank is installed in the generator terminal for local reactive power compensation.
Fig. 7 Power characteristics of wind turbine
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(a) (b) (c)
Fig. 8 Electrical characteristics of WF under study: (a) Wind speed profile; (b) Active power of WF (10MVA base); (c) Reactive power of WF (10MVA base).
(a) (b) (c)
Fig. 9 Simulation results of WF interfaced by conventional transformer: (a) PCC voltage; (b) PCC current; (c) PCC voltage RMS value (25KV base).
(a) (b) (c)
(d) (e) (f)
Fig. 10 Simulation results of WF interfaced by SST: (a) 38KV DC voltage; (b) 1.2KV DC voltage; (c) PCC voltage; (d) PCC current; (e) PCC voltage RMS value (25KV base); (f) Reactive power of SST (capacitive, 10MVA base).
Table I Parameters of a single SCIG Nominal power 3.3MVA Nominal voltage 575V Nominal frequency 60HZ Stator resistance (pu) 0.048 Stator inductance (pu) 0.075 Rotor resistance (pu) 0.018 Rotor inductance (pu) 0.1791 Mutual inductance (pu) 6.77 Inertia constant (s) 5.04 Pole pairs 3
Table II Parameters of 120KV generator equivalent impedance
Table III Parameters of transmission line (three-phase PI section) Positive sequence Zero sequenceResistance ( Ω /km) 0.1153 0.413 Inductance (H/km) 1.05e-3 3.32e-3 Capacitance (F/km) 11.33e-9 5.01e-9
Fig. 6(b) on the other hand shows the proposed WF with a
10MVA three-phase SST, using the circuit topology depicted in Fig .4 and control method presented in Fig .5. The high voltage DC bus of SST is regulated to 38KV and the low voltage DC bus is regulated to 1200V. It is worth to remind
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again that although the power and voltage of the studied system is high, the simple topology depicted in Fig. 4 is adopted for demonstrating the concept proposed in the paper and this will not affect the conclusion drawn from the simulation results. In addition, the average modeling approach is adopted for this large time scale simulation, which is also valid for the verification purpose.
The simulation conducted for the conventional WF system is to demonstrate the natural power flow from the conventional WF to the distribution system in order to explore its impact on the PCC in terms of voltage regulation especially. The simulation for the proposed WF system on the other hand demonstrates the capability of proposed SST-interfaced WF system to perform reactive power compensation in order to suppress voltage fluctuation.
Fig. 8 shows the electrical characteristics of the WF under study, where the wind speed profile shown in Fig. 8(a) is continuously changing around 8m/s. Accordingly, the active power transferred by the wind generator follows the same trend as shown in Fig. 8 (b). Fig. 8 (c) illustrates the reactive power at the induction generator terminal, which indicates a significant amount of reactive power consumption by the SCIG.
Fig.9 shows the simulation results of the WF interfaced by the conventional transformer, where the wind speed profile is the same as shown in Fig. 8(a). Fig. 9(a) and Fig. 9(b) are the PCC voltage and current in the presented system. Due to the fluctuation of the wind speed, the magnitude of current is changed accordingly. As a result, the PCC voltage is also affected, as shown in Fig.9(c), which is less than the rating value and fluctuated.
As a comparison, Fig. 10 shows the simulation results of the proposed SST-interfaced WF with integrated active power flow, reactive power compensation and voltage-conversion functions. Fig. 10(a) and Fig. 10(b) show the 38 KV and 1.2KV DC voltage of three-phase SST respectively. It can be seen that the DC voltage is regulated well although there are some dynamic responses caused by the active power fluctuation. Fig. 10(c) and Fig. 10(d) show the PCC voltage and current in the proposed system. Compared with Fig. 9(b), the current is a little smaller since the PCC voltage is higher in the SST-interfaced system. In Fig. 10(e), the RMS value of PCC voltage is illustrated, which is within 1% of the nominal value as expected. Fig. 10(f) demonstrates the reactive power sent by the SST. The trend of reactive power is similar with that of the wind profile, thus compensates for the voltage fluctuation at PCC.
The results in Fig .8-Fig .10 evince the effectiveness of the SST as a potential WF interface for the integration of wind energy into the grid with less voltage fluctuation. It is clear that the proposed wind energy conversion system can fulfill the tasks of active power transfer, reactive power compensation and voltage-step.
The reactive power compensation capability of the SST is mainly limited by the power rating of it since both active and reactive power flows through the converter. Based on this consideration, the power stage should be designed so that the maximum current stress is within the range of the power device and passive components selected. Furthermore, since the proposed system can provide reactive power
compensation, it is also expected that the presented system can ride through the fault by injecting reactive power to the system as that of the STATCOM. However, this topic is out of the focus of the paper and will not be discussed here.
IV. A HIGH VOLTAGE AND HIGH POWER SST In order to realize the proposed wind energy system, the high voltage and high power SST is required, and this is a challenge considering the capability of power semiconductor devices. In this section, a promising high voltage and high power modular type SST topology is presented and analyzed for achieving the proposed wind energy system.
A. Topology of a Modular Three-phase SST Fig. 11 demonstrates the presented modular, high voltage
and high power three-phase SST topology in the proposed application. Three single-phase SSTs are connected to compose a three-phase SST, in which a high voltage and high power modular type single-phase SST is adopted as the basic phase building block. In the single-phase building block, the cascaded multilevel converter is utilized in the high voltage side with identical (low voltage) H-bridges, thus low voltage power device can be adopted. Several DAB converters with relatively high switching frequency (in the range of KHz to more than ten KHz decided by the power devices) are then parallel connected to each high voltage DC link regulating a common low voltage DC bus. In the last stage, conventional low voltage high power inverter technologies, such as device parallel or converter parallel, can be used to integrate with WFs. It is worth to point out that there is no limitation for the number of modules since this is the key feature of the cascaded type multilevel converter, the number of the modules is depended on the operating voltage and power device adopted [13].
The above converter structure represents a highly modular design, and as such opens the path to redundant operation, for reliability improvement. For instance, the N+1 redundant design can be incorporated to achieve fault tolerant operation [27]. In addition, it opens the path to modular manufacturing with its potential cost reductions due to economies of scale. In this way it tackles two of the main problems haunting the development of the SST, namely high voltage and high power operation, and reliability.
Converter Cell No.2
Converter Cell No.N
Phase APhase B
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Fig. 11 Proposed high voltage high power three-phase SST
The control of the proposed three-phase converter can
actually be simplified to the control of single-phase converter
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shown in Fig .12 due to the phase building block configuration.
Fig .12 Single phase SST building block Due to the circuit complexity of the single-phase SST in
each phase, the following control goals need be met except for the ones mentioned in the part III-A: that (a).DC voltage in each rectifier should be balanced. (b).Current in each DAB converter should be balanced.
Practical controllers for the rectifier stage and DC/DC stage of SST have already been presented in the previous works [28]-[29], where the voltage and current in the SST can be effectively balanced. The method can be modified to incorporate the reactive power compensation functions by adding the PCC voltage loop for the rectifier stage for the proposed wind energy system application which is similar as shown in Fig. 5. The detail of the control method is not repeated here since it is not the focus of the paper.
B. Simulation results A 20KVA single-phase SST with three H-bridges in the
cascaded front-end rectifier and three DAB converters as the intermediate stage has been considered in this paper, with the topology that shown in Fig. 12. The SST is rated as single-phase input voltage 60Hz, 7.2KV, and output voltage 60Hz, 120V. The 7.2KV distribution voltage is firstly rectified to three 3.8KV DC by the cascaded seven-level rectifier, then converted to 400V DC by three DAB converters, and finally inverted to 120V AC. The circuit parameters are shown in Table IV.
Table IV Circuit parameters of presented SST Line inductance 280 mH High voltage DC capacitance 37.5 Fμ Rectifier switching frequency 1080 Hz Low voltage DC capacitor 2 mF Transformer linkage inductance 68 mH Transformer turns ratio 9.5:1 DAB switching frequency 3000 Hz Inverter LC filter 1 mH 5 FμInverter switching frequency 10000 Hz
Simulations have been run as well for the SST converter in
question. Some results are shown in Fig. 13, Fig. 14, Fig .15 and Fig.16, where four scenarios have been considered. The
first one with reactive current set to 0.4pu, the second one set to -0.4pu, the third one changes from -0.4pu to 0.4pu, and the fourth one with active power changed from 0.6pu to 0.9pu. The active power transferred from distribution system to the low voltage side of the system is set to 0.8pu for the first three scenarios and the reactive power is set to 0 for the fourth scenario. The results for demonstrating the integrated functions of active power transfer, reactive power compensation, and voltage-conversion are recorded.
Fig. 13(a) illustrates the PCC voltage at the distribution system side, PWM voltage generated by the seven-level rectifier, and the current at the input terminal of the SST. As observed, the SST current leads the voltage, which indicates the capacitive operation mode. The three high voltage DC links are regulated to 3.8kV, as shown in Fig. 13(b). Fig. 13(c) finally depicted the regulated 400V DC bus and 120V AC output.
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Fig. 13 Operation waveforms of SST under capacitive mode operation: (a) PCC waveforms (7.2KV, 20KVA base); (b) High voltage DC link; (c) Low voltage side waveforms
Similarly, the inductive operation mode is also tested and
documented, as shown in Fig. 14, and similar conclusion can be obtained.
Fig. 15 demonstrates the operation of the SST with reactive power changing from -0.4pu to 0.4pu. Shown in Fig. 15(a), the phase of current is regulated with satisfactory dynamic when changes the reactive current reference, and this change
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is reflected in the high voltage DC links since they also couple with the high voltage AC side, as shown in Fig. 15(b). Fig. 15 (c) shows the low voltage side DC voltage and output AC voltage, which are all regulated well without any disturbance because of the transformer isolation.
Fig .16 shows the SST dynamics in the load change condition. In this scenario, the load is changed from 0.6pu to 0.9pu at 1 sec. It is observed from the simulation results that there is a dynamic response for both high and low voltage DC bus. The output voltage is regulated well with only a little sag when load changes. Current at PCC increases because of the load power increasing. The whole system exhibits a good dynamic performance.
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Fig. 14 Operation waveforms of SST under reactive mode operation: (a) PCC waveforms (7.2KV, 20KVA base); (b) High voltage DC link; (c) Low voltage side waveforms.
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Fig. 15 Operation waveforms of SST with reactive power change: (a) PCC waveforms (7.2KV, 20KVA base); (b) High voltage DC link; (c) Low voltage side waveforms.
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Fig. 16 Operation waveforms of SST with load change: (a) PCC waveforms (7.2KV, 20KVA base); (b) High voltage DC link; (c) Low voltage side waveforms.
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It is clear that the presented single-phase SST can regulate the reactive power to a desired value as well as transfer the active power from the distribution system to the low voltage AC side with high conversion ratio. Due to the bidirectional characteristics of the topology, the reverse power flow is also easily achievable, thus the proposed application is feasible.
C. Experimental results A laboratory prototype of the presented 7.2kV, 20kVA
single-phase SST was built for validation purposes [30]. This unit is shown in Fig. 17. The system parameters are the same with the simulation model and hence given in Table IV. 6.5kV, 25A silicon based dual insulated gate bipolar transistor (IGBT) has been customized and adopted for the rectifier and primary side of DAB stages. It is packaged by POWEREX with chip supplied by ABB. For this package design, the minimum clearance distance in air is 19mm and creepage distance is 78mm which complies with IEC-60077-1 standard. 600V commercialized intelligent power modules (IPM) were adopted for the secondary side of DAB and inverter stage. Thermal management of the power device is also introduced in [30], which is based on forced air convection method. Eight inductors are connected in series to distribute the high voltage and as such the normal copper wire can be used for winding the inductor. Metglas AMCC 1000 core was used for the high voltage and high frequency transformer. The transformer is naturally cooled with the maximum temperature around 450C [31]. Based on the detailed test results of the power device used in the power stage, PLECS simulation tool is used to estimate the efficiency of the converter, and the efficiency for this prototype at full power rating is about 88.06% [30]. However, with the power rating increasing and an optimized design, we are expecting the efficiency higher than 95% with the presented topology for a MVA level SST prototype. In the experimental setup, TI series 28335 DSP is used for each power stage, generating the PWM signals for each building block with desired synchronization and phase shifting. However with the number of building modules increasing, FPGA can be used to generate the signals. It is also needed to point out that the experiments presented in this paper below are carried out in a relatively low power and voltage rating for the verification purpose. The full power test is still carried on. However this will not affect any conclusion made from the paper for the proposed wind energy application.
A: Cascaded seven-level rectifier B: Primary side of DAB
C. High frequency transformer D: Secondary side of DAB and inverter Fig. 17 Prototype of single-phase 20KVA, 7.2KV SST
(a) Voltage balancing of cascaded multilevel rectifier
Fig.18 shows the dynamic voltage balancing process for
the cascaded seven-level rectifier. The input voltage is set to 600V, and the total high voltage DC bus is set to 1100V. The load connected to the first H-bridge is 1800Ω and the load connected to the second and third H-bridges is 1000Ω . Before the PWM control mode, the system operates in diode rectifier mode with voltages diverge to different value naturally. When PWM mode control is triggered, the three DC voltages converge into the same value immediately, thus the voltage balancing is achieved.
Fig. 18 Voltage balancing of cascaded rectifier stage
(b) Current balancing of parallel operated DAB
The prototype is also tested for verifying the current sharing among paralleled DAB converters. The primary side DC voltage of each DAB is 633V (total 1900V) and secondary side is regulated to 60 V with a 6Ω load.
Fig. 19(a) shows the three high voltage side DC link voltages, which are identical and regulated to 633V. The 60V low voltage side DC link is also captured in the figure. Fig .19(b) depicts the 60 V low voltage side DC link and three DAB inductor currents. It can be observed that the magnitude of currents is the same, thus the current balancing is achieved.
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Fig. 19 Current sharing of DAB converter: (a) High voltage side and low voltage side DC voltages; (b) Low voltage side DC voltage and DAB current.
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(c) Integration of active power transfer, reactive power compensation, and voltage-conversion
Since the wind turbine is not available for the testing, only the SST experimental results are demonstrated to validate its functions as a WF grid-interface device. Scaled down experiments are carried out for verifying the function of SST in integrating active power transfer, reactive power regulation, and voltage-conversion, which are valid and sufficient to validate the effectiveness of the proposed concept. In the experiments, the PCC voltage is set to 1000 V, and
high DC voltage link is regulated to 1900V (633V each). The low voltage DC link is set to 60 V, and the inverter output is set to 23 V (same conversion ratio with the presented power system). A resistive load of 3.6 Ω is connected to the low voltage AC terminals. Experimental results are shown in Fig. 20 and Fig. 21,
depicting capacitive and inductive operation modes respectively under a power factor of ± 0.866. Fig. 20(a) shows the waveforms at PCC as well as the high voltage DC links for the capacitive case, including the PCC voltage and current, the rectifier seven-level PWM voltage, and the three high-voltage DC link voltages. As observed, the three DC voltages are identical and all regulated to 633 V. Fig. 20(b) shows the 60 V low-voltage DC bus and 23 V regulated AC output. Fig. 21(a) shows the operating waveforms under inductive operating mode, depicting the PCC voltage and current, the SST PWM voltage and the high voltage DC links. Just as shown in the previous case, the three DC voltages are identical and all regulated to 633 V. Fig. 21(b) shows the 60 V low-voltage DC bus and 23 V regulated AC output. These results show that the developed SST can indeed
transfer power between the grid and load terminal while providing the required VAr to compensate for the fluctuating active power load demand. In this way the SST is expected to be capable of suppressing all voltage fluctuation in WFs using SCIG under the proposed system architecture.
(a)
(b)
Fig. 20 Operation waveforms of SST under capacitive mode operation: (a) PCC voltage, current, PWM voltage, three DC link voltages; (b) Low voltage DC link voltage and AC output voltage.
(a)
(b)
Fig. 21 Operation waveforms of SST under inductive mode operation: (a) PCC voltage, current, PWM voltage, three DC link voltages; (b) Low voltage DC link voltage and AC output voltage.
D. Cost analysis of the proposed wind energy system Although SST has caught tremendous attention in the recent years, its cost issue is still a major concern which utilities always question. It is not easy to compare the accurate cost of two transformers since the SST is still not commercially available and the cost of the traditional distribution transformer is not open to the academic community. In order to provide some useful information of the cost of SST, this section gives the cost estimation of the established prototype. Fig .22 shows the cost breakdown of the laboratory prototype, and the total cost of the prototype is around $10726. The major cost comes from the high voltage power device, high voltage high frequency transformer, and DC capacitors. It is worth to point out that it is not a surprise to see a 5 to 10 times cost reduction when the estimated cost is based on the large-quantity production, thus the cost of the 20KVA, 7.2KV-120V SST is about $1000-2000. As the comparison, the market price for a single phase 25KVA, 7.2KV-120/240V pole mount transformer is around $1500. The cost of it should be much lower than the market price. It is concluded that the cost of traditional transformer is lower than that of SST.
6.5KV IGBT and driver32%
IPM and driver7%
High frequency transformer
16%
DC capacitors16%
Heatsink and Fan
9%
Sensors4%
Controller boards8%
Auxi‐power supply1% Filter inductors
5%
Others2%
Cost breakdown of prototype: total cost ($10726)
Fig .22 Cost breakdown of the SST laboratory prototype
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It is admitted that the cost of traditional transformer is much lower than the SST. The cost gap between the two technologies can only be filled if advanced features of SST are explored. That is the reason why we look into the application of SST in the wind energy system. As shown in Fig .3(a), the SST can replace two transformers and one STATCOM, with the power rating larger than single transformer depended on the reactive power compensation requirement. In this condition, the cost difference between the traditional wind energy system and the proposed one is much smaller or the SST based wind farm maybe even cheaper, and thus presenting a promising market for the SST technology.
V. CONCLUSION Energy crisis calls for a large penetration of renewable
energy resources, among which wind energy is a promising one. Voltage and frequency regulation is vital to meet the grid code. This paper has covered many key issues to compose the proposed wind energy system, including the system architecture, control objective, and component design. Specifically: 1. A family of wind energy system with integrated active power transfer, reactive power compensation, and voltage-conversion capabilities was proposed. Compared with the previous applications which utilize only the active power transfer and voltage-conversion functionalities, reactive power compensation capability is fully investigated. 2. The proposed family of wind energy system was demonstrated in the presence of squirrel-cage induction generators, by far the most demanding case in terms of voltage fluctuation and reactive power demand. Under the SST interface, the WF was rendered free of distribution power transformer and mandatory passive and active static power compensators. 3. A modular type, high voltage and high power three-phase SST topology was presented and its control strategy were investigated and shown to successfully carry out the tasks needed to interface WFs to the grid. 4. A single phase solid state transformer building block prototype rated at 20KVA, 7.2KV to 120V, was built and tested for verification purposes. However, there are still lots of issues needed to be addressed and thus opens the possible research opportunities: 1. The high voltage and high power system operation is still not fulfilled yet, although there is no technique limitation in this topic. 2. Fault operating condition is not studied yet, which is a key issue in wind energy system. Similar issues, such as how to realize the fault-ride through of the traditional wind energy system, can also be studied in the proposed SST-interfaced wind system.
ACKNOWLEDGEMENT This work is supported by National Science Foundation under award number of EEC-0812121. The authors would like to thank all the SST group members in the FREEDM systems center at North Carolina State University, US for contribution in hardware prototype design and project
discussion.
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Xu She (S’08) received the B.S.c and M.S.c degree in electrical engineering from Huazhong University of Science and Technology, Wuhan, China, in 2007 and 2009 respectively. He is currently working toward the Ph.D degree in FREEDM Systems Center, department of Electrical and Computer Engineering, North Carolina State University, Raleigh, US, where his research focus is solid state transformer topology, control, and integration with renewable energy
resources. From May to August, 2012, he was an R&D Intern with GE Global Research Center, US, conducting research on HVDC systems. His research interests are high voltage and high power converters for utility and distribution system, digital control techniques, high power magnetic design, and renewable energy system architecture and integration.
Alex Q. Huang (S’91–M’94–SM’96–F’05) received the B.Sc. degree from ZheiJiang University, Hangzhou, China, in 1983 and the M.Sc. degree from Chengdu Institute of Radio Engineering, Chengdu, China, in 1986, both in electrical engineering, and the Ph.D. degree in electrical engineering from Cambridge University, Cambridge, U.K., in 1992. From 1992 to 1994, he was a Research Fellow at Magdalene College, Cambridge, U.K. From 1994 to 2004, he was a Professor at the
Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg. Since 2004, he has been with North Carolina State University, Raleigh, and is currently the Progress Energy Distinguished Professor of Electrical and Computer Engineering and directs the NSF FREEDM Systems ERC, NCSU Advanced Transportation Energy Center (ATEC), and NCSU Semiconductor Power Electronics Center (SPEC). Since 1983, he has been involved in the development of modern power semiconductor devices and power integrated circuits. He fabricated the first IGBT power device in China in 1985. He is the inventor and key developer of the emitter turnoff thyristor technology. His current research interests are utility power electronics, power management microsystems, and power semiconductor devices. He has published more than 200 papers in the international conferences and journals, and has 14 U.S. patents. Dr. Huang is the recipient of the National Science Foundation (NSF) CAREER award and the prestigious R&D 100 Award. Fei Wang (S’11) received the B.Sc. degree from North China Electric Power
University, Beijing, China, in 2005 and the M.Sc. degree from the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China, in 2008. From 2006 to 2008, he was a visiting student at Shanghai Maglev Transportation Development Co., Ltd, Shanghai, China, where he was involved in the development of high speed linear motor drive systems. He is currently pursuing the Ph.D. degree in FREEDM Systems center, department of
Electrical and Computer Engineering, North Carolina State University, Raleigh, NC, USA. His current research interests include solid state transformer, Microgrid communication and distributed grid intelligence.
Rolando Burgos (S’96–M’03) received the B.S. on Electronics Engineering, the Electronics Engineering Professional Degree, and the M.S. and Ph.D. degrees in Electrical Engineering from the University of Concepción, Chile, in 1995, 1997, 1999, and 2002 respectively. In 2002 he joined as Postdoctoral Fellow the Center for Power Electronics Systems (CPES) at Virginia Polytechnic Institute and State University (Virginia Tech), in Blacksburg, VA, becoming Research Scientist in
2003, and Research Assistant Professor in 2005. In 2009 he joined ABB Corporate Research in Raleigh, NC, first as Scientist, and since 2010 as Principal Scientist. In 2010 he was also appointed Adjunct Associate Professor in the Electrical and Computer Engineering Department at North Carolina State University (NCSU). His research interests include multi-phase multi-level power conversion, stability of ac and dc power electronics systems, hierarchical modeling, control theory and applications, and the synthesis of power electronics conversion systems. Dr. Burgos is Member of the IEEE Power Electronics Society where he currently serves as Associate Editor for the IEEE Transactions on Power Electronics, and IEEE Power Electronics Letters.
