FLEXIBLE POWER ELECTRONIC TRANSFORMER (FPET)

Europe is currently making a great effort in order to improve the sustainability and reduce the environmental impact of its energy and transportation systems. A key role on these initiatives is played by efficient generation systems, like cogeneration, and clean or renewable energies, like wind or solar energy, as well as, by efficient and improved transportation technologies. In the evolution of these energy and transport systems, the development of Power Electronic Converters with greater functionality, higher reliability, higher efficiency, lower cost, and more sophisticated control will be essential. The main goal of future Power Electronic Converters will be to increase power density, reduce cost and improve reliability. This way, volume, weight and material reduction as well as reliability will gain the future market. A great contribution of these goals will be made by new high-power semiconductor devices, which permit the extension of the frequency range of power converters, and consequently the reduction of magnetic components. A good example of one of these systems are medium-frequency power conversion systems, also known as Power Electronic Transformers, which are able to convert electric power as convectional transformers but with increased features: volume and weight reduction, power transfer and quality control etc. The present work introduces a complete characterization of a medium-frequency power transformer, suitable for efficient Power Electronic conversion systems. The motivation of the present work stems out from the need to evaluate the constraints of conventional transformer characterization and design methodologies. The proposed expressions are able to successfully address the problematic related to non-sinusoidal waveforms, typical of medium-frequency power transformers. Moreover, a design methodology for the optimal design of medium-frequency power transformers is introduced. The characterizations, as well as the design methodology, are verified by means of finite element simulations and measurement results.

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction. When the output voltage of transformer is higher than its input voltage, it is called the step up transformer and when it lowers the voltage it is called the step down transformer. Since its basic construction requires no moving parts, so it is often called the ‘static transformer, and it is very rugged machine requiring the minimum amount of repair and maintenance. Owing to the lack of rotating parts there are no friction or windage losses. Further, the other losses are relatively low, so that the efficiency of a transformer is high. As there are no teeth, slots or rotating parts, and the windings can be immersed in oil, it is not difficult to insulate transformer for very high voltages.

                     In recent years, significant advances in power semiconductor device technology, low-cost, high-speed control processors, and matured PWM algorithms have led to a number of modern power converter topologies. A new type of transformers based on Power Electronics PE) has been introduced, which realizes voltage transformation, galvanic isolation, and power quality enhancements in a single device. The PE based transformer provides a fundamentally different and more complete approach in transformer design by using power electronics on the primary and secondary sides of the transformer. Several integrated PQ features such as instantaneous voltage regulation under load dynamics and transients, voltage sag compensation, power factor correction, and harmonic suppression can be incorporated into PET, thanks to the application of power electronics technology.

The PET can compensate both the active and reactive powers, and remove the power quality disturbances such as sag, swell, under voltage, over voltage and voltage flicker. In comparison to the conventional transformers, it has low weight, compact volume, extended functionality, and eliminates the necessity for toxic dielectric coolants. The PE-based transformer is a multi-cellular step-down converter that can directly connect to medium voltage levels on the primary side and provide a low voltage, highly stable interface for consumer applications. PET replaces conventional transformers and performs better voltage regulation.

PRINCIPLE OF OPERATION

                The line side AC waveform is modulated with a static converter to a high-frequency square-wave and passed through a HF transformer and again with a synchronous converter, it is demodulated to AC form. Since the transformer size is inversely proportional to the frequency, the high frequency transformer will be much smaller than the line frequency transformer. So, the transformer size, weight and stress factor is reduced noticeably. This scheme can be utilized to mitigate power-line disturbances such as voltage sags and swells in low voltage equipments.

Mathematical relation between transformer size and frequency

The transformer equation E = 4.44(f)(B)(N)(a) where E is the voltage in the winding either primary or secondary, f is the frequency, B is the flux density in the core, N are the number of turns on the winding, and a is the cross area of the core. Power devices operated at magnetic saturation so B is constant. Then it is clear that at higher frequencies the number of winding turns N is reduced.

ADVANTAGES OVER CONVENTIONAL TRANSFORMERS

1.      Active and reactive powers compensation: The power actually consumed in an ac circuit is called active power (VI cosφ). The lagging reactive power is responsible for the low power factor (VI sinφ). Main disadvantages of reactive power are:

a) Large KVA rating of the equipment. Electrical machinery is always rated in KVA.

b) Increase in electricity bill.

c) Losses increases.

d) Decrease in power factor, there by penalty in electricity bill.

                    Devices such as shunt, series, synchronous condenser, DVR, UPQC are widely employed for reactive power compensation. In a weak network is very sensitive to load changes. A sudden change in active load will cause both a phase jump and a magnitude fluctuation in the bus voltage, whereas reactive load changes mainly affect the voltage magnitude. With the addition of energy storage to a static synchronous compensator (StatCom), it is possible to compensate for the active power change as well as providing reactive power support. The PET can compensate both the active and reactive powers.

2.      Flexible regulation of the voltage and power: Regulation means the change in secondary terminal voltage from no load to full load at any particular load (about 4%). On the consumer point it should be as minimum as possible.

3.      Remove the power quality disturbances such as sag, under voltage, over voltage and voltage flicker

4.      Compact: low weight, compact volume.

5.      Eliminates the necessity for toxic dielectric coolants: Mineral oil, beta oil, silicone, envirotemp are widely used coolant materials, cost of coolant higher and its replacement also difficult.

CONSTRUCTION OF PET

Figure 1

n  THREE STAGES

                                                          

n  Cascaded H-bridge (CHB) rectifier (Input stage)

n  Isolation stage

n  Output stage

INPUT STAGE

                  Figure shows the basic block diagram of a power electronic based transformer (PET) which includes three parts. First part or the input stage is an AC/DC converter which is utilized to shape the input current, to correct the input power factor, and to regulate the voltage of primary DC bus. Input stage is an active rectifier to ensure that the input current is sinusoidal.

CASCADED H-BRIDGE CONVERTER

Figure 2

                The CHB converter is the best choice for working in high-voltage and high-power applications due to the extreme modularity, simple physical layout, and low losses. CHB converter has N H-bridge cells connected in series. Each H-bridge consists of four power switches (with anti-parallel diodes) and a DC bus capacitor. Each capacitor feeds a high-frequency DC/DC converter. It is worth noting that the unidirectional rectifier can be realized from the bidirectional rectifier by turning off the upper switches of the H-bridge cells (or by replacing the upper switches with relatively fast diodes). The utilized rectifier contains two parts. At the input side, there is a cascaded H-bridge rectifier which connects directly to the medium voltage levels and corrects the input power factor. The second stage is a modular parallel-output converter which provides a low-voltage and highly-stable DC interface with the consumer applications.

            The modular converter in Figure composed of N individual converter cells, connected in series on the primary side and in parallel on the secondary side. Each converter cell consists of an H-bridge AC-to-DC chopper on the line side to rectify the AC voltage and to stabilize the DC link voltage on the primary side of the individual converter sections. The second part of the converter cells is formed by a DC-to-DC converter. This part of the converter cell contains the HF transformer with the high insulation capability. On the secondary side conventional voltage source inverters or motor drives (which can be part of the respective load) are connected. This stage generates the symmetric three-phase voltages with the desired amplitude and frequency.

SECOND STAGE (ISOLATED DC/DC CONVERTER)

                      Second stage which prepare galvanic isolation between the primary and secondary system. In this part, the DC voltage is converted to a high-frequency square-wave signal, coupled to the secondary of the HF transformer and is rectified to form the DC link voltage.

                      The isolated DC-DC converter constitutes the front-end of two-stage distributed power architectures (DPA) with an intermediate bus voltage feeding non-isolated point-of-load (POL) converters. Usually two-stage DPA schemes do not require a tightly regulated intermediate bus voltage, since the POL will typically accept a relatively wide input voltage, and the POL provides the needed regulation to the load. In general, the topologies not based on bridge configurations are used in single-stage DPA for driving loads directly.

 THIRD STAGE (THREE PHASE INVERTER)

 Figure 7

3-phase inverter with wye connected load

               Three-phase inverters are used for variable-frequency drive applications and for high power applications such as HVDC power transmission. A basic three-phase inverter consists of three single-phase inverter switches each connected to one of the three load terminals. For the most basic control scheme, the operation of the three switches is coordinated so that one switch operates at each 60 degree point of the fundamental output waveform. This creates a line-to-line output waveform that has six steps. The six-step waveform has a zero-voltage step between the positive and negative sections of the square-wave such that the harmonics that are multiples of three are eliminated as described above. When carrier-based PWM techniques are applied to six-step waveforms, the basic overall shape, or envelope, of the waveform is retained so that the 3rd harmonic and its multiples are cancelled.

3-phase inverter switching circuit showing 6-step switching sequence and waveform of voltage between terminals A and C

           To construct inverters with higher power ratings, two six-step three-phase inverters can be connected in parallel for a higher current rating or in series for a higher voltage rating. In either case, the output waveforms are phase shifted to obtain a 12-step waveform. If additional inverters are combined, an 18-step inverter is obtained with three inverters etc. Although inverters are usually combined for the purpose of achieving increased voltage or current ratings, the quality of the waveform is improved as well.

FPET

                The proposed FPET is flexible enough to meet future needs of power electronic centralized systems. The main feature of the FPET is the independent operation of modules each contains one port. Each port can be considered as input or output, because bidirectional power flow is provided. The modules are connected to a common dc link that facilitates energy transfer among modules as well as ports. So a multi-port system is developed that the ports can operate independently. This merit is important for applications, where input and output voltages are different in many parameters. Also the measurement results of a laboratory prototype are presented to verify the capabilities of FPET in providing different output waveforms and controlling load side reactive power. Power Electronic Transformers (PETs) are proposed to replace conventional transformers and perform voltage regulation and power exchange between generation and consumption ports by electronically conversion]. The previous researches show that PETs have a great capacity to receive much more attention due to their merits such as high frequency link transformation and flexible regulation of the voltage and power. Although many studies have been conducted on application and control of PET in power systems, less attention is paid to the areas of the circuit topologies. The topology of PET can be developed in such a way to achieve multi-port electrical system that converts variable input waveform to the desired output waveform. In addition, for higher voltage applications or three phase systems the topology is expandable as it is modular. It is constructed based on modules and a common dc link, which is used to transfer energy between ports and isolate all ports from each other. In this bidirectional topology, each port can be considered as an input or output. Each module consists of three main parts, including modulator, demodulator and High Frequency Isolation Transformer (HFIT). The modulator is a dc-ac converter and the demodulator is an ac-ac converter both with bidirectional power flow capability. Each module operates independently and can transfer power between ports. These ports can have many different characteristics, such as voltage level, frequency, phase angle and waveform. As a result, FPET can satisfy almost any kind of application, which are desired in power electronic conversion systems and meet future needs of electricity networks.

               Each port of FPET is composed of a Full-Bridge DC link Inverter (FBDCI), HFIT and a cycloconverter. This topology consists of independent and similar modules and each port can work independently. Thus analysis of one port is sufficient to introduce whole topology. The FBDCI (modulator) can operate as an inverter when it converts the dc link voltage to an ac waveform at the HFIT side. It can operate as an active rectifier when it converts the ac waveform of the HFIT to the dc link voltage.

The modulator description is expressed as follows:

a. Bidirectional power flow capability

b. Adjustable switching frequency which feet voltage pulses frequency it into the pass-band of HFIT, and

c. Stored energy in the dc link (if the modulator is in active rectifier mode).

For cycloconverters, several circuit topologies can be proposed using unidirectional or bidirectional switches. In this paper, a typical cycloconverter with two bidirectional switches operates as the demodulator. The demodulator converts high frequency voltage (i.e. Vs) to low frequency voltage (i.e. Vpr1) and vice versa. The specifications of the demodulator are listed as follows:

 a. Bidirectional power flow capability

b. Providing zero voltage switching by turning the switches of cycloconverter on/off, while voltage of HFIT riches to zero.

APPLICATIONS

1.      Dynamic Voltage Restorer (DVR) and Active Filter (AF):  Dynamic Voltage Restorer (DVR) and Active Filter (AF) [16] applications can be satisfied by the FPET, because it can connect to the grid in series or/and in parallel. Desired voltage and current can provide by the flexibility of FPET in providing various waveforms. Voltage sag is one of the power quality issue and DVR is using for mitigation of voltage sag.

2.      Universal Power Quality Conditioner (UPQC): FPET can provide desired waveform in each phase (or port) independently, so this can be used in Universal Power Quality Conditioner (UPQC).

3.      Interline Power Flow Controller (IPFC): FPET can transfer active and reactive power from one port or phase to another port or one phase. This in power distribution system is very useful for Interline Power Flow Controller (IPFC).

4.      Uninterruptible Power Supply application (UPS): FPET can provide symmetrical three phase voltage from an asymmetrical ac source in the form of an Uninterruptible Power Supply application (UPS).

5.      Renewable energy applications: FPET can play a role in providing useful power from variable Low voltage dc sources. That is suitable for renewable energy applications such as photovoltaic and fuel cell.

6.      Aircraft and shipboard: PET will have a major impact on the utility industry and the places such as aircraft and shipboard where the high quality power conversion is very desirable.