Distribution Electronic Power Transformer with ...

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The converter uses two power switches for converting DC voltage to high frequency. AC square voltage in the primary side of the transformer. A diode bridge has ...
Distribution Electronic Power Transformer with Reduced Number of Power Switches Farshid Najaty Mazgar

Mehrdad Tarafdar Hagh

Ebrahim Babaei

Department of electrical engineering, Islamic Azad University, Ahar Branch Ahar, Iran Email: [email protected]

Department of electrical engineering, Islamic Azad University, Ahar Branch Ahar, Iran Email: [email protected]

Department of electrical engineering, Islamic Azad University, Ahar Branch Ahar, Iran Email: [email protected]

Abstract— Nowadays power transformers are considered among the huge and expensive equipment because of their massive iron core and heavy copper windings. Since the size of the transformers and the maximum power transition are reversely related, a new family of electronic power transformer has come into existence Which increase the transformer frequency, using power electronic converters. In addition to voltage transformation and good isolation which they bring about, these transformers are also associated with significant advantages, including considerable reduction in the size, power quality improvement, voltage profile improvement, etc. This paper attempts to introduce a new structure for Distribution Electronic Power Transformer (DEPT), using fewer power switches while maintaining the previous capabilities of the system. System performance and control strategies were successfully simulated in PSCAD/EMTDC and the simulation results are given below. Keywords; distribution electronic power transformer, high frequency, voltage sags and swells, Push-Pull converter

I.

INTRODUCTION

Power transformers are widely used in electrical power systems for voltage transforming and isolation purposes. As a result of the massive iron core and heavy copper windings this transformers have come to be among the huge and expensive power system equipment. On the other hand, with the increase in nonlinear loads in the distribution systems the power quality does not fare well in such systems and custom power equipment has long been felt. Since transition power of transformer and their size are reversely related to the frequency, increasing the frequency provides for using magnetic steel core in the transformers, thereby considerably reducing its size. Since there is no energy storage source in the conventional transformers, when there is disturbance in the transformer input the output loads are disturbed as well. Similarly when the load is troubled with transient states, harmonics and power disturbances the conventional transformers reflect all this problems toward the grid. In order to overcome such problems power electronic technology could be an option, which act as energy buffer, thereby preventing the mutual effect of grid and load on one another. That’s why the new family of electronic power transformers have come into existence which, using electronic converters, increasing the frequency of AC signals and thus reduce the transformer

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sizes. They can also improve the voltage and power quality when equipped with a DC link. The Distribution Electronic Power Transformer(DEPT) are reduction converters connected to medium voltage lines in their primary sides, and provide lower and more stable voltages in their outputs for consumption. In [1-3] references using high frequency AC link in a power electronic transformer has been studied. In this method input AC voltage turns to high frequency square waves with high voltages which are converted to low level high frequency square voltages using high frequency transformer, which finally change to AC voltages in the output through synchronic converters. In any event this method cannot be credited with too many advantages as it is not capable of controlling or improving suitable power factor. And since there is no storage source it cannot protect sensitive loads against high voltage flickers. Other types of electronic power transformers are divided to two categories. The first include the transformers which do not use DC links in their structures [48]. However the second type comprises the transformers which make use of DC links which in addition to smaller size brings about many advantages such as suitable input and output voltage and current control with more flexibility. Even though electronic power transformers are much more expensive than the conventional ones, this increase in the expenses is well justified for their special capabilities (for the distributed generation, feeding sensitive loads, providing the quality of desirable power for special loads, etc.). Among the power quality equipment such as UPS which connected to the grid in parallel and has problems synchronizing with the grid on the return of the network voltage to the its nominal value, electronic power transformers do not need to be synchronized to the grid as a result of being connected in series. In [9-13] references two DC links have been used in the electronic power transformer structures, these links are connected to each other through the isolation stage. In the isolation stage, one to three high frequency transformers have been used with two Hbridges in their primary and secondary sides. In the structure presented in this paper a Push-Pull converter is used in the isolation stage. The converter uses two power switches for converting DC voltage to high frequency AC square voltage in the primary side of the transformer. A diode bridge has been used in the directional power flow conditions. And if required an H-bridge rectifier would be used

in bidirectional power flows. The system in question was successfully tested in PSCAD/EMTDC software and the simulation result illustrates voltage quality improvement for disturbed inputs.

Figure 1

II.

Proposed structure for DEPT.

THE STRUCTURE EMPLOYED FOR DEPT

As it can be seen in Fig.1 the suggested EPT is composed of three main stages; the input stage, isolation stage and the output stage. Input three-phase voltage of the transformer is converted to the DC voltage in the DC link by the input threephase rectifier. The DC voltage is converted to high frequency AC voltage by a Push-Pull converter in the isolation stage. With the voltage frequency increasing there is a decrease in the transformer size. And through a high frequency reduction transformer the high voltage AC signal is transformed to the high frequency low voltage AC signal in the secondary side of the transformer. At the end of the isolation stage this voltage is converted to DC voltage. In fact the isolation stage acts like an isolated DC-DC converter. This structure makes it possible for the transformer to feed several isolated single phase loads in the output of the transformer or to feed AC and DC loads simultaneously adding multiple parallel cells to the secondary side of the high frequency transformer. In the output stage the DC voltage is converted to three single phase AC voltage or a set of three-phase voltage with desired amplitude and frequency by means of three single phase H-bridge inverters. In the structures for the power electronic transformer 16 to 24

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power switches in the isolation stage have been used to provide three single phase isolation loads [9-13]. However in the new system presented here the same quality and performance of the previous system can be achieved through using two power switches and 12 power diodes in directional power flow while 14 switches are employed for bidirectional power flow. The maximum voltage of the power switches and the secondary diodes of the high frequency transformer in the isolation stage are as follows;

VS max = 2VDC

VD max =

2VDC n

(1)

(2)

Where VSmax is the maximum voltage of the S7, S8 power switches that is equal to VDC1, and VDmax is the maximum voltage of the transformer secondary diodes and n represent the transformer ratio.

III.

DEPT CONTROL

DEPT is reduction transformer and considering the low voltage of the DC link compared to the input voltage, with the help of a suitable control system one can eliminate voltage sags, swells, flickers and harmonics in addition to controlling principle power transformer functions.

VDC1*

+

VDC1 _

ed PI

ild*

PI

+

+

ild Ia,b,c

ω L1

abc dq

PWM Control

ilq

ω L1 _

ilq*

_

_ PI

+

eq Figure 2

Vld*

_ +

Vlq*

+

Control diagram of the input stage rectifier.

A. Controlling Input Rectifier Fig.2. shows control diagram of the input stage of the DEPT this stage performs two major tasks. The first one is to sinusoid and keep the input current of each phase of the transformer in-phase with the voltage of that phase. As a result of which the input power factor of transformer equals one. This is done to reduce the amplitude of the current flowing from the grid. The other task of the input rectifier is to stabilize the DC link voltage at the desired value against the variable and imbalanced input. As the Fig.2 illustrate an outer voltage loop and an inner current loop are used to sinusoid input current and stabilized DC link. The reference value for the reactive power is set to zero so that the power factor equal one. Through using this control method the DC link voltage is stabilized even when there are voltage sags, swells, imbalances and harmonics [5]. B. Isolation stage control As Fig.1 illustrates the isolation stage, in fact, functions as an isolated DC-DC converter and is composed of the inverters, high frequency transformers and the rectifiers. The inverter use in the isolation stage of the DEPT proposed structure is of Push-Pull type. The DC link voltage in the input of the stage is converted to high frequency square AC voltage by means of two power switches. To this end two power switches with equal duty cycles and high frequencies are implemented. Accordingly in the half of the switching period S7 switch is turned on and with the S8 switch being turned off and conversely in the other half of the switching period cycle the S7 switch is turned off while the S8 is turned on. In this way,

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considering the equal duty cycles of the switches the AC current (without DC component) passes through the transformer, preventing it from saturation. Now if the duty cycle of one of the switches happens to exceed that of the other, a certain amount of DC current will flow through the transformer, leading to transformer saturation and its current excess. In practice, high-speed pulse-to-pulse current sensors are used in both switches in order to prevent this from happening. This way if more current happens to pass through either of the switches the sensors reduces its current through reducing its duty cycle thereby maintaining the AC current in the transformer [14]. The resulting high frequency square AC voltage is transformed to high frequency square voltage with lower amplitude. In this paper diode bridge is use to transform the high frequency AC voltage to DC voltage. On occasions when power flow might be required in both directions 4 power switches in H-bridge could replace the diode bridge. The DC voltage transition function for this converter can be calculated through the following equation;

M=

VDC 2 2 D = VDC1 n

(3)

Where M is the converter voltage transition function, D the duty cycle of the S7 and S8 switches and n is the ratio of the transformer. With D increased from 0 to 0.5 the M value increased from 0 to 1/n.

C. The output stage control In this stage the DC voltage from the isolation stage is converted to AC voltage with the desired amplitude and frequency through the three single phase H-bridge inverters. These inverters may provide three isolated single phase loads or a balanced three-phase load, depending on the consumer needs. Either the Space Vector Modulation (SVM) or the Pulse Width Modulation (PWM) method can be used to control the inverters, while in the present work PWM method has been employed to control the output inverters. IV.

Load

C1 , C2 Capacitors Input Inductance Push-Pull Switching Frequency Inverters LF Output Filter CF

Ea

Eb

0.00

0.10

0.20

(b)

30.00k

(a) The input voltage of the transformer(V), (b) input current of the transformer(A). Ea

Eb

Ec

20.00k 10.00k 0.00

0.60

0.50

(a) 30.00k

Ea

Eb

Ec

20.00k 10.00k

Ec

10.00k 0.00 -10.00k -20.00k -30.00k 0.10

-1.00

0.40

20.00k

0.00

-0.50

-30.00k

In the Fig.3 (a), (b) the input three-phase voltage and current of the transformer are shown. As it can be seen in the figure the purposed structure, using the employed control method, has managed to make the voltage and current of the each phase completely in-phase, keeping the input power factor equal to one. Fig 4(a) shows the system input voltage when there is sag or swell in the grid voltage. In order to demonstrate the performance and capability of the control method and the purposed structure voltage sag of 15 percent of the maximum input voltage in the 0.45 second was created. This voltage returned to its initial value in the 0.5 second. And in the 0.55 second a 20percent voltage swell was created in the grid voltage. Fig.4 (b) shows transformer input voltage with harmonics. 30.00k

0.00

-20.00k

Value 60Ω 60mH 1000µF 19mH 2KHz 3KHz 1mH 40µF

R L

Ic

-10.00k

PRINCIPAL PARAMETERS OF THE SYSTEM. Parameter

Ib

0.50

Figure 3

THE SIMULATION RESULTS

The system illustrated in Fig.1 is simulated in PSCAD/EMTDC software and the results have been reported to show the performance and the capabilities of the purposed structure. The principal parameters of the system are shown in Table I. TABLE I.

Ia

1.00

0.20

(a)

321

0.00 -10.00k -20.00k -30.00k 0.40

0.50

0.60

(b) Figure 4 (a) Input voltage of the transformer with sags and swells(V), (b) input voltage of the transformer with harmonics(V).

Fig.5 shows DC link voltage where control system has managed to keep DC link voltage stable despite the sags and the swells in the input voltage. Fig 6(a), (b) shows the high frequency primary and secondary voltages of the transformer in the isolation stage, respectively. As it can be seen the transformer input and output voltages have high frequencies (5 kHz) and are completely square. In Fig. 7 (a), (b) also the output three-phase voltage and current are shown and as it can be seen despite the sags and swells or the harmonics in the grid voltage the purposed structure for DEPT has successfully maintained a balanced and perfectly sinusoid voltage in the output with the help of the control method.

balanced voltage in the output despite prolong sags and swells of the input voltage (Fig.8 (a), (b)).

V_dc

14.0k 12.0k 10.0k

Ea

30.00k

8.0k 6.0k

20.00k

4.0k

10.00k

2.0k

0.00

0.0

-10.00k

0.00

0.10

0.20

Figure 5

0.30

0.40

0.50

0.60

Eb

-20.00k -30.00k

DC link voltage(V).

0.00

0.20

0.40

V1

15.0k 10.0k 5.0k 0.0 -5.0k -10.0k -15.0k

Ec

0.60

0.80

1.00

0.80

1.00

(a)

Main : Graphs

0.4400

0.4450

0.4500

0.4550

0.4600

0.4650

1.3k 1.0k 0.8k 0.5k 0.3k 0.0 -0.3k -0.5k -0.8k -1.0k -1.3k

0.4700

(a) V2

1.0k

0.00

Vc

Vb

Va

0.20

0.40

0.60

(b) Figure 8

(a) Input voltage with prolong sag(V), (b) output voltage of the DEPT(V).

-1.0k

V.

0.4350 0.4400 0.4450 0.4500 0.4550 0.4600 0.4650 0.4700 0.4750

In the present paper a new structure using Push-Pull converter for DEPT transformer was purposed which, as a result uses fewer power switches compared to the previous structures. The simulation results indicate that this transformer is capable improving the power quality and providing a balanced three-phase voltage or feeding sensitive loads in the output in despite of sags and swells in addition to voltage transforming, isolation the input and output and power transition.

(b) Figure 6 (a) Voltage of the primary side of the high frequency transformer(V), (b) voltage of the secondary side of the high frequency transformer(V). 1.3k 1.0k 0.8k 0.5k 0.3k 0.0 -0.3k -0.5k -0.8k -1.0k -1.3k

Va

Vb

Vc

REFERENCES [1]

0.40

0.50

0.60

[2]

(a) 20.0 15.0 10.0 5.0 0.0 -5.0 -10.0 -15.0 -20.0

CONCLUSION

Ioa

0.40

Iob

Ioc

[3] [4] [5] [6]

0.50

0.60

(b) Figure 7

[7]

(a) Output voltage of the DEPT(V), (b) output current of the DEPT(A).

Additionally, thanks to the control system and the structure employed the purposed transformer is capable of maintaining

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[8]

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