WO2009123469A1

WO2009123469A1 - Buck boost topology

Info

Publication number: WO2009123469A1
Application number: PCT/NO2009/000119
Authority: WO
Inventors: Christian M. Hartmann
Original assignee: Magtech As
Priority date: 2008-03-31
Filing date: 2009-03-31
Publication date: 2009-10-08

Abstract

The invention relates to a system for voltage stabilization of a power supply line. The system comprises an autotransformer (T1) comprising a series winding (S) and a parallel winding (P), where the series winding comprises a first terminal (S1) for connection to a line input terminal (LI). The system further comprises a variable inductance (LR) connected to the autotransformer, the variable inductance comprising a magnetic core, a first main winding (MW1) wound around a first axis, and a control winding (CW) wound around a second axis. The first axis and the second axis are orthogonal axes so that when the first main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core. A control system is arranged to control the permeability of the magnetic core to compensate automatically for voltage variations in the power supply line. The variable inductance (LR) comprises a second main winding (MW2) wound around the first axis of the core, where the second main winding comprises a first terminal (10) connected to a second terminal (S2) of the series winding (S) of the autotransformer (T1) and a second terminal (12) for connection to a line output terminal (LO).

Description

Buck boost topology

FIELD OF THE INVENTION

The present invention relates to voltage stabilization of power supply lines. More particularly, the invention relates to methods and systems that employ a variable inductance to compensate for voltage variations that may arise in power supply lines.

BACKGROUND

EP 1 576 437 Bl describes a system for voltage stabilization of power supply lines. Fig. Ia is an illustration of the system of this publication, where it is shown that an autotransformer Tl and a variable inductance LR is connected between a line input LI and a line output LO.

The autotransformer Tl comprises a series winding S and a parallel winding P, where the series winding comprises a first terminal Sl for connection to a line input terminal LI. The series winding S has Ns turns, while the parallel winding P has Np turns.

The variable inductance LR is connected to the autotransformer and comprises a magnetic core, a first main winding MWl wound around a first axis, and a control winding CW wound around a second axis. The first axis and the second axis are orthogonal axes so that when the first main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core. A control system is arranged to control the permeability of the magnetic core to compensate automatically for voltage variations in the power supply line.

Figure Ib shows the relationship between the voltage phasors (phase vectors) for the system described in figure Ia when the control winding is energized with a dc current near its nominal value. This results in a large increase in output voltage relative to input voltage, i.e. that the voltage Uout > the voltage Uin.

Fig. Ic shows the voltage phasors for the same system. Here, the control winding is energized with a dc current near zero. As a result, the voltage Uout is ~ Uin.

Hence, the voltage rise from input to output voltage is small. At zero dc control current the output voltage will be nearly equal to the input voltage. Since the system's behaviour is the same regardless of the polarity of the dc control current, there exists no controlled manner to reduce the output voltage relative to the input voltage in the system described in figure 1 a. It should be noted that multiphase systems can comprise several such voltage stabilisation systems to enable individual voltage regulation on each phase and/or line voltages. Such systems have been found to have some deficiencies. This is especially evident in three-wire, delta coupled systems. Here, one such voltage stabilization system is normally connected between two and two phases. If the load is skew, or unsymmetric, there can be situations where the voltage over the light load is too high due to the influence of the neighboring line voltage. There can also be cases when it is desirable to reduce the output voltage relative to the input voltage regardless of the load symmetry. The object of the present invention is to provide a system for voltage stabilization of power supply lines where the output voltage is controlled to a more suitable level for all phases when needed, also for unsymmetrical loads. Moreover, it is an object of the invention to improve the symmetry of the output voltage from such stabilization systems.

SUMMARY OF THE INVENTION

The invention relates to a system for voltage stabilization of a power supply line, the system comprising: an autotransformer comprising a series winding and a parallel winding, where the series winding comprises a first terminal for connection to a line input terminal; a variable inductance connected to the autotransformer, the variable inductance comprising a magnetic core, a first main winding wound around a first axis, and a control winding wound around a second axis; wherein the first axis and the second axis are orthogonal axes so that when the first main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core; and a control system arranged to control the permeability of the magnetic core to compensate automatically for voltage variations in the power supply line; characterized in that the variable inductance comprises a second main winding wound around the first axis of the core, where the second main winding comprises a first terminal connected to a second terminal of the series winding of the autotransformer and a second terminal for connection to a line output terminal. In an aspect of the invention, a first terminal of the parallel winding is connected to the line input terminal and a second terminal of the parallel winding is connected to a first terminal of the first main winding of the variable inductance.

In an aspect of the invention, a second terminal of the first main winding is connected to the common input/output terminal. In an aspect of the invention, the control system further comprises: a controller unit; a sensor unit comprising one or several sensors in electrical communication with the controller unit, wherein the sensor unit measures relevant process variables for input to the controller unit; a setpoint reference unit for setting one or more setpoint references for the controller unit; wherein the controller unit controls one or more control currents supplied to the control windings.

The invention also relates to a a three-phase system for voltage stabilization, comprising a system according to any of claim 1 - 4 for voltage stabilization of each phase.

In an aspect of the invention, control windings for three phases are connected in series or parallel and regulated together.

In an aspect of the invention, control windings for the three phases are controlled independently of one another.

The invention also relates to a method of stabilizing a voltage, the method comprising the steps of: connecting an autotransformer comprising series winding and a parallel winding to an line input terminal; connecting a first main winding of a controllable inductance between the parallel winding and a common input/output terminal, where the first main winding is wound around a magnetic core; connecting a second main winding between the series winding and an output line terminal, where the second main winding is wound around the magnetic core; generating orthogonal magnetic fields in the magnetic core; sensing an output voltage; adjusting at least one of the orthogonal magnetic fields to control a permeability of the magnetic core to adjust the voltage in response to the output voltage sensed.

In an aspect of the invention, the method further comprises the step of controlling the permeability further comprises adjusting a control current supplied to a control winding of the controllable inductance.

DETAILED DESCRIPTION Embodiments of the invention will now be described with reference to the enclosed drawings, where:

Fig. Ia illustrates a prior art system for voltage stabilization of power supply lines.

Fig. Ib illustrates a phasor diagram for fig. Ia for control current near nominal value. Fig. Ic illustrates a phasor diagram for fig. Ia for control current near zero.

Fig. 2 illustrates an embodiment of the system for voltage stabilization according to the invention.

Fig. 3 illustrates a general block diagram of the embodiment in a power supply system. Fig. 4a illustrates the system of fig. 2 in a three phase delta configuration. Fig. 4b illustrates the system of fig. 2 in a three phase star configuration.

Fig. 5 illustrates the buck and boost modes of operation in relation to the control current. Figs. 6 - 9 illustrate different phasor diagrams for the embodiment shown in fig. 2 and fig. 4a.

It is now referred to fig. 2, illustrating an embodiment of a system for voltage stabilization of a power supply line according to the invention. In fig. 1, the system is generally denoted with reference number 1. Many of the elements of the system in fig. 2 is similar to elements of fig. 1 a, and the reference numbers are used where suitable.

The system comprises an autotransformer Tl, a variable inductance LR and a control system. The autotransformer Tl (indicated by upper dashed box in fig. 2) comprises a first, so-called series winding S and a second, so-called parallel winding P. The series winding S comprises a first terminal Sl and a second terminal S2. The parallel winding P comprises a first terminal Pl and a second terminal P2. The series and parallel windings S, P are wound around the same core. Consequently, a change in the voltage over the parallel winding P will cause a change in the voltage over the series winding S. The series winding S has Ns turns, while the parallel winding P has Np turns. In the present embodiment, Ns/Np = 1/4,5. It should be noted that these numbers may vary from application to application and may depend on the system voltage level etc. The variable inductance LR (indicated by lower dashed box in fig. 2) comprises a magnetic core, a first main winding MWl wound around a first axis of the core, and a control winding CW wound around a second axis of the core. The first axis and the second axis are orthogonal axes so that when the first main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core. The first main winding MWl has N_MWI turns, while the second main winding MW2 has NMW2 turns. In the present embodiment, N_MW₂/N_MW_I ⁼ 1/10.

The control system is arranged to control the permeability of the magnetic core of the variable inductance LR to compensate automatically for voltage variations in the power supply line. 4. The control system may comprise a controller unit, a sensor unit and a setpoint reference unit. The sensor unit comprises one or several sensors in electrical communication with the controller unit, wherein the sensor unit measures relevant process variables for input to the controller unit. The setpoint reference unit is used to set one or more setpoint references for the controller unit, i.e. as reference signals. Input to the control system may be process variables such as output voltage Uout and the setpoint reference may be a corresponding voltage reference. Output from the control system is a dc current supplied to the control winding of the variable inductance LR. Based on the measured output voltage Uout, the control current is increased or decreased to achieve an output voltage Uout being as close as possible to the reference voltage. The autotransformer Tl, the variable inductance LR and the control system are described in detail in the abovementioned publication EP 1 576 437 Bl and are hence considered known for a skilled person.

The first terminal Sl of the series winding S and the first terminal Pl of the parallel winding P are connected to the line input terminal LI. The first main winding MWl is connected with its first terminal 14 to the second terminal P2 of the parallel winding P and with its second terminal 16 to a common input/output terminal L. This corresponds to the system illustrated in fig. Ia.

In the present embodiment, the variable inductance LR comprises a second main winding MW2 wound around the first axis of the core. The second main winding comprises a first terminal 10 connected to a second terminal S2 of the series winding S of the autotransformer Tl. Moreover, the second main winding MW2 comprises a second terminal 12 for connection to the line output terminal LO.

It is now referred to fig. 3. Here it is shown a power source and a line impedance representing power transmission. A three phase voltage stabilization system with control system, again denoted with reference number 1, is connected between the line impedance and the load, for stabilizing the voltage supplied to the load.

It is now referred to fig. 4a, where a three phase system according to the invention is illustrated. Here, three systems Ia, Ib and Ic, each corresponding to system 1 in fig. 2, are connected between the respective phases A, B C (illustrated by voltages Ua, Ub, Uc respectively). The system in fig. 4 is connected as a delta connection, where the first main winding MWl of a first system Ia is connected to the first terminals Sl, Pl of a second system Ib, the first main winding MWl of the second system Ib is connected to the first terminals Sl₅ Pl of a third system Ic, and the first main winding MWl of the third system Ic s connected to the first terminals Sl, Pl of the first system Ia.

It is now referred to fig. 4b, where an alternative embodiment of fig. 4a is illustrated. Here, the system is connected as a star connection, where the first main winding MWl of the respective first, second and third systems Ia, Ib, Ic are connected directly to each other.

It should be noted that in the three phase system, the control current can be controlled independently for each phase. It is now referred to fig. 5. Here the function of the system according to the invention is shown. The horizontal axis is illustrating the control current from Icontrol = 0 to Icontrol = nominal value.

A first interval is denoted with "buck", indicating that in this interval the system is bucking or decreasing the output voltage Uout, or, in case of a three phase system, the output voltage for one phase Ua, Ub or Uc.

A second interval is denoted with "boost", indicating that in this interval the system is boosting or increasing the output voltage Uout, or, in case of a three phase system, the output voltage for one phase Ua, Ub or Uc.

It should be noted that the 6 the control windings for a three phase stabilization system may be connected in series or in parallel and hence be regulated together by one common control system. Alternatively, the control windings for the three phase stabilization system are controlled independently of one another.

It is now referred to figs. 6 - 9.

Fig. 6 and 7 show the phasor relationships in the single phase topology from figure 2. Figure 6 shows the system in boost mode, whereas figure 7 shows the system in buck mode.

In boost mode, figure 6, the phasor U_MW2, representing the voltage over the second main winding Mw₂, has the same direction as U_MWI, assuming ideal transformation. U_MWI represents the voltage over the first main winding Mwi-The magnitude of phasor U_MW₂ is determined by the winding ratio N_MW₂/N_MW_I- In the same manner, Us, representing the voltage over the series winding S is related to Up, representing the voltage over the parallel winding Np, by the winding ratio Ns/Np.

In buck mode, figure 7, the voltages U_MW_I and U_MW₂ are large compared to the voltages Up and Us which results in the output voltage U_OUT being reduced relative to the input voltage U_IN-

In boost mode, figure 6, the voltages Up and Us are large compared to the voltages U_MW_I and U_MW2 which results in the output voltage Uout being increased relative to the input voltage Uin. The amount of reduction is proportional to the ratio Ns/Np.

The ability to buck the output voltage is mainly governed by the winding ratio N_MW₂/N_MW_I while the ability to boost the output voltage is mainly governed by the winding ratio Ns/Np. However, the two winding ratios N_MW2/N_MWI and Ns/Np, also affect each other: the ability to buck is reduced by increasing the ability to boost and vice versa. Therefore, the winding ratios N_MW2/N_MWI and Ns/Np must be carefully selected. Between buck and boost mode there will be a neutral mode where the reducing voltage U_MW₂ and the increasing voltage US cancel each other.

Consequently, it has been shown that the output voltage Uout for one or several phases may be reduced or increased individually. This increases the ability to control the output voltages between the respective phases, which again results in improved symmetry between the voltages especially at skew or unsymmetric loads.

Figure 8 shows the voltage phasors for a skew loaded three phase system based on prior art technology from EP 1 576 437 BL. This system consists of three delta- connected voltage stabilisation systems, each as shown in figure Ia. In situations where the load is unsymmetric, this system will be unable to boost the voltage of the heavy loaded lines without increasing the voltage of the line with light load at the same time. This is exemplified by an 1.1 p.u. output voltage in figure 8.

Figure 9 shows the voltage phasors for the delta-connected system described in figure 4a. The system is based on three regulators, each equal to the system shown in figure 2. Since these regulators have the ability to buck the output voltage, it is now possible to avoid the problem described in figure 8 at unsymmetric loads.

Figure 9 shows this: all output voltages are now equal to 1.0 p.u. for the same load as in figure 8.

Alternative embodiments

It should be noted that other winding polarities for Tl and LR apart from what is shown in the figures are possible. For example, by reversing the polarity of both the series winding S of Tl and the second main winding MW2 of LR in figure 2, the relationship between control current and operation modes shown in figure 5 will be reversed. Consequently, boosting will occur for low control currents, while bucking will occur for higher control currents.

Claims

1. A system for voltage stabilization of a power supply line, the system comprising: an autotransformer (Tl) comprising a series winding (S) and a parallel winding (P), where the series winding comprises a first terminal (Sl) for connection to a line input terminal (LI); a variable inductance (LR) connected to the autotransformer, the variable inductance comprising a magnetic core, a first main winding (MWl) wound around a first axis, and a control winding (CW) wound around a second axis; wherein the first axis and the second axis are orthogonal axes so that when the first main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core; and a control system arranged to control the permeability of the magnetic core to compensate automatically for voltage variations in the power supply line; c h ar ac t e r i z e d i n that the variable inductance (LR) comprises a second main winding (MW2) wound around the first axis of the core, where the second main winding comprises a first terminal (10) connected to a second terminal (S2) of the series winding (S) of the autotransformer (Tl) and a second terminal (12) for connection to a line output terminal (LO).

2. System according to claim 1, where a first terminal (Pl) of the parallel winding (P) is connected to the line input terminal (LI) and a second terminal (P2) of the parallel winding is connected to a first terminal (14) of the first main winding (MWl) of the variable inductance (LR).

3. System according to claim 2, where a second terminal (16) of the first main winding (MWl) is connected to the common input/output terminal (L).

4. System according to claim 1, where the control system further comprises: a controller unit; a sensor unit comprising one or several sensors in electrical communication with the controller unit, wherein the sensor unit measures relevant process variables for input to the controller unit; a setpoint reference unit for setting one or more setpoint references for the controller unit; wherein the controller unit controls one or more control currents supplied to the control windings.

5. A three-phase system for voltage stabilization, comprising a system according to any of claim 1 - 4 for voltage stabilization of each phase.

6. A three-phase system according to claim 5, wherein control windings for three phases are connected in series or parallel and regulated together.

7. A three-phase system according to claim 5, wherein control windings for the three phases are controlled independently of one another.

8. A method of stabilizing a voltage, the method comprising the steps of: connecting an autotransformer (Tl) comprising series winding and a parallel winding to an line input terminal (LI); connecting a first main winding (MWl) of a controllable inductance (LR) between the parallel winding and a common input/output terminal (L), where the first main winding is wound around a magnetic core; connecting a second main winding (MW2) between the series winding and an output line terminal (LO), where the second main winding is wound around the magnetic core; generating orthogonal magnetic fields in the magnetic core; sensing an output voltage; adjusting at least one of the orthogonal magnetic fields to control a permeability of the magnetic core to adjust the voltage in response to the output voltage sensed.

9. Method according to claim 8, wherein the step of controlling the permeability further comprises adjusting a control current supplied to a control winding of the controllable inductance.