WO2007035111A1

WO2007035111A1 - Stabilizing winding for mvb in tn and tt grids

Info

Publication number: WO2007035111A1
Application number: PCT/NO2006/000328
Authority: WO
Inventors: Christian M. Hartmann; Bjørnar S. JOHANSEN; Reidar Tjeldhorn
Original assignee: Magtech As
Priority date: 2005-09-23
Filing date: 2006-09-22
Publication date: 2007-03-29
Also published as: NO20054428L; NO324259B1; NO20054428D0

Abstract

The present invention relates to a power supply system with voltage stabilisation comprising, for each phase an autotransformer (TA, TB, TC) with a parallel winding (4, 5, 6) and a series winding (7, 8, 9), the autotransformer (TA, TB, TC) further comprising a tertiary winding (22, 23, 24) galvanically separated from the parallel (4, 5, 6) and series (7, 8, 9) winding of the autotransformer; a magnetic controllable inductor (MCIA, MCIB, MCIC) where the parallel winding (4, 5, 6) of the autotransformer (TA, TB, TC) is series connected to a first winding (13, 14, 15) of the magnetic controllable inductor MCIA, MCIB, MCIC between the input voltage (Ua, Ub, Uc) and the neutral point, and the tertiary winding (22, 23, 24) of the autotransformer (TA, TB, TC) is connected to a second winding (16, 17, 18) of the magnetic controllable inductor (MCIA, MCIB, MCIC) through their respective first terminals forming pairs of windings, where these pairs of windings are mutually connected.

Description

Stabilizing winding for MVB in TN and TT grids

TECHNICAL FIELD

The present invention relates to a method and a system for voltage symmetrisation and for increasing short circuit capacity. In particular, the present invention relates to symmetrisation and increase of short circuit capacity in a device for voltage control and/or voltage stabilization for a three-phase power supply.

BACKGROUND OF THE INVENTION

Undersized lines for electric power transmission, also referred to as "weak lines", have too small conductor cross section in relation to the load requirements, moreover they have relatively high impedance. Excessive voltage drop will result from the losses caused by undersized conductors. The excessive voltage drop results in inadequate voltage levels for the electric power connected to the lines. WO 2004/053615 describes a system for voltage stabilization of such power lines.

In general, the TN- and TT-system comprises four current carrying lines - three phases and neutral. For this system, a single phase load will draw current from one supply line only and return it through the neutral conductor. This causes voltage unbalance when a system with weak lines has an unsymmetrical load. When the load is far downstream, this disadvantage is even more noticeable. It shall be noted that for any given point on the power transmission lines, "upstream" means any point closer to the distribution transformer and "downstream" means any point further away from it.

Another problem related to weak grids in TN- and TT-networks called neutral point displacement. The problem is that the impedance in the neutral conductor causes a substantial voltage difference between the neutral point at the distribution transformer and a neutral point far downstream when the system is non- symmetrically loaded. This increases the unbalance and also causes the system to be very unpredictable. For this reason, the voltage stabilizing system such as described in WO 2004/053615 will not be sufficient to stabilize the voltages at very nonsymmetrical loads in TN- and TT-networks. Therefore, an additional means to stabilize the phase voltages is necessary.

In standard three-phase transformer theory there exist winding arrangements to stabilize the phase voltages, i.e. make the phase voltages equal in magnitude and with an internal displacement of 120 electrical degrees. One such method is to use a tertiary delta winding. Such an arrangement is not suitable for the voltage stabilizing system described in WO 2004/053615. This is because the system uses three single-phase autotransformers, one for each phase, where each transformer must have the ability to have an individual voltage level.

The European standard EN50160, "Voltage characteristics of electricity supplied by public distribution systems" sets, among other things, limits for permissible deviations from the nominal system voltages.

Another problem related to weak grids, is that the one-poled short circuit capacity is low because of the high impedance in the grid. Hence, the currents that occur at faults are in some cases not sufficiently high to release the over-current protection within a satisfactory time-frame. This can result in high temperature in the fault- spot which in worst case causes fires.

The object of the present invention is to provide an improved method and system for voltage symmetrization in a three phase power supply system which overcomes the above-mentioned disadvantages. Consequently, one object of the invention is to increase the short circuit capacity at single-pole faults.

SUMMARY OF THE INVENTION

The present invention relates to a power supply system with voltage symmetrisation and increased short circuit capacity, comprising for each phase: an autotransformer (TA, TB, TC) with a parallel winding (4, 5, 6) and a series winding (7, 8, 9), where the series winding (7, 8, 9) is connected between the input voltage (Ua, Ub, Uc) and the output voltage (Ua_out, Ub_out, Uc_out), the autotransformer (TA, TB, TC) further comprising a tertiary winding (22, 23, 24) galvanically separated from the parallel (4, 5, 6) and series (7, 8, 9) winding of the autotransformer; - a magnetic controllable inductor (MCI_A, MCI_B, MCIC) comprising a first winding (13, 14, 15) wound around a core, and a second winding (16, 17, 18) wound around said core, and a control winding (10, 11, 12), which is adapted to create a magnetic field mainly orthogonal to the field created by the first winding and the second winding; - the parallel winding (4, 5, 6) of the autotransformer (T_A, T_B, Tc) is connected to the first winding (13, 14, 15) of the magnetic controllable inductor MCIA, MCIB, MCI_C between the input voltage (Ua, Ub, Uc) and the neutral point; the tertiary winding (22, 23, 24) of the autotransformer (T_A, T_B, Tc) is connected to the second winding (16, 17, 18) of the magnetic controllable inductor (MCIA, MCIB, MCI₀) through their respective first terminals, forming pairs of windings, and where these pairs of windings are mutually connected; where the system further comprises a control circuit, which based on measured voltages and/or currents, controls the control current of control windings (10, 11, 12), thereby controlling the inductance of the magnetic controllable inductor MCI_A, MCIB, MCIC. The method and system above provide an improved voltage symmetrization in three phase power supplies and also an increase in the short circuit capacity at single-pole faults. This is because it enables the device to draw current from all input phases to supply a single phase load at the output.

DETAILED DESCRIPTION The present invention will now be described in detail by means of a preferred embodiment with reference to the enclosed drawings, where:

Fig. 1 shows a system according to the preferred embodiment of the invention; Fig. 2 shows the system in fig. 1, where various voltages and currents are shown; Fig. 3 shows a phasor diagram of the voltages in fig. 2; Fig. 4 shows the relationship between the voltage on the input and the stabilizing winding;

Fig. 5 shows how the voltages over the stabilizing winding form a closed polygon.

Fig. 6 shows an alternative embodiment of the autotransformer in the system in fig. 1. Fig. 7 shows the results when the short-circuit performance was tested.

The system according to the invention comprises, for each phase: an autotransformer TA, TB, TC with a parallel winding 4, 5, 6 with Np turns and a series winding 7, 8, 9 with Ns turns. The series winding 7, 8, 9 is connected between the input voltage Ua₅ Ub, Uc and the output voltage U_a>Out, Ub,_out, and U_C;0Ut. The autotransformer TA, TB₅ TC also comprises a tertiary winding 22, 23, 24 with NT turns. This tertiary winding is galvanically separated from the parallel 4, 5, 6 and series winding 7, 8, 9 of the autotransformer.

The system further comprises three magnetic controllable inductors MCI_A, MCI_B5 MCIc, one for each phase. The magnetic controllable inductors comprise a first winding 13, 14, 15 with N_MC_I,_P turns wound around a first core, and a second winding 16, 17, 18 with N_Mci,s turns wound around the core, and a control winding 10, 11₅ 12. The control winding is adapted to create a magnetic field mainly orthogonal to the field created by the first winding and the second winding, thereby controlling the inductance of the magnetic controllable inductor MCI_A, MCI_B, MCIc. The magnetic controllable inductor is described in the prior art. The inductance of the magnetic controllable inductor MCI_A5 MCI_B, MCI_C is controlled by providing a control current to the control winding. This also means that the magnetic permeability of the mutual flux path for the first and second winding 13, 14, 15, 16, 17, 18 of MCI_A, MCI_B, MCI_C is controlled by providing a control current to the control winding. The control current is controlled by a control circuit as e.g. described in the applicant's Norwegian patents NO 319367 and NO 318397.

The parallel winding 4, 5, 6 of the autotransformer T_A, T_B, T_C is connected between the input voltage Ua, Ub, Uc and the first winding 13, 14, 15 of the magnetic controllable inductor MCI_A, MCI_B, MCI_C- The other terminal of the first winding 13, 14, 15 is connected to the neutral point.

The tertiary winding 22, 23, 24 of the autotransformer TA, TB, TC is connected to the second winding 16, 17, 18 of the magnetic controllable inductor MCI_A, MCI_B, MCIc through their respective first terminals. These pairs of windings are also mutually connected. The second terminal of the tertiary winding 22 of the first autotransformer TA is connected to the second terminal of the second winding 18 of the third magnetic controllable inductor MCIc, the second terminal of the tertiary winding 23 of the second autotransformer TB is connected to the second terminal of the second winding 16 of the first magnetic controllable inductor MCI_A, the second terminal of the tertiary winding 24 of the third autotransformer TC is connected to the second terminal of the second winding 17 of the second magnetic controllable inductor MCI_B. In this way the tertiary windings 22, 23, 24 are connected in series with the secondary windings 16, 17, 18 to form a delta connection. The delta connection 22, 16, 23, 17, 24, 18 establishes a stabilizing winding of the system according to the invention. The function of the stabilizing winding will be further explained below.

The transformation ratios n between the autotransformers' parallel windings 4, 5, 6 and tertiary windings 22, 23, 24 are designed to equal the transformation ratios between the MCFs primary windings 13, 14, 15 and secondary windings 16, 17, 18. That is, Np/Nχ = N_MCI,P/N_MCI,_S ⁼ n. It shall be noted that the transformation ratio n defines a relation between the transformation ratios for the autotransformers and the magnetic controllable inductors.

The operation of the system will now be described with reference to fig. 2 and 3 where some of the voltages in the system are illustrated. All voltages except Uc₁, Uc₂ and Uc3 are referred to the neutral point, N.

In Figure 3, the phase diagram for the circuit in Figure 2 is shown. The phasors are related to the voltages in Figure 2. It is seen that the voltage over the series windings, U_A,OUT-UA, UB,OUT-UB, UC,OUT-UC is added to the input voltages UA₅ U_B and Uc such that the output voltages UA,OU_T> UB,_OU_T and U_C._OUT are larger in magnitude compared to the input voltages. The magnitude of the voltage over the series windings, U_A,OUT-U_A, U_B,_OU_T-U_B, U_C,O_UT-UC is determined by the voltage division between the voltages over the autotransformers' parallel winding U_A-U₁, U_B-U₂, U_C-U₃ and the voltages U₁, U₂, U₃ over the controllable inductances' first winding.

In Figure 4, the voltage over the transformer parallel winding, U_A-U₁ and the voltage over the primary winding of the magnetic controllable inductor U₁ are shown. Correspondingly, the voltages U_B6-U_B1 and U_B1-U_B2, which are the voltages over the transformer's tertiary winding and the controllable inductance's secondary winding is shown. The voltage transformation for both the transformer and the controllable inductance are assumed to be ideal. Also, the transformation ratio, n, is substantially equal. Consequently, the voltage pair U_BO-U_BI, U_B1-U_B2 is a mapping of the voltage pair U_A-U_1, U₁. The mapping is scaled by the transformation ratio, n.

Although the transformed voltages are a mapping of the primary voltages, they differ in the way the three phases are interconnected. Figure 5 shows that the transformed voltages form a closed polygon of phasors because of the delta connection of the stabilizing winding.

The sum of voltages in the delta connection must always equal zero. Hence, if the voltage transformations are ideal, the sum of the voltages on the primary sides must also be zero. Further, the sum of the three input phase voltages must be zero. This can be seen in Figure 2. Therefore, the delta winding imposes an important constraint on the input voltages: A necessary but not sufficient requirement for a symmetric three phase voltage is that the sum of the phase voltages is zero.

Tests of the system verify that the stabilizing winding helps stabilizing the input and output voltages as well as increasing the fault current at single-pole short circuits.

A test of the system according to the invention has been performed. The table below shows test results when various non-symmetric three phase loads are connected to a weak TN -network. For each triplet of load impedances, three different cases are investigated:

1. The network as-is, i.e. with no voltage stabilizing unit connected. This case is referred to as "lines only" in the table.

2. The network with a voltage stabilizing unit as described in WO 2004/053615. This unit does not have a stabilizing winding. The voltages on both the input and the output are measured.

3. Same as above, but here a stabilizing winding is included in the topology.

The following should be observed: in the first triplet of impedances, having an extreme dissymmetry, all output voltages are within acceptable boundaries (ca 220 - 240V). This is not the case for the prior art system. - in more normal load situations, the system according to the invention is better than or as good as the prior art system.

The autotransformers' series windings 7, 8, 9 are used to increase the output voltages at normal operation. However, practical tests have shown that the short circuit capacity of the device will increase if the series windings 7, 8, 9 are bypassed when a fault occur. Therefore, a bypass circuit 1 is preferably included, as shown in fig. 6. The switch 100 in the bypass circuit is controlled by the control circuit. At a predefined voltage level, the bypass circuit is activated to ensure maximum fault current.

Figures 7a - d show test results of the performance at single pole short circuit for four cases. The parameters of interest are:

1. the system's ability to deliver a high fault current at the short circuited phase 2. the system's ability to keep the voltage of the two phases that are not short circuited close to their nominal voltage (ca 230 V).

The four cases studied are:

a) The network as-is, i.e. with no voltage stabilizing unit connected. (Figure

7a). b) The network with a voltage stabilizing unit as described in WO 2004/053615. This unit does not have a stabilizing winding. (Figure 7b) c) Same as b), but here the stabilizing is included in the topology. (Figure 7c) d) Same as c), but here, the series windings, 7, 8, 9 in figure 1, are short circuited as shown in figure, as shown in fig. 6. (Figure 7d)

The test results show a clear improvement in the system's behavior when the device according to the invention is used.

Claims

1. Power supply system with voltage symmetrisation comprising for each phase: an autotransformer (TA, TB, TC) with a parallel winding (4, 5, 6) and a series winding (7, 8, 9), where the series winding (7, 8, 9) is connected between the input voltage (Ua, Ub, Uc) and the output voltage (Ua_out₅ Ub_out, Uc_out), the autotransformer (TA, TB, TC) further comprising a tertiary winding (22, 23, 24) galvanically separated from the parallel (4, 5, 6) and series (7, 8, 9) winding of the autotransformer; a magnetic controllable inductor (MCI_A, MCI_B, MCI_C) comprising a first winding (13, 14, 15) wound around a core, and a second winding (16, 17, 18) wound around said core, and a control winding (10, 11, 12), which is adapted to create a magnetic field mainly orthogonal to the field created by the first winding and the second winding; the parallel winding (4, 5, 6) of the autotransformer (T_A, T_B, T_C) is series connected to the first winding (13, 14, 15) of the magnetic controllable inductor MCI_A, MCI_B, MCI_C between the input voltage (Ua, Ub, Uc) and the neutral point, the tertiary winding (22, 23, 24) of the autotransformer (T_A, T₅, T_c) is connected to the second winding (16, 17, 18) of the magnetic controllable inductor (MCI_A, MCI_B, MCIC) through their respective first terminals forming pairs of windings, where these pairs of windings are mutually connected; where the system further comprises a control circuit, which based on measured voltages and/or currents, controls the control current of control windings (10, 11, 12), thereby controlling the inductance of the magnetic controllable inductor (MCI_A,

2. System according to claim 1, where the tertiary windings (22, 23, 24) are connected in series with the secondary windings (16, 17, 18) to form a delta connection.

3. System according to claim 2, where the delta connection of the tertiary windings (22, 23, 24) and the second windings (16, 17, 18) of the magnetic controllable inductor (MCI_A, MCI_B, MCI_C) comprises connection of the second terminal of the tertiary winding (22) of the first autotransformer (TA) to the second terminal of the second winding (18) of the third magnetic controllable inductor MCIc, connection of the second terminal of the tertiary winding (23) of the second autotransformer (TB) to the second terminal of the second winding (16) of the first magnetic controllable inductor (MCI_A), and connection of the second terminal of the tertiary winding (24) of the third autotransformer (TC) to the second terminal of the second winding (17) of the second magnetic controllable inductor (MCI_B).

4. System according to claim 1, where each series winding (7, 8, 9) comprises a bypass branch with a switch controlled by the control circuit, where the switch is closed when a fault is detected to increase the fault current.

5. System according to one of the claims above, where the parallel winding (4, 5, 6) has Np turns, the series winding (7, 8, 9) has Ns turns, the tertiary winding (22, 23, 24) has NT turns, the first winding (13, 14, 15) of the magnetic controllable inductor has N_MCI,P turns and the second winding 16, 17, 18 of the magnetic controllable inductor has N_MC_I,_S turns, where the transformer ratio n = Np/Nτ = NMCI,P/NMCI,S-