FIELD OF THE INVENTION

[001] The present invention relates to the field of electrical equipment, and more specifically to traction transformers used for railway electrification.

BACKGROUND OF THE INVENTION

[002] At present, at most electrified railways, a v/v coupling traction transformer with AT power supply is used. The structural design of a traditional V/V coupling transformer mainly adopts the following three kinds of schemes:

Scheme 1: A transformer consisting of two single -phase transformers with separate oil tanks and externally connected by V/V;

Scheme 2: A transformer consisting of two single-phase transformers with their cores and windings connected by V/V inside a common oil tank; and

Scheme 3: Respectively wound two single -phase transformer coils on two side limbs of a three-column core, connecting two single-phase transformers by V/V inside a common oil tank.

[003] A transformer of scheme 1 consumes more materials, occupies a large area and has high manufacturing costs. Compared with scheme 1, a transformer of scheme 2 consumes slightly less materials, occupies a slightly smaller area and has a slightly lower manufacturing cost. Based on the first two schemes, a transformer of scheme 3 reduces more materials and costs.

[004] It is desirable to have transformers which require lesser materials and costs. Reduction in materials however can cause the core to get saturated. Accordingly, the core should be designed to limit saturation. Along with optimizing the design of the core, the transformer should have additional components necessary to monitor the saturation levels.

SUMMARY OF THE INVENTION

[005] The present invention relates to a traction transformer. The traction transformer has a core and a plurality of windings.

[006] The core is a four-limb core having four core limbs arranged side by side. The four core limbs include two main limbs located at the center and two side limbs located on sides of the two main limbs. The four core limbs form a closed magnetic circuit through a top yoke and a bottom yoke.

[007] In an embodiment, cross-sectional area of each side limb of the two side limbs, is less than cross-sectional area of each main limb of the two main limbs. Further, cross-sectional area of each of the top and bottom yoke is less than cross-sectional area of each main limb. Accordingly, the side limbs and the yokes have lesser cross-sectional area (e.g. half) of that of the main limbs.

[008] The plurality of windings include one or more primary and secondary windings and one or more measurement windings. The primary and secondary windings are for power transfer in one or more phases (e.g. phase A, phase B etc.). In case of two phases, the primary and secondary windings of phase A are wound on one of the four core limbs, and the primary and secondary windings of phase B are wound on another one of the four core limbs.

[009] In an embodiment, the primary and secondary windings of phase A are wound on one main limb of the two main limbs, and the primary and secondary windings of phase B are wound on another main limb of the two main limbs. The terminals of the primary and secondary windings of phase A are connected with corresponding terminals of the primary and secondary windings of phase B. Further, the primary and secondary windings of phase A are wound opposite to the primary and secondary windings of phase B. In this arrangement, the two side limbs form a return path for the magnetic flux through in the two main limbs.

[0010] Each measurement winding is wound on at least one of the top yoke, the bottom yoke and a core limb of the four core limbs, to monitor at least one of flux and harmonic components thereof. For example, there are two measurement windings, where each measurement winding is wound on one of the four core limbs. Taking another example, there are two measurement windings, wherein one measurement winding is wound on a core limb, and another measurement windings is wound on the top yoke.

[0011] In an embodiment, the one or more measurement windings are provided on one of the two side limbs of the traction transformer.

[0012] The monitoring with the one or more measurement windings is utilized for regulating a voltage at one or more windings of the plurality of windings, for limiting saturation of the core. In an embodiment, the voltage at the one or more windings is regulated with a Static Var Compensator (SVC).

[0013] An On-load Tap Changer (OLTC) can be provided, either as a part of the traction transformer or external to the transformer. The OLTC position is controlled based on measurements made with the one or more measurement windings.

[0014] The traction transformer can also have one or more sensors for sensing oil level, oil pressure, gases dissolved in oil and oil temperature. In such a case, the traction transformer is monitored based on measurements made with the one or more sensors and the one or more measurement windings.

BRIEF DESCRIPTION OF DRAWINGS [0015] The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in attached drawings in which:

[0016] Fig. 1 shows core structure of a traction transformer, in accordance with various embodiments of the present invention;

[0017] Fig. 2A and Fig. 2B respectively show voltage and current vector diagrams of primary and secondary windings of the traction transformer, in accordance with an embodiment of the present invention;

[0018] Fig. 3 shows potential vector diagram of the traction transformer, in accordance with an embodiment of the present invention; and

[0019] Fig. 4 shows a simplified representation of a system for monitoring the traction transformer in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0020] The present invention relates to a traction transformer that has an optimized core design, as well as sensors for monitoring core saturation. The traction transformer provided by the present invention is composed of two single-phase transformers that are v/v coupled.

[0021] The capacity of these two single-phase transformers may or may not be equal. The transformer can be regulated on the primary side or secondary side respectively. The voltage regulation mode can be either on-load or off-load. The traction transformer can also meet a variety of voltage regulation structures, such as single-bridge, double-bridge, linear regulation, Plus-Minus, coarse-fine etc. [0022] In accordance with some embodiments, the primary side of the transformer is a symmetric three-phase system, and the secondary side can be symmetrical or asymmetrical two-phase system. Here, High Voltage (HV) and Low Voltage (LV) windings are connected with VV, by applying a three-phase sine voltage on the HV side.

[0023] Although many embodiments are described here, other schemes that can be thought of by technicians in this field are also within the scope of the present invention.

[0024] The following describes the present invention with reference to the drawings that illustrate some embodiments of the present invention.

[0025] Figure 1 shows core structure of the traction transformer, in accordance with various embodiments of the present invention. As shown, the transformer has a four-core limb composed of four core limbs set side-by-side: limb 1 , limb 2, limb 3 and limb 4. Two main limbs of the four core limbs, limbs 1 and 2, are located at the center. Two side limbs of the four core limbs, limbs 3 and 4, are located on both sides of two main limbs. The adjacent four core limbs (1 , 2, 3 and 4) form a closed magnetic circuit through by a top yoke 5 and a bottom yoke 6. Preferably, the two side limbs 3 and 4 form a return path for the magnetic flux through two main limbs 1 and 2.

[0026] As shown in Figure 1 , the two main limbs have identical cross-sectional areas, the two side limbs have identical cross-sectional areas and the top and bottom yokes have identical cross-sectional areas. In the embodiment of Figure 1 , the cross-sectional area of side limb 3 or 4 is less than that of the main limb 1 or 2, and the cross-sectional area of the top yoke 5 or the bottom yoke 6 is less than that of the main limb 1 or 2. [0027] Through the above structure, the height of the core limb is shortened, and the weight of the core is reduced. This reduces the height of the transformer, and makes the transformer structure more compact, which significantly reduces the manufacturing cost of the transformer.

[0028] The transformer has a plurality of windings. These include one or more primary and secondary windings and one or more measurement windings.

[0029] The transformer windings of one phase, e.g. phase A, including the primary winding and the secondary winding, are both wound on one of the four core limbs. The windings of another phase, e.g. phase B, including the primary winding and the secondary winding, are both wound on another one of the four core limbs.

[0030] In case of two phases as shown in Figure 1, the primary and secondary windings of phase A are wound on one of the two main limbs, such as main limb 1 , and the primary and secondary windings of phase B are wound on another one of the two main limbs, such as main limb 2. Also, as shown, one terminal of the primary winding of phase A is connected with one terminal of the primary winding of phase B, and one terminal of the secondary winding of phase A is connected with one terminal of the secondary winding of phase B, thereby connecting corresponding terminals of phases A and B. The winding direction of the transformer windings can be chosen according to the design requirement. For example, the winding direction of the primary side winding of phase A is opposite to that of the winding direction of the primary side winding of phase B, and the winding direction the secondary side winding of phase A is opposite to that of the winding direction of the secondary side winding of phase B. As the primary and secondary windings of phase A are wound opposite to the primary and secondary windings of phase B, in this arrangement, the two side limbs form a return path for the magnetic flux through in the two main limbs. [0031] The measurement windings are wound on different locations on the core for measurement of voltages and / or currents at the respective locations. These measurements can be used to monitor various electrical parameters such as flux, harmonic components etc. For instance, the measurements made with the measurement windings can be used to limit core saturation. The number and location of measurement windings can be determined according to the rating of the transformer.

[0032] In the embodiment of Figure 1 , three measurement windings are provided (Aul, Au2, and Au3). One measurement winding (Aul) is wound on the top yoke, one (Au2) is wound on the bottom yoke and one (Au3) is wound a side limb of the four core limbs. This allows for monitoring the voltages at the side limb 3 and the top and bottom yokes (5,6). It should be noted that the measurement windings can be provided in other configurations. For example, measurement windings may be provided on side limbs only, or on both side limbs and yokes. This depends on the transformer design, as well as the monitoring desired.

[0033] Figures 2A and 2B respectively show voltage and current vector diagrams of primary winding and secondary windings of the traction transformer, in accordance with an embodiment of the present invention; The primary winding of transformer is connected with three-phase power supply, and the three-phase voltage depends on actual load demand. Assuming that the transformer is in normal operation, the primary voltage is balanced and has a phase difference of 120 degrees, then the modules of the primary voltage U_AC and U_BC are equal and 120 degrees each other in the vector. Similarly, the induced voltage U_ac and U_bc of the secondary winding have the same modulus and 120 degrees each other in the vector. Assuming that the load of two side windings is equal and the impedance of the transformer winding is zero, then U_AC=U_ac, U_BC=U_bc. As a result, h— h h—

[0034] As shown in the figures, the primary current I_A and I_C are 120 degrees to each other on the vector, that is, the line current of phase A and phase B is

[0035] Then line current of C phase is [0036] Thus, the primary line current is unbalanced.

[0037] Figure 3 shows potential vector diagram of the traction transformer of the present invention, in accordance with an embodiment of the present invention. By properly selecting the connection mode and winding direction of the winding, the voltage vector U_AC and U_BC flows through two main limbs 1 and 2 form of the magnetic flux 0_AC and 0_BC with same modulus and 120 degree phase difference. The vector addition of 0_AC and 0_BC return through by the side limbs 3 and 4, the magnetic flux is calculated as follows:

[0038] That is, the modulus of magnetic flux in the two side limbs is equal to that in the main limb. That is, the sum of the cross-sectional area of the two side limbs is equal to cross-sectional area of a main limb, and the sum of the cross- sectional area of the top and bottom yokes is equal to cross-sectional area of a main limb. For example, the cross-sectional area of the two side limbs or the top and bottom yokes can be designed as half of cross-sectional area of a main limb, thereby reducing the cross-sectional area and height of the entire core. Of course, the cross-sectional area of the side limb and the upper and lower yoke should also be considered in combination with other factors such as core losses and hot spots.

[0039] The core design of the traction transformer in accordance with the present invention is optimized to reduce the transformer size and accordingly the cost. Additionally, the core can be monitored with the measurement windings. For example, voltages measured with the measurement windings (e.g. with the measurement winding Au2 (8)) can be used to monitor the core saturation.

[0040] In cases where the core gets saturated, or close to saturation, effects of the saturation are detected from electrical measurements such as from analysis of waveforms (e.g. voltage waveforms). As an example, deviations in the shape of waveforms indicate a possible condition of saturation. As another example difference between expected and actual levels of a representative parameter (shape) derived from analysis of the waveforms, indicates a possible condition of saturation. Thus, the sensed electrical measurements obtained from measurement windings such as Au2 (8), are monitored to detect changes in flux levels at the core (or saturation levels). The monitoring with the one or more measurement windings is utilized for regulating a voltage at one or more windings of the plurality of windings, for limiting saturation of the core. For example, Static Var Compensators (SVC) can regulate the voltages at the primary windings, according the measurements made with the measurement windings.

[0041] Some transformers can have On-load Tap Changers (OLTCs), either as a part of the traction transformer or external to the transformer. In such cases, the OLTC position is controlled based on measurements made with the one or more measurement windings. [0042] In addition to the measurement windings, the traction transformer can also have one or more sensors as shown in Figure 4. As shown, there can signals (402) for sensing oil level, oil pressure, gases dissolved in oil and oil temperature. The oil level, oil pressure, gases dissolved in oil and oil temperature can be monitored with one or more sensors for monitoring the oil.

[0043] The signals are transmitted to an external equipment, such as an intelligent equipment (406). In accordance with the embodiment of Figure 4, the signals are from the different measurement windings as well as sensors that are placed in different locations within the transformer. As shown in Figure 4, sensors (406) such as core earthing sensor, bushing dielectric loss sensor, temperature rise sensor, partial discharge sensor, harmonic sensor, no load current sensor etc. can be provided on the transformer.

[0044] The measurements with the measurement windings and the sensors of the transformer are obtained at the intelligent equipment. For example, there can be a main IED which processes the signals to have the transformer measurements.

[0045] The traction transformer can be monitored with a computer such as 410 or an online monitor 408, which devices receive the measurements from the intelligent equipment. For example, the measurements can be used to monitor the core saturation. Taking another example, the oil level, pressure, and / or temperature can be controlled based on the measurements.

[0046] The computer or smart unit can control devices such as SVCs and OLTC based on the measurements. Thus, by actively regulating the input / output at the transformer windings, and additionally as needed the oil parameters, the performance of the traction transformer is optimized. [0047] Thus, the traction transformer has an optimized core design and the core as well as other transformer components are monitored to limit core saturation and improve transformer performance.