AU2014252950A1 - Transformer provided with means for adjusting the in-load transformation ratio - Google Patents

Abstract

In order to allow adjustments in the voltage on the basis of the charge fluctuations, an MV/LV transformer (10) is combined with a second booster transformer (20), comprising a plurality of sockets (22, 24, 26) on the primary winding of same, the connection of said sockets with the LV network being controlled by low voltage contactor switching means (32, 34, 36). The modification in the voltage thus produced by the second transformer (20) modifies the transformation ratio of the assembly (10, 20) and allows voltage variations. The invention is particularly suitable for three-phase oil immersed transformers.

Description

WO 2014/167205 PCT/FR2014/050719 1 TRANSFORMER PROVIDED WITH MEANS FOR ADJUSTING THE IN-LOAD TRANSFORMATION RATIO TECHNICAL FIELD 5 The invention relates to electrical distribution and notably transformation ratio adjustments in order to maintain the circulating voltage within acceptable limits despite possible load fluctuations. 0 In particular, the invention relates to a medium-voltage (MV) / low-voltage (LV) transformation device in which the main transformer is associated with a device for modifying the transformation ratio, without requiring the main transformer to be de energized. 5 PRIOR ART Electrical networks are generally structured in several levels separated by transforming substations: after a first very-high-voltage and high-voltage transport and distribution network, an HV-A or MV medium-voltage distribution network, ?0 usually between 1 and 35 kV, and more specifically 15 or 20 kV in France, provides for the smaller scale transport to industrial type customers or to LV low-voltage networks (in particular 0.4 kV in France) which supply customers having a low energy demand. The MV/LV distribution transformers are thus designed with a ratio between the input and output voltage, or transformation ratio, determined to ensure a voltage ?5 level compatible with the user-end standards. Changes in loads, the physical structure of the network and notably the cable lengths and impedances of the components between the transformer and the user can lead to voltage fluctuations, and it is important to keep these as low as possible in the LV 30 network and at the premises of the end users. At the MV/LV transforming substations, regulating systems are thus installed, which compensate for voltage WO 2014/167205 PCT/FR2014/050719 2 variations caused by changes in the network by modifying the transformation ratio without load, for example as described in the document FR 2 787 248. However, the installation of decentralized production means, with photovoltaic 5 panels, wind farms and small power stations, considerably increases the parametric fluctuations. In fact, a local production, far from the source transformer, can contribute to making the voltage nearer the production increase sharply due to the impedance of the line; conversely, in the absence of production, the impedance of the line creates a lowering of the voltage locally. It is hence important to increase, or 0 respectively decrease, the transformation ratio to remain within recommended limits. It is therefore appropriate to equip the MV/LV distribution transformers, conventionally oil-filled, with dynamic means of regulation of the transformation ratio, if possible on load, i.e. without having to cut off the power supply. Various technical solutions have been proposed for these "On-Load Tap Changers" (OLTCs). 5 In particular, the conventional mechanical solution performs the change of transformation ratio by modifying the number of primary side (MV) turns, by virtue of the separation of the primary winding into a main part and an adaptable part. However, such a mechanical switching system is complex and costly, both to ?0 manufacture and to maintain: for dielectric isolation requirements, the switches of the adaptable part are placed in oil which is contaminated during electrical arcs generated by the change from one tap to the other. To eliminate the electrical arc problem, it has been proposed to use vacuum chambers (see WO 2012/062408), which however increase the cost and size of the initial transformer. 25 Another option to address the mechanical switching problems, which is presented for example in FR 2 873 489 or WO 2010/072623, comprises the use of semiconductors for the change from one tap to the other. Since this type of component cannot be placed on the medium-voltage side without difficulties and extra costs related to the 30 presence of oil and to the electrical isolation requirements, the adjustment is hence performed on the LV side, thereby reducing the precision of the regulation given that the low voltage and the significant level of the current lead to a low number of voltage WO 2014/167205 PCT/FR2014/050719 3 adjustment turns. Moreover, the control for such a solution is complex and, considering the current flowing, these semiconductor components cannot be standard, furthermore having significant losses: the cost of the solution is high. 5 SUMMARY OF THE INVENTION Among other advantages, the invention aims to address drawbacks of existing adjustment devices, and notably to provide a reliable and inexpensive system for selecting taps, which is suitable for oil-filled MV/LV transformers. 0 In one of its aspects, the invention thus relates to a voltage transformation device comprising a first main transformer, the ratio of which is determined by the desired voltages of the MV primary and LV secondary circuits. Preferably, this is an oil-filled transformer. Preferably, the device is equipped with means for regulating the no-load 5 main transformation ratio, prior to connecting the electrical device to the network and according to the characteristics of said network. The device additionally comprises a second adjustment transformer, preferably also oil-filled and located in the same tank as the main transformer, the secondary ?0 winding of which is connected in series with the primary circuit of the main transformer. The primary winding of the adjustment transformer comprises, for its part, at least two taps, in particular at the ends, and preferably three. The transformation ratio of the adjustment transformer is advantageously chosen to correspond to a predetermined percentage N, conventionally between 0.5% and 5%, ?5 notably 2.5%, of that of the main transformer when it is connected to the main transformer by one of the end taps; if a third central tap is provided, it is desired that it is at the center of the primary winding, in order to generate an adjustment transformation ratio hence equal to double the stipulated regulating value, notably at 5%, of the main transformation ratio. 30 At least two taps, preferably the end taps, of the adjustment transformer are associated with three connection lines: a first connection line to the secondary circuit WO 2014/167205 PCT/FR2014/050719 4 of the main transformer equipped with first switching means, a second connection line to another phase or the neutral of the secondary circuit equipped with second switching means, and a third connection line, or "bypass" line, to the secondary circuit of the main transformer equipped with third switching means and with a load 5 impedance. Preferably, the two bypass lines have their load impedance in common. The other taps of the adjustment transformer can comprise only one connection line equipped with means of switching to the secondary circuit of the main transformer. The switching means operate in air, and are preferably grouped together on the 0 same plate that is directly accessible to an operator. The transformation device according to the invention also comprises a control system for the tap switching means, which control system can be secured to the device or be positioned away, and preferably supplied via the secondary circuit of the main 5 transformer. In order to prevent any problem during the on-load tap change, the device according to the invention is equipped with means for ensuring that at least one of the connection lines of two different taps is closed, so that the primary of the adjustment ?0 transformer is not in open circuit. Advantageously, a safety circuit, notably equipped with switches similar to the switching means, is installed between the first and second lines of each tap; alternatively, the switching means can be chosen to be in a bistable position and/or associated with position-holding devices. ?5 In another aspect, the invention relates to a corresponding three-phase electrical installation. Notably, an installation according to the invention comprises three devices as defined previously, the transformers of which are preferably housed within the same oil tank and the switching means of which are also preferably housed on the same plate. The three devices of the three-phase electrical installation are 30 advantageously of identical characteristics and parameters, and notably the three transformers, main and/or adjustment, can themselves be formed of unitary blocks covering the three phases. The control system is suitable for controlling the switching WO 2014/167205 PCT/FR2014/050719 5 means of the various taps simultaneously, and preferably, the switching means are electromechanical contactors, operating in air, which can be associated with position holding devices. 5 BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and features will become clearer from the following description of particular embodiments of the invention, which are given by way of illustration and are not at all limiting, and which are represented in the appended drawings. 0 Figure 1 shows a three-phase electrical installation according to a preferred embodiment of the invention. Figure 2 represents the electrical circuit diagram of an installation according to the 5 invention. Figures 3A to 3F illustrate adjustment steps in a device according to the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 20 A three-phase electrical installation 1 according to the invention can have the conventional components and characteristics of a commercially available three phase transformer assembly, as illustrated in Figure 1. In particular, the electrical installation 1 advantageously comprises a tank with fins 3 filled with oil or other ?5 dielectric fluid, in which tank the actual transformation components are housed, and from which tank bushings and other connection devices 5 emerge on the MV network and to the enclosure 7 and the LV network. As main transformation component, according to the invention, it is possible to use a 30 main transformer unit housed in the tank 3, for example suitable for 630 kVA, with a primary circuit at 20 kV - 18.2 A and a secondary circuit at 410 V - 887 A, or three identical single-phase transformers 10, or any other system. Preferably, means are WO 2014/167205 PCT/FR2014/050719 6 provided for regulating and adjusting on site the transformation ratio(s) of the main transformation component, before placing it in operation, according to the local MV/LV usage conditions, the location of the installation 1, etc, for example a conventional de-energized tap changer. 5 To allow the voltage to be adjusted while limiting costs, the invention uses an adjustment transformer unit, notably with a second adjustment transformer 20 associated with each main transformer 10. The second adjustment transformer 20, smaller in size than the main transformer 10 but which can be of the same kind, can 0 be installed in the same oil tank 3. The second adjustment transformer 20 is equipped with taps which enable it to be linked or not linked with the main transformer 10. Advantageously, the adjustment transformer 20 comprises three taps 22, 24, 26 enabling a variation in voltage of N % and of 2-N % (i.e. notably, for the case presented above, where N % = 2.5 %, 500 V - 22.2 A and 1 kV - 43.3 A taps). 5 However, the invention does not use the principle of the "booster" transformer as voltage transformer, but as current transformer. In fact, the addition of a simple booster transformer as presented in CN 20158428 does not address the problem of complexity of implementation if the tap change is performed in oil by complex devices ?0 or via semiconductors. The invention relies, for its part, on the use of a proven tap switching system 30, with low-voltage switching of devices themselves located in air and not submerged in the tank 3: thus, the oil in the tank 3 is not contaminated and the parameters of execution ?5 are easy to implement, given that submerged transformers 10, 20 are concerned, which have been known for a very long time. Furthermore, maintenance is easy, since the switching components 30 can be directly accessible from the outside. Thus, according to the invention, a second transformer 20 is associated with the 30 main transformer 10, said second transformer 20 taking the voltage of the secondary circuit, processing it, and injecting it in series on the primary voltage in order to adjust WO 2014/167205 PCT/FR2014/050719 7 it. This adjustment of the primary MV voltage automatically brings about a modification of the LV voltage. The switching means 30 see only the current flowing in the adjustment transformer 5 20 and not in the complete network. Preferably, the switching means 30 are hence commercially available electromechanical contactors and used commonly for example for controlling motors, the costs of which are low and the performance levels predictable, and which additionally offer the advantage of grouping together the switching means for the three phases into the same device. 0 In the embodiment illustrated in Figure 2, for each phase U, V, W, the primary windings of the main transformer unit and the secondary windings of the adjustment transformer unit are placed in series on the MV network, the secondary winding of each main transformer 10 corresponding to the phases of the LV network. The 5 primary windings of the adjustment transformer unit comprise for their part several taps 22, 24, 26 which may or may not be connected to the corresponding winding of the main transformer unit on the LV network. Thus, depending on the tap 22, 24, 26, the current flows through all or part of the primary winding of the adjustment transformer 20, and the transformation ratio of the set 10, 20 housed in the tank 3 is ?0 modified, thus modifying the voltage of the secondary LV network (starting from the assumption that the LV voltage is imposed by the generator 6). To connect or not connect a tap 22, 24, 26 of a primary winding of an adjustment transformer 20 to the LV network, connection lines equipped with switching means ?5 are implemented. In particular, each tap 22, 24, 26 of each phase U, V, W is connected by a line equipped with a first switch 322, 324, 326 on the corresponding phase U, V, W of the LV network. It is also appropriate to provide the current return, and at least two taps 22, 24 are connected by a line equipped with a second switch 34, on another phase V, U, W of the LV network. It is to be noted that a connection to 30 neutral could also be provided. In the preferred embodiment illustrated, only the two end taps of the winding 22, 24 are connected to said second connection line, which turns out to be in fact sufficient and reduces the number of contactors required.

WO 2014/167205 PCT/FR2014/050719 8 Since the adjustment of the transformation ratio can, according to the invention, be carried out on load, additional lines of temporary connection to a load impedance Zu, Zv, Zw are provided. According to the invention, at least two taps 22, 24 are 5 connected to a third line equipped with a third switch 362, 364 and of load impedance Zu, Zv, Zw on the corresponding phase U, V, W of the LV network. In fact, this solution offers the advantage of limiting the magnetizing current with respect to a series-placing of the center of the primary winding and therefore reducing the magnetizing requirements for the adjustment transformer 20: furthermore, the voltage 0 and current requirements are lower at the end taps, such that both the impedances and the associated contactors are protected. Moreover, and as explained above, the adjustment transformer is used as current transformer, while the tap change can be performed on load. It is therefore 5 appropriate to ensure that the current flow is never interrupted in the adjustment transformers 20, due to the risk of overvoltages in their windings and damage to components of the installation 1. Means for making sure of the closure of at least one of the contactors of two different taps 22, 24 are therefore provided, in this case, in the embodiment illustrated, by the presence of contactors on a safety circuit 38 which ?0 closes automatically in the event of a fault. Any other option can be considered, for example the use of switching means other than industrial contactors designed to be normally open to meet motor startup safety standards. The plate 30 of contactors is associated with a control system 40 for the switching ?5 means. Preferably, the control system is powered directly on the LV network via suitable means. The control system 40 can be placed permanently on the oil tank 3, or it can be placed in a panel positioned away, for example for off-site control via a wired link. 30 The control system 40 is designed to enable a switchover from one tap to the other according to a predefined closure/opening sequence for the contactors depending on the degree of adjustment of the transformation ratio required. In particular, the WO 2014/167205 PCT/FR2014/050719 9 adjustment principle will be illustrated with reference to Figures 3 for a single-phase device and the change from a zero adjustment, i.e. only the main transformer 10 is used, to an adjustment of + N % and + 2-N %. 5 In the initial position (Figure 3A), the three taps 22, 24, 26 are disconnected from the phase of the LV network, i.e. the five contactors 32, 36 of the first and third lines are open, and only the contactors 34 of the second lines of the end taps 22, 24 are closed; these two contactors 342, 344 are held mechanically closed in order to ensure safety during a possible loss of voltage. To prepare the switchover of the righthand 0 end tap 24 onto the low-voltage phase, first, as illustrated in Figure 3B, it is appropriate to proceed by closure via the load impedance Z 4 by driving the associated contactor 364; therefore an intermediate step takes place in which two contactors 344, 364 are closed for the tap 24 concerned. Once this closure of the third line is acquired, it is possible to open the second contactor 344 corresponding to 5 placing the tap off-circuit, in order to have the voltage pass only through the impedance Z 4 . The first contactor 324 is then driven to be closed, and once this closure is acquired (Figure 3D), the load contactor 364 is opened, and the position corresponding to an adjustment of + N % is obtained (Figure 3E). Here again, the first contactor 324 is held in this closed position by suitable means until a switchover ?0 is required. The adjustment tap 24 is disconnected by following the steps in the reverse order, and notably via the intermediate step of the closure of the third contactor 36 of the load impedance Z during any switchover between first and second contactors 32, 34. 25 For an adjustment at 2-N % corresponding to the connection of the central tap 26, a similar process is implemented from the position corresponding to Figure 3C in which the second and third contactors 344, 364 of the end tap are closed: the contactor 326 associated with the central tap 26 is closed (Figure 3F), then the load contactor 364 is 30 opened (not illustrated).

WO 2014/167205 PCT/FR2014/050719 10 For negative adjustments, in the same way, the lefthand tap 22 is connected, according to a defined sequence involving the contactors 322, 342, 362 (and 326); it is in fact advantageous to proceed with the switching via the load impedance Z 2 corresponding to the tap 22. 5 It would be possible to add other taps on the same winding of the adjustment transformer 20 to increase the number of modifications of the transformation ratio of the installation 1. Alternatively, in order to facilitate the electrical connections and prevent damaging interactions, for a five-tap adjustment (and therefore nine 0 adjustment values), it is preferable to implement in series two adjustment transformers 20 similar to that presented with reference to Figures 2 and 3. In fact, in the majority of cases and specifications, five taps are more than enough, and the use of two similar adjustment transformers additionally provides for limiting storage and maintenance costs. 5 The solution according to the invention, while using additional components, the cost and size of which are minimal with respect to the requirements on the transformers related to the power of the installation 1, therefore provides for regulating the nominal power with degrees of variation which can be adjusted precisely on site, before ?0 commissioning, depending on the location of the transformer and the nature of the local network. The variations in transformation ratio of the installation 1 are then carried out on load, in complete safety notably in the case of the use of bistable contactors, without the ?5 need for a vacuum chamber, power electronics or even a voltage or current measurement. Although it is performed on the LV side, the adjustment of the transformation ratio is precise, with an interval that can be fine, notably 2.5 and 5%. The variations in transformation can be controlled remotely, or directly on site, via a standard controller provided to operate in a restrictive industrial environment, and 30 notably without voltage and/or current measurement. Additionally, maintenance is equivalent to that of existing transformers, since the contactors are themselves of proven, reliable and easily accessible solutions.

WO 2014/167205 PCT/FR2014/050719 11 Although the invention has been described with reference to a 630 kVA MV/LV oil filled three-phase distribution transformer assembly driven by industrial type contactors, it is not limited thereto: the invention can relate to other transformation 5 devices and other switching means. In particular, the primary circuit of the adjustment unit 20 can be connected in series with the primary windings of the transformer unit 10, the secondary of the adjustment unit 20 being driven with one phase and neutral. In addition to the electromechanical contactors, it is possible to use semiconductor type switching means, or others. And of course, the adjustment according to the 0 invention can relate to the entire range of MV/LV transformers, in particular 100 to 2500 kVA, for an MV network of 5 to 36 kV and an LV network of 220 to 440 V.

Claims (14)

1. An electrical transformation device comprising: - a main transformer (10) with a medium-voltage primary circuit and a low 5 voltage secondary circuit; - an adjustment transformer (20) for the main transformer (10) with a secondary winding connected to the medium-voltage primary circuit and a primary winding comprising at least two taps (22, 24), each tap being associated with 10 o a first line equipped with first switching means (32) connected to the low-voltage secondary circuit, o a second line equipped with second switching means (34) connected to another phase or to the neutral of the low-voltage secondary circuit, 15 o a third line equipped with third switching means (36) and with a load impedance (Z) connected to the low-voltage secondary circuit; - a control system (40) for controlling the first, second and third switching means (32, 34, 36) of each of the two taps (22, 24). 20

2. The electrical transformation device as claimed in claim 1, in which the load impedance (Z 2 , Z 4 ) is common for the third lines of the two taps (22, 24).

3. The electrical transformation device as claimed in either claim 1 or claim 2, in which the primary winding of the adjustment transformer (20) comprises a 25 third tap (26) connected via switching means (326) to the low-voltage secondary circuit, the control system (40) being suitable for additionally controlling the switching means (326) of the third tap (26).

4. The electrical transformation device as claimed in claim 3, in which the third 30 tap (26) is located between the first and the second tap (22, 24) on the primary winding of the adjustment transformer (20), notably at the center of the two taps (22, 24). WO 2014/167205 PCT/FR2014/050719 13

5. The electrical transformation device as claimed in one of claims 1 to 4, comprising means for ensuring that at least one of the switching means of at least two different taps is closed. 5

6. The electrical transformation device as claimed in one of claims 1 to 5, comprising means for regulating the transformation ratio of the main transformer (10).

7. The electrical transformation device as claimed in one of claims 1 to 6, in 10 which the main and adjustment transformers (10, 20) are located in an oil tank (3).

8. The electrical transformation device as claimed in claim 7, comprising a plate (30) housing the switching means (32, 34, 36) of the taps (22, 24, 26) of the 15 primary winding of the adjustment transformer (20), said plate (30) being directly accessible outside the oil tank (3).

9. The electrical transformation device as claimed in either claim 7 or claim 8, in which the control system (40) is positioned away from the oil tank (3). 20

10. The electrical transformation device as claimed in one of claims 1 to 9, in which the control system (40) is supplied directly from the secondary winding of the transformer (10). 25

11. A three-phase electrical installation comprising a device as claimed in one of claims 1 to 10 for each phase, and in which the control system (40) is suitable for simultaneously controlling the respective switching means (32, 34, 36) of each device. 30

12. The three-phase electrical installation as claimed in claim 11, in which the load impedances (Z) and the transformation ratio of each transformer (10, 20) are identical for the three devices. WO 2014/167205 PCT/FR2014/050719 14

13. The three-phase electrical installation as claimed in either claim 11 or claim 12, in which the two transformers (10, 20) of the three devices are housed in the same oil tank (3), and the switching means of the three devices are 5 housed in the same plate (30) outside the oil tank (3).

14. The three-phase electrical installation as claimed in one of claims 11 to 13, in which, for each phase, the switching means of each line of each tap are grouped together and formed by an electromechanical contactor.