GB2132836A - Transformer providing continuous alternating current source - Google Patents

Abstract

A transformer (15) has at least two primary coils (1, 2) and at least one secondary coil (4), the primary coils being mutually separated galvanically. The primary coils (1, 2) are connected to separate 6 kV sources to maintain a continuous supply to a load (17). If undervoltage or lack of synchronism is detected, one of a pair of switches (13.1, 14.1) is opened to disconnect one of the sources from the transformer (15). <IMAGE>

Description

SPECIFICATION Transformer formed as a continuously operative current source with alternating voltage The invention relates to a transformer which is expediently formed as a continuously operative current source with alternating voltage.

In recent years the range of equipments requiring continuous energy supply has widened to an ever-increasing extent. Such equipments are e.g. electronic computers, control and regulating equipments for different technologies demanding a continuous operation, control centres on airfields, operating rooms in hospitals and special medical equipments, as e.g. iron-lungs.

These consumers require in general standardized three-phase alternating voltage 50 Hz frequency.

By "a continuously operative current source" is meant that the consumers are able to tolerate the interruption of failure only for some msecs. A longer interruption may lead to considerable disturbance in operation, and if long enough, it may result in a complete standstill.

To ensure continuous energy supply several soiutions for energy storage have been developed. Of these the system operating with a static inverter is considered the most modern. The essence of this solution lies in that after having rectified the alternating voltage of the mains staying at disposal, an accumulator battery is charged, thereafter the voltage of the accumulator is inverted to alternating voltage by means of the inverter and the outputs thereof are connected to the consumer requiring the continuous supply.

The disadvantage of the solution with the inverter lies in that investment and operational costs are very high. In addition, accumulators of large capacity are required. It is a well-known fact that accumulators have a most disadvantageous efficiency, their maintenance requirements are considerable, at the same time their useful life is rather short. The costs of the accumulator may be reduced, if in the establishment direct voltage is required for other purposes too, e.g. for protective systems.

Investment costs of inverters are very high, e.g. the costs of an inverter with an output of up to 10 kW may reach the order of magnitude of 1 0,000,00. These high costs are partly due to the high price of the components of the power electronics, partly to the high development costs of the individually produced apparatuses. Accordingly, in the near future reduced costs cannot be expected.

The aim of the invention is to develop a continuously operative current source with alternating voltage which is expediently a transformer and which is able to assure energy supply without the intermediate energy storing system so that simultaneously with the reduction of costs quality of energy supply may be improved.

The invention is based on the recognition that for larger consumers, in general, sources of alternating voltage which are in practice independent of each other but have the same phase remain available, and by means of these a transformer can be developed which is well suitable as a continuously operative safety current source.

The transformer according to the invention contains primary and secondary coils and iron core means.

The essence of the transformer according to the invention lies in that it is provided with at least one iron core, at least two primary coils and at least one secondary coil for the continuous supply of consumers connected to the secondary side, and the primary coils are galvanically separated from each other and the secondary coil.

One of the preferred embodiments of the invention may be characterized in that all the coils are arranged on one common iron core.

In a further preferred embodiment of the invention the primary coils are arranged on separate iron cores and to each of the primary coils at least one secondary coil is connected, while the secondary coils connected to identical consumers are parallel-connected.

In a further preferred embodiment of the transformer according to the invention a switching contact each of a switch is connected in series with the primary coils which are also connected to the output of power amplifier means supplied from a respective control circuit; the inputs of said control circuits are connected to the outputs of undervoltage relays arranged in the primary circuits as well as with a sensing circuit for sensing the asynchronous state of the two primary nets, while the inputs of said sensing circuit are connected to the outputs of the primary and secondary coils.

Preferred embodiments of transformers according to the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 shows a schematic arrangement of one preferred embodiment of a transformer according to the invention; Figure 2 shows a further possible embodiment thereof; Figure 3 shows a transformer system according to the invention; and Figure 4 shows a control unit for a transformer system according to the invention.

In the embodiment to be seen in Figure 1 two primary coils 1 and 2, as well as a secondary coil 4 are arranged on a common iron core 3 and they are galvanically separated from one another.

In the embodiment according to Figure 2 two primary coils 1 and 2 are arranged on separate iron cores 3 and 3', while secondary coils 4, 4', connected to identical consumers, are parallel-connected.

Supposing that the synchronous state of the supply voltages as mentioned above is assured, the energy supply of a consumer 17 (seen in Figure 3) is assured by the primary coils 1 and 2, in a 50-50% distribution. However, taking the possibility of a simple supply- and in a permanent manner- into consideration, e.g. if one primary voltage Up1 falls out, it is expedient to choose the same capacity for the primary coils 1,2 and the secondary coil 4.

In the case of a double supply, i.e. in the normal operative state of the transformer according to the invention the primary coils 1, 2 are not fully loaded. Their heating reserve allows a reactive power, possibly an efficient power current of a certain restricted magnitude representing the result of the synchronous difference between the two systems, i.e. resulting from the difference in the voltage and in the phase-angle of the two primary voltages.

Figure 1 shows a three-coiled transformer forming part of the invention, wherein, by choosing an approximately 20% drop between the primary coils 1 and 2, the primary voltages Up1 and Up2 restrict the compensating current between the two supply systems and in this case a permanent difference in voltages amount to 10% does not result in an overload of the coils.

Evidently, the current distribution between supplies may be further improved if the background impedances are not too different. Where two supplies are used i.e. the consumer 17 on the secondary coil 4 is supplied with the primary voltages Up1 and Up2, the secondary voltage will be less sensitive to the voltage fluctuations on the supply side, as in the majority of cases the voltages Up1 and Up2 do not simultaneously fluctuate, while this phenomenon is sensed so by the transformer as if the exciting voltage represented the mean value of the voltages Up1 and Up2.

Figure 3 shows the hierarchically built-up net of a supply system. The two voltages, which are practically independent of each other but are of the same phase, are derived from the 6 kV mains of a power plant. The 6 kV mains is connected to two 6kV/0.4 kV transformers 6,9 via circuit-breakers 5,8. From there, a connection is established with buses 25,26 via further circuit-breakers 7,10. On these buses the voltage is 0.4 kV.A connection between the two 0.4 kV nets and a transformer 15 embodied in the invention is effected via fuses 11,12 and contacts 13.1, 14.1 of respective switches, wherein the voltage Upi is connected to the primary coil 1 and Up2 to the primary coil 2; the basic values of the voltages are identical and as a matter of fact, the secondary voltage Us on the secondary coil 4 of the transformer 15 represents the desired continuously operative voltage source.

A difference of not more than 10% between voltages Up, and Up2 does not lead to overloading of the coils.

Accordingly, this difference may be permanently allowed. Voltage regulation of the middle-voltage net does not allow the difference to exceed permanently 5%.

Atransient change in voltage of 20% at the buses 25,26 of the distributor, which occurs when starting 0.4 kV motors with a higher output, results in an 1.5-fold current on the primary coils of a higher voltage.

However, the coils are able to bear this for some minutes. After the expiration of the period of acceleration lasting some seconds, transversal or cross-flow of reactive power ceases automatically.

Under the effect of the difference of 30% between the supply voltage Up1 and Up2, respectively, one of the primary coils is subjected to a double load. This can be observed when starting 6 kV motors. In general, thermic protection of the primary coils switches off the double overload after a minute; however, acceleration of the motors occurs even within a shorter time, so an approximately identical current distribution will be resotred. When starting motors, a voltage surge exceeding this value scarcely ever occurs in practice.

A difference in primary voltages reaching 40 to 50% may occur in power plants, in the course of block-stop, i.e. the so-called "interrupted change-over". The duration of breaking down lies with one second; accordingly, the about threefold transient current does not cause any detrimental overheating.

At such a low voltage, an undervoltage relay, the switch of the primary coil, will most probably release, thus stopping transversal compensating current. Consequently, the secondary voltage will be also restored.

Development of the consumer voltages A Ups { /0) Us { /0) 10 95 20 90 30 85 40 80 50 75 wherein A Ups stands for the differential voltage of the two supply voltages Up1 and Up2, and Us stands for the consumer voltage.

Reliable operation of electronic circuits is assured even at voltage levels deviating by - 15% from the nominal value, accordingly only a primary voltage breakdown above 30% requires intervention.

Switching-off of the supply source broken down with the reduced voltage without delay is controlled by means of the under-voltage relay which was previously set to 70% of the nominal voltage Unom. The releasing voltage of the relays should be set above the releasing voltage of the switch in order to be able to obtain definite release.

The secondary voltage of a simple supply is lower by about 2.5% than the nominal value to be measured with a double supply.

Figure 4 illustrates the operation of the under-voltage relays and the control circuits associated with them.

The embodiment according to Figure 4 has a three-phase supply, i.e. Up1 and Up2 are three-phase voltages and the transformer is a three-phase transformer. It goes without saying that the system can be realized as a one-phase version also. Accordingly, with this system to all voltages UR, Us, UT of both primary supplies a respective under-voltage relay, 18, 19,20;; 18', 19', 20' is connected, the outputs of which are coupled to respective circuits 21,21'. The task of said control circuits 21,21' lies not only in observing whether the single voltage levels have not fallen below a given value, i.e. below the switchoff voltages, but via the sensing circuit 24, they also observe the synchronous state of the voltages Upl and Up2 of the two primary nets.Depending on whether the voltage is dropping below the given level or whether the synchronous state between the two primary nets is destroyed, the output of the control circuit 21, ' disconnects the corresponding primary net through respective power amplifiers 22, 22', involving the fact that - resulting from the mode of operation of the under-voltage controller device - following the restored state of the voltage, the corresponding relay repeatedly restores the connection of the related primary net.

Keys 23,23' are designed for the first start, as in course of first start contactors 13 and 14 sense automatically the lower voltages from the under-voltage relays 18,18'; 19,19'; 20,20'.

From the Figure 3 it is clear- despite illustrating schematically only, as Up1 and Up2 can be either one-phase or three-phase voltages - that the contacts of the contactors 13,14 coupled to the outputs of the respective control circuits 21,21' (as seen in Figure 4) are connected to the primary net of voltages Up1 and Up2.

Below we shall detail the effect of short-circuits occurring in the other branchings of the two primary nets.

In case of electrical short-circuits, far from each other, if the voltage of the distributor does not fall below the releasing value of the under-voltage relay, the contactor of the short-circuited primary side does not disconnect. The transversal flow between the two primary nets stops only if the short-circuit is eliminated by the fuse in the branching.

If the impedance of the short-circuit is so low that the voltage of the buses 25,26 of the distributor drops below the releasing voltage of the under-voltage relay independently of melting of the fuse in the branching - the contactor of the short-circuited primary supply disconnects prior to the cessation of short-circuit. In such a manner transversal flow between primary nets, i.e. transient voltage break-down on the secondary side, are stopped.

If there is a short-circuited voltage on one of the buses 25,26 the consumer voltage will be reduced to 50% instead of zero, by virtue of the symmetry between the drops. This break-down lasting for only some msecs can be easily bridged over by the storing units with condensers contained in electronic equipments.

Where one of the paths of energy flow of the transformer with the double supply is interrupted due to protective operation or voluntary disconnection, the net part disconnected in such a manner remains under voltage from the other supply by virtue of the magnetic coupling of the two primary coils 1 and 2. Now, the primary coil connected with the interrupted path becomes a secondary coil. The terminal voltage will be determined by the resultant impedance of the consumers remaining in a connected state in this part of the net.

Two basic cases are to be differentiated: a) the resultant impedance of the consumers remaining connected to the distributor not being supplied from the mains is lower than one half of the nominal impedance of the transformer. In this case the under-voltage relays adjusted to 70% release and by disconnecting the contactor of the primary coil with the voltage break-down they stop trans-supply. In practice, impedance closing the unsupplied primary coil is always less than the critical value, and accordingly, quick cessation of trans-supply is guaranteed.

b) If for any reason the closing impedance exceeds the critical value, trans-supply can be stopped by 'reverse power' protection.

Trans-supply may also be prevented in that, by means of the auxiliary contacts of the circuit-breakers 7, 10 of the supplying transformers 6, 9, the actuating circuits of the contactors 13, 14 are blocked. Of course, this solution can be realized with short cables only.

If the remote common point of two supply sources, which are practically independent of each other, is destroyed (one of the supply sources passes into an 'island' operation), the primary coil connected to the supply source and separated from the mains is to be disconnected. With equipments within power plants disconnection may be performed by blocking the auxiliary contacts of the interrupter or by sensing the voltage.

The resulting field of primary coils which are excited from voltages of identical effective value but with a slightly different frequency floats with the differential frequency. Accordingly, the effective value of the secondary voltage changes slowly between zero and the nominal value. By including the secondary side into voltage detection this state of operational trouble can be well distinguished from other disturbances between the primary coils and the order for disconnecting the relevant primary coil can be unambiguously formed.

Output of the secondary coil 4 is to be selected in dependence of the effective power requirement of the consumer.

Considering the possibility of simple supply, as well as inevitable length between the vectors of the two voltages Up1 and Up2 of the supplies and angular deviations, it is expedient to choose identical outputs for the primary coils 1, 2 and the secondary coil 4, respectively, e.g. 25/25/25 kVA.

Consumer voltage Us supplies consumers with a nominal voltage of 3 x 380/220 V. Supplies have the same nominal voltages.

Due to the voltage fluctuations in the mains, as well as voltage drops arising on the series impedance of the transformer it is expedient to select a voltage which is higher by 5% E.g. transmission should equal to 400/400/400 V. Theoretically the two primary coils 1,2 can be matched to the mains with different nominal voltages (so e.g. 380 V and 550 V or 300 V and 660 V). If no higher supply voltage-losses are to be reckoned with, the primary coils 1 and 2 should be prepared without tappings. These tappings can be reconnected in a voltage-free state only.

Coils are to be arranged on the iron core 3so that the drop between the voltages of the primary coil 1 and the secondary coil 4, as well as the primary coil 2 and the secondary coil 4 should amount to approx. 10-10%, while the drop between the voltages of the primary coil 1 the primary coil 2 should be about 20%. In practice, transformers with divided secondary coils are made in such a manner.

When choosing the drop of main relations for 10%, with a 231 V no-load phase-voltage, at the nominal voltage and with an inductive power factor of cos ey = 0.8 output phase will amount with a good approximation to 220 V. The loose coupling (a drop of 20%) between the two primary coils 1 and 2 enables in case of short-circuits on the buses of the distributor compensating currents to flow through the coils which cannot exceed a 5..6-fold of the nominal current.

Claims (8)

1. Transformer for the continuous energy supply of a consumer or consumers connected to the secondary side, comprising at least one iron core, at least two primary coils and at least one secondary coil, the primary coils as well as the secondary coil(s) being mutually separated galvanically.

2. Transformer as claimed in claim 1, wherein all the coils are arranged on a common iron core.

3. Transformer as claimed in claim 1, wherein the primary coils are arranged on separate iron cores, and each primary coil is connected to at least one secondary coil, and the secondary coils connected to identical consumers are connected in parallel.

4. Transformer as claimed in any of claims 1 to 3, wherein a switch is connected in series with each primary coil, which switches are connected to the output of respective power amplifiers, each supplied from a respective control circuit and inputs of said control circuits are connected to the outputs of under-voltage relays arranged in the primary circuits, as well as to a circuit for sensing the asynchronous state of said primary circuits and inputs of said sensing circuit are connected to the primary and secondary coils.

5. A transformer substantially as shown in and as hereinbefore described with reference to Figure 1 or 2 ofthe accompanying drawing.

6. An electrical energy supply system comprising a transformer as claimed in any one of claims 1 to 3 or 5, in which each primary coil is supplied in use by a respective source of AC voltage which sources normally have substantially the same phase, the system further including control means for interrupting the voltage supply to any one or more of the primary coils whose supply voltage falls below a predetermined value.

7. An energy supply system as claimed in claim 6, wherein the control means is also arranged to interrupt the voltage supply to at least one of the primary coils in the event that the phase difference between the voltages supplied to the primary coils exceed a predetermined value.

8. An electrical energy supply system substantially as shown in and as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.