Method of operating parallel transformers

Abstract

Transformers are in a parallel circuit in which a tap changer with a control sensor is associated with each of the transformers and all control sensors are connected together by a communications connection. Each control sensor generates a measurement that is transferred by the communications connection. A measurement is generated with each of the control sensors, and at least one of the measurements of the control sensors of the transformers by the communications connection is transferred to N?1 control sensors. Then a controlling deviation caused by a circuit reactive current is calculated for each control sensor on the basis of the measurements of the control sensors. Finally the tap changer associated with each transformer is actuated by the respective control sensor as a function of the calculated controlling deviation such that minimization of the circuit reactive current is carried out for the respective transformer.

Claims

1. A method of controlling parallel operation of N transformers in a parallel circuit in which a respective tap changer of each transformer has a respective control sensor and all of the control sensors are connected together by a communications connection, N being equal to or greater than 2, the method comprising the steps of: generating a measurement with each of the control sensors; transferring at least one of the measurements of the control sensors of the transformers by the communications connection to the N?1 remaining control sensors; calculating a controlling deviation caused by a circuit reactive current for each control sensor on the basis of the measurements of the control sensors; actuating the tap changer associated with each transformer by the respective control sensor as a function of the calculated controlling deviation such that minimization of the circuit reactive current is carried out for the respective transformer; detecting absence of at least one signal of at least one of the control sensors caused by interruption of the communications connection at an instant and reporting the absence of the at least one signal of the at least one control sensor to all remaining control sensors; and determining the controlling deviation required for minimization of the circuit reactive current of the at least one control sensor affected by the interruption of the communications connection on the basis of the measurements from at least one further control sensor of at least one further transformer of the N transformers not affected by the interruption of the communications connection and received by the communications connection prior to the instant; and including with the at least one further control sensor affected by the interruption the individual instantaneously measured measurements of its respective transformer in the determination.

2. The method defined in claim 1, wherein the measurements measured at each control sensor of the parallel circuit, of the respective transformers are each an active current changing with time and a reactive current changing with time.

3. The method defined in claim 1, further comprising the step of: using the instantaneously measured measurements of the control sensors still connected together by the communications connection and of the remaining control sensors still connected by the communications connection for calculation of the required controlling deviation for minimization of the circuit reactive current.

4. The method defined in claim 1, further comprising the step of: using the instantaneously measured measurements of the control sensors still connected together by the communications connection and of the remaining control sensors still connected by the communications connection and the measurements regarded as a constant of the control sensors treated as isolated by the interruption of the communications connection at the instant for calculation of the required controlling deviation for minimization of the circuit reactive current.

5. The method defined in claim 1, wherein the number of transformers provided in the parallel circuit is at least two and less than or equal to sixteen.

6. The method defined in claim 1, wherein the at least one signal transferred by the communications connection from the control sensors consists of a readiness signal of the respective control sensor and the measurements of the respective control sensors.

7. The method defined in claim 6, wherein the absence of the readiness signal and/or the absence of the measurements from the respective control sensor indicates or indicate the interruption, and the affected control sensor is isolated.

8. The method defined in claim 6, wherein the readiness signal is transmitted from the control sensors at a frequency higher than or equal to a transmission frequency of the measurements from the control sensors.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention and the advantages thereof are described in more detail in the following with reference to the accompanying drawings in which:

(2) FIG. 1 is a time plot of the voltage measured at a transformer of a parallel circuit and the controlling voltage;

(3) FIG. 2 is a schematic illustration of a parallel circuit of three transformers known from the prior art;

(4) FIG. 3 is a schematic plot of the method, which is known from the prior art, for parallel control of several transformers with tap changers;

(5) FIG. 4 is a schematic illustration for determination of the angle ?? of the vector of total effective current and total reactive current in the electrical vector diagram;

(6) FIG. 5 is a schematic illustration of the load relationships and the respectively resulting controlling deviation in the case of parallel operation of two transformers; and

(7) FIG. 6 is a schematic illustration of the parallel circuit of three transformers of FIG. 3, in which the method according to the invention is used and only the control sensor of the tap changer of the second transformer is affected by an interruption.

(8) Identical reference numerals are used in the figures for the same or equivalent elements of the invention. In addition, for the sake of clarity only reference numerals required for description of the respective figure are illustrated in the individual figures.

(9) FIG. 1 is a time plot of the measured voltage U.sub.M of a transformer of a parallel circuit of transformers, which as seen over time lies in a range 3 defined by an upper voltage level 5 and a lower voltage level 6. Lying between the upper voltage level 5 and the lower voltage level 6 is a target value 1 of the voltage about which the voltage U.sub.Regel to be regulated can fluctuate without the tap changer switching the secondary side of the transformer to be one or more steps higher or switching the secondary side of the transformer to be one or more steps lower. The tap changer switches only when the voltage U.sub.Regel to be regulated exceeds the upper voltage level 5 or falls below the lower voltage level 6 for a pre-defined time period 7. The voltage U.sub.Regel to be regulated is brought back to the range 3 by actuation of the tap changer, as shown in FIG. 1. As similarly shown in FIG. 1, the controlling voltage U.sub.Regel is composed of the measured voltage UM, a voltage component ?U.sub.KBS due to the circuit reactive current and a voltage compensation component ?U.sub.KOMP. For the controlling voltage U.sub.Regel there applies:
U.sub.Regel=U.sub.M+?U.sub.KBS+?U.sub.KOMPEquation (1)

(10) FIG. 2 is a schematic illustration of a parallel circuit 10 of three transformers T1, T2 and T3. Although the following description for the method according to the invention relates to three transformers, this is not to be taken as restrictive. It will be obvious to an expert that the invention can also be used for any multiple of transformers T1, T2, . . . , TN of substantially the same type. According to a preferred embodiment at least two and at most sixteen transformers are connected in parallel. A respective control sensor 12 is associated with each output 9 of each transformer T1, T2 and T3. The control sensor 12 performs a current measurement 11 and a voltage measurement 13 at the output 9 of each transformer T1, T2 and T3. In addition, the control sensors 12 of the individual transformers T1, T2 and T3 are connected together by a common communications connection 14. The communications connection 14 can be, for example, a CAN bus.

(11) The measurement of current and voltage by the respective control sensors 12 is not carried out on the basis of the actually present voltage, for example 230 kV and the flowing current of approximately 100 A. For measurement of the voltage there is thus used a voltage transformer (not illustrated) which lowers the voltage from, for example, 230 kV to, for example, 100 V. For measurement of the current use is made of a current transformer (not illustrated) which lowers the current from, for example, 100 A to, for example, 1 A. The voltage transducer secondary nominal voltage U.sub.VT.sub._.sub.SEC and the current transducer secondary nominal current I.sub.CT.sub._.sub.SEC are included in the further calculation.

(12) Initially, the method illustrated in FIG. 3 shall be explained in its entirety. Parallel control is carried out by the parallel circuit, which is illustrated on the basis of FIG. 2, of three transformers T1, T2 and T3 so as to keep the reactive current as small as possible, approximately at zero. The reactive current is zero when the angles of the currents of all transformers are equal to the angle of the total current. The method consists of several individual method steps.

(13) Initially, in a first method step 100 individual measurements for the active current I1W, I2W and 13W and for the reactive current I.sub.1B, I.sub.2B and I.sub.3B are determined by each control sensor 12 and communicated to the other control sensors 12 of the other transformers T1, T2 and T3.

(14) In a second method step 200, the measurements of all transformers T1, T2 and T3 connected in parallel are cyclically collected and evaluated. It is critical for this step of the method that an individual control sensor 12 is associated with each transformer T1, T2 and T3 connected in parallel and that all control sensors 12 are connected together by the common communications connection 14, for example a CAN bus, for information exchange. This so-called CAN (Controller Area Network) bus offers, apart from high transmission speed with simplest installation, a high measure of transmission security. All control sensors 12 can accordingly exchange data with the other control sensors 12 of the parallel connected transformers T1, T2 and T3.

(15) Subsequently, in a third method step 300, from the collected measurements the vector 20 of total active current and total reactive current of all transformers is recorded in the electrical vector diagram (see FIG. 4). The vector 20 includes an angle ?? with the X axis of the electrical vector diagram.

(16) Each control sensor 12 determines the sum ?I.sub.W of all active currents and the sum ?.sub.IB of the reactive currents of all parallel connected transformers T1, T2 and T3.
I.sub.1W+I.sub.2W+I.sub.3W+ . . . +I.sub.NW=?I.sub.WEquation (2)
and
I.sub.1B+I.sub.2B+I.sub.3B+ . . . +I.sub.NB=?I.sub.BEquation (3)

(17) Subsequently, in a fourth step 400 the individual target reactive current I.sub.1BSOLL is determined from the individual active current I1.sub.W and the ratio of the sum ?.sub.IW of all active currents to the sum ?.sub.IB of all reactive currents by each control sensor 12.
I.sub.1BSOLL/I.sub.W=?I.sub.B/?I.sub.WEquation (4)
I.sub.1BSOLL=I.sub.W*?I.sub.B/?I.sub.WEquation (5)

(18) On the assumption that, when a tap changer is switched, at one of these parallel connected transformers T1, T2, . . . , T3, for example, only the reactive current I1B through the corresponding transformer T1 changes (the connected load in fact remains constant) and the control sensor 12 knows the active current I1W of the corresponding transformer T1, it is now possible to calculate the level of the reactive current I1BSOLL which would be necessary in order together with the measured active current I1W to make it parallel with the vector 20 of total active current and total reactive current.

(19) Calculation of the individual circuit reactive current I1B_KBS of each control sensor 12 from the calculated target reactive current I1B_SOLL and the individual reactive current I1B is carried out in the fifth method step 500. It may be mentioned again at this point that the calculation here and also in the following is, in fact, specifically described merely for i=1, thus for the transformer T1, but the calculation is carried out analogously for all transformers T1, T2, . . . , TN, i=1, N of the parallel circuit.

(20) As a result of the preceding method step 500, the control sensor 12 knows the target reactive current I1B_SOLL, which conveys the load, and the contribution of the reactive current I1B. The me the associated transformer T1 delivers for that purpose.

(21) The circuit reactive current can now be calculated from the difference of the target reactive current I1B_SOLL and the reactive current I1B of the respective transformer with consideration of the signs of the two currents:
I.sub.1B.sub._.sub.KBS=I.sub.1B?I.sub.1B.sub._.sub.SOLLEquation (6.1)
I.sub.2B.sub._.sub.KBS=I.sub.2B?I.sub.2B.sub._.sub.SOLLEquation (6.2)
I.sub.3B.sub._.sub.KBS=I.sub.3B?I.sub.3B.sub._.sub.SOLLEquation (6.3)

(22) The above equations make clear the calculation of the respective circuit reactive current I.sub.1B.sub._.sub.KBS, I.sub.2B.sub._.sub.KBS and I.sub.3B.sub._.sub.KBS for the three parallel connected transformers T1, T2 and T3 (see FIG. 2).

(23) This difference of the target reactive current I1B_SOLL and the reactive current I.sub.1B of the transformer T1 is the circuit reactive current I.sub.1B.sub._.sub.KBS and shall be minimized by actuation of the tap changer at the respectively associated transformer T1, T2, . . . , TN, here actually T1. I.sub.1B.sub._.sub.KBS is the controlling deviation for the transformer T1.

(24) The voltage difference ?U.sub.KBS is derived in a sixth step 600 from I.sub.1B.sub._.sub.KBS by recalculation.

(25) If the controlling deviation is not equal to zero and exceeds the level of the controlling deviation of a predetermined limit value then the control sensor 12 will have the effect on the tap changer that this moves to a position or tap of the respective transformer at which the reactive current I1B through the transformer T1 is minimal, at the best zero. Through actuation of the tap changer essentially the inductive component of the current flowing through the respective transformer T1, T2, . . . , TN is influenced. This means that an increase and a decrease of the longitudinal impedance of the respective transformer T1, T2, . . . , TN oppose the circuit reactive current I.sub.1B.sub._.sub.KBS.

(26) When the tap changer is actuated, windings of the controlling winding are connected with or disconnected from the main winding.

(27) Since this controlling deviation is calculated on a sign basis by each of the parallel connected control sensors 12 for the respective transformer T1, T2, T3, all control sensors 12 cause their tap changers, which are associated with the transformers T1, T2, T3, to move to a tap-changer position at which the respective circuit reactive current I.sub.1B.sub._.sub.KBS, I.sub.2B.sub._.sub.KBS or I.sub.3B.sub._.sub.KBS is minimal, at the best zero.

(28) In that case, one tap changer can indeed be moved to a higher tap changer setting whilst the other tap changers move to a lower position.

(29) Reference is made to the electrical vector diagram disclosed in FIG. 4 for illustration of the angle ?? of the vector 20 of the collected measurements of total active current and total reactive current with respect to the X axis. For that purpose, measurements for the active current for the three transformers T1, T2 and T3 assume the following values (see Table 1) for the respective active current I.sub.W and the respective reactive current I.sub.B.

(30) TABLE-US-00001 TABLE 1 Transformer 1 Transformer 2 Transformer 3 I.sub.W [active current] 1A 2A 3A I.sub.B [reactive current] 3A 2A 1A

(31) Thus, as apparent from FIG. 4, for the sum ?I.sub.W of all active currents and the sum ?.sub.IB of all reactive currents in each instance the value 6 A arises. The object of parallel regulation of the transformers T1, T2 and T3 is to change the components of the respective active currents I.sub.1W, I.sub.2W, I.sub.3W and the components of the respective reactive currents I.sub.1B, I.sub.2B, I.sub.3B in such a way that the angle thereof adopts the same value with respect to the active current axis W in the electrical vector diagram 25. In the case of the illustration shown in FIG. 4, ?1>??, ?2=?? and ?3<??. Since by the tap changers of the transformers T1, T2 and T3 the taps at the transformers are connected in dependence on the measured values, adaptation or minimization of the circuit reactive current is achieved. A controlling deviation of a current D.sub.IN.sub._.sub.KBS for each control sensor 12 of a transformer T1, T2, . . . , TN can be calculated. This controlling deviation thus results from consideration of the individual currents I.sub.1, I.sub.2 and I.sub.3, particularly the vectors thereof in the vector diagram.

(32) As a result thereof, a minimum circuit reactive current always flows through all parallel connected transformers after the end of the controlling process.

(33) FIG. 5 is a schematic illustration of the load relationships and the respectively resulting controlling deviation in the case of parallel operation of two transformers T1 and T2. The phase angle ?.sub.LOAD (corresponds with ?? of FIG. 4) of the load 15 of the parallel circuit of the two transformers T1 and T2 is predetermined by the characteristics thereof and cannot be influenced by the control sensor 12 and the associated transformer T1 or T2.

(34) The tap changer, which is associated with each transformer T1 and T2, as longitudinal regulator influences substantially only the inductive component (reactive current) of the total current flowing through the transformer T1 or T2. This is due to the relationship that the inductive component (reactive current) is substantially larger than the active component (active current).

(35) The controlling deviation ?I.sub.1B.sub._.sub.KBS or ?I.sub.2B.sub._.sub.KBS for the respective control sensor 12 of the associated and parallel operating transformer T1 or T2 is calculated by consideration of the reactive currents for each individual transformer T1 or T2:
?I.sub.1B.sub._.sub.KBS=I.sub.1B?I.sub.1BSOLL

(36) In that case, I.sub.1BSOLL is calculated from
(I.sub.1B+I.sub.2B)/(I.sub.1W+I.sub.2W))*I.sub.1W
and the inductive component (reactive current component) of the first transformer T1.

(37) Analogously:
?I.sub.2B.sub._.sub.KBS=I.sub.2B?I.sub.2BSOLL

(38) In that case, I.sub.2BSOLL is calculated from
((I.sub.1B+I.sub.2B)/(I.sub.1W+I.sub.2W))*I.sub.2W
and the inductive component (reactive current component) of the second transformer T2.

(39) The controlling deviation will then be smallest (ideally equal to zero) if the measured phase angle ?1 or ?2 at the first transformer T1 or second transformer T2 is equal to that of the load.sub.load (corresponds with ?? of FIG. 4) of the parallel circuit of the first transformer T1 or second transformer T2.

(40) This is achieved by changing the longitudinal impedance of the respective transformer T1 or T2 by the tap changer with which each of the transformers T1 and T2 is associated. Through actuation of the tap changer, windings of a controlling winding are connected with or disconnected from a main winding.

(41) FIG. 6 is a schematic illustration of the parallel circuit 10 of three transformers of FIG. 3, in which the method according to the invention continues notwithstanding an interruption 16 of the communications connection 14. By the communications connection 14 each of the control sensors 12 of the respective transformers T1, T2 and T3 obtains, at regular time intervals, data about the measurements I.sub.W (active current) and I.sub.B (reactive current) of the remaining transformers T1, T2 and T3. In continuing operation, thenas already mentioned abovethe parallel circuit 10 of the transformers T1, T2 and T3 is so controlled by the measurements of the parallel circuit 10 that the circuit reactive current of the individual transformers T1, T2 and T3 is minimal, at the best equal to zero. Each of the control sensors 12 obtains by the communications connection 14 the information of the other control sensors 12 that are still reachable by the communications connection 14. For that purpose, all control sensors 12 transmit a signal (readiness signal) at specific intervals in time. If a signal no longer comes from one or more control sensors 12, then this means that the one or more control sensors 12 is or are no longer reachable and that an interruption 16 of the communications connection 14 is present.

(42) In the case of the illustration shown in FIG. 6, the interruption 16 of the communications connection 14 is present at, for example, the control sensor 12 of the second transformer T2. This means that no information can be transmitted from the control sensor 12 of the first transformer T1 and from the control sensor 12 of the third transformer T3 to the control sensor 12 of the second transformer T2. On the other hand, the control sensors 12 of the first transformer T1 and of the third transformer T3 do not receive any information from the control sensor 12 of the second transformer T2. This means that the two control sensors 12 of the first transformer T1 and of the third transformer T3 continue to set a minimum circuit reactive current without then taking into consideration the values of the control sensor 12 of the second transformer T2, since no measurements can be delivered by this due to the interruption 16.

(43) The control sensor 12 of the second transformer T2 continues the method for setting a minimum circuit reactive current with use of the last communicated values of the control sensors 12 of the first transformer T1 and the third transformer T3.

(44) The advantage of the present invention is that the measurements of the control sensor 12 which has failed due to the interruption 16 are assumed as a constant by the other control sensors 12. Thus, the transformer at which the interruption 16 of the control sensor 12 occurred continues to be taken into consideration in the parallel operation. That control sensor 12 which no longer has communication with other control sensors 12 assumes the last values of the other control sensors 12 as a constant and continues the method of circuit reactive current minimization. Control sensors 12 which still have communication with at least one other control sensor 12 continue to perform the method of circuit reactive current minimization and, in particular, only with the control sensors 12 participating in the communication.

(45) In other words, in the present, concretely described case this means that the second (isolated) transformer T2 is controlled by the last measurements, which were transmitted by the communications connection 14, by the associated control sensor 12.

(46) The control sensor 12 at the second transformer T2 should not be blocked by the above-proposed and improved solution (see FIG. 6). In the case illustrated here, the component of the sum of the part currents (the reactive current and also the active current component) is to be considered as a constant by the control sensor 12 of the first transformer T1 and by the control sensor 12 of the third transformer T3.

(47) As a consequence thereof, risk of creation of circuit reactive currents is significantly reduced, additionally because all other control sensors 12 can operate in accordance with the same calculation rule and a partial dynamic of the controlling circuit is maintained notwithstanding the interruption.

(48) As apparent from the above equations, the values. The me were determined by the control sensors 12 before the interruption, for active current IW and reactive current IB are included in the calculation of ?U.sub.KBS in the event of interruption of the communications connection 14. These values remain constant until reinstatement of the communications connection 14, so that the sum ?I.sub.W of all active currents and the sum ?I.sub.B of all reactive currents can be calculated for the duration of the interruption.

(49) The invention was described with reference to one embodiment. It will be obvious to an expert that changes and modifications can be carried out without in that case departing from the scope of protection of the following claims.