JPH01259719A - Protective method for transformer - Google Patents

Abstract

PURPOSE:To perform protection of a transformer with high accuracy and high speed, by determining the phase currents of a transformer having delta- connection windings based on the differential currents obtained on the basis of line currents fed from respective terminals of the transformer and the differential currents obtained on the basis of the phase currents, and employing the phase currents for judgement of inner fault of the transformer based on the parallel admittance of the transformer. CONSTITUTION:Differential currents are obtained with respect to line currents i1a, i1b, i1c, i2ac, i2ba, i2cb. Absolute values of these differential currents are compared with a first detection level in order to obtain the phase currents i2a, i2b, i2c of delta-connection windings. Then it is judged whether the absolute values of the differential currents concerning to these phase currents i2a. i2b, i2c are larger than a second detection level or not. Initial setting is made for the first judgement while parallel admittance is operated for second and following judgements, then the operated parallel admittance is compared with a judgement value thus judging an inner fault. If the condition continues, it is judged that an inner fault exist in the transformer and a signal for allowing trip of circuitbreaker is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a transformer protection method, particularly in three-phase transformers with triangular connected windings and in three-phase connecting banks of single-phase transformers, to prevent internal accidents. This article relates to a transformer protection method that can be detected with high speed and high sensitivity.

[Conventional technology]

Conventionally, protection of a transformer in the event of an abnormality such as a winding short-circuit accident of the transformer has been performed by detecting the differential current of each terminal current of the transformer. Differential currents are generated not only by winding faults but also by excitation inrush of transformers. Conventionally, the so-called second harmonic suppression method was used to utilize the property that the excitation inrush current has a high content of second harmonic components. Prevents transformer protection relays from malfunctioning due to inrush current.

However, depending on the ground capacitance, reactance, and transformer impedance of the power transmission system, secondary
Low-order harmonics near the harmonics may be included, and the conventional second harmonic suppression method may cause a delay in the operation of the transformer protection relay, or even a malfunction of the transformer protection relay, resulting in a serious disaster.

As a countermeasure to this problem, a digital method has been proposed in Japanese Patent Laid-Open No. 62-89424. This is because when considering a transformer as a multi-terminal network and expressing it using an admittance equation, the transfer admittance at the time of excitation rush and internal fault is almost the same as when it is healthy, whereas the drive point admittance or the transfer admittance derived from this is almost the same. This is based on the fact that the parallel admittance differs significantly between inrush and internal fault conditions. Since there is no appropriate word to express the reciprocal of inductance, this is referred to as admittance here. The specific method is shown below.

(1) Determine from the transfer admittance and leakage inductance of the transformer to be protected, and store it in advance as a constant inside the relay.

(2) Sample the voltage and current at each terminal of the transformer to be protected at appropriate time intervals.

(3) Find the differential current from each sampled terminal current.

(4) When the differential current reaches a predetermined detection level, there is an internal fault state or an excitation inrush state. Therefore, the drive point admittance or parallel admittance is determined from each voltage, current, and previously stored transfer admittance, and it is determined whether the internal fault state is an excitation inrush state according to the magnitude of the value.

(5) When the determination of the internal fault state continues for a predetermined number of samples m, a command is given to open the circuit breaker in order to disconnect it from the transformer.

This transformer protection method can be calculated with high accuracy because the transfer admittance, which is a coefficient in the calculation formula, is a known constant, and can be applied to transformers of various structures.It is also possible to determine whether an internal fault or inrush is occurring regardless of the current waveform. Therefore, it can be applied to transformers of all systems regardless of the size of ground capacitance, etc., and since it is possible to determine whether an internal fault or an excitation inrush occurs from the first wave of current, relay operation in the event of an internal fault can be performed at high speed. It has very superior features compared to conventional second harmonic suppression methods, such as the ability to

[Problem to be solved by the invention]

Driving point admittance, parallel admittance, etc. exist for each phase of a three-phase transformer, and these values differ depending on the phase in an internal fault state or excitation inrush state.
To determine its value, it is necessary to know the phase current. That is, the terminal current of the transformer must be a phase current.

On the other hand, power transformers used in three-phase circuits generally have windings that are connected in a triangular manner, but in the case of a three-phase transformer, the wires are connected in a triangular manner inside the transformer tank, and only the connection terminals have bushings. Phase currents in the triangularly connected windings are often not detectable because they are drawn out of the transformer tank through the triangularly connected windings. Therefore, the transformer protection method using the transformer admittance cannot be applied to a three-phase transformer having triangularly connected windings.

SUMMARY OF THE INVENTION An object of the present invention is to provide a transformer protection system using transformer admittance, which eliminates the drawbacks of the prior art described above and is devised to be applicable to a three-phase transformer having triangularly connected windings.

[Means to solve the problem]

The above-mentioned problem arises in a method for protecting a transformer that has star-shaped connected windings and triangular connected windings and is connected in three phases. The differential current corresponding to each terminal current of the connection winding is calculated, the abnormal phase that caused the abnormality in the transformer is estimated according to the magnitude of the differential current, and based on the estimation, the hJ triangular connection winding is Calculate the estimated phase current of each phase of the line, calculate an admittance parameter that relates the estimated phase current and the terminal voltage of the triangularly connected winding from the estimated phase current and the terminal voltage of the triangularly connected winding, The problem is solved by a transformer protection method that determines that the abnormal phase is in an internal fault state based on the value of an admittance parameter and issues a command to open the circuit breaker of the transformer.

[Operation] The present invention establishes a specific correlation between the differential current obtained from the line current of each terminal of the transformer and the differential current obtained from the phase current in the internal fault state or excitation inrush state of the transformer. By focusing on this, we are trying to estimate and find the phase current of the triangularly connected windings. The outline of the procedure is shown below.

(1) Three differential currents are obtained from the line currents of the three phase terminals of the transformer to be protected, and it is determined from the magnitude that the protected image transformer is in an internal fault state or an excitation inrush state.

(2) Identify the phase that is in an internal fault state or excitation inrush state from the magnitude relationship of the differential currents regarding the three line currents, and determine the phase current from the specific relationship between the differential currents regarding the three line currents. Find the differential current for

(3) Find the phase current of the triangularly connected winding from the differential current regarding the phase current and the phase current of the star-shaped connected winding.

(4) Find the riJAIjJ point admittance admittance from the phase current of each winding, terminal voltage, and transfer admittance stored in advance, and depending on the magnitude of the value, determine whether it is an internal fault or an excitation inrush state. Determine.

(5) When the determination of the internal fault state continues for a predetermined number of samples, a circuit breaker opening command is given to disconnect the transformer from the system.

[Embodiments of the invention]

The basic configuration of the computer used in the transformer protection system according to the present invention, including the main circuit of the transformer to be protected, is shown in FIG. 1, and an example of the calculation flow is shown in FIG. 2, and the following explanation will be added.

In Figure 1, 11.12 is the primary and secondary winding of the transformer to be protected, 21.22 is the primary and secondary current transformer, and 31.32 is the primary and secondary voltage transformer. vessel, 41.4
2 is a primary side and secondary side circuit breaker, 43.44 is a trip coil of each circuit breaker, 5 to 9 are a computer, 5 is an input section, 6 is an arithmetic processing section, and 7 is a storage section. , 8 is an output section, and 9 is a coefficient setter. The input section 5 receives the voltage of the main circuit,
Auxiliary voltage and current transformer Aux that converts the current level.

PCT, filter FIL that removes harmonic components that are not necessary for protection, sample holder SH that samples and holds the instantaneous value of the input, multiplexer MPX that sequentially switches the output of SH and inputs it to A/D, analog/digital conversion It consists of a device A/D. The arithmetic processing section 6 includes a main processing unit CPU that executes control and arithmetic operations, a data bus, and an address bus. The storage unit 7 is a ROM (Read Only Me) that stores programs.
RAM for storing data, (Rando
m Access Me+++ory).

Furthermore, there may be a nonvolatile memory, such as a semiconductor nonvolatile memory EAROM, which is rewritable for storing and changing coefficients. The output section 8 is a digital output section for calculation and determination results, and the circuit breaker trip permission signal is output from this section.

The coefficient setter 9 sets the coefficients of the calculation formula, relay setting values, etc. 1
Although it is for displaying, it displays the calculation 2 judgment result,
It may also be output.

Next, before explaining the calculation flow diagram in Figure 2, we will give an overview of the transformer inrush phenomenon, how to estimate the phase current of the triangularly connected windings, and how to determine whether it is an internal fault or a magnet inrush based on the parallel admittance of the transformer. This section explains how to do this.

First, an outline of the excitation inrush development of a transformer will be explained using FIGS. 3 and 4. Figure 3 is a route map of a single-phase two-winding transformer.
11 is the primary winding, and 12 is the secondary winding.

41 indicates a circuit breaker. For simplicity, it is assumed that the number of turns of the primary and secondary windings is equal. if and i2 are the primary and secondary currents, respectively, and Vn is the primary or secondary winding voltage. When the circuit breaker 41 is closed at time 1=0, the magnetic flux density B in the transformer core changes according to equation (1).

B = BR+−f ovnd t−(
1) nS However, BR is the residual magnetic flux density in the transformer core before the circuit breaker is closed, No is the number of turns of the primary or secondary winding, and S is the cross-sectional area of the core. The magnetic flux density in the transformer core during steady operation is lower than the saturation magnetic flux density BS, but when the circuit breaker is closed as described above, the magnetic flux density may vary depending on the size of BR, the voltage phase when the circuit breaker is closed, etc. B may exceed Bs and the core may become magnetically saturated. The magnetically saturated state of the iron core is equivalent to that of an air core, and the excitation current increases rapidly, causing what is called an excitation inrush current. Such a magnetizing inrush current is (2
) is detected as a differential current Σi shown by the equation. Furthermore, the first order,
The secondary currents it and iz are converted values to the same order side9
The addition and subtraction of the primary current and secondary current below are also conversion values for either one.

Σ1=it+iz...(2)
Voltage V16! The relationship between i flux density B and differential current Σ1 is shown in FIG.

Next, the case of a three-phase transformer will be explained with reference to FIGS. 5 and 6.

In Fig. 5, 1 is a two-turn vA three-phase transformer, lla, l, whose primary and secondary windings are star-shaped and triangularly connected, respectively.
lb and llc are star-connected primary windings, 12a, 1
2b and 12c indicate triangularly connected secondary windings. it
a, i ib and ilc are the line currents on the primary side and are also the phase currents of the star connected windings. i2a, iZb and izc are the phase currents of the triangularly connected windings.

i 2aC+ i zba and 12cb are line currents. , □8゜vtb and VIC are primary terminal voltages, V 2a
, V 2b and VZC are the secondary terminal voltages, and are the phase voltages of each winding, respectively. For simplicity, it is assumed that the number of turns of the primary winding and the secondary winding are equal. In the excitation inrush state a
.

The relationship between the voltage of each phase b and c, core magnetic flux density, and differential current is the same as in the case of the single-phase transformer described above.

However, since the voltage phase of each phase is different when the circuit breaker is closed, and the residual magnetic flux density is also generally different for each phase, the situation in which the magnetizing inrush current is generated also differs for each phase. FIG. 6 shows an example of the relationship among voltage, core magnetic flux density, and differential current in the excitation inrush state. Ba, Bb, and Bc indicate the magnetic flux densities of the a, b, and a phase cores. When the magnetic flux density of the iron core exceeds the saturation magnetic flux density BS, a differential current is generated in the island phase. Σ14. ΣBlue,
Σ1c is a differential current regarding the phase currents, as shown in equation (3).

Σ+a: i 1a+ i 2a Σlb= l 1b+ i zb
...(3) ΣLC: i tc+ i zc Σl&c, Σlha+ Σlcb are differential currents regarding the line currents, and have the relationship of equation (4).

Σ+ac= (ila 1lc) + 12ac=
(i+a-izc) -(isc-izc)=Σ18-
ΣIc Σ+ba= (ixb 1ta) +12ha” (
itb iZb) (ila 12a)=Σl
b−ΣI＆ΣIcb= (iic 1tb)+1zcb=
(ixc 12c) (itb 1zb
) = Σ1C - Σlb ... (4) As shown in Figure 6, the period during which the differential current is generated is 5
Divide into sections (1) to (5) and examine the differential current generation situation and the magnetic saturation situation of the iron core in each section, and the results will be as follows.

Case 1: This is the case for magnetic saturation sections (1) and (5) for only one phase. Differential currents are generated with opposite polarity for the two line currents associated with the magnetically saturated phase. For example, in section (1), the b phase is magnetically saturated, and Σ1cb=-Σiba≠O=-(5).

Case 2: This is the case where two phases are simultaneously magnetically saturated in sections (2) and (4). In this case, all three differential currents regarding the g current are no longer zero. 3
In this case, the two phases are magnetically saturated with opposite polarities in most cases due to the relationship between the fla pressure phases in the phases and the state of the residual magnetic flux density. For example, in section (2), the a phase is magnetically saturated on the positive side, and the b phase is magnetically saturated on the negative side. In such a state, the differential current Σiba related to the line currents related to the two magnetically saturated phases has a polarity opposite to that of the other two differential currents, and has a maximum absolute value.

Case 3: This is the case where three phases are simultaneously in the magnetic saturation section (3). Due to the voltage phase relationship and residual magnetic flux density in the three phases, it is rare for all three phases to reach a magnetic saturation state at the same time, and even if such a state does occur, it will only last for a very short time. It is.

By the way, in the three-phase two-winding transformer shown in FIG. 5, a current transformer is generally not installed in the transformer tank 7, so the current that can be detected is the primary side line current i1a.

i sb, i lc and secondary line current i 2ac,
i zha, i 2cb. The above-mentioned difference iI at the time of excitation inrush! A method for estimating the phase current of the triangularly connected winding from these line currents based on the IJ current relationship will be described below.

(a) Detect each line current and find the difference current regarding the line current from equation (6).

Σ+ac= (ita 1tc) +1zacΣ+b
a= (i 1b i ta) + i xbc
...(6)ΣIcb= (itc 1tb)
+1zcb (b) If the currents related to all line currents are smaller than the predetermined detection level εl, it is a steady operating state, so there is no need to find the phase current of the triangularly connected windings. If not, consider the excitation inrush state and estimate the phase current of the triangularly connected windings as follows.

(c) When two differential currents among the three differential currents related to the line current exceed the detection level El, one phase common to the two differential currents is magnetically saturated. It is determined that the For example, in the case of section (1) shown in Figure 6,
Since Σlba and Σicb exceed the detection level El, it is determined that the b phase, which is common to both, is magnetically saturated. (d) Three differences regarding the line currents 1llll currents all exceed the detection level εl , it is determined that the opposite polarities of the two phases related to the absolute value and the largest differential current are saturated. For example, in the case of section (2) in FIG. 6, the absolute values of Σ, 5& are the largest, so it is determined that the b-phase and a-phase are magnetically saturated.

(c) Set the differential current related to the phase current of the phase not magnetically saturated to zero, and find the differential current related to the phase current of the magnetically saturated phase from the relationship of equation (4). For example, in the case of section (1) in Figure 6, Σia = Σic"O... (7) If we set Σja"ΣLba or Σi,, = -Σjcb
...(8). Note that Σiba and Σlcb have the same absolute value.

It is of opposite polarity.

In the case of section (2) in Figure 6, Σ1c=O...(9) And then.

It is.

(f) The phase current of the triangularly connected winding is expressed as (
11) Obtained from formula.

12a=Σ1a-ita ixb=Σ几-1tb
...(11) izc=Σjc Llc In addition, when two phases have the same polarity and are magnetically saturated, or when three phases are magnetically saturated at the same time, the above method will change the phase current of the triangularly connected winding. Since it cannot be estimated correctly, errors may occur in calculation results such as parallel admittance, which will be described later, and an erroneous internal accident determination may occur. However, as mentioned above, even if such a situation exists, it will only last for a very short time.

Since the circuit breaker opening command is output when internal fault determination continues for multiple samples, there is no problem in actual operation.

Next, we will explain internal accidents in transformers. FIG. 7 shows an example of the differential current related to the phase current and the differential current related to the line current when an internal fault such as a short circuit occurs in the a-phase winding in the three-phase transformer shown in FIG. 5. A differential current Σia is generated in the a-phase, which is the fault phase, and the differential currents Σiac and ΣLba related to the two line currents related to the a-phase have opposite polarities and exceed the detection level El.

Such a relationship between differential currents is the same as a state in which only one phase is magnetically saturated at the time of excitation inrush. Therefore, the phase current of the triangularly connected windings can be estimated from each line current by the same procedure as in the case of excitation inrush described above.

If an internal fault occurs in two phase windings at the same time, depending on the nature of the internal fault, the magnitude relationship between the fault phase and each differential current may be different from the state in which the two phases are magnetically saturated with opposite polarity at the time of excitation inrush. There is. Therefore, in the same procedure as in the case of excitation inrush described above, it may be erroneously determined that a differential current is generated in a healthy phase in which no fault has occurred. However, as described below, the values of parallel admittance, etc. when the core is not magnetically saturated in a healthy state are different from the values at the time of magnetic inrush when the core is magnetically saturated, so depending on the calculation result of parallel admittance, etc., it is not magnetic inrush. In other words, it is determined that it is an internal accident, and as a result, the circuit breaker opening command can be correctly output.

Next, we will show how to determine the parallel admittance of the transformer to be protected from the phase current and terminal voltage of each winding to determine the fault. The details are described in Japanese Patent Application Laid-Open No. 62-8942, so only an outline thereof will be shown here.

Figure 8 is a schematic diagram of a two-winding transformer, where 10 is an iron core.

11.12 indicates the primary and secondary windings. 11°12 in the figure
are primary and secondary currents, vl and v2 are primary and secondary currents)
The general admittance equation derived from the relationship is (1
2), the morphic equivalent circuit is shown in FIG.

y12 is the primary-secondary leakage inductance L1 of the transformer
2 to (13), and is called a transfer admittance.

y+z=-1/L12 °-(
13) y11+y22 is an unknown quantity whose value differs depending on the state such as steady state, excitation inrush state, internal fault state, etc., and is called driving point admittance. The driving point admittance is
It can be calculated using equation (14) derived from equation (13) using the sampled voltage, current, and constant transfer admittance.

yto and y2o in FIG. 9 are obtained by the following equation (15).

Varies depending on the internal state of the transformer. This is called parallel admittance.

3/20" yzi+ y22 The parallel admittance during normal operation corresponds to the excitation inductance. Since the excitation current during steady operation is generally negligible compared to the load current, yto≠y2o'=o...
・(16). At the time of excitation entry, it becomes a constant non-zero value determined by the air-core self and mutual inductance of each winding. In the case of an internal accident, for example, in the case of a total short-circuit accident in the primary winding 11.

y20 valve O, and in the case of a total short-circuit accident of the secondary winding 12, y2o doo■. That is, since the value of the parallel admittance is different between the time of excitation inrush and the time of an internal fault, it is possible to distinguish between the two based on this.

This concludes the explanation of the overview of the inrush phenomenon of transformers, the method of estimating the phase current of the triangularly connected windings, and the method of distinguishing between internal faults and inrush by the parallel admittance of the transformer. The example calculation flow shown in the figure will be explained in detail for each step shown in the figure.

FIG. 2 is a calculation flow diagram when the three-phase two-winding transformer shown in FIG. 5 is targeted for protection. First, in step S1, ``time t''
Voltage at Vlay Vlb, VIC*V Za
+ V 2t+ , V 2c and currents iLa, il
b, ilc.

Sample 12aC+ l 2ha, l ZCb.

Differential current Σiac regarding line current in step S2.

Σiba, Σ1cb are obtained from equation (6).

In step S3, the magnitudes of the three differential currents are checked.

When it is determined in step S4 that all three of the absolute values of the differential currents are greater than the predetermined detection level ε1, the process proceeds to step S6. This is an excitation inrush state in which the two phases are magnetically saturated with opposite polarity, or an internal fault state in the two phases, so in order to identify the two phases, first in step S5, the find something It is determined that a differential current is generated in two phases corresponding to the differential current related to the line current with the maximum absolute value. In step S6, among the differential currents Σia and Σlb1 Σic regarding the phase currents, the differential current of one phase for which it is determined that no differential current is generated is set to zero, and the differential current of the remaining two phases is set to zero. The differential current is determined from the differential current regarding the line current using the relationship of equation (4). For example, in section (2) of FIG. 6 in the case of excitation inrush, each differential current regarding the phase current is obtained by equations (9) and (10) as described above.

In step S7, two of the differential currents related to the three line currents are
If it is determined that the absolute value of ε is greater than the detection level ε1, the process proceeds to step S6, otherwise the process proceeds to step S9.

The process proceeds to step S8 when one phase is in an excitation inrush state where it is magnetically saturated or when one phase is in an internal fault state. Therefore, in step S8, it is first determined that a differential current is generated in a phase related to both of the differential currents related to the two line currents whose absolute values exceed the detection level ε1. Among the differential currents Σia, Σi, . . . The piezoelectric current is determined from the differential current regarding the line current using the relationship of equation (4).

For example, in section (1) of FIG. 6 in the case of excitation inrush.

As mentioned above, the differential current regarding the phase current is (7).

It is obtained by equation (8).

Since the process proceeds to step S9 when the transformer is in steady operation, the differential current regarding the phase currents is set to zero for all three phases.

In step Szo, the phase current i2a of the triangular connected winding
, i2b, and 12c are obtained from equation (11).

After step Sll, K is a variable indicating each of the three phases, and K=1.2.3 represents phase a, phase A, and phase C, respectively. Step Sll.

The flow after identifying the phase in S12 is described in Japanese Patent Application Laid-Open No. 62-89.
Since it is the same as Publication No. 42, its contents will be briefly described below. For simplicity, the symbols for each phase have been omitted.

In step S1g, it is determined whether the absolute value of the difference current regarding the phase current is larger than a predetermined detection level ε2, and in step S
14, it is determined whether this is the first time.

At the first time, initial setting is performed in step S15, and at the second time or later, the process proceeds to step si8 to obtain the driving point admittance 3'lly 3'2Z. Letting to be the time at the first step when the absolute value of the differential current exceeds the detection level ε2, the driving point admittance at time t is determined by modifying equation (14) in step 51B.
19) can be obtained using the formula.

...(19) Current i z(t o) p i z( at time to
to) is stored during initialization in step 315. The voltage integration is performed by discretizing the trapezoidal side, for example. .

In step SIT, parallel admittance ylO,
3'20 is calculated using equation (15), and step sia
Determine its value at .

Next, a method for determining parallel admittance in steps Slδ to S21 will be explained. As mentioned above, 3'IO+ y2. at the time of excitation inrush. Since the value of O is almost a constant value, its approximate value is predicted in advance and the judgment values larger and smaller than this are memorized. For example, ylo
As a judgment value larger than the value at the time of excitation entry, αl,
βl is stored as a small value. Judgment formula (20)
to (23).

yio≦βl...(20
)y10≧αtechnique...(2
1) y20≦β2...(
22) y20≧α2...
(23) By executing the determination formulas (20) to (23), ylo and y20 are determined in step S δ, and when at least one of these determination formulas is satisfied, it is determined that it is outside the healthy range in step S19. It is determined that this is the case, and a determination flag is set in step Sx1. (20) to (23)
If none of the judgment expressions in the expressions are satisfied, step 819
In step s20, it is determined that the condition is within the healthy range, and the determination flag is reset in step s20. It should be noted that all of the determinations of equations (20) to (23) are not necessarily required to determine if the condition is outside the healthy range. All internal accidents can be determined to be outside the healthy range even if only some of the determination formulas are used.

In order to determine an internal accident without error and output continuous trip permission signals, the continuity of the determination flag is checked in steps 322 and S23.

When it is determined that there is continuity, an accident is determined in step S26, and a circuit breaker trip permission signal is output.

If not, a health determination is made in step S24.

The trip permission signal is a signal that trips the circuit breaker either alone or in combination with other fault detection elements.

Finally, in step S2 [1, calculation 2 for all three phases
It is checked whether the determination has been completed, and if it has not been completed, the process returns to step S12 to calculate the next phase.

Make a judgment. This concludes the explanation of the example calculation flow shown in FIG.

The driving point admittance may be used instead of the parallel admittance for determining an internal accident. In this case, there is no need to calculate the parallel admittance in the calculation flow example of FIG. Also, instead of these parallel admittances,
In the two-winding transformer circuit shown in FIG. 9, the parallel admittance currents ixo and izo can be calculated and used for fault determination. Furthermore, if a differential type admittance equation is used instead of the integral type admittance equation shown in equation (12), the differential value of the parallel admittance current can also be used for fault determination.

These various accident determination methods are detailed in Japanese Patent Laid-Open No. 62-8942.

Next, it will be shown that the present invention can be applied to a three-winding transformer.
In Fig. 10, 1 is a three-phase three-winding transformer 11a, ll
b, lle are star-connected primary windings, 12a, 12b
, 12c is a star-connected secondary winding, 13a, 13b,
13c shows a triangularly connected secondary winding. ixa, i
lh, ilc are primary line currents, 12a+ iZb+ i
zc is the secondary line current, i3a.

i3b, ia1: are tertiary phase currents, l 3 & Cr l
aba+l ach is the tertiary line current. For simplicity, it is assumed that the number of turns in each winding is equal. The differential current regarding the phase current is expressed by equation (24).

Σ1a=ita+i2a+i3a Σ1b=itb+i2b+iab...
(24) Σ1c=itc+izc+iac The differential current regarding the line current can be expressed by equation (25).

Σi ac ”(i ia i tc) + (i z
a-i 2C)+i 5ac=(ita-i x
c) + (i za -i 2c) + (i 3a -i
3C)=Σi＆−Σic Σiba”Σ11.−Σia ・・
・(25) Σ1cb = Σic - Σ11° The relationship between the differential current related to the line current shown in equation (25) and the differential current related to the phase current is 2 shown in equation (4).
The same is true for wire-wound transformers. Therefore, as in the case of a two-winding transformer, the differential currents related to the phase currents can be determined from the differential currents related to the line currents at the time of excitation rush or in the event of an internal fault. The phase current of the triangularly connected winding is obtained by equation (26), which is a modification of equation (24).

1aa=Σia −(ita+1za)i3b=Σi
b-(i ib+ 1211)...
(26) i3e=Σj,c (ile+12c) The method of calculating parallel admittance and determining an accident is the same as in the case of a two-winding transformer.

There are cases where the terminals of the triangularly connected windings cannot be drawn out of the tank of the transformer due to built-in diagonal windings, and the present invention can be applied to such cases as well.

For example, in the three-winding transformer shown in Fig. 10, if the terminal of the tertiary winding is not drawn out, simply i aac"i
It is sufficient to set aba=i3cb=O...(27).

〔Effect of the invention〕

According to the present invention, the phase current of a transformer having triangularly connected windings is determined by using a specific correlation between the differential current determined from the line current at each terminal of the transformer and the differential current determined from the phase current. This can be used to determine internal faults in the transformer based on the parallel admittance of the transformer, etc. Therefore, transformer protection can be performed with high precision and high speed.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of an embodiment of the present invention, Figure 2 is a calculation flow diagram of an embodiment of the present invention, Figure 3 is a route diagram of a single-phase two-winding transformer, and Figure 4 is a diagram of a single-phase two-winding transformer. Voltage and magnetic flux density at the time of excitation inrush of the transformer shown in 3 times. Figure 5 shows waveforms of differential current, etc. Figure 5 is a route map of a three-phase two-winding transformer Figure 6 shows waveforms of voltage, magnetic flux density, current, etc. at the time of excitation inrush of the transformer shown in Figure 5 Figure 7 is a diagram showing the current waveform at the time of an internal fault in the transformer shown in Figure 5, Figure 8 is a route diagram of a two-winding transformer, and Figure 9 is a diagram showing the transformer shown in Figure 8. Figure 10 is a route diagram of a three-phase, three-winding transformer. DESCRIPTION OF SYMBOLS 1... Transformer to be protected, 5... Input section, 6... Arithmetic processing section, 7... Storage section, 8... Output section, 9...
Coefficient setter, 41.42... Circuit breaker, 43.44...
・Trip coil of circuit breaker.

Claims (1)

[Claims] 1. In a method for protecting a three-phase connected transformer having a star-shaped connected winding and a triangular connected winding, each terminal wire current of the transformer is detected and the wire current is adjusted to protect the triangular connected winding. calculate the differential current corresponding to each terminal current, estimate the abnormal phase that caused the abnormality in the transformer according to the magnitude of the differential current, and based on the estimation, calculate the estimated phase current of each phase, calculate the admittance parameter that relates the estimated phase current and the terminal voltage of the triangularly connected winding, A method for protecting a transformer, characterized in that the abnormal phase is determined to be in an internal fault state based on the value of a parameter, and a command is issued to open a circuit breaker of the transformer.