JPH0435969B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field to Which the Invention Pertains] This invention provides a method for solving problems that occur when an accident occurs inside a transformer.
This accident is detected and the transformer is disconnected from the circuit.
A protective relay device that protects a transformer from burnout, comprising: 1
The difference between the secondary current and the current obtained by converting the secondary current to the primary side exceeds a predetermined value, and the ratio of the second harmonic component to the fundamental component included in this difference current is less than a predetermined value. Sometimes, it is determined that there is an internal fault in the transformer and a signal is output to disconnect the transformer from the circuit, and if the ratio of the second harmonic exceeds a predetermined value, the differential current is caused by the magnetizing inrush current rather than the internal fault. The present invention relates to a ratio differential relay for a transformer that determines that a fault has occurred and does not output a signal to prevent the transformer from being disconnected from the circuit.

[Prior art and its problems]

Figure 2 shows the basic circuit configuration when using a ratio differential relay to protect a transformer from internal accidents.
When the current transformer secondary winding currents of the three-phase current flowing from the primary side of the transformer 1, in which the primary side is connected to Y and the secondary side is connected to Δ, are respectively ia, ib, and ic, the current transformer The output currents of are respectively ia−ic, ib−ia, and ic−ib, and the current transformer output currents on the secondary side of the transformer are respectively
2 of the current transformer so that ia−ic, ib−ia, ic−ib.
If the Y winding side of the transformer is connected to Δ, and the Δ winding side is connected to Y, and the current transformation ratio is adjusted according to the transformer's transformation ratio, then during normal operation of the transformer, transformer 1
Since the current flowing into the ratio differential relay 2 from the current transformer 3 on the next side and the current flowing out from this ratio differential relay toward the current transformer 4 on the secondary side of the transformer are equal in magnitude, Operating coil 2a of ratio differential relay 2
No current flows and the relay does not operate. However, if a short circuit occurs inside the transformer, for example between layers of the same winding, the current flows out from the current transformer 3 and the current flows from the ratio differential relay (hereinafter referred to as relay) 2 towards the current transformer 4. A current equal to this difference flows into the operating coil 2a, operating the relay 2, and interrupting the circuit breakers 5 and 6 to disconnect the transformer 1 from the circuit.

However, on the other hand, if you try to keep the transformer 1 alive by closing the circuit breaker 5, the closing phase (the position on the power supply voltage waveform at the point when current begins to flow through the transformer)
Depending on the magnitude of the residual magnetic flux in the transformer core, an excitation current, that is, an excitation closing current several times the rated negative current flows in the primary side of the transformer. Since this current is an exciting current, it does not flow out from the secondary side of the transformer.
The current flows into the operating coil of the relay as a difference current between the first and second order and attempts to operate the relay. Figure 3 shows the relationship between the current and voltage observed on the primary side of the transformer and the magnetic flux in the transformer core at this time. In the figure, e is the power supply voltage waveform, Em is the power supply voltage peak value, and i
is the excitation inrush current waveform, φr is the residual magnetic flux in the transformer core at the time of interrupting inrush, φs is the saturation magnetic flux in the core, φt is the transient magnetic flux waveform, φ is the steady magnetic flux waveform, φ
m is the steady magnetic flux peak value. As can be seen from the figure, when the point at which the excitation current begins to flow coincides with the zero point of the power supply voltage waveform, the magnetic flux changes by 2φm during 1/2 cycle after turning on. If residual magnetic flux φr exists in the transformer core at the time of turning on, the magnetic flux will be φr + 2φ at 1/2 cycle after turning on.
m, which far exceeds the saturation magnetic flux φs of the iron core,
This results in a large excitation inrush current. That is, the rush current suddenly increases from the point where the transient magnetic flux φt exceeds the saturation magnetic flux φs.

However, since the current flowing through the operating coil of the relay is an excitation current and not a fault current, the relay must maintain the transformer connected in the circuit without operating. For this reason, conventionally, a time-limiting method has been used that locks the operation of the relay for a time until the magnetizing inrush current attenuates and the difference current between the primary and secondary of the transformer becomes less than the sensitivity of the relay, and a Since this includes waves, harmonic suppression methods have been adopted in which the harmonic components are passed through a suppression coil installed in the relay to suppress the movement of the relay contacts that are about to be driven by the operating coil. .
The principle of harmonic suppression is shown in FIG. In the figure, id is the difference current between transformer 1 and secondary, and 10 is the difference current
A fundamental wave passing filter that passes the fundamental wave included in the ID, 11 a fundamental wave blocking filter that blocks the fundamental wave, 12 a full wave rectifier for all harmonics except the fundamental wave, and 13 rectified in the same direction. A suppression coil that tries to prevent the movement of the relay contacts when all harmonics pass through, 14 is the operation coil of the relay, and 15 is for backup protection for the relay contact control circuit configured using the above 10 to 14. is the overcurrent element.

At present, focusing on the fact that among the harmonic components contained in the excitation inrush current, there are many second harmonic components and few components of the third harmonic and above, a ratio differential relay with a second harmonic suppression method is being developed. Widely used. In this case, the second harmonic suppression ratio, that is, the percentage of the peak value of the second harmonic component of the fundamental wave component peak value at which the operation of the relay is suppressed can be determined based on the actual measurement results of the excitation inrush current in the past. Based on the results of computer simulation studies, a value of about 15% is generally adopted.

Recently, with the increase in capacity, noise reduction, and efficiency of transformers, the iron core material has changed to cold-rolled silicon steel sheets.
High magnetic permeability grain-oriented silicon steel plate is used. High magnetic permeability silicon steel sheets tend to have larger excitation inrush currents and lower second harmonic component content in the inrush currents than conventional cold-rolled silicon steel sheets. Under extremely severe overexcitation conditions (110% of rated voltage) during use of a transformer, the content of second harmonic components tends to further decrease by several percentage points. For this reason, if the second harmonic suppression ratio is set to about 15% as in the past, the suppressing power is insufficient and malfunctions are frequently observed.

Therefore, measures such as (1) turning on low excitation and (2) changing the second harmonic suppression ratio are taken to prevent malfunctions even under severe overexcitation conditions. Here, low excitation closing means that the closing operation is performed using a winding tap that minimizes the magnetic flux density in the iron core for the same power supply voltage, and the maximum magnetic flux density generated in the iron core is The time width exceeding the saturation magnetic flux density is reduced to prevent the generation of excitation inrush current that causes the second harmonic component to fall below the conventional second harmonic suppression ratio (15%), and Changing the second harmonic suppression ratio means changing the design of the suppression coil to lower the suppression ratio by several percent.

However, when low excitation is applied, the voltage appearing on the secondary side of the transformer is different from the normal operating voltage, so in reality this countermeasure may not be possible, and even if it is possible, this measure may not be possible. There is a drawback in that there must be no omissions in the communication of information that such operations will be carried out and the precautions taken when operating the variable station associated with these operations, and that it cannot be a universal countermeasure. Furthermore, since the change in the suppression ratio is a change that strengthens the suppression force of the suppression coil, the distortion current caused by an internal fault in the transformer may cause the suppression to occur, and the relay may not operate properly. This distortion current at the time of an internal fault in a transformer is caused by the minute harmonic components contained in the internal fault current, for example, the short circuit current caused by a short circuit between layers within the same winding. This is caused by the fact that the system has a large capacitance, for example, when it includes underground cables, and the malfunction of the relay due to this distorted current is
This is an extremely serious problem from the point of view of transformer protection. Further, even if the relay does not become inoperable, the operation of the relay tends to be delayed.

In recent digital ratio differential relays, the second harmonic suppression ratio is changed by changing the constant K in FIG. That is, the excitation inrush current i ₁ (actually the transformer primary current) as shown in FIG. 6 is changed as shown in FIG.
It is input in parallel to the fundamental wave extraction filter 21 and the second harmonic filter 22 via the current transformer 20, and the levels of the respective peak values of the fundamental wave and the second harmonic extracted in each filter are analogized. Digitally detected by a level detector 23 using a goo digital converter, and the detected digital amount
X ₂ /X _{1 is} calculated using X ₁ and Prevent signal output to. Therefore, the operation or non-operation of the relay is determined only by whether the ratio of the second harmonic component to the fundamental wave component is smaller or larger than the preset second harmonic suppression ratio K. As there is no need for a suppression coil to determine whether the relay operates or not, there is no time delay in relay operation in the event of an internal fault, and in this respect, the drawbacks of conventional analog ratio differential relays have been eliminated. become. However, if you lower the second harmonic suppression ratio K to avoid malfunctions during excitation inrush current, depending on how this suppression ratio is set, the second harmonics contained in the distorted waveform during an internal fault may component becomes larger than the suppression ratio K, and the relay may become inoperable in the event of an internal accident.

[Purpose of the invention]

The present invention aims to improve the suppression ratio over a wide range without causing delay in operation time or non-operation in the event of an internal fault in the transformer, which occurs when the second harmonic suppression ratio is lowered. The purpose of the present invention is to provide a ratio differential relay device that can freely change the ratio.

[Key points of the invention]

First, the principle of this invention will be explained.

The phenomenon of magnetizing inrush current is that when the magnetic flux in the transformer core is saturated, a large amount of magnetic flux in the core no longer changes over time, and therefore, after saturation, the windings wound around the core act as an air-core reactor. By this,
This is a phenomenon in which the excitation current rapidly increases after the saturation point. On the other hand, when the magnetic flux in the transformer core is not saturated, the large amount of magnetic flux in the core changes over time and the transformer winding exhibits a significantly large reactance.
The magnitude of the excitation current is substantially close to zero. 7th
The figure shows the results of harmonic analysis of this magnetizing inrush current. The figure shows the analysis results for the second harmonic. In Figure 7, the horizontal axis represents the time A during which the excitation inrush current is generated in terms of electrical angle during one cycle, as shown in Figure 9. It is something. This time A is substantially the same as the time width A ₁ during which the magnetic flux density waveform shown in FIG. 8 exceeds the saturation magnetic flux density Bs. As can be seen from Figure 7, the longer the inrush current generation time A, that is, the wider the saturation time width _A1 of the magnetic flux, the lower the second harmonic content, and the narrower the saturation time width _A1 , the lower the second harmonic content. The content rate of will increase. In other words, as the inrush current increases its generation time A and peak value and forms a large loop, the content of the second harmonic decreases, and conversely, when the inrush current forms a small loop, the content of the second harmonic increases. In the end, the content of the second harmonic is determined by the ratio of A and B shown in FIG.

On the other hand, in general, in the current waveform that is monitored by a protective relay, if we look at whether there is a current that has a section where the current value is substantially zero, such as an excitation inrush current, It will look like the figure. FIG. 10 shows the waveform of a fault current, such as a short circuit current, occurring in the system. Depending on the voltage phase at the time the short circuit occurs, this waveform may include an attenuating value current and become an asymmetric waveform, but it will not produce a zero value section of the current, and usually the short circuit occurs at the zero value of the voltage. Since there is very little to do, the waveform of the short circuit current will be symmetrical as shown in the figure. Figure 11 shows the load current, and although this load current waveform may be distorted when a relatively large nonlinear load is applied to a small capacity transformer, In the case of a substation, a symmetrical waveform is shown as shown in the figure. However, in neither case does a zero-value section of the current occur. FIG. 12 shows the waveform of a short-circuit current when an internal fault occurs in a transformer, such as a short circuit between layers within the same winding. As already mentioned, the presence of large ground capacitances in the external system connected to the transformer, such as underground cables, accentuates harmonics in the short circuit current, which causes may cause distortion in the current waveform. However, in this case as well, no current zero value section occurs. FIG. 13 shows the waveform of the excitation inrush current.

In this way, among the currents that have the operating frequency of the system, the only current that has a clear zero value section is the magnetizing inrush current, and therefore, by detecting the width of this zero value section, it can be determined that the current is the magnetizing inrush current. can be determined. Furthermore, since there is a relationship between the width of this zero value section and the second harmonic content as shown in FIG. The current difference between the side current and the secondary side current is sampled at certain minute time intervals, for example, at a time interval of several tenths of one cycle of the operating frequency, and the sampled currents are compared one after another in chronological order. The number of samplings in which the line difference is continuously substantially zero is counted, and when the counted number of samplings exceeds a predetermined value, the operation of the relay is suppressed or the output signal is blocked. It aims to achieve the objectives of

[Embodiments of the invention]

FIG. 1 shows an embodiment of the circuit configuration of a transformer ratio differential relay device constructed based on the present invention. When a difference occurs between the transformer primary and secondary currents taken out by the current transformers 3 and 4, this difference current is transferred to the analog-to-digital conversion circuit via the operating coil 2a. (hereinafter referred to as an A/D conversion circuit) 31. This A/D converter circuit is controlled by the microcomputer 32 in the subsequent stage to sample the difference current at fixed minute time intervals, for example, at intervals of 6 degrees in electrical angle, and the sampled current value is digitized and transmitted to the microcomputer 32 in the subsequent stage. input into the computer 32. Inside the microcomputer 32, the input current values are compared one after another to find a difference, and while this difference is less than a predetermined value, a counter built into the microcomputer is continuously incremented. When this difference exceeds a predetermined value, the count up to that point is erased, and the counter is incremented from the time when the difference becomes less than the predetermined value again. In this way, when the count becomes equal to the predetermined value, that is, when the predetermined value of time width B in FIG. 9 is obtained, the microcomputer 3
2 outputs a signal that blocks the output signal of the relay device. This signal is guided, for example, to the gate of the thyristor 33, turns on the thyristor, and excites the operating coil 34a of the auxiliary switch 34 connected in series with the thyristor, thereby forming the second contact in the relay device of the present invention. The auxiliary contact 34b is opened. Therefore, even if the difference between the primary and secondary currents of the transformer exceeds a predetermined value and the first contact 35a is closed by the operating coil 2a, the second contact 35a is closed by the operating coil 2a. The time is set so that the operating time of the contact 34b, that is, the time from when the opening command is given to the second contact until the contact is opened, is longer by at least one cycle of the operating frequency. The contact 34b has already been opened, and therefore no tripping current flows through the tripping coil 36 of the circuit breaker, keeping the circuit breaker in the closed state.

In this way, by replacing the second harmonic suppression ratio in a ratio differential relay with the time width of the zero value section of the magnetizing inrush current and detecting this time width, it is possible to block the output signal of the relay. In addition, since the suppression coil operated by the harmonic current can be omitted, when the second harmonic suppression ratio is lowered, the relay operating time may be delayed or the relay may not operate in the event of an internal fault in the transformer. Therefore, it becomes possible to freely set the second harmonic suppression ratio without affecting the operation of the relay in the event of an internal accident.

In the above embodiment, the contact of the auxiliary switch, that is, the auxiliary contact is used as the second contact 34b that blocks the output signal of the relay, but instead of using this auxiliary contact, the operating coil 34a is Even if it acts as a coil, the operation of the relay can be suppressed. In this case, the suppressing force of the suppressing coil is not the difference current between the primary and secondary currents of the transformer, but the control power buses P and H of the substation.
Since it is obtained by using a constant DC current obtained from the current, it is possible to always reliably suppress the operation without being affected by the waveform of the differential current.

Note that in the above embodiment, the set second
Harmonic suppression ratio, that is, the time width B of the current zero value section
If is small, the time change in the current value is small near the peak value of the excitation inrush current. Therefore, the digital values input to the microcomputer 32 are compared one after another to determine the difference, and this difference is less than or equal to a predetermined value. If only detecting whether or not a predetermined time interval has been reached by continuously incrementing a counter during the interval, there is a risk of misunderstanding that a predetermined current zero value interval width has been obtained. Therefore, if you want to set the time width B small, the absolute value of the digital quantity input to the microcomputer should be, for example, 30% of the rated load current of the transformer.
% or less, the increment of the counter is determined by the logical sum of the result of determining the absolute value of the digital quantity and the determination result obtained from the comparison of the digital quantity, that is, the difference calculation. By doing so, it becomes possible to set the second harmonic suppression ratio in an extremely wide range.

In addition, since a plurality of contacts of the auxiliary switch, that is, the second contacts 34b shown in FIG. 1 can be provided as shown in the figure, these contacts can be connected to overcurrent relays or If the system is a directly grounded system, if it is installed in series with the contact of the protective relay installed on the neutral point side of the transformer, it will be possible to avoid malfunctions at the time of excitation inrush current, and smooth operation of the variable station will be possible. It is possible to contribute to the development of

〔Effect of the invention〕

As described above, according to the present invention, (1) by replacing the second harmonic suppression ratio with the zero value interval interval of the magnetizing inrush current and detecting whether this width is greater than or equal to a predetermined value; , it is determined whether the difference current between the primary side current and the secondary side current of the transformer obtained through the current transformer is caused by the excitation inrush current, and harmonic current as in the conventional case is determined. Since we omitted the suppression coil operated by
Even if the zero value interval width is varied over a wide range, the relay will not operate delayed or fail due to the distorted current contained in the fault current in the event of an internal fault in the transformer.
It becomes possible to always normally handle a transformer internal fault regardless of the second harmonic suppression ratio.

(2) In the waveform of the magnetizing inrush current obtained through the current transformer, the zero point may shift as shown in Figure 14 due to the saturation phenomenon of the current transformer core, but it is difficult to detect the current zero value section. Since the difference between the current values obtained by sibling is used, correct detection is possible regardless of this shift of the zero point.

(3) Determining whether the difference between the primary and secondary currents of the transformer obtained through the current transformer is due to the magnetizing inrush current of the transformer This is done by detecting the current zero value interval width within one cycle of the frequency, so the judgment time is 1
20 when the cycle or operating frequency is 50Hz
ms, at 60Hz, it is a short time of 16.7ms,
This is significantly shorter than about 30 ms in the conventional system, so the lower limit of allowable relay operation time in the event of an internal accident can be further shortened and the protection effect of the transformer can be enhanced.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram according to an embodiment of a ratio differential relay device for a transformer constructed based on the present invention, and FIG. 2 illustrates the operating principle of a conventional ratio differential relay including a transformer. Overall circuit diagram, Figure 3 is transformer 1
An explanatory diagram showing the relationship between the current and voltage on the primary side of the transformer and the magnetic flux in the transformer core when the magnetizing inrush current flows into the next side. Figure 5 is a functional block diagram illustrating the principle of harmonic suppression that suppresses the operation of relays. Waveform diagram of magnetizing inrush current input to the relay shown in Figure 7 to 9
The figures are diagrams and waveform diagrams for explaining the principle of the present invention, and Fig. 7 shows the relationship between the time during one cycle in which the excitation inrush current is generated and the content rate of the second harmonic component. 8 is a waveform diagram showing the relationship between the magnitude of the magnetic flux in the core to explain the time during which the magnetizing inrush current is generated or the width of the zero value section, and FIG. 9 is a waveform diagram showing the relationship between the magnitude of the magnetic flux in the core and FIG. 3 is a waveform diagram showing definitions of time A during one cycle and zero value interval width B of current. 10 to 13 are waveform diagrams showing the types of operating frequency currents flowing through the transformer, and FIG. 14 is a waveform diagram showing the shift of the zero point of the current transformer secondary current when the current transformer core is saturated. 1...Transformer, 2...Ratio differential relay, 10...
... Fundamental wave passing filter, 11 ... Fundamental wave blocking filter, 13 ... Suppression coil, 14 ... Operating coil, 21 ... Fundamental wave extraction filter, 22 ... Second
Harmonic extraction filter, 31...Analog-digital conversion circuit, 32...Microcomputer, 3
4...Auxiliary switch, 34b...Second contact, 35
a...First contact, 100...Ratio differential relay device.

Claims (1)

[Claims] 1 The difference between the primary current and the current obtained by converting the secondary current to the primary side exceeds a predetermined value, and the ratio of the second harmonic component to the fundamental component included in this difference current exceeds a predetermined value. In the following cases, it is determined that there is an internal fault in the transformer, and a signal is output to disconnect the transformer from the circuit. If the ratio of the second harmonic exceeds a predetermined value, the difference current is caused by an excitation inrush current rather than an internal fault. A ratio differential relay for a transformer that determines that a current difference has occurred and does not output a signal to prevent the transformer from being disconnected from the circuit; and a second contact that is opened when the ratio of the second harmonic component included in the difference current to the fundamental wave component exceeds a predetermined value, in series. The operating time of the first contact is set to be longer than the time from when the opening command is given to the second contact until it is opened, by one cycle or more of the operating frequency of the circuit, and The opening causes the difference current waveform to be one of the operating frequency of the circuit.
The sampled currents are sampled at a certain minute time interval that is sufficiently smaller than the cycle, and the number of samplings in which the difference is continuously equal to or less than a predetermined value is counted while comparing the sampled current one after another in time series. 1. A transformer ratio differential relay device, characterized in that the relay is operated by a command that occurs only when the number exceeds a predetermined value.