JPH07110102B2 - Protection / control device with disconnection detection function - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a device that detects a zero-phase voltage of a power system and protects and controls the power system based on the detected value, and particularly, the power system. With a disconnection detection function that has the function of discriminating between the ground fault and the disconnection of the input circuit.
Regarding the control device.

(Prior Art) Conventionally, as shown in FIG. 8, the protection / control for the ground fault of the power system, as shown in FIG. 8, detects the phase voltage of each three-phase from the tertiary circuit of the transformer PT for an instrument, and detects it. The values are vector-synthesized to calculate the zero-phase voltage, and the digital relay or the like is operated based on the zero-phase voltage.
In other words, when the combined value of each phase voltage vector exceeds a certain value, it is judged that an abnormality such as a ground fault has occurred in the system, the relay system is activated, or an alarm is issued to protect the system.・ I was controlling.

Here, this kind of protection / control device is aimed at maintenance-free, and it is generally equipped with an "always monitoring function" for constantly detecting the presence or absence of an abnormality due to a disconnection of the input circuit. Has become.

In FIG. 8, the phase voltages of the input circuit of the protection / control device composed of digital relays are shown as _a , _b ,
_c, when the zero-phase voltage is _{0, 0,} 3 ₀ = is represented by _a + _b + _c, in a normal operating condition, since it is _a + _b + _c = 0, ₀ 0, strains ₀ does not occur unless a ground fault or the like occurs. Therefore, in the past, protection /
It was difficult to constantly monitor the input circuit of the control device for abnormalities.

Therefore, an inspection power supply is separately prepared, and the input circuit is disconnected from the power system and then switched to this power supply at regular intervals to inspect whether or not the circuit is abnormal.

However, this type of automatic inspection device generally has a complicated circuit configuration, which increases the cost of the device itself, and it is necessary to lock this circuit and disconnect it from the power system during inspection of the input circuit of the protection / control device. Therefore, it was impossible to protect and control the system from a ground fault during the inspection period.
Furthermore, since the inspection by the automatic inspection device is usually performed every 1 to 10 days for a long period of time, once an abnormality occurs in the input circuit, the abnormality can be detected until the next inspection. However, the input circuit remains defective, and it becomes impossible to detect the ground fault that occurred during this period, which is an incomplete protection of the power system.

Against this background, the zero-phase voltage generated in the grid is not taken from the tertiary circuit of the transformer PT for the instrument, but rather the phase voltage _a ,
_A method has been proposed and adopted in which _b and _c are separately taken, these phase voltages are combined to derive 3 ₀ , and this is ⅓ to extract the zero phase voltage ₀ . According to this method, since each phase voltage _a , _b , _{c of the system} is taken in from the secondary circuit of the transformer PT for instrumentation, the input voltage of the protection / control device is always rated voltage (normally ) Is being applied.

Then, it becomes possible to constantly monitor the abnormality of the input circuit by a method such as comparing the values of these respective phase voltages, and as a result, the zero-phase circuit (note that in the case of carrying out the arithmetic processing of the microprocessor, It does not exist as an independent circuit), but will always be monitored.

In the protection / control device having such a configuration, the zero-phase voltage input circuit is simplified or unnecessary, so that the hardware is simplified and the automatic inspection device is also unnecessary. .

(Problems to be Solved by the Invention) However, in the case where this system is adopted, if a 1-wire or 2-wire disconnection failure occurs on the secondary side of the instrument transformer PT, the input circuit of the protection / control device Zero phase voltage ₀ is detected at. As a result, there is an inconvenience that the protection / control device determines that a ground fault has occurred in the system, and the protection relay outputs a trip command.

Here, when a one-line complete ground fault occurs in the system, the zero-phase voltage V ₀ generated in the system has a value substantially equal to the power supply voltage Ea, so the zero-phase voltage v ₀ generated in the protection / control device is 2 Rated voltage of next winding On the other hand, the zero-phase voltage ₀ ′ when there is a one-wire or two-wire disconnection failure on the secondary side of the instrument transformer PT is
When the zero-phase voltage of 1-wire full ground fault and _{0, 0} '= _0/3 is expressed in 33% of the zero-phase voltage of 1-wire full ground fault in the secondary circuit of the instrument transformer PT ₀ Zero-phase voltage is generated.

When the secondary circuit has a line voltage load (Δ load), the zero-phase voltage ₀ ′ is as shown in FIG.
Healthy phase phase) (b, since the voltage of the c-phase) is applied goes around through the _{load, v 0 '> v 0/} 3 , and the more than 32% of the zero-phase voltage v ₀ 1 line complete ground fault magnitude It becomes

In general, the ground fault that occurs in the system is more than 30% (a ground fault in which 30% of the zero-phase voltage V _{0 when} one line is completely grounded occurs), so such a ground fault occurs in the system. In this case, the protection / control device detects the zero-phase voltage _{0 of the} system,
The ground fault protection or control function will be activated.

However, as described above, instrument transformer secondary circuit is more than 30% of the zero-phase voltage secondary circuit zero-phase voltage v ₀ 1 line complete ground fault even when disconnected v of PT ₀ 'is generated Therefore, the protection relay outputs the trip command and shuts off the system even though no ground fault has occurred in the system.

As described above, in the conventional protection / control device that detects the zero-phase voltage by synthesizing the phase voltages by the instrument transformer, there is a disadvantage that the instrument transformer malfunctions when the secondary circuit is disconnected.

The present invention has been proposed in order to solve the above problems, and an object of the present invention is to provide each phase before and after generation of a zero-phase voltage when a zero-phase voltage is generated in a secondary circuit of an instrument transformer. A disconnection detection function that immediately discriminates from the voltage ratio whether there is a ground fault in the system or a disconnection failure in the secondary circuit to prevent malfunctions during disconnection and to realize highly reliable protection and control functions. It is to provide a protection and control device.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides that a zero-phase voltage is represented by 1/3 of a combined value of each phase voltage, and an input circuit of a protection / control device. This was done by paying attention to the fact that the zero-phase voltage generated in the secondary circuit of the transformer for an instrument when the wire is disconnected is smaller than the zero-phase voltage generated when the system is grounded.

That is, the present invention, means for detecting the occurrence of zero-phase voltage,
Means for storing the magnitude of each phase voltage, means for calculating the ratio of the magnitude of the phase voltage after the generation of the zero phase voltage to the magnitude of the phase voltage before the generation of the zero phase voltage in each phase, and the setting of these ratios Outputting a signal that locks the protection / control function by considering the disconnection of the secondary circuit of the instrument transformer that constitutes the input circuit when both of the means for comparing with the value and each ratio are smaller than the set value And means for doing so.

(Operation) In the present invention, the voltage for each phase of the system is always taken in by the transformer for the instrument, and the magnitudes of these phase voltages are sequentially stored in the storage means. When the occurrence of the zero-phase voltage is detected, the ratio between the phase voltage before the zero-phase voltage generation and the phase voltage after the zero-phase voltage generation stored in the storage means is calculated.

Then, these ratios are compared with preset values by a comparison means, and if any of the ratios exceeds the set value, it is considered that a ground fault has occurred in the system. If none of the ratios exceeds the set value, it is regarded as a disconnection of the secondary circuit of the instrument transformer, and a signal that locks the function of the protection / control device is output to prevent malfunctions during disconnection. To do.

(Example) An example of the present invention will be described below with reference to the drawings.

First, FIG. 1 is a functional block diagram showing the basic configuration of this embodiment. In the figure, 1 is a detection means connected to the secondary side of an instrument transformer (not shown) to detect the presence or absence of zero-phase voltage generation, 2 is a sample hold circuit (SH circuit), and 3 is an analog / digital conversion circuit. (A / D converter circuit) 4, storage means for inputting each phase voltage as a digital signal and sequentially storing these voltage values, and 5 for the zero phase in each phase when the occurrence of the zero phase voltage is detected. Computing means for computing the ratio of the magnitude of the phase voltage before the voltage generation to the magnitude of the phase voltage after the zero phase voltage generation, 6 is a comparing means for comparing each of these ratios with a preset set value k, Reference numeral 7 is a lock signal output means for outputting a signal for locking the protection / control function according to the comparison result.

Next, this operation will be described. First, each secondary side phase voltage of the instrument transformer is sequentially stored as a digital signal in the storage means 4 via the SH circuit 2 and the A / D conversion circuit 3 at a constant cycle.

Now, when the generation of the zero-phase voltage is detected by the detection means 1, the detection means 1 outputs a calculation command to the calculation means 5. The calculating means 5 reads out each phase voltage value before generation of the zero-phase voltage and each phase voltage value after generation of the zero-phase voltage from the storage means 4, and for each phase (voltage value after generation of zero-phase voltage) / (Voltage value before generation of zero-phase voltage) is calculated, and the ratio of these is output to the comparison means 6. The comparing means 6 compares these ratios with a set value k, and when any of the ratios for each phase is smaller than the set value k, it is considered that a disconnection failure has occurred in the secondary circuit of the instrument transformer. The lock signal output means 7 outputs a signal for locking the protection / control function.

Maximum FIG. 2, in this embodiment, the phase voltage v of the secondary circuit of the instrument transformer (v _a, v _b or v _c), the zero-phase voltage v _0, v of ₀ phase voltages after the occurrence 3 is a timing chart showing a ratio v _p ′ / v _{p of} the voltage v _p ′ of the above and the phase voltage v _p before generation, and a lock signal.

In the figure, it is assumed that a zero-phase voltage is generated at time t ₀ .
At this time, the calculation means 5 calculates the ratio v _p ′ / v _p based on the values of v _p ′ and v _p in each phase stored in the storage means 4, and then the comparison means 6 calculates the above ratio. Set value k
Compare with.

When any one of the phase voltages is v _p ′ / v _p > k, it is determined that a ground fault has occurred in the system, and the lock signal output means 7 does not output the lock signal (see FIG. 2). (I)),
When each of the phase voltages is v _p ′ / v _p <k, it is determined that the secondary circuit of the instrument transformer is broken, and a lock signal is output ((b) in the same figure).

Hereinafter, a method of determining the set value k will be described in detail. Originally, the judgment of whether the ground fault occurred in the system or the disconnection failure occurred in the secondary circuit of the instrument transformer was made when the magnitude of any of the phase voltages after the generation of the zero-phase voltage exceeded a certain value. It may be regarded as a system ground fault, and when it does not exceed the predetermined value, it may be considered as a disconnection fault. However, the above-mentioned constant value cannot be determined in consideration of an error such as a voltage fluctuation.

Therefore, in order to avoid an error due to voltage fluctuation, the present invention considers this value to be a ground fault in the system when any of the ratios of the phase voltages after and before the generation of the zero-phase voltage exceeds a predetermined value, and determines this value. If it does not exceed the limit, it is considered as a disconnection failure of the secondary circuit of the instrument transformer and the protection / control function is locked.

Below, the maximum phase voltage at the time of system ground fault is calculated, the maximum phase voltage at the time of the secondary circuit disconnection is calculated, the voltage value K ₁ that does not consider the error due to the voltage fluctuation is calculated, and the error due to the voltage fluctuation is considered. A value is calculated and this value is set as a set value k.

In the order of, the method of determining the set value will be described with reference to the drawings.

《Calculation of maximum phase voltage during system ground fault》 There are 1-line ground fault and 2-line ground fault in the system ground fault, and 1-line disconnect fault in the secondary circuit of meter transformer. There is a 2 wire disconnection failure.

<About 1-line ground fault> First, calculate each phase voltage on the secondary side of the transformer for instrument in case of 1-line ground fault of the system.

First, FIG. 3 (a) is a system diagram when a 1-line ground fault occurs in the system, and FIG. 3 (b) shows an equivalent circuit when a 1-line ground fault occurs in the a phase. .

In the figure (a), G is a generator, T is a transformer, R _N is the ground resistance of the transformer T, m is the point where the instrument transformer is grounded, and Z _A ′ is the power source side from the m point. Line impedance,
Z _A is the line impedance on the load side from the point m, point f is the ground fault accident point, and _Ra is the fault point resistance that occurs during _a ground fault.

In addition, in the figure (b), Ea is a transformer T as a power source.
Phase voltage, ▲ ^′ ₀ ▼ _A , ▲ ^′ ₁ ▼ _A , ▲ ^′ ₂ ▼ _A are the zero-phase, positive-phase, and negative-phase impedances on the power supply side from the point m, and _0A , _1A , _2A are points m Zero-phase, positive-phase, and negative-phase impedances on the load side, _0f , _1f , and _2f are zero-phase voltage, positive-phase voltage, negative-phase voltage at point f, ₀ , ₁ , and
₂ indicates the zero-phase voltage, the positive-phase voltage, and the negative-phase voltage at point m, respectively.

Generally, in a power system, R _N , R _a >> Z _0A , ▲ Z ^′ ₀ ▼ _A , Z _1A , ▲ Z ^′ ₁ ▼ _A , Z _2A ,
▲ Z ^' ₂ ▼ _A is established.

In the case of 1-line ground fault, the following relation holds between ₀ , ₁ , and ₂ . ₀ = ₁ = _{2 0, 1, 2} is represented by ^{_{0 = 3R n + ▲ '0}} ▼ A + 0A ≒ 3R N 1 = ▲' 1 ▼ A + 1A 2 = ▲ '2 ▼ A + 2A, 0 _{is, 0 = a / (0 +} 1 + 2 + 3R a) ≒ a / (3R N + 3R a) = a / {3R N 1 + k a)} ( where, a _{k a = R a / R N} ).

In order to obtain each phase voltage at the relay installation point when there is a one-wire ground fault, first find ₀ , ₁ , and ₂ at the relay installation point.

As apparent from FIG. 3 (b), _{0, 0} = _0f + but _0A is _{0 ≒ 0f + 0A a / 3R} N, 0f = - (3R N + ▲ '0 ▼ A + 0A) 0 = - because it is _{a / (1 + k a)} , 0 = - a _{a / (1 + k a)} . _{1, 1} = _1f + _1A · ₁ is a _{≒ 1f, 1f = a - (} 1A + 1A) 1 because it is ≒ _a, a ₁ = _a.

Also, ₂ is a _{2 = 2f = 2A 2 ≒ 2f} , 2f = - for _(2A + _2A) is a ₂ ≒ 0, a ₂ = 0.

Therefore, each phase potential _a , _b , _{c at the} relay installation point
Is And k _a = 0 when there is _a complete ground fault, so _a = 0 Becomes

In the case of 30% ground fault, _a = -0.3 _a + _a = 0.7 _a Becomes

In addition, Fig. 3 (c) is a vector diagram at the time of 30% ground fault,
At the time of 1-wire ground fault, the ground fault phase voltage is always reduced, and the other 2
The phase voltage rises.

As is clear from the figure, the secondary side phase voltage of the instrument transformer is v _a = 44.45 [V], v _b = v when the a-phase 1-line 30% ground fault occurs.
_c = 74.87 [V].

<Two-wire ground fault> Next, the voltage of each phase on the secondary side of the transformer for an instrument at the time of two-wire ground fault is calculated.

First, FIG. 4 (a) shows a system diagram when a two-wire ground fault occurs in the system, and FIG. 4 (b) shows an equivalent circuit when two-line ground faults occur in the b-phase and the c-phase. There is.

At the time of a two-wire ground fault, the following relationship is established between the zero-phase voltage _0f , the positive-phase voltage _1f , and the negative-phase voltage _{2f at} the ground fault point. _0f = _1f = _{2f For} zero-phase impedance ₀ , positive-phase impedance ₁ and negative-phase impedance ₂ of the system, as in the case of 1-wire ground fault, R _N , R _a >> Z _0A , ▲ Z ^' ₀ ▼ _A , Z _{1 A} , ▲ Z ^′ ₁ ▼ _A , Z _{2 A} , ▲
Z _^'2 ▼ _A is satisfied.

In the case of 2-wire complete ground fault, positive phase current ₁ is Here, ₁ When _{≒ 2, 1 = a / 2} 1 0 ≒ 0 2 ≒ - 1 = - a _a / ₍₁ + _2).

In order to obtain each phase voltage at the relay installation point when a two-wire ground fault occurs, first find ₀ , ₁ , and ₂ at the relay installation point. _{For 0} , ₀ = _0f + _0A · ₀ ≈ _0f , but _0f = − (▲ ^′ ₂ ▼ _A + _2A ) · ₂ = _a / 2, so ₀ = _a / 2.

Also, ₁ is ₁ = _1f + _1A · ₁ ≈ _1f + _{1A a} / 2 _{1 but 1f} = _a / 2, so Becomes

Furthermore, ₂ becomes _{2 = 2f + 2A · 2 ≒} 2f + 2A a / 2 1.

However, since _2f = _a / 2, _1A = _2A , Becomes

Therefore, each phase voltage _a , _b , _{c at the} relay installation point
Is _a = ₀ + ₁ + ₂ = 3 _a / 2 Becomes

It should be noted that FIG. 4C is a vector diagram when a two-line complete ground fault occurs.

The characteristics of the two-wire incomplete ground fault are the resistance at the fault point (arc resistance,
The phase voltage varies depending on how the tower pedestal resistance, etc. are applied, but the fault phase (b, c phase) voltage decreases in both the complete ground fault and incomplete ground fault. The voltage rises.

When a ground fault occurs with R _f = 0, R _1f = R _2f , and a zero-phase voltage equivalent to 30% of one-line complete ground fault occurs, the sound phase voltage _a is _a = 3 _a / 10 + _a = 13 _a / 10 Becomes

In this case, the secondary side phase voltage of the instrument transformer is v _a = 82.6
[V], v _b = v _c = 12.7 [V].

At this time, the phase ψ ₀ of the zero-phase voltage _{0 is} the phase ψ of the phase voltage V _a .
Although become almost the same phase as _a, phase ψ ₀ is in a different spot out-of-phase ground fault
And ψ _a cause _a phase difference. In this case, the secondary side phase voltage of the instrument transformer will be v _a <82.6 [V], but in the actual system, it will almost never drop to the voltage rise value (74.87 [V]) at the time of 1-line ground fault. Absent.

"Calculation of the maximum phase voltage in the disconnection" Next, each phase voltage _a of the secondary circuit of the instrument transformer when disconnection failure ', _b', and calculates the _c '.

Here, Fig. 5 shows the protection and protection of digital relays.
The input circuit of the control device is roughly input. In the figure, PT is an instrument transformer, _a , _b and _c are digital relay loads, and _ab , _bc and _ca are line voltage loads ( Δ load), F ₁ and F ₂ are failure points (disconnection points).

<About 1-wire disconnection> FIG. 6 (a) is an equivalent circuit when the 1-wire (a phase) is disconnected,
In the figure, _0B , _1B , and _2B are zero-phase, positive-phase, and negative-phase impedances when the current side is viewed from the fault point, and _0L ,
_1L and _2L represent the zero-phase, positive-phase, and negative-phase impedances of the relay viewed from the fault point.

At the time of one wire disconnection, when the Δ load is not connected to the secondary side of the instrument transformer, that is, when the phase voltage load (Y load), the disconnection phase voltage _a is shown in the vector diagram of Fig. 6 (b). As shown, v _a ≈0. In addition, even when there is a Δ load, F _{2 in} FIG.
When an accident occurs at a point, the zero-phase voltage is not affected by the Δ load, and therefore can be treated in the same manner as when the Δ load is not connected.

On the other hand, if there is a Δ load or if an accident occurs at point F _{1 in} Fig. 5, the sound phase voltage also flows into the disconnection phase through the Δ load, so the disconnection phase voltage _a must have a certain value. become.

6 (c) is one showing an example of a vector diagram of the respective phase voltage when Δ load is present, _a is also changed by the ratio of ratios and Δ load and Y loads between Δ load mutual But the sound phase voltage Never exceeds.

Thus, v _a at the time of a phase disconnection is Becomes

However, if there is an LC resonance phenomenon between the Δ load and the Y load, the sound phase voltage may be exceeded, but in the actual system there is no such impedance distribution.

<About 2-wire disconnection> In the case of 2-wire (a, b phase) disconnection, there is only one healthy phase, and the basic tendency of each phase voltage is the same as when 1 wire is disconnected. FIG. 7 (a) is an equivalent circuit at the time of disconnection of two wires, FIG. 7 (b) is an example of a vector diagram when there is no Δ load, and FIG. 7 (c) is a vector diagram when there is a Δ load. It shows an example of.

Also in this case Always holds.

《Calculation of voltage value K ₁ when the error due to voltage fluctuation is not taken into account》 The secondary side phase voltage of the transformer for the instrument at the time of 1-line ground fault or 2-line ground fault From the maximum value and the maximum value of the secondary side phase voltage of the transformer for the instrument at the time of 1-wire disconnection or 2-wire disconnection failure, the system ground fault and the instrument transformer by the following phase voltage comparison method The secondary circuit disconnection failure can be discriminated.

That is, when the protection / control circuit detects an abnormality in the phase voltage, all of the phase voltages have a constant value K ₁ If it is smaller, it is regarded as a wire break, and if it is larger, it is regarded as a system ground fault.

Here, the value of K ₁ needs to be smaller than the maximum phase voltage v _m at the time of a ground fault in the system and larger than the maximum phase voltage v _m ′ at the time of disconnection. Therefore, 63.5 <K ₁ <74.9.

However, the voltage of the system has a voltage fluctuation of about ± 10%,
Considering the detection error of 6% (device error ± 5%, instrument transformer error 1%), the total error is (0.9 × 0.94-1) × 100 = -15.4% from (1.1 × 1.06-1) × it will take 100 = 16.6% of the range, setting the value of K ₁ as a fixed value is difficult.

For example, if K ₁ is set as the center value between 63.5 and 74.9 described above, that is, K ₁ = (63.5 + 74.9) /2=69.2 [V], the maximum phase voltage at the time of disconnection is the highest, 6
It may be detected as 3.5 × 1.1 × 1.06 = 74.01 [V], or the maximum phase voltage at the time of a ground fault may be detected as 74.9 × 0.9 × 0.94 = 63.3 [V]. You will get it.

<< Calculation of set value k considering error due to voltage fluctuation >> Therefore, in order to make the logic unaffected by the error (± 10%) due to voltage fluctuation of the power supply, each phase voltage after and immediately before the occurrence of zero-phase voltage Take the ratio of.

63.5 / 63.5 <k <74.9 / 63.5 That is, the value of k is set as 1.0 <k <1.18 (where k = K ₁ /63.5), and the ratio after the zero-phase voltage is generated and before it is generated is k. If the ratio is larger than k, it is considered as a disconnection failure of the secondary circuit of the instrument transformer, and if this ratio is larger than k, it is considered as a ground fault in the system.

By defining the range of k in this way, the error (10%) due to the voltage fluctuation of the system can be avoided, and only the detection error (± 6%) of the instrument needs to be considered. Can be set. The value of k is 1.0
The optimal value is 1.09, which is an intermediate value between 1.18 and
For example, it may be in the range of 1.06 to 1.11.

If the detection error of the equipment exceeds ± 5%, the ground fault sensitivity of 1-line ground fault may be restricted or disconnection may not be detected, but k is the initial setting value. Depending on whether it is larger or smaller than (for example, 0.09), the disconnection failure detection and the ground fault detection are weighted, and either of them can be prioritized.

(Effects of the Invention) As described above, according to the present invention, when a zero-phase voltage is generated in the secondary circuit of the instrument transformer, it is a ground fault of the system or a secondary circuit disconnection failure of the instrument transformer. Since it is possible to discriminate between them, it is possible to reliably prevent a malfunction at the time of disconnection and provide a highly reliable protection / control device.

In addition, the method of deriving the zero-phase voltage by synthesizing the input phase voltages can be applied, and the zero-phase input circuit can be eliminated, and the automatic inspection device is not required. The device can be supplied.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing an embodiment of the present invention, and FIG.
The figure shows the timing chart of the operation.
Is a system diagram for a system 1-line ground fault, FIG. 4B is also an equivalent circuit, FIG. 4C is a vector diagram, and FIG. 4A is a system diagram for system 2-line ground fault. (B) is the equivalent circuit,
The same figure (c) is also a vector diagram, FIG. 5 is an explanatory diagram of the input circuit of the protection and control device, and FIG. 6 (a) is the instrument transformer 2.
Equivalent circuit of the following circuit when one wire is broken, (b) and (c) in the same vector diagram, and (b) of FIG. 7 is equivalent circuit when two wires of the transformer secondary circuit for measuring instrument are broken. (B) and (C) are also vector diagrams, and FIGS. 8 and 9 are explanatory diagrams of the input circuit of the protection / control device for explaining the conventional example. 1 ... Phase voltage generation detection means 2 ... Sample and hold circuit 3 ... Analog / digital conversion circuit 4 ... Phase voltage storage means, 5 ... Calculation means 6 ... Comparison means, 7 ... Lock signal output means

Claims (1)

[Claims] 1. A device for protecting and controlling a system by detecting a zero-phase voltage from each phase voltage in a three-phase power system, a means for detecting generation of the zero-phase voltage, and a magnitude of each phase voltage stored. Means for calculating the ratio of the magnitude of the phase voltage after the zero-phase voltage is generated in each phase, and the magnitude of the phase voltage before the zero-phase voltage is generated, and means for comparing these ratios with a set value, When any of these ratios are smaller than the set value,
A protection / control device with a disconnection detection function, comprising means for outputting a signal that locks the protection / control function by regarding it as a disconnection of the secondary circuit of the instrument transformer that constitutes the input circuit.