JPH0561856B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is a digital protective relay for detecting faults in a power system, and relates to an undervoltage relay with current compensation.

[Prior art] Figure 9 shows, for example, the Conference published by IEE.
This is a circuit configuration diagram of a conventional undervoltage relay with current compensation (hereinafter referred to as UVZ) shown on pages 50 to 54 of publication No. 249 April 1985. In the figure, 1 is a voltage transformer, 2 is a current transformer, 3,4
is an input converter that converts voltage and current into UVZ input, 5 is a phase shifter, 6 is a setting device that sets the amount of current compensation, 7 and 8 are half-wave rectifiers that only pass the AC half-wave period, and 9 is a A setter provides a detection level, a comparator 10 compares and determines the magnitude, and a timer circuit 11 constitutes a UVZ indicated by 12.

Next, the operation will be explained with reference to FIG.
The voltage V and current I obtained from the power system are introduced in isolation by respective input converters 3 and 4. The current I is phase-shifted by an angle φ in the phase shifter 5, and this angle is selected to correspond to the phase difference between voltage and current at the time of a power transmission line fault, and is approximately 75°.
When this phase-shifted current I∠φ is applied to the setting device 6 that determines the current compensation amount Z, the current compensation amount of ZI is obtained. The voltage V and current compensation ZI are passed through half-wave rectification paths 7 and 8, respectively, and the set value Ko is
When added to the comparator 10, as shown in FIG.
An oval locus surrounded by a size Ko is obtained with the ZI axis as the origin, and if the voltage V is within the range of this oval, the comparator 10 outputs an output. This output is checked for a predetermined period of time by a time limit circuit 11, and the determination result is output to the OUT terminal.

[Problem that the invention seeks to solve]

Since the conventional UVZ is configured as described above, it can perform only one judgment process every half cycle of AC, so the maximum missed time is half a cycle from the time an accident occurs until it is detected. The problem was that the judgment time was half a cycle, but one cycle was required, and the accident detection time was slow.

In addition, since the circuit configuration is an analog circuit, it is likely to be affected by drift and aging of components, resulting in problems such as time-consuming adjustment and the inability to achieve high detection sensitivity.

Furthermore, since it is not possible to receive digital signals from digital transformers and digital current transformers, which have recently been developed, a problem has arisen in that the range of use is limited.

This invention was made to solve the above-mentioned problems, and because numerical calculations are performed using a microprocessor, UVZ can be determined at high speed, and there is no need to consider drift or aging.
The purpose is to obtain.

[Means for solving problems]

The UVZ according to this invention realizes an undervoltage relay that converts voltage and current into digital values and adds current compensation through arithmetic processing.It reduces the number of sampling data used for arithmetic processing,
It not only enables high-speed detection, but also achieves high reliability by reducing the effects of drift and aging, which are problems with analog circuits.

[Effect]

In order to achieve this characteristic, the UVZ of this invention is provided with a current compensation coefficient, and the sum of the magnitude of the difference calculated from the voltage and the magnitude of the voltage itself is the magnitude of the difference multiplied by a constant. a first determination element that determines the magnitude of the voltage by comparing it with a predetermined value; and a second determination element that determines the magnitude of the voltage itself by comparing it with a predetermined value; Digital calculation processing is used to detect an accident when any of the judgment elements is determined, which has the effect of making judgments possible at high speed.Furthermore, digital calculation processing allows for long-term stable characteristics with little drift or aging. can be retained.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. In Fig. 1, 13 and 14 are filters, which remove frequencies higher than 1/2 of the sampling frequency among harmonics contained in voltage and current, and 15 and 16 are sample and hold filters. 17 is a multiplexer that sequentially switches the outputs of the sample holds 15 and 16 and transmits them to the analog-to-digital converter 18; 19 is a microprocessor;
The calculation is performed using a program stored in advance in the computer, and the results are outputted to the output circuits 21 and 22.

Figure 2 is a waveform diagram showing the operation of sample hold, in which voltage V and current I are sampled at the same time as sample signals and held until the next sample signal arrives. , it will be converted into a digital value and processed.

FIG. 3 is a vector diagram for explaining means for realizing the characteristics of an embodiment of the present invention, which is actually realized by digital calculation. The same applies when a vector diagram appears in the following explanation.

When a vector ZI is obtained by shifting the phase of the current I by φ and multiplying the magnitude by Z, and finding the difference between the vector ZI and the voltage V, a vector (V-ZI) is obtained. If we calculate the sum of the magnitude of the vector (V-ZI) and the magnitude of the voltage vector V, and make this value a constant value, then the locus of the voltage V will become an ellipse, as is well known. Become.

However, as the current I increases, the vector
Since ZI becomes larger and (V-ZI) becomes larger,
In the equation where the sum with V is a constant value, the ellipse becomes gradually thinner.

Therefore, instead of this constant value, k×
When expressed as |ZI|, the equation becomes the property inside the ellipse.

|V|+|V-ZI|≦k・|ZI|... The characteristic inside this ellipse is expressed as characteristic 23 as the first determination element.

On the other hand, find the magnitude of voltage V, and find that this is a constant value
If we find a range smaller than Ko, we get the formula,
It becomes a circle centered at the origin.

|V|≦Ko... This is expressed as characteristic 24 as a second determination element. Therefore, by taking the logical sum of the first and second determination elements, the characteristic shown by the solid line in FIG. 3 is obtained, and by outputting the determination output from the output circuit 21 in FIG. It turns out that UVZ has detected it.

FIG. 5 is a flowchart illustrating an example of an arithmetic processing method for realizing the UVZ of the present invention shown in FIG. 4;
In the following description, the sampling frequency is 12 times the frequency of the voltage and current, that is, an electrical angle of 30° with respect to the frequency of the voltage and current, and the symbol is indicated as T. The data at the current sampling point should be indicated as suffix (t), and the previous data should be indicated as (t-T), (t-2T), (t-3T), etc. shall be.

Therefore, voltage and current, if the phase difference is θ, V(t-nT) = Vsin {(wt+θ)-30°×n} = Vsin{(wt+θ)-nT} ...... i(t-nT )=Isin(wt−nT)... However, n=1, 2, 3... In the following explanation, (t), (t-T), (t-3T), (t
−4T), is used.

First, the first determination element 23 will be explained.

Using the data of i(t)26 and i(t-t)27,
When the current I is shifted by the phase angle φ = 75°, i (t + 75°) = Isin (wt + 75°) = I (0.2588 sinwt + 0.9659 ceswt) ... On the other hand, i (t - T) = Isin (wt - 30°) =√3/2Isin wt−1/2Icos wt =√3/2I(t)−1/2Icoswt …… Substituting into i(t+75°)=0.2588i(t)+0.9659｜√3(t) -2i(t-T)} =1.9319{i(t)-i(t-T)}... is obtained and can be realized by subtraction 28 and multiplication 29.
This is multiplied by the current compensation amount Z30 to obtain Zi(t+
75°). When subtracting V(t)31 and Zi(t+75°) 32, the difference D(t)=V(t)−Zi(t+75°) =V(t)−1.9319Z{i(t)−i( t-T)}... This means that the following calculation has been executed.

Similarly, i(t-3T)33, i(t-4T)
34, V(t-3T) 38 = 90° from the previous data, i(t++75°-3T) = Isin(wt+75°-3T) = 1.9319 {i(t-3T)-i(t-4T) } ... is calculated as the current compensation amount Z37, and D (t-3T) = V (t-3T) - 1.9319Z {i (t-3T) - i (t-4T)} ... is calculated by subtraction 39 .

In the case of a sine wave, it is known that the square root of the sum of squares of data having a phase difference of 3T=90° represents the magnitude of the sine wave. Therefore, finding the square root 43 of the sum 42 of the square of subtraction 32 40 and the square 41 of subtraction 39, we get {D ² (t) + D ² (t-3T)}1/2 = [{V(t) −Zi (t+75°)} ² + {V (t−3T) −Zi (t+75°−3T)} ² }]1/2 = [V ² (t)−2×1.9319ZV(t) {i(t )−i(t−
T)} +1.9319 ² Z ² {i(t)−i(t−T)} ² +V ² (t−
3T) −2×1.9319ZV(t−3T)}i(t−3T)−i
(t-4T)} +1.9319 ² Z ² {i(t-3T)-i(t-4T)} ² ]1/2 = {V ² −2VZ Icos (75°-θ) + (ZI) ² }1/2
......

The voltage V is V(t)31 squared 44 and V(t-3T)
Finding the square root 47 of the sum 46 of 38 squared 45 is {V ² (t)+V ² (t-3T)}1/2 = {V ² sin ² (wt+θ)+V ² sin ² (wt+θ-3T)}
1/2=V...

The sum 48 of this equation and the equation corresponds to the left side of the equation.

On the other hand, the magnitude of the current compensation amount is the square root 52 of the sum 51 of Z30 squared and Z37 squared 50, and this multiplied by constant k53 is k{Z ² i ² (t+75°) +Z ² i (t+75°-3T)}1/2 = kZI {sin ² (wt+75°) sin ² (wt+75°-3T)}1/2 = kZI... The equation corresponds to the right side of the equation.

Therefore, by applying the formula, formula, and formula to the formula, the comparison operation 54 obtains the following result: V+{V ² −2VZIcos (75°−θ) + (ZI) ² }1/2≦K21 .

Expanding the equation, V ² −2VZIcos (75°−θ) + (ZI) ² ≦k ² (ZI) ² −2Vk (ZI) + V ² ∴V≦ZI (k ² −1)/2 {k−cos75゜-θ) ..., and the relational expression between the current compensation amount ZI and the voltage V can be expressed as a function of their phase difference (75゜-θ).

The phase shift angle of the current compensation amount is set to 75°, but if we generalize it to φ, it can be expressed as V≦ZI(k ² −1)/2{k−cos(φ−θ)}... I can do it.

In Figure 5, with k set to k = 1.2 and k = 1.5,
The phase difference between the vector ZI of the current compensation amount and the voltage V (φ
-θ) is varied from 0° to 330°, the coefficient of the magnitude of V with respect to ZI (k ² -1)/2 {k-cos
(φ−θ)}.

In addition, A and B in Fig. 6 are illustrations of Fig. 5, and k = 1.2, φ = 75°, and k = 1.5, respectively.
This figure shows the range of V in the equation when φ=75°, and ellipse retention is obtained.

Therefore, if the voltage V is inside this ellipse,
The determination condition for the calculation is satisfied, and the first determination element 23 is obtained.

Next, the second determination element 24 will be explained.
To determine if the voltage V has decreased below a predetermined value ko, a comparison operation 56 is made between the square root 47 and a constant ko 55, and {V ² (t) + V ² (t-3T)} 1/2 = V≦ko . . . Then, this becomes a circle passing through the origin and having a radius ko, and if the voltage V is within this circle, the determination condition for the calculation is satisfied and the second determination element 24 is obtained.

Therefore, by obtaining the logical sum 57 of the first and second determination elements, the characteristic shown by the solid line in FIG. 3 can be obtained at the OUT terminal.

In addition, in FIG. 3 of the above embodiment, the width of the ellipse (ZI
and 90° direction) is narrow, please refer to Figures 6 and 7.
As can be seen from the figure, by increasing the value of k, the width can be increased, but the vertical width (in the same direction as ZI) is also expanded at the same time. If you _want to enlarge only the _width , as _shown in _FIG . This can be handled by dividing it into two parts.

FIG. 8 is an explanatory diagram for calculating the UVZ detection time according to the present invention. A is the voltage V, B is the current I with a phase delayed by 75° from V, C is the current compensation amount with φ = 75°, and D is the voltage V with current compensation and set as (V-ZI). , E is the amplitude value calculation result of the equation, F is the sampling time, and it is assumed that the accident occurred at point F immediately after time (t-5T).

Since the current compensation amount C after the accident occurs is determined by the currents i(t-4T) and i(t-3T) after the accident occurs, it becomes an accurate value from time (t-3T) onwards.

Therefore, the amplitude calculation of E will be the correct value after the accident after the time t when i(t-4T) becomes the established value, so the maximum detection time for which the formula holds true is { (t-4T) - (t-5T)} + {t-(t-4T)} + {(t+T)-t} = 6T, that is, the time equivalent to 30° x 6 = 180°, which is It becomes 1/2 cycle. Specifically, in a 50Hz power system, (1/50) x (1/2)
sec=10msec.

In addition, in the above example, a method using data around 3T = 90° was shown as a method for determining the magnitude of the sine wave, but data at every 30° or 60° can be used as {V ² (t) +V ² (t-T)-√3(t)V(t-T)}1/2 =V{sin ² (wt+θ)+sin ² (wt+θ-T) -√3sin(wt+θ)sin(wt+θ-T) }1/2 = 1/2 ... {V ² (t)−V ² (t−T)+V ² (t−2T)}1/2 = V{sin ² (wt+θ)−sin ² (wt+θ−T ) +sin ² (wt + θ−2T)}1/2 = 1/√2V ... The same calculation can be performed using the following formula.

In the case of this formula, since data at times t and (t-1) are used, the calculation of the current compensation amount is also (t-
Assuming that data of i(t-T) and i(t-2T) are used at time 1), the detection time is {(t-2T)-(t-3T)}+{t-(t- 2T)} +{(t+T)-t}=4T In other words, if the power system is set to 50Hz, high-speed detection of (1/50) x (30° x 4/360°) sec = 6.67 msec is possible.

〔Effect of the invention〕

As described above, according to the present invention, in order to realize UVZ characteristics using a digital arithmetic processing method, a current compensation coefficient is provided, and the magnitude of the difference between the voltage and the voltage itself is calculated. A first determination element that determines the magnitude by comparing the sum with the magnitude of the difference amount multiplied by a constant, and a first determination element that determines the magnitude by comparing the magnitude of the voltage itself with a predetermined value. Since the second determination element is configured to include a second determination element, it is possible to detect an accident at a high speed, and it is also possible to maintain stable characteristics over a long period of time.

[Brief explanation of the drawing]

Fig. 1 is a circuit configuration diagram showing a UVZ according to an embodiment of the present invention, Fig. 2 is an explanatory diagram of sample and hold operation, Fig. 3 is an explanatory diagram showing the characteristics of this invention, and Fig. 4 is an arithmetic processing method. FIG. 5 is a relationship diagram between voltage and current compensation amount, FIG. 6 is a characteristic diagram of the first determination element, and FIG. 7 is a flowchart diagram explaining
FIG. 8 is an explanatory diagram for calculating detection time, FIG. 9 is a circuit diagram of a conventional UVZ, and FIG. 10 is a diagram showing characteristics of a conventional UVZ. In the figure, 1 is a voltage transformer, 2 is a current transformer, 3 and 4 are input converters, 5 is a phase shifter, 6 is a setting device, 7 and 8 are half-wave rectifiers, 9 is a setter, and 10 is a comparison 11 is a time limit circuit, 12 is UVZ, 13,1
4 is a filter, 15 and 16 are sample holds,
17 is a multiplexer, 18 is an analog-to-digital converter, 19 is a microprocessor, 20 is a memory, 21, 22 are output circuits, 23, 24, 25 are
UVZ characteristics, 26, 27, 31, 33, 34,
38 is input data, 28, 32, 35, 39 are difference calculations, 29, 30, 36, 37, 53 are multiplication calculations, 40, 41, 44, 45, 49, 50 are square calculations, 42, 46, 48 , 51 is an addition operation, 4
3, 47, and 52 are square root operations, 54 and 56 are comparison operations, 55 is a setting value, and 57 is a logical sum. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 1. In a digital protective relay that detects faults in the power system by sampling the voltage and current of the power system at regular intervals, converting them into digital data, and performing calculations based on the values, the current is converted to a predetermined phase by a phase shift calculation. means for phase shifting; means for converting the phase shift calculation result into a predetermined size by a magnification calculation; means for obtaining a difference between the voltage and the magnification calculation result by a difference calculation; means for calculating the magnitude by an amplitude value calculation of the voltage; means for calculating the magnitude of the voltage by a second amplitude value calculation; and combining the first and second amplitude value calculation results by an addition calculation. means, a first determination element that determines the magnitude by a comparison operation between the result of multiplying the magnification calculation result by a first constant and the result of the addition operation; a second determination element that determines the magnitude by a comparison operation with a constant, and an accident determination is made by a logical sum of the first determination element and the second determination element. Protective relay.