JPS6366138B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a ratio differential relay provided for detecting a failure occurring inside an object to be protected.

[Technical background of the invention and its problems]

Generally, in differential protection systems, errors in current transformers used therein, particularly errors in the overcurrent region, pose a problem, and ratio differential relays are used to avoid this problem. As an example, a conventional ratio differential relay used in a two-winding transformer will be described with reference to FIGS. 1 to 4.

In FIG. 1, numeral 1 is a two-winding transformer to be protected, and a single phase out of three phases is shown. 2 and 3 are current transformers, which are provided to transform the currents I ₁ and I ₂ across the transformer 1 to supply current to the secondary load. 4 is a ratio differential relay body (hereinafter simply referred to as a relay),
Input the secondary currents I ₁ ′ and I ₂ ′ of current transformers 2 and 3.

In FIG. 2, 20 is a vector synthesis circuit as a differential current detection circuit, which vector synthesizes the currents I ₁ ' and I ₂ ' and outputs a differential current I _s =I ₁ '+I ₂ '. 21
is an addition circuit as a sum current detection circuit, and the current
Scalar addition of I ₁ ′ and I ₂ ′ gives sum current I _a = |I ₁ ′|+|
Outputs I ₂ ′|. 22 is an overcurrent element, and its operation is determined based on the magnitude of the differential current _Is . 23 is a ratio differential element, and the operation is determined based on the ratio between the difference current I _s and the sum current I _a . 24 is an OR circuit, which uses the logical sum of the outputs of the overcurrent element 22 and the ratio differential element 23 as a relay output.

In such a device, if a failure occurs inside the transformer 1, the difference current I _s = I ₁ ′ + I ₂ ′≠0, and the magnitude or the ratio of the difference current I _s to the sum current I _a When the operating range shown in FIG. 3 is reached, a relay output is generated to protect the transformer 1.

By the way, when the currents (current transformer primary currents) I ₁ and I ₂ become very large, the current transformers 2 and 3 become saturated, and the current transformer secondary currents I ₁ ′, I ₂ ′ becomes a distorted waveform containing harmonics as shown in FIG. Therefore, due to variations in the saturation characteristics of current transformers 2 and 3, even if the current transformer primary current difference I ₁ + I ₂ = 0, the secondary current difference I _s = I ₁ ′ + I ₂ ' is no longer 0. That is, even if there is an external failure, there is a possibility that the relay 3 will generate a relay output. In general, the ratio differential element 23 has a harmonic suppression function and therefore has a low possibility of malfunction, but the overcurrent element does not have a harmonic suppression function and therefore has a possibility of malfunction. Therefore, the detection level of the overcurrent element 22 needs to be set higher in consideration of harmonics caused by the saturation of the current transformers 2 and 3. However, setting this detection level high reduces the sensitivity of the relay. In this manner, in order to prevent malfunctions due to saturation of the current transformers 2 and 3, the inherent function of the ratio differential relay to detect internal failures with high sensitivity has been impaired.

[Purpose of the invention]

The present invention has been made to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide a ratio differential relay that can prevent malfunctions due to current transformer saturation and can detect internal failures with high sensitivity.

[Summary of the invention]

In order to achieve the above object, the present invention uses a bandpass filter to remove the harmonics of the difference current that is the output of the difference current detection circuit, leaving only the fundamental wave component. As an input, the operation of a low level overcurrent element causes a relay output.

[Embodiments of the invention]

An embodiment of the present invention will be described with reference to FIGS. 5 and 6.

In FIG. 5, 20 is a vector synthesis circuit as a differential current detection circuit, and current transformers 2 and 3 detect currents I ₁ and I ₂ at both ends of the transformer 1 to be protected.
Input the secondary currents I ₁ ′ and I ₂ ′ and perform vector synthesis,
Outputs the difference current I _s =I ₁ ′+I ₂ ′. 21 is an addition circuit as a sum current detection circuit, and 21 of current transformers 2 and 3
The next currents I ₁ ′ and I ₂ ′ are input, scalar addition is performed, and the sum current I _a =|I ₁ ′|+|I ₂ ′| is output. 23 is a ratio differential element, which operates when the ratio of the difference current I _s to the sum current I _a is in the operating range shown in FIG. In addition,
The ratio differential element 23 has a harmonic suppression function. Reference numeral 50 denotes a high-level overcurrent element (HOC.RY), which operates when the value of the differential current _Is is in the operating range (hatched area) shown in FIG. Note that the operating range shown in FIG. 6 is set so that malfunctions do not occur due to harmonics generated when the current transformers 2 and 3 are saturated. 51 is a band pass filter, and the difference current I _s
Removes the harmonics contained in the signal and outputs only the fundamental wave. 52 is a low-level overcurrent element (LOC・RY), and the difference current I _s output by the bandpass filter 51
It operates when the value of the fundamental wave is in the operating range (hatched area) shown in FIG. Note that this operating range is set so that it can operate even at a lower value than the operating range of the high-level overcurrent element (HOC・RY) 50. 53 is an OR circuit which outputs the logical sum of the outputs of the high level overcurrent element (HOC・RY) 50 and the low level overcurrent element (LOC・RY) 52. 24 is an OR circuit which outputs the logical sum of the outputs of the OR circuit 53 and the ratio differential element 23 as a relay output.

Next, the operation of this embodiment will be explained.

Due to an external fault, a large current flows through current transformers 2 and 3, causing them to become saturated and secondary currents I ₁ ′ and I ₂ ′ containing harmonics as shown in Figure 4. Due to the distorted waveform and variations in the characteristics of current transformers 2 and 3,
Even though I ₁ +I ₂ =0, I _s =I ₁ ′+I ₂ ′≠0. At this time, the high-level overcurrent element 50 does not operate with the difference current I _s due to the saturation of the current transformers 2 and 3. Also, the band pass filter 5 of the low level overcurrent element 52
It does not operate because the fundamental wave of the difference current I _s from which harmonics have been removed by 1 is input. Therefore, the inputs of the OR circuit 53 are both "0", that is, the output is "0",
No relay output occurs.

Conversely, if an internal failure occurs in transformer 1 while current transformers 2 and 3 are not saturated, I ₁ + I ₂ ≠ 0, and the difference current I _s = I ₁ ′ + I ₂ ′≠ 0. . This difference current
Since _Is does not contain harmonics, it is not attenuated by the bandpass filter 51. Therefore, the low level overcurrent element (LOC・RY) 52 operates and a relay output is generated.

Additionally, if an internal failure occurs when the current transformers 2 and 3 are saturated, the low-level overcurrent element (LOC・RY) 52 may become inoperable due to waveform distortion of the secondary currents I ₁ ′ and I ₂ ′. However, high-level overcurrent elements (HOC・
RY) 50 operates and a relay output is generated.

According to this embodiment, when the current transformers 2 and 3 are saturated due to a large current due to an external failure and the difference current I _s ≠0, no relay output is generated, so malfunction can be prevented. Further, when the differential current I _s ≠0 due to an internal failure, the fundamental wave of the differential current I _s falls within the operating range of the low-level overcurrent element 52, and a relay output is generated. Therefore, the sensitivity of the relay can be improved compared to the conventional one. Furthermore, since the influence of saturation of the current transformers 3 and 4 can be alleviated, the performance of the current transformers 3 and 4 can be reduced, and inexpensive and small current transformers can be used.

Next, another embodiment of the present invention will be described with reference to FIG.

In FIG. 7, 70 is an operating delay circuit that delays the output of the low level overcurrent element (LOC・RY) 52 for a set time period. If you do this,
For major accidents that require high-speed operation, high-speed operation is possible by operating the high-level overcurrent element (HOC/RY) 50 to output a relay output.

[Effects of the Invention] According to the present invention, as described above, it is possible to provide a ratio differential relay that can prevent malfunctions due to current transformer saturation and can detect internal failures with high sensitivity.

[Brief explanation of drawings]

Figures 1 and 2 are block diagrams showing conventional ratio differential relays, Figure 3 is a diagram of the operating characteristics of the device shown in Figure 2, Figure 4 shows the primary current at saturation of the current transformer,
A waveform diagram showing the secondary current, FIG. 5 is a configuration diagram showing one embodiment of the present invention, FIG. 6 is an operating characteristic diagram of the device shown in FIG. 5, and FIG. 7 is a diagram showing another embodiment of the present invention. FIG. 2, 3...Current transformer, 20...Difference current detection circuit (vector synthesis circuit), 21...Sum current detection circuit (summation circuit), 23...Ratio differential element, 24...OR circuit, 50 ...High level overcurrent element, 51...
Band pass filter, 52...Low level overcurrent element, 53...OR circuit.

Claims (1)

[Claims] 1 A current transformer that transforms the current at both ends of the object to be protected, a difference current detection circuit that inputs the secondary current of this current transformer and outputs the difference current, and a current transformer that inputs the secondary current of the current transformer and outputs the difference current. , a sum current detection circuit that outputs the sum current, a high-level overcurrent element that inputs the difference current and operates when the value is greater than or equal to a first predetermined value, and a high-level overcurrent element that inputs the difference current and operates its basic circuit. a bandpass film that outputs wave components; and a low-level overcurrent that operates when the output of the bandpass filter is input and the value is equal to or higher than a second predetermined value that is lower than the first predetermined value. a first OR circuit that inputs the outputs of the low-level overcurrent element and the high-level overcurrent element and outputs the logical sum thereof; and inputs the difference current and the sum current and operates based on the ratio thereof. 1. A ratio differential relay comprising: a ratio differential element; and a second OR circuit which inputs the outputs of the ratio differential element and the first OR circuit and outputs the logical sum thereof.