JPS638692B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection device for a high frequency filter.

In recent years, with the remarkable spread of thyristor-applied electrical equipment, the waveform distortion of the voltage of power transmission and distribution lines and the current flowing therein has become large, and the occurrence of various failures due to this has become a problem. In order to solve this problem, the most effective means is to install a harmonic filter at the load point that generates the harmful harmonics, and this harmonic filter is installed in most cases with large load capacity. has been done.

A harmonic filter generally comprises a power capacitor 1 and a series reactor 2, as shown in FIG. 1a, and forms a series resonant circuit for harmonics of a target order. Further, as a harmonic filter for high-order harmonics, a resistor 3 is connected in parallel to a series reactor 2 as shown in FIG. 1b. Such a harmonic filter is provided for each required harmonic.

These harmonic filters are directly connected to power transmission lines, and if a short circuit, disconnection, or ground fault occurs in the capacitor 1 or reactor that makes up the filter, it will cause serious damage not only to the filter but also to the power transmission and distribution system. It will have an impact. Therefore, in the event of a filter failure, the failure can be quickly detected and
It is necessary to protect the faulty filter by disconnecting it from the power transmission and distribution lines.

To protect harmonic filters, it is necessary to be able to reliably protect equipment without being affected by harmonic currents that vary greatly depending on the load condition. Among the various protection methods for conventional phase advance capacitors, Double star-shaped neutral point current detection method shown in Figure 2a, Figure 2b
The voltage differential method shown in Fig. 3, the open delta method shown in Fig. 3c, etc. are applicable. In addition, in the figure, 4
5 indicates an instrument current transformer, 5 indicates an instrument transformer, and 6 indicates a detection device. Of these, the double star-shaped neutral point current detection method is the most economical method as it requires few accessories, but it requires dividing the capacitor 1 into two, and in addition, it is recommended to install the series reactor 2 on the neutral point side. In this case, it is necessary to divide the series reactor 2 into two as shown in FIG. 3, which has the disadvantage that the structure of the filter itself becomes complicated. Further, the voltage differential method and the open delta method require a dedicated instrument transformer 5 for detecting the voltage between the terminals of the capacitor 1, which has the disadvantage of increasing the number of attached devices.

The present invention focuses on the characteristics of harmonic filters provided for each harmonic order, and by applying a double star-shaped neutral point current detection method between filters of different orders, the parallel reactor To provide a harmonic filter protection device which can be connected to a neutral point side, can protect filters of different orders at the same time, and is simple in configuration and economical.

The present invention will be described below with reference to the embodiments shown in FIGS. 4 and 5.

FIG. 4 is a circuit diagram of a double star-shaped neutral point current detection system according to an embodiment of the present invention.
The series reactor 2, the capacitor 1', and the series reactor 2' are set to resonate at different harmonic frequencies, and each constitute a harmonic filter for a specific harmonic. The neutral point of the series reactors 2 and 2' provided on the neutral point side is drawn out by a bushing 7, and this bushing 7 connects the neutral points a and b of each filter via a current transformer 4. do. An overcurrent relay 8 is connected to the secondary side of the current transformer 4 to detect a failure and protect the harmonic filter. That is, when the capacitors 1, 1' and series reactors 2, 2' of each filter are normal, there is no potential difference between neutral points a and b, and therefore no current flows through the overcurrent relay 8. Overcurrent relay 8 does not operate. However, if a fault such as a short circuit, disconnection, or ground fault occurs in any of the components of each filter or in the wiring between them, a potential difference will occur between neutral points a and b, and overcurrent relay 8 It operates when current flows through it. As a result, the filter is protected by tripping the circuit breaker connecting it to the power transmission and distribution lines.

Now, considering the above-mentioned detection operation, when the neutral points of star-shaped connections of filters with different resonance frequencies, that is, filters with different values of C and L are connected, and a three-phase voltage is applied to this, , the current I ₀ flowing there can be expressed by equation (1), assuming that the voltage that appears when a and b are opened in FIG. 4 is E ₀ .

Under normal conditions, the potentials at neutral points a and b are both zero, so E ₀ becomes 0 in equation (1), and I ₀ does not flow no matter what value W, that is, the frequency. E ₀ appears in C ₁ or C ₂ or L ₁ or
This occurs only when the value of one phase of _L2 changes, and protection is possible without being affected by harmonics.

In addition, in arc furnaces, etc., there is a significant load imbalance,
The filter is in the same state as if a single-phase harmonic voltage E was momentarily applied. The equivalent circuit in this case is as shown in Figure 5, where the potential difference between a and b is E ₀
is expressed by equation (2). If Z ₁ and Z ₂ connected in a star shape are each balanced, equation (2) becomes 0, and no current flows even if a and b are connected.

E ₀ =Z ₁ /Z ₁ +Z ₁ /2・E−Z ₂ /Z ₂ +Z ₂ /2・E=0……
(2) Therefore, protection is possible no matter what unbalanced harmonic voltage is applied.

It should be noted that the present invention can be applied to a combination of the filter configuration shown in FIG. 1a and the configuration filter shown in FIG.

As described above, according to the present invention, harmonic filters having different resonant frequencies can be commonly protected, thereby reducing the number of fault detection transformers.
Furthermore, since the series reactor can be installed on the neutral point side, no short circuit current flows through the windings of the series reactor, making the series reactor less expensive.

[Brief explanation of the drawing]

Figures 1a and b are basic circuit diagrams of a harmonic filter, Figure 2 is a circuit diagram of a conventional filter capacitor protection system, and a is a circuit diagram showing a double star-shaped neutral point current detection system. , b is a circuit diagram showing a voltage differential method, c is a circuit diagram showing an open delta method, FIG. 3 is a circuit diagram of a conventional double star-shaped neutral point current detection method, and FIG. 4 is a circuit diagram according to the present invention. A circuit diagram of the double star-shaped neutral point current detection method, and FIG. 5 is an equivalent circuit diagram when a single-phase voltage is applied. 1, 1'... Capacitor, 2, 2'... Series reactor, 3... Resistor, 4... Current transformer, 5... Instrument transformer, 6... Detection device, 7... Bushing, 8 ...Overcurrent relay.

Claims (1)

[Claims] 1 A series reactor and a capacitor are connected in series to form harmonic filters of each order, and the neutral point side lead terminals of the harmonic filters of different orders are connected via a detection device. harmonic filter protection device.