The directional component of the three-stage directional current protection can select the forward or reverse direction action mode through the soft pressure plate. Here we only take the positive direction as an example to illustrate the principle of directional components.
In order to ensure that the directional components can operate reliably and sensitively in various phase-to-phase short circuit faults, the "wiring method" (referring to the software wiring method) of the directional components of microcomputer protection still adopts the 90° wiring method. For example, the A-phase directional element (called DA element) takes the current and the voltage, and the phase difference between the current and the voltage is. In order to make the directional element have maximum sensitivity, similar to the analog circuit type directional protection, the transfer vector =e is introduced, and the angle is the internal angle of the directional element. And ×e is called a vector, then the positive direction action equation of the absolute value comparison directional element is expressed by the following formula
When it falls in the direction of the maximum sensitivity line, the vector falls near the direction, takes the maximum value, takes the minimum value, and the directional component is in the most sensitive state. The vector diagram is shown in Figure 3-1(a).
From the vector diagram analysis, it can be seen that if the reference vector is taken as the reference vector, if the critical action of the protection positive direction element expressed by Equation (3-1) is required, the phase difference angle with the vector is -. It should be earth 90, and the protection will take action when the following formula is met:
That is, when it falls within the obliquely shaded area in the figure, the direction element will act.
If the inner angle a of the directional element is 30, and the 35kV line impedance angle = 60, it is obvious that the above directional element takes the same direction as the vector when =90-==30, and the protection has maximum sensitivity. Since microcomputer protection can use software to easily complete phase shifting and phase comparison, it is much simpler to use phase comparison directional elements in microcomputer protection than absolute value comparison. In microcomputer protection, the phase comparison type directional element uses the sampling calculation results to compare the phase difference angle of the directional element current and voltage, and check whether the phase difference angle is within the value range of the positive direction.
According to the relationship between phase comparison and absolute value comparison, the above action equation of absolute value comparison can be changed into an action equation of phase comparison. Because the two vectors comparing absolute values are + and -, the vectors comparing phases are sum, so the action equation of the phase comparison directional element is:
In order to facilitate debugging in microcomputer protection, a certain phase voltage is usually selected as the reference vector and it is agreed that clockwise rotation is the angle increasing direction. For example, if phase A voltage is selected as the reference vector and the direction is 0, then the range of the oblique shadow area in Figure 3.1(a) can be expressed as
This is the action range of the A-phase directional element (=;=); the action range of the B-phase directional element (I; one person; Uj=Uc.) is 90”~270”; the action range of the C-phase directional element (I=person; Uj=U .) The action range is 210~30. The above action range is shown in Figure 3-1(b). During microcomputer protection debugging, the angle value of the action range can be displayed on the LCD screen.
In fact, the directional elements implemented by digital operation logic judgment are equally sensitive in the action area, because the action of the directional element in the action area consumes almost no power from the input loop. In addition, it is also very convenient to implement some special functions in microcomputer protection. For example, after the direction element determines the positive direction, set the DA flag to "1", and after the direction state is memorized, its effective time can be controlled by software. For example, if the DA flag is set to 0 after 3 seconds, its memory function will "disappear" "Yes.
In this way, the device may operate for a fault in the opposite direction (such as the fault cannot be removed). This arrangement just allows the protection to have a back-up protection function in the opposite direction.