1 Reasons for Errors in Measuring the Direction of Zero-Sequence (Including Negative-Sequence) Current in Relay Protection Device Wiring To date, relay protection that responds to zero-sequence current remains the main method for detecting ground faults in three-phase systems. When a system ground fault occurs, the flow of zero-sequence current is determined by the distribution of zero-sequence impedance in the system. Even in the case of a simple single-side power supply, direction discrimination is sometimes required. Years of practice have shown that due to the wiring of the zero-sequence circuit, the direction of zero-sequence current is often mismeasured, which can cause maloperation of the protection device. The reasons are as follows: (1) The phase relationship between zero-sequence voltage U0 and zero-sequence current I0 is opposite to the phase relationship between general voltage and the current formed. In the case of asymmetrical three-phase system, zero-sequence voltage U0 is generated at the fault point. Under the action of U0, zero-sequence current I0 appears in the system. U0 is expressed as voltage drop according to regulations, while the direction of I0 can be determined by the specified fault current direction. Since the fault current is specified to flow to the fault point, I0 should also be specified to flow to the fault point in the positive direction. Only in this way can the following substitution relationship be satisfied. [img=300,142]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jsdjgc/2000-2/46-1.jpg[/img] Due to this stipulation, when Z0Σ is inductive, ID0 is capacitive current, and vice versa. This is different from the usual practice and can easily lead to errors. (2) The current direction specified by the power system (i.e., the defined current direction) and the direction specified by the zero-sequence current (i.e., the actual zero-sequence current direction) are not distinguished and are confused. The power system stipulates that the current flowing out of the bus is positive. This definition of the current direction determines the polarity connection of the current transformer, relay, etc. in the measuring protection circuit. It is unrelated to the actual current direction sent into the secondary circuit. The actual zero-sequence current flowing from the bus to the line should be determined according to the direction specified above. If the current direction specified by the power system is considered to be the positive direction of the zero-sequence current, an error may occur. (3) In some materials and textbooks, there are errors in the direction of zero-sequence current, or the statements are inconsistent. It should be pointed out that the direction of zero-sequence current is not a theoretical issue, but a practical application issue. In some cases, the concept of current direction may not need to be emphasized, but it is an important issue in relay protection. Incorrect direction judgment will lead to serious consequences. [b]2 Analysis of zero-sequence current direction and phase angle in several typical cases[/b] The phase angle is the angle by which I0 lags U0, i.e., ArgU0/I0. (1) Figure 1 is a zero-sequence network diagram of a high-current grounding system. Zero-sequence voltage UD0 appears at grounding fault point D. The dashed line in the figure is the specified I0 flow path. Assume that the impedance angle in the zero-sequence network is φ0 (inductive), then the zero-sequence currents I0Ⅰ and I0Ⅱ lead UD0 by an angle of (180° - φ0). If φ0 is inductive 90°, then I0Ⅰ and I0Ⅱ are capacitive 90°. [img=333,208]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jsdjgc/2000-2/46-2.jpg[/img] On the other hand, according to the system current polarity specification, the current flowing into the relays of line I and II is: In the intact line, IMoⅠ lags UD0 by 180°－（180°－φ0）＝φ0, which is inductive; In the faulty line, IM0Ⅱ leads UD0 by (180°－φ0), which is capacitive. Therefore, in order to correctly determine the fault direction, the maximum sensitivity angle of the zero-sequence directional relay installed on the line should be 180°－φ0. (2) The system with the neutral point not grounded is shown in Figure 2. One phase at point D is grounded, and the zero-sequence voltage at the fault point is still UD0. Since the neutral point is not grounded, the zero-sequence current is only generated through the phase-to-ground capacitance. Therefore, I0Ⅰ and I0Ⅱ in the figure are both inductive. [img=270,246]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jsdjgc/2000-2/47-2.jpg[/img] To correctly determine the direction, the maximum sensitivity angle of the zero-sequence directional relay installed on the line should be inductive 90°, i.e., +90°. The distribution of zero-sequence current in a compensated grounding system can be deduced by analogy. [b]3 Definition of Zero-Sequence Current and Zero-Sequence Voltage Polarity[/b] In order to connect the correct phase of zero-sequence voltage and current to the zero-sequence directional relay, the polarity of the filter output needs to be correctly defined. According to the definition [img=271,227]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jsdjgc/2000-2/47-1.jpg[/img] Your downloads and evaluations will become a powerful force influencing public opinion; we look forward to your insightful comments! (Download requires 1 point. How to earn points) Previous Page Next Page