Principle of Distributed Bus Protection Based on Directional Impedance Component Comparison

Distributed bus protection is one of the main tasks of smart substation construction to improve the secondary system because it is economical, simple, easy to layout, and easy to realize double. This paper mainly considers that distributed current differential protection is limited by high precision synchronization and high bandwidth and a distributed bus protection based on the directional impedance element comparison principle is proposed. The bus impedance protection constructed by reverse current calculation is proposed, which the impedance protection with positive sequence polarization and the impedance protection with memory polarization are the main components. Further considering the influence of system oscillation, the power frequency variable distance protection is introduced to form a distributed bus protection system, and the setting and time setting principles are given. The principle of the protection system is simple and the action is reliable. The effectiveness of the protection system is verified based on PSCAD simulation software.


Introduction
As a key component of the smart grid, digital substation has developed rapidly in China and abroad. The busbar in the substation undertakes the important task of current convergence and distribution, and its reliability plays a vital role in the safe and stable operation of a power system [1][2]. Traditional microcomputer busbar protection mainly adopts the centralized protection mode. The secondary wiring is complex and costly. The refuse operation of busbar protection may cause damage to power equipment and the disintegration of the system, and the misoperation of busbar protection will cause large-scale blackout.
With the rapid improvement of computer and communication technology, distributed busbar protection has become the trend of future development due to its simple structure, local layout, and easy realization of double configuration [3][4]. The principle of current differential protection is simple and reliable, which is the main protection principle of traditional microcomputer busbar protection and distributed bus protection. Cross-interval sampling value synchronization is one of the difficulties in the process bus communication of digital substations. There are mainly two solutions to synchronization issues. One is to improve the reliability of the synchronous clock by improving the synchronization technology. References [5][6] propose to use the difference between the time benchmarks of the busbar Compared with the bus-type distributed bus protection, the ring-type distributed bus protection is more in line with the technical characteristics of the new generation of smart substation. In addition, when the local control unit of the busbar protection is judged as a busbar fault, it only disconnects the corresponding circuit from the busbar without affecting other circuits. Therefore, even if a busbar protection unit of the ring-network distributed busbar protection is misjudged and jumped by mistake, it is only to jump out of the corresponding circuit, without causing the power outage of the entire busbar.
At present, most busbar protections are centralized current differential protections. After sample value has been summarized in the central unit, central unit executes calculation and sends commands. Distributed busbar protection uses current differential protection in the same principle as centralized protection. This requires each merge unit to accept sample values sent by other units and perform a differential calculation to determine whether the failure occurred, so it is necessary to achieve high consistency in communication and synchronization. However, in the case of more intervals, the realtime data of busbar differential protection is required and the amount of data communication is larger. Therefore, considering the technical problems of current differential protection in distributed protection, this paper proposes a distributed busbar protection based on directional impedance element comparison principle.

The Analysis of Fault Characteristics
Based on the distance protection, we can get (1), the working voltage op U is usually expressed as the linear combination of the measured voltage m U and the measured current m I at the protection installation, In (1), set Z is the setting impedance, which corresponds to the line impedance from the protection installation to the set point.
In the traditional line distance protection, different reference voltage ref U and different action boundary 1  and 2  are selected first. Based on the phase difference between op U and ref U under different fault conditions of inside and outside the area, the location of the fault point can be distinguished, and the line forward protection can be realized. The size of the protection range is determined by the protection setting impedance set Z . The busbar is located at the back of the line protection installation, which is the opposite position of the line. Referring to the principle of traditional line impedance protection, busbar impedance protection can be constructed by current reverse calculation. The protection schematic diagram is shown in figure 3. When a transmission line fault occurs, the measured impedance at the installation will only fall into their respective line impedance protection zone. When the busbar fault occurs, the reverse impedance component of each line will act, as shown in figure 3, the overlapping area is the busbar impedance protection area.

Z3/Z4 Protection Principle.
The busbar is a confluence element, and the voltage transformers are generally installed on the busbar or at the line outlet. When the busbar fault occurs, the voltage dead zone generally occurs. Therefore, when considering the principle of busbar impedance protection, it is necessary to select a variety of reference voltages according to different fault types to deal with the voltage dead zone issues. According to the line protection configuration, this paper selects the impedance protection based on positive sequence voltage polarization and voltage memory polarization as the main components. Considering the characteristics of impedance protection may misoperate when the system oscillates, the busbar impedance protection is equipped with power frequency variation distance protection at the same time. The voltage component of the busbar impedance protection takes the voltage of the bus voltage transformer, and the current takes the

Protection Setting Calculation.
Busbar fault is mainly phase-to-phase fault, for grounding fault is line to insulator or earth discharge short circuit, generally will not happen high resistance grounding fault. Therefore, only the arc resistance is considered in the calculation of the setting value when the busbar fault occurs. It is recommended that the resistance is not greater than 25  under different voltage levels, and then the impedance value in the direction of the line impedance angle is calculated according to the characteristics of the direction circle. The protection setting value can be obtained as (2),  is the line impedance angle.

Time Setting Caculation.
The busbar is a node of the transmission system, and the protection range includes the circuit breaker area. Compared with the system impedance, the bus impedance is very small. When the system oscillates, the possibility of the oscillation center falling on the bus node is very small. Even if it falls into the busbar protection area, as shown in figure 3, when the system oscillates, the time of oscillation trajectory passing through the overlapping area is related to the overlapping range. It can be seen from figure 3 that the oscillation impedance trajectory passes through the maximum area of the busbar impedance protection. The angle formed by the two points of the regional boundary and the protection installation point is  and the oscillation period is T , the time of the impedance trajectory staying in the busbar protection area can be obtained as (3): In order to avoid the influence of system oscillation on bus impedance protection, it is only necessary to set the setting time greater than t  , so as to ensure the reliability of the protection criterion.
Therefore, according to the above analysis of the inverse impedance element bus protection principle, it can be known that when the busbar fault occurs, the criterion for the action of the reverse impedance element of the distance protection is met, and the busbar protection control unit on each line will act. When the fault occurs on line, the line does not meet the action criterion of the anti-directional impedance element, so at least one line does not act.

Action Criterion of Busbar Protection
The distributed busbar protection adopts the ring network busbar protection structure, and each line interval needs to form the communication of the ring network. To prevent the data transmission error caused by communication, the above three directional impedance components are applied to the protection units of each line interval in this paper. These three criteria are not affected by each other. Even if bad data appear in the communication process, this method can also ensure certain fault tolerance.
Firstly, through the measured voltage and current obtained at the protection installation, a point difference is carried out to filter the decaying DC component.

Simulation Verification
The simulation model is built in PSCAD, and the 500kV bus simulation model is used as shown in figure 4. The S1 side system parameter is 1

Internal Bus Fault
Reference to figure 4, busbar fault occurs in 1s and lasts 150ms. The action of three kinds of antidirectional protection elements of each line in the case of phase A grounding fault is shown in figure 5. Limited to the length of the article, other fault types of action as shown in table 2. Through the protection action of three lines, the criterion can correctly identify the internal busbar fault. Also, the criterion can still correctly identify the internal busbar fault under the condition of system oscillation.     All the simulation tests show that the distributed busbar protection based on the comparison of directional impedance components can correctly identify the internal and external fault of the busbar, and is not affected by the fault type, the length of the fault distance from the outlet, and the system oscillation. At the same time, through the simulation data of table 3, it is found that an anti-directional impedance element of DP2 may act when a three-phase symmetrical short circuit occurs at the outlet of the line, resulting 1 M G  . However, the anti-directional impedance element of DP3 is correct. Therefore, these three impedance elements can be configured at the same time in each local control unit of bus protection to form redundant configuration to deal with different fault scenarios.

Conclusion
In this paper, a distributed busbar protection principle based on the comparison of directional impedance components is proposed. This principle uses three kinds of anti-directional impedance components to determine the fault direction, and comprehensively compares the fault directions of each line to identify the internal and external fault of the busbar. Theoretical analysis and PSCAD simulation results show that the principle can effectively identify the internal and external fault of the busbar, and the protection action is fast, which is less affected by the system oscillation. There is no need for sampling synchronization between different intervals, and it is less affected by data transmission delay. Using state information as an algorithm criterion can effectively solve the problem of hardware synchronization in data acquisition of different interval protection units in existing distributed busbar protection, so that each line can be logically judged independently and the reliability of distributed busbar protection is improved.