A new metric for reliability assessment of distribution networks

Power system reliability is an important indicator of power system operation. Traditional power system reliability indicators reflect the availability of power system components or networks, but do not reflect their economic components. This paper proposes a new indicator for evaluating power system distribution networks - economic reliability, which not only reflects the reliability of the power system, but also reflects the economy of the power system. This is of great significance for improving the economic benefits of the power system.
Keywords : power system, economy, reliability

A New Index of Reliability Evaluation of Power Systems

Zhang x in-yong, Huang Qun-gu, Ren Zhen

(1. Electrical Power College, South China University of Technology, Guangzhou 510640, China; 2. Zhongshan Power Bureau, Zhongshan 528400, China )

Abstract : Reliability of Power Systems is an important evaluation index for Power System operation. Traditional reliability index reflects availability of equipment or power networks in Power Systems, and does not relate to its economic factors. A new index: economic reliability that evaluates reliability of power systems presented in this paper, not only reflects availability of Power Systems but also deals with economic properties of power systems. This is of importance for improving economic results.
Key words : Power Systems; economy; reliability

0 Introduction
The research on power system reliability has been involved in many literatures ^[1-3] , and the research contents are also very extensive: reliability of high-voltage power network, reliability of high-voltage direct current transmission, reliability of high-voltage switchgear, etc.
The distribution system is an important part of the power system. Distribution system reliability assessment is to evaluate the power supply reliability of the distribution network in operation or the newly designed distribution network, so as to determine the pros and cons of the power supply reliability of the distribution network. Through the reliability assessment of the distribution network, the impact of planned power outages and fault power outages on power supply reliability can be determined, thereby determining the technical measures to improve power supply reliability and seeking management methods to improve power supply reliability.
Generally speaking, in conventional reliability assessment, a distribution network with high reliability is not necessarily good in economy, and a distribution system with good economy is not necessarily high in reliability. The distribution network with high "economic reliability" index proposed in this paper has both high reliability and economy. 1 Conventional reliability assessment of distribution network system When conducting reliability assessment of distribution network system, traditionally, the more commonly used indicators are: failure rate λ and inspection rate of transmission and transformation equipment. The failure rate λ is generally expressed in units of "times/year·unit" or "times/year·km", indicating the average number of failures per device in a year, or the average number of failures per kilometer of line in a year. In addition, the following reliability indicators are also commonly used: (a) System Average Interruption Frequency Index SAIFI: The above formula is also called power supply availability or power supply reliability. (f) Average unavailability index ASUI: Complex distribution networks are composed of radial networks and ring networks. Obviously, in terms of reliability, ring networks are better than radial networks, and networks with segment or branch switches are better than networks without switches. This is mainly determined by the scope of power outages when the network fails, that is, the number of households × hours of power outages. However, using the indicators "number of households × hours/year" or "times/km" or "times/year" to evaluate the reliability of distribution systems only reflects the scope of power outages, but cannot reflect the economic losses caused by power outages. A large user (with large installed capacity) and a small user (with small installed capacity) have the same contribution to the indicator "number of households × hours/year" or "number of times/km" during the same power outage time, but obviously, the economic losses of the two are very different. The following "economic reliability" indicator is mainly used to evaluate the reliability of the distribution network system from an economic perspective. 2 Economic reliability indicators of distribution network system In the power system, the power supply is directly related to the economy. When the system fails and the power is cut off, the power supply is also affected. When the system fails, the first thing is to reduce the power outage time, the second is to reduce the scope of the power outage, that is, the number of households that are out of power, and the third (and more importantly) to minimize the power supply loss due to the power outage. In other words, we should not only consider "number of households × hours", but also "kilowatt × hours" or "megavolt-ampere × hours". The "MVA (hours)" index here highlights the weight of large users and reduces the status of small users, so that the distribution network reliability index can not only reflect the scope of fault power outages, but also reflect the economic losses during power outages. Based on the above concepts, when evaluating the reliability of distribution network systems, the following economic reliability indexes are proposed: (a) Failure rate index λ: The following economic reliability index is used to evaluate the reliability of a distribution network system in a certain urban area.

3 Application Examples
For the sake of clarity, the examples here are limited to radial distribution networks, and ring distribution networks can be analyzed in the same way.
Suppose a city has an independent 10 kV medium-voltage distribution network system, with a total of 620 households, a total installed transformer capacity of 359 MVA, a total line length of 245.53 kilometers, and 55 10 kV lines. The power outage data statistics are shown in Table 1.

In the following analysis, the original reliability index is marked with the subscript Y, and the economic reliability index is marked with the subscript W. The failure rate λ and the average time of each power outage T of the system are shown in Table 2.

Taking a 10kV line in the city as an example (as shown in Figure 1), a reliability assessment is conducted using both traditional reliability and economic reliability.

For convenience, all branches are equivalent to a line segment with an arrow, and all capacities on the branch line are equivalent to its total capacity. The capital letters A, B, C, etc. in Figure 1 are user numbers, the arrows point to the installed capacity (MVA) of all users on the branch line, the number next to the line segment is the length of the line segment (km), and the number in the circle is the number of all users on the branch line. Assume
that the substation is completely shut down for 8 hours, so all 39 users on the network in Figure 1 are affected; since there is no switch installed on the line, when a permanent fault occurs at any point on the line, it will cause a power outage on the entire line shown in Figure 1, and the power outage time is the fault elimination time, which is set to 4 hours. According to Table 1, when the substation is completely shut down, the household × hours h _Y1 and h _W1 of the network , the household × hours h _Y2 and h _W2 of planned power outages , and the household × hours h _Y3 and h _W3 of fault power outages can be calculated. The ASAI of total outage, planned power outage and accidental power outage, as well as the ASAIF when only fault power outages are considered, can be calculated. The results are shown in Table 3.

It can be seen from Table 3 that the original reliability and economic reliability indicators of the system in Figure 1 are close.
When the line is equipped with a section switch and a branch switch, as shown in Figure 2, the original reliability and economic reliability indicators are calculated respectively, and the results are shown in Table 4.

It can be seen from Table 2 that, in the case of section switches and branch switches, the original power supply availability of the network shown in Figure 2 is higher, while the economic power supply availability is lower.
Assume that the network structure in Figure 2, the number of users and numbers on each branch line remain unchanged, but the installed capacity of some branches changes, which is equivalent to the exchange of some large users and small users, as shown in Figure 3.

Recalculating the reliability indexes of the network structure in Figure 3, we found that the original reliability index of the network after the swap remains unchanged, while the economic reliability index changes, and the economic availability increases, as shown in Table 5.

The installed capacity of some users in the network of Figure 2 is swapped again, and the corresponding reliability indicators are calculated. The results are shown in Figure 4 and Table 6.

Comparing Table 3 and Table 4, it can be seen that when the network is installed with segment and branch switches, the original reliability index and economic reliability index are improved, that is, the power supply reliability rates ASAI and ASAIF of the two methods are improved. It can be seen that segment and branch switches play an important role in improving the reliability of the distribution network.
Comparing Table 4, Table 5 and Table 6, it can be seen that when the network structure and the number of users remain unchanged, and only the user-installed transformer capacity on the branch line changes, the original reliability rates ASAI and ASAIF remain unchanged, that is, whether the network is connected to a large user or a small user, the reliability rates ASAI and ASAIF are unchanged. However, the economic reliability rate has changed. The closer the large user is to the power supply end of the line, the higher the economic reliability rate.
Comparing the MVA (hours/year of power outages due to faults in Table 4, Table 5 and Table 6, the network in Figure 2 is 95.291 MVA (hours/year, the network in Figure 3 is 83.22 MVA (hours/year, and the network in Figure 4 is 78.286 MVA (hours/year. From the network in Figure 2 to the network in Figure 4, the number of households (hours) of power outages due to faults decreased by 17.8%, and thus the economic losses decreased by 17.8%. If the distribution network of the entire urban area can take into account both economic efficiency and medium and low voltage distribution transformation, it will not only improve reliability, but also have considerable economic benefits. 4 Conclusions This paper proposes a new indicator for evaluating the distribution network of the power system - economic reliability, which not only reflects the reliability of the power system, but also reflects the economy of the power system. The improvement of this indicator is of great significance to improving the reliability and economy of the operation of the power system distribution network.