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Power network structure evaluation based on voltage sag and network loss.

1. Introduction

With the increase of sensitive load in industrial load, the problems caused by voltage sag become more and more serious, and the voltage sag has become the most complaints of power quality problem in the home and abroad. The voltage sag can make low voltage protection of frequency converter mis-operation, programmable logic controller failure, contactor tripping, computer storage data loss, motor shutdown, may even make the entire production line interrupt, it is difficult to estimate the economic losses caused by voltage sag for enterprises. Mitigation of voltage sag (Ambra et al., 2000; Su et al., 2013; Li et al., 2004; Li et al., 2011; Naidu et al., 2012; Xiao et al., 2009) has been widely concerned by researchers. A new effective measure to alleviate voltage sag in literature (Shun, 2008) is that closed loop design and open loop operation with multi chip and multi power supply is implemented in power distribution system. The different operation modes of transformer neutral point in the system and the influence of generator unit start and stop on voltage sag are studied in literature (Chen and Zhao, 2014). In the view of the voltage sag caused by busbar fault, the evaluation index of power network structure is proposed in literature (Ma et al., 2015). How to ease the voltage sag by optimizing the power network structure has been widely concerned by researchers.

The traditional network reconfiguration is to find the optimal power network structure in order to reduce the network loss and improve the economic benefit of power system. According to the normal operation condition, the dispatcher achieves the optimal network structure by adjusting the switch states. The purpose is to balance the load, eliminate the overload, improve the quality of the power supply voltage, on the other hand, to reduce the network loss and improve the economic performance of the system.

Power network structure not only affects the economic operation of the system but also affects the security and stability of the system, Literature (Nesrallh et al., 2010) combined network reconfiguration with static var compensator to alleviate voltage sag. Genetic algorithm in literature (Sanjay et al., 2007) is used to optimize network structure to reduce the economic losses caused by voltage sag. Therefore the analysis and evaluation of the multi objective power grid structure considering the mitigation of voltage sag and the reduction of the network loss is needed to be paid attention.

The evaluation index of power grid structure is put forward in this paper to alleviate the voltage sag and reduce the grid loss. Firstly, based on the fault point method, four types fault are set up at each busbar of the power grid, and the vulnerability area matrixes of voltage sag are established, the evaluation index of power network structure based on voltage sag is proposed. The weight of voltage sag and the weight of network loss are introduced to evaluate the power network structure, the evaluation index of power network structure is proposed, which can reduce the network loss and alleviate the voltage sag. The proposed index can evaluate the superiority of grid structure. On the basis of the IEEE14 bus system, the different network structures constructed by power supply access node changed are evaluated by the proposed index. It is proved that the proposed evaluation approach is feasible and guides the grid safe operation, planning and transformation effectively, and achieves the economic and security of power grid operation (Hernandez et al., 2016).

2. The Evaluation Index of Power Grid Structure Based On Voltage Sag Caused by Busbar Faults

It is assumed that there are 4 kinds of faults on each busbar of the power grid, and the voltage sag value of the busbar is calculated respectively. The busbar voltage sag matrix [U.sup.(m)] (Ma et al., 2014) is established accordingly. (m=1, 2, 3, 4 respectively, on behalf of single phase grounding short circuit, two-phase short circuit, three-phase short circuit, two phase grounding short circuit)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

In which [U.sup.(m).sub.ij] represents the A, B, C three-phase voltage value of busbar j when m type fault occurs on busbar i. Set the threshold value of the voltage sag as [U.sub.thre], As long as there is any one voltage value of [U.sup.(m).sub.ij] below the threshold value, it is considered that voltage sag occurs on the bus j, order [Z.sup.(m).sub.ij] = 1, otherwise, [Z.sup.(m).sub.ij] = 0, thus the depression domain matrix Z(m) is acquired.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (2)

Similarly, for different power network structures, the corresponding m type voltage sag matrix and its depression domain matrix can be established.

m type power network structure evaluation index [D.sup.(m)] based on voltage sag caused by m type fault on basbar is as follow.

[D.sup.(m)] = [n.summation over (i = 1)][n.summation over (j=1)] [Z.sup.(m).sub.ij] (3)

The average evaluation index of power grid structure considering four types fault on busbar is as follow.

D = [4.summation over (m=1)][[lambda].sub.m][D.sup.(m)] (4)

In which [[lambda].sub.m] is the probability of the m type busbar fault.

The evaluation index [D.sup.(m)] and D can be reflected the voltage sag in the power network structure, the bigger the index data, the more serious the voltage sag is in the power network structure. These indexes can be used to analyze and evaluate the power network structure changed by various factors, and can be used as one of the indexes to judge the merits of power grid structure, effectively guide the safe operation of the power grid, planning and transformation.

3. The Calculation of Power Network Loss

Power network loss consists of the active power loss and reactive power loss, reducing the power network loss is to reduce the active power loss. The active power loss of the network includes the active power loss of the line and the active power loss of the transformer.

Total active power loss of network line

[DELTA][P.sub.LINE] = [L.summation over (i=1)][I.sup.2.sub.i] [R.sub.i] (5)

In which [I.sub.i] is the current flowing through the i line, [R.sub.i] is the resistance of the i line, L is the total number of line.

The total active power loss of the network transformer includes the winding resistance loss and no-load loss of the transformer.

[DELTA][P.sub.T] = [H.summation over (j=1)] ([I.sup.2.sub.j][R.sub.Tj] + [DELTA][P.sub.0j]) (6)

In which [I.sub.j] is the current of the j transformer, [R.sub.Tj] is the winding resistance of the j transformer, [DELTA][P.sub.0j] is the no-load loss of the j transformer, H is the total number of transformer.

The active power loss of the network [DELTA]P is as follow.

[DELTA]P = [DELTA][P.sub.LINE] + [DELTA][P.sub.T] (7)

4. Multi Objective Evaluation Index of Power Network Structure

Multi objective evaluation index S for network structure which consider of mitigating voltage sag and reducing network loss is as follow.

S = [alpha]D + [beta][DELTA]P '([alpha] + [beta] = 1 (8)

In which [alpha] is the weight of the network structure evaluation index D based on voltage sag, [beta] is the weight of the network loss [DELTA]P, the value of [alpha] and [beta] can be set according to the actual needs. The smaller the evaluation index D, the better the effect of power network structure to alleviate the voltage sag. The smaller [DELTA]P, the smaller the network loss of the power network structure. Therefore, the index S can be considered as one of the indicators of power grid planning, operation and improvement of the power network structure to mitigate the voltage sag and reduce the network loss.

5. Example analysis and evaluation

In order to verify that the proposed indicators can be analyzed and evaluated for different power grid structures, based on the IEEE14 node system, the different structure of the power network is constructed by the change of the power supply access point.

5.1. The IEEE14 Node System and Parameters

The IEEE14 busbar system is shown in Figure 1, and the system parameters are found in the literature (Zhang and Chen, 1996). The system contains 110 kV and 23 kV voltage levels, and consists of 4 generating units, 14 buses, 17 lines and 3 transformers, all transformers with Y0/Y0 connection mode.

The threshold of the power network voltage sag is set as 0.7. Assuming the weight of voltage sag is 0.6, the weight of network loss is 0.4, the probabilities of the four type busbar fault are shown in Table 1.

In order to analyze the influence of power access point on voltage sag and network loss, assume that the active power of the supply is 20MW, the voltage is 1.022, [Q.sub.max] = 30MVar, [Q.sub.min] = -15MVar. The power supply is the PV node, the power supply is respectively accessed to 10, 11, 12, 13, 14 node, five different power network structures are obtained, the five network structures are analyzed and evaluated by the above proposed network structure evaluation index. The evaluation data are shown in Table 2.

The above data shows that evaluation index S of the network structure whose power supply access to the 10 node are less than those of the network structure power to access to the remaining nodes. In spite of the network loss of the network structure whose power supply access to the 10 node is not the least, by the weight values, the network loss is considered, at the same time the mitigation of voltage sag is payed more attention, the network structure evaluation index based on the voltage sag of the power supply access to 10 node is the smallest. Therefore select the 10 node as the power access point.

6. Conclusions

The evaluation method and evaluation index are proposed in this paper, which can evaluate the structure of power network with the consideration of voltage sag and the network loss. Based on the IEEE14 node system, the different structures of the power network constructed by the change of the power supply access node are evaluated by the proposed evaluation method and evaluation index. It is proved that the proposed evaluation index is feasible and can be used as one of the index to assess the power network structure, and guides the grid safe operation, planning and transformation effectively, and the economic and security of power grid operation are achieved.

Recebido/Submission: 03/05/2016

Aceitacao/Acceptance: 28/08/2016

References

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Li Ma (1), Qian Zhou (2)

710849937@qq.com, 13991849689@163.com

(1) Country School of Electrical Engineering, Xi' an University of Science & Technology, 710054, Xi' an, China

(2) Shaanxi Electric Power Research Institute, Xi' an, China

Table 1--The probabilities of the four type busbar fault

Fault Type    M=1    M=2    M=3    M=4
probability   0.83   0.08   0.04   0.05

Table 2--Evaluation results of different network structure
caused by power access point changed

Index      D       [DELTA]P   S

busbario   81      0.124      48.65
busbarll   89.08   0.133      53.5
busbarl2   94.7    0.121      56.87
busbarl3   86.28   0.147      51.83
busbarl4   83.81   0.121      50.33
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Author:Ma, Li; Zhou, Qian
Publication:RISTI (Revista Iberica de Sistemas e Tecnologias de Informacao)
Date:Oct 15, 2016
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