Evaluation Index of Equipment Utilization Efficiency in Substation Considering Distributed Generation

With the development of Distributed Generation (DG), traditional evaluation index of equipment utilization efficiency is not objective in some situations. To solve this problem, a comprehensive evaluation method of equipment utilization efficiency in substation is proposed in this paper. Based on the characteristics of distribution network including DG, two indexes, the burden rate and the load rate, are considered. Then, the indexes are further analyzed that they are specified into the injected indexes and the bidirectional indexes. From the operation data of transformers, the specified indexes are calculated representatively and it is indicated that bidirectional indexes could reflect the equipment utilization efficiency in substation more objectively. The presented method provides a new perspective of evaluating the equipment utilization efficiency in substation considering DG.


Introduction
In the rural areas with low load density, 35kV and 10kV distribution systems play an important role in power distribution, and they are significant connection between Distributed Generation (DG) and the power Grid in rural areas.Evaluating transformer equipment utilization in 35kV substation scientifically is the base of operation economy and power system planning for power supply enterprise.
At present, power supply reliability, voltage profile and line loss are the most concerning indexes in evaluation of distribution network for power supply enterprise.Most of the equipment utilization efficiency research is carried on by other industries [1]- [3], and research in power grid is still relatively fewer.Indexes like maximum and average burden rate of the main transformer are chosen to reflect the equipment utilization efficiency in some papers.Reference [4] studies on how to improve the equipment utilization efficiency in mediumvoltage distribution network from the perspective of optimal burden rate of equipment.Based on the burden rate, the evaluation index system of equipment utilization efficiency in distribution network is established in [5].Reference [6] proposes that the burden rate and the load rate should be considered simultaneously in evaluation of equipment utilization efficiency in distribution network.So the burden rate and the load rate could be used to analyze the transformer utilization efficiency in substation, but in distribution network including DG, like small hydropower, the traditional evaluation index is controversial.As electricity seller, the power supply enterprises generally focus on the injected load.But in rural distributed power grid, if only evaluate the transformer equipment utilization efficiency by injected burden rate and load rate, the index can't reflect the real contribution of the transformers when DG in operation.
Therefore, it is necessary to specify the indexes of burden rate and load rate to reflect the transformer utilization situation objectively when DG transmits power reversely.

Definition of burden rate and load rate
Burden rate is defined as the ratio of practical load to transformer capacity.Since there is evaluation period of equipment utilization efficiency in substation, the index of burden rate is specifically defined as the ratio of maximum active load to transformer capacity.The transformer loading capability is represented by burden rate, which shows the utilization level of equipment.However, burden rate only reflects the best utilized situation at a certain time point, and the utilized situation in a long period is unknown.Therefore, using the burn rate as the sole evaluation index would lead to imperfect and unilateral problem.So the evaluation indexes including load rate are needed.
Load rate is defined as the ratio of average load in a regulation period divided by maximum load, which reflects load variation in the regulation period and transformer utilization level.Under the consideration of economic operation, the transformer utilized situation Therefore, burden rate and load rate are complementary indexes in the evaluation of transformer utilization efficiency, in which burden rate represents the ultimate transformer utilized situation at a time point, and load rate represents the transformer utilized situation in a long period.

Analysis of equipment utilization efficiency in substation including DG
Traditionally, it is net load that power supply enterprises focus on.In substation, net load is given by subtracting DG output from overall load, which is injected load from substation and the equation is shown in (1).
where P eq is net load in substation, P Li is load in power grid, P DG is the output of DG, S L is sets of loads in power grid and S DG is sets of DGs.When DG inversely transmitting power to main network, the uncertainty of net load is increase because of the DG fluctuation, which requires more reserve capacity of transformer and reduces the utilization efficiency.

Specify the burden rate
With DG in operation, it may result in inverted power transmission and the effect on burden rate is analyzed in Figure 1.Assuming in evaluation period, the burden rate of transformer is referred to the ratio of maximum |P eq | to transformer capacity S T in traditional way.But when the DG output reaches a relatively large value like small hydropower in wet season, it may occur in max(|P eq |) > max(P eq ).So it is not objective to evaluate the equipment utilization efficiency in substation by traditional burden rate.
However, from the perspective of power supply enterprises, considering the burden rate of transmission power in forward direction is reasonable in some way because only injected power could bring economic profit.So the burden rate is specified into the injected burden rate α 1 representing the economic profit and the bidirectional burden rate α 2 representing the equipment utilization efficiency in substation.
The formula of the injected burden rate is shown in (2).

D
where P ud-max is the maximum value of injected load and S T is transformer capacity.The formula of the bidirectional burden rate is shown in (3).

D
where P bd-max is the maximum load when considering the bidirectional load.
The specific algorithm of the burden rate index is shown in Table 1.

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drawn by Figure 1, it is pointed out that the fluctuation of P eq would be reduced if the characteristic of operating DG is complementary with the load.However, the DG output is influenced greatly by season and weather.Moreover, the characteristic of DGs is usually inverse with the strategy of peak shaving.So the fluctuation of P eq would be larger with DG in operation.And the load rate would be smaller according to the definition, which means the index of transformer utilization efficiency would be smaller.Therefore, it is necessary to specify the load rate into injected load rate β 1 which represents the economic profit and the bidirectional load rate β 2 which represents the equipment utilization efficiency in substation.
The formula of the injected load rate is shown in (4).

Application analysis
Simulation cases are chosen from Shaoguan, a city located in south of China.Shaoguan is a typical city including distributed generation in China.In 2012, the average burden rate of Shaoguan is 36% while it is 35% in and the average load rate of Shaoguan is 79% while it is 80% nationwide.The management of power grid operation and electric equipment in Shaoguan is in average level of China.Therefore, the equipment utilization efficiency in Shaoguan is representative.

Comparison of burden rate indexes
According to the operation data of 37 transformers with 35kV class in Shaoguan in 2013, the calculating results of injected and bidirectional burden rate are given by means of Table 1.
The comparison result is shown in Figure 2 and Table 3.

Table 3. Comparison between the proportion of two burden
rates when in different interval.

Conclusion
The equipment utilization efficiency is a significant index in economic evaluation of distribution network construction.However, the importance of it didn't take seriously before and the power supply enterprises only consider the contribution of transformer from the perspective of injected load, which is not objective to the substation including DGs.This paper concentrates on the transformer utilization efficiency in substation.The burden rate and load rate are selected as the evaluation indexes, and specify them under the consideration of the particularity of distribution network including DGs.Then, compare the traditional injected burden rate α 1 and the bidirectional burden rate α 2 , as well as the traditional injected load rate β 1 and the bidirectional load rate β 2. Compared with traditional evaluation index α 1 and β 1, the application results show that the equipment utilization efficiency in substation is reflected more objectively by bidirectional calculation.Based on the proposed method, the bidirectional calculation and specific indexes should be considered in evaluation of equipment utilization efficiency in distributed network including DGs, which represent the contribution of equipment when the power transmission of DG is inverse.

Figure 1 .
Figure 1.The inverse power flow with DG in operation.Neglecting the line loss, it is obtained that 4
ICMIT 2016would be better when load rate is closer to 1, which means the load is smoother.

Table 1 .
Algorithm of injected and bidirectional burden rate.

Table 2 .
where P bd-avg is the average bidirectional load.The specific algorithm of the load rate index is shown in

Table 2 .
Algorithm of injected and bidirectional load rate.