Evaluation Method of the Power Grid Unbalance Factor under Cooperative Distribution of Photovoltaic and Energy Storage

With the rapid evolution of electrification, a number of asymmetric loads appear in the power system, making three-phase unbalanced pollution of the power grid increasingly serious. However, there are currently no comprehensive evaluation indexes to assess the unbalanced factor between power grids and customers, which makes it tough to locate and control unbalanced pollution sources. To solve the conundrum, the paper first summarizes the traditional assessment indexes and points out their shortcomings. Then, according to IEEE Std 1459-2010 power theory, the apparent fundamental power is represented by sequence components and propose a set of new evaluation indexes and methods of unbalanced factor. Finally, to verify the practicability and accuracy of the new indexes, simulation models of different wiring forms are constructed in MATLAB, and the simulation experiment results are analyzed. The results show that compared with the existing indexes, the new indexes can comprehensively reflect the unbalanced pollution factor of the system and the customer side under different wiring forms. They can be used to locate the unbalance sources and divide the unbalanced responsibilities.


Introduction
With the process of the fourth industrial revolution, the global level of science and technology is developing rapidly, and the adjustment of energy structure is also continuing to advance towards the goal of low carbon.The new-type power system structure and load characteristics under the integrated form of source network load storage have become increasingly complex.A number of asymmetric loads appear in the power system, resulting in three-phase imbalance, harmonics, voltage sags, and other power quality problems becoming increasingly serious, which has a certain impact on the reliability and safe working of the power system [1-2] .
The negative sequence voltage, current, and zero sequence components in unbalanced power systems not only have a significant impact on electrical equipment but also affect the accurate measurement of electric energy [3].Xu et al. [4] analyzed the impact of negative-sequence power on the accuracy of electric energy metering methods.They advocated the adoption of a new metering method with positivesequence active electric energy as the main metering body.Still, this method does not provide a reasonable solution for the negative sequence energy injected into the power grid by asymmetric loads.At present, researchers mainly consider the impact of harmonics and negative sequences on the energy metering methods and the accurate location of harmonic sources [5][6].There are few studies on the positioning of unbalanced sources and the evaluation index of unbalanced pollution.
To ensure the reliable and safe working of the electric network and the consumer's electricity quality, it is essential to accurately assess the nonlinear unbalance factor in the power system, thereby laying the foundation for the formulation and implementation of pollution control plans.The Chinese national standard defines for negative sequence voltage and current unbalance, zero sequence voltage and current unbalance [7].To facilitate engineering applications and simplify the calculation process, Zhang et al. [8] defined the voltage unbalance based on the effective value of the phase voltage.In practical engineering measurements, only line voltages can be measured in the three-phase power network without a neutral line.Given this, some scholars have proposed a method for calculating voltage unbalance and load current imbalance at common connections based on sequence capacity [9].In [10][11], a method to calculate the three-phase balance factor stems from the ratio of the square of the effective value of the positive sequence component to the sum of the squares of the total amount was put forward to tackle the question of inconsistency in the existing definition of the unbalanced factor.Zhu et al. [12] took non-sinusoidal unbalanced systems as the research object.They proposed a set of pollution assessment indicators that comprehensively consider harmonics and unbalances in view of the shortcomings of traditional pollution evaluation indexes.However, this method is too complex and not conducive to practical engineering applications.To sum up, the existing unbalanced pollution assessment indexes are a single assessment of voltage or current imbalance.There is not a set of comprehensive evaluation indicators suitable for the power network both with and without neutral lines at the same time.
To reasonably and accurately evaluate the factor of unbalanced pollution in power systems, this paper put forward a set of novel indexes for evaluating the unbalanced factor applicable to different connected systems through a detailed decomposition of apparent power to address the problems existing in existing methods and indicators for evaluating unbalanced pollution.With the help of specific simulation results, the rationality and practicality of the new indicators are analyzed.

Traditional unbalanced factor assessment indexes
For the three-phase power network, the calculation expression of three-phase voltage and current unbalance factor are as follows [7].
where U11, U12, and U10 are the root-mean-square values of the positive, negative, and zero sequence components of the fundamental voltage, respectively; I11, I12, and I10 are the root-mean-square values of the positive, negative, and zero sequence components of the fundamental current, respectively.
In three-wire power networks, we can only measure three-phase line voltage and are unable to directly calculate the sequence component.Therefore, IEC61000-4-30 recommends using the effective value of line voltage to calculate the voltage unbalance factor [10].
U and CA U are the effective values of three-phase line voltage, respectively.Some scholars have proposed a current unbalance factor calculation method that can be applied to both systems with and without neutral lines [11].The expression of the current balance factor is: Therefore, the expression of the current unbalance factor is: 12 10 From Expressions (1) to (7), it can be seen that traditional unbalanced pollution assessment indexes are based on the voltage or current sequence components to assess the unbalanced factor of the system or customers.They cannot integrate the three-phase comprehensive unbalance of the system or customers.When there are multiple unbalanced sources in the regional power grid, the electric power department first needs to address the largest pollution source, which requires assessing the harm level of many unbalanced sources.If traditional indicators are used to identify pollution sources, there may be conflicting identification results corresponding to different indexes.At this time, how to choose the order of treatment?To solve this problem, this paper attempts to put forward a set of new evaluation indexes which can comprehensively reflect the unbalanced pollution factor of the system and customers.

The sequence component expression of apparent power under unbalanced conditions
IEEE Std.1459-2010 provides expressions for equivalent voltage U e1 and equivalent current I e1 for systems with different wiring types.In three-phase power networks with neutral lines, the expressions for the U e1 and I e1 are as follows [13].
In three-phase power networks without neutral lines, their expressions are as follows.
For a three-phase power system with the neutral line, the equivalent apparent power can be further expressed as: where e1 where, except that the fundamental unbalanced apparent power is meaning represented by the other physical quantities is the same as that in Expression (12) and will not be discussed here.

A new index for evaluating the factor of unbalanced pollution
Based on the sequence component expression method of equivalent apparent power in Section 3.1, this paper attempts to propose a new set of comprehensive unbalanced pollution evaluation indexes from the perspective of apparent power.In three-phase power networks with neutral lines, the calculation expression for the new unbalanced pollution factor evaluation index is: In three-phase power networks without neutral lines, the calculation expression for the new unbalanced pollution factor evaluation index is: Compared to the existing evaluation indexes, the new unbalanced pollution factor evaluation index not only considers the voltage and current unbalances as a whole but also comprehensively considers the negative sequence and zero-sequence unbalances.Using this index in practical projects can more comprehensively and accurately monitor and evaluate the factor of unbalanced pollution at PCC points or three-phase customers, facilitating the power supply department to develop an unbalanced source control plan.

Simulation model of three-phase power networks with neutral line
Figure 1 shows the designed simulation circuit model of the three-phase power network with the neutral line.The three-phase voltage on the system side is balanced, and the effective value of the power supply phase voltage is 220 V, and The customer side is connected to two unbalanced three-phase power customers.The parameter values of each component are shown in Table 1, and all parameter values are obtained based on relevant information and actual engineering experience.According to the data in Table 2 and relevant calculation expressions, the percentage values of each assessment index are obtained, as shown in Table 3.  indexes, the corresponding values for customers 1 and 2 are equal, and it is impossible to determine which party is the main responsible party for pollution.When using the 2I  index, the value corresponding to customer 1 is smaller than customer 2, indicating that customer 2 is the main unbalanced pollution source.When using the 0I  evaluation index, the value corresponding to customer 1 is greater than customer 2, indicating that customer 1 is the main source of unbalanced pollution.4. The percentage values of each evaluation index calculated according to Table 4 and related expressions are shown in Table 5.As can be seen from the data in Table 5, unbalanced pollution in the grid is mainly caused by the negative sequence component, and the "contribution" of the zero-sequence component is zero, which is consistent with the actual situation.When using traditional evaluation indexes, it can be determined that customer 1 is the main pollution source.When using the new evaluation method put forward in this paper, it can also accurately determine that customer 1 is the main pollution source.

Conclusions
This paper proposes a new set of unbalance factor assessment indexes suitable for different wiring forms from the perspective of apparent power, aiming at the problems existing in existing unbalance factor assessment indexes.The main contributions of this paper are: (1) Traditional evaluation methods of unbalanced factors are summarized and analyzed.The results indicate that they lack comprehensiveness, that is, the corresponding evaluation results are inconsistent or contradictory when different evaluation indexes are used.
(2) According to the IEEE Std 1459-2010 standard, the apparent fundamental power is represented by sequence components, the sequence component calculation expression of each apparent power is defined, and the meaning of each physical quantity is clear.
(3) A new unbalance evaluation method is proposed from the perspective of apparent power to solve the problems in traditional evaluation methods.Besides, the calculation expressions of the new evaluation indexes in a three-phase power network with a neutral line and a power network without a neutral line are derived, respectively.
(4) The proposed new method can not only accurately assess the factor of unbalanced pollution of the system and customers but also can be used to locate unbalanced sources, thereby laying the foundation for precise treatment of the unbalance sources in the future.

Figure 2 Figure 2 .
Figure 2. Simulation circuit model of a three-phase power network without a neutral line Through simulation, we get the measured values of each phase voltage and current for each user.Then we get the sequence component values of voltage and current through sequence component decomposition (a power network without a neutral line does not have the zero-sequence).Their effective values are shown in Table4.
In three-phase power networks without a neutral line, since there is no zero-sequence component, the equivalent apparent power can be further expressed as:

Table 2 .
A1 /mH L B1 /mH L C1 /mH L A2 /mH L B2 /mH L C2 /mH Through simulation, we get the measured values of each phase voltage and current for each user and then get the sequence component values of voltage and current through sequence component decomposition.Their effective values are shown in Table 2. Sequence component value of each customer a /mH L n /mH L

Table 3 .
Each pollution assessment index value Object

Table 4 .
Sequence component value of each customer

Table 5 .
Each pollution assessment index value Object