Pilot protection based on novel comprehensive impedance of positive sequence fault component for distribution network

The integration of distributed power sources into the distribution network has changed the topology of traditional distribution networks, resulting in changes in fault characteristics, and brought severe challenges to conventional current protection of distribution network. This article proposes a protection method based on the comprehensive impedance of a new positive sequence fault component. Analyze the fault characteristics of positive sequence fault voltage and current during faults inside and outside the line area connected to different types of distributed power sources. The positive sequence fault components of voltage and current at both ends of the section are used to calculate the estimated line impedances. A novel comprehensive impedance is obtained by their subtraction. The ratio of comprehensive impedance to line impedance is constructed. The protection criterion for short circuit fault is given. The simulation experiments illustrate the effectiveness of proposed protection algorithm, not affected by the fault types, locations, permeabilities of distributed generations, has strong ability to withstand transition resistance.


Introduction
The goal of double carbon points out the direction for China's green transformation.The active distribution network that makes full use of clean energy has low energy consumption [1][2][3].With the access of large-scale distributed power sources, the single-power radiation network becomes multiterminal network, the fault characteristics also change.Conventional current protection loses selectivity and brings harm to distribution network operation, vulnerable to various natural disasters [4].Different types of DG have different characteristics when the distribution network fails, and the increase in DG penetration rate brings certain challenges to the safeguarding of the distribution network.IIDG is greatly affected by power electronic devices and control strategies, and the output of inverter DG such as photovoltaic has intermittent characteristics, which brings difficulties to protection setting and fault power direction judgment [5][6].
In recent years, the scholars have given a lot of research on active distribution network protection.Reference [2] utilized the magnitude and phase disparity of the positive sequence current components at both terminals of the line as the control parameter.Reference [7] established an additional network equivalent model for internal and external negative sequence faults.Different model expressions on the protected line under internal faults and external faults are analyzed and extracted, and a protection scheme based on negative sequence fault additional network model identification is constructed.References [8][9] proposed adaptive current protection and combined it with traditional pilot protection, but it needs to install directional elements to dynamically adjust the threshold in the protection system.Reference [10] suggested a pilot protection strategy for a distribution network with photovoltaics (PV) based on a comprehensive comparison of sequence currents.The protection criterion is constructed by positive sequence current, negative sequence current and zero sequence current, which has low requirement for communication conditions.With the zero-sequence current at the bus side and the zerosequence parameters of the line, Reference [11] establishes the ranging equation whenever there is a single-phase ground fault in each section.The pseudo-root is excluded by the length of the section line, the real root is determined.The location is realized by traversing each section of the system.In Reference [12], the information distortion correction method based on phasor correction is proposed for the situation that the FTU upload information in the distribution network is prone to distortion.The positioning result is corrected by the phase difference between the fault current and the normal operating current, and the influence of fault information distortion on the positioning result is reduced.
In view of the fact that most existing literatures focus on short-circuit faults and do not fully consider high permeability and high transition resistance fault scenarios, this paper suggests a novel comprehensive impedance-based pilot protection scheme for an active distribution network.Through utilizing the positive sequence components of voltage and current on both sides of the line during a fault.the calculated line impedance on both sides is constructed, and then the new comprehensive impedance and its coefficient are constructed.The fault characteristics of short-circuit faults of different types of DG connected to distribution network are analyzed, and the protection criteria of short-circuit faults are constructed.where Es is the system power supply, DG refers to the distributed power generation, which serves as the decentralized power supply in the distribution network, and Load refers to the electrical devices or equipment that consume electrical power in the distribution network.According to the interface of grid connection, the distributed power supply is divided into motor type Distributed Generation (DG) directly interconnected with the grid.and inverter type DG connected to the grid through the inverter.

The construction of new positive sequence fault component comprehensive impedance
This paper defines new calculated impedance of line on M side and N side ZM' and ZN': A novel comprehensive impedance is developed for the positive sequence fault component.
From formula (1)(2)(3), the impedance coefficient k = 0 can be obtained for the external fault in the distribution network with DG.When an internal fault occurs, the downstream of the line is connected to either an Inverter-Interfaced Distributed Generation (IIDG) or a Motor-Generator-Distributed Generation (MTDG) respectively.the influence on the impedance coefficient k is also different.

Characteristic analysis of internal faults with IIDG
With the control strategy, IIDG is solely present in the positive sequence network and only provides positive sequence current.The additional fault network of internal fault with IIDG on downstream is shown in figure 2, where α is the fault location, which represents the of the distance between the fault point and bus M to the total length of the line, ( ) Z represents the equivalent impedance of the downstream line and load of the distributed generation (DG) system.
For the virtual power branch, IIDG is represented by a current source and a resistor in the fault component network.The internal impedance is infinite to the outside, and the constant impedance is equivalent to the back-end impedance n Z , that means . (1 ) , Substituting Eq. ( 4) into Eq.( 3), the impedance coefficient k for internal fault is obtained: Where F is the amplitude ratio of the fault component of the positive sequence current Set at different α, make F change within 3~30, mn  change within 0~180°, calculate the impedance coefficient k with Eq. ( 5).The k curves for IIDG are shown in Figure 3. From Figure 3, it can be seen that for an internal fault in the downstream with IIDG, the impedance coefficient k gradually increases with the increase of the fault location α.In most cases, k is greater than 2. k is larger when it is close to the load side, which makes k have certain advantage.

Characteristic analysis of internal faults with MTDG
When a short-circuit fault happens in a distribution network with a motor-like MTDG., which is similar to a synchronous power supply and equivalent to a linear system, the positive sequence fault additional network of the distribution network with MTDG is shown in Figure 4. ( ) From Eq. ( 6), the amplitude ratio F with m I  to n I  is obtained: ( ) When the fault location is close to the load side (α is close to 1), F is less than 1.For the fault location close to the system side, because the DG capacity is much smaller than the system capacity, nm ZZ  , F is greater than 1.For MTDG, the amplitude ratio F is generally between 0.5 and 2 from the experiments of the literature [13] and this paper.
Set at different fault locations, F changes from 0.5 to 2, mn (0,18 )   , the respective k is calculated.
The k curves for MTDG are shown in Figure .5.From Figure 5, as the fault location α increases, k decreases from large to small and then increases, that is, the k value on both sides of the section is larger, and the k value in the middle section is smaller.
The k values at the beginning and end of the line are higher.basically greater than 1.In the middle of the line, there are 0.4 < k < 1.

Protection criterion
The protection criterion of short-circuit fault in the zone is: When k satisfies Eq. ( 9), it can be judged as an internal fault.After starting the protection by using the protection starting criterion, the voltage and current fault components are calculated, then the impedance coefficient k is obtained.If k > Kset, it is judged to be an internal short-circuit fault.

Simulation experiment
PSCAD/EMTDC software is utilized to construct a model of a 10kV active distribution network, as depicted in Figure 6.The reference voltage is 10.5kV, the transformer capacity is 50MVA, The line parameters for the active distribution network model are (0.17 + j0.34) Ω/km. the length is 4km, the capacity of IIDG is 2.5MW, the capacity of MTDG is 5MW.The active power of the load in the active distribution network model is 0.8 MW, and the reactive power is 0.4 MVA.

Figure 1
illustrates the simplified equivalent diagram of the distributed power distribution network.

Figure 1 .
Figure 1.Simplified diagram illustrating the distribution network incorporating distributed generation.

ZZ
positive sequence voltage fault components at both ends of the line, m I  and n I  represent the positive sequence current fault components at both ends of the line.For external faults or normal status, ' are equal in size, opposite in direction.Therefore, the two calculated impedances of the line under external faults are is the line impedance.For the internal faults, 'M Z and 'N Z are not equal to line impedance due to the influence of fault location, fault type and DG type, and their difference reflects the occurrence of internal faults.

Figure 2 .Z
Figure 2. fault additional network of internal fault with IIDG on downstream.The calculated impedance ' M Z and ' N Z of M and N side during internal faults can be derived.
Eq. (5), k is affected by the fault location α, amplitude ratio F and phase difference mn  .

Figure 4 .
Figure 4. Fault additional network of the internal fault with MTDG on downstream.According to Figure4.The fault component of positive sequence current at both ends of the section is:
Where set K is the protection threshold, and θmn (unit: radian) represents the phase disparity of the fault components in the positive sequence current at both terminals.