Increment of Fault Current due to integration of Distributed Generation and its impact on Distribution Network Protection

Background One of the new technologies in generating power near the distribution system is called distributed generation which has supportive and destructive characteristics to the power system protection. One of the destructive characteristics of distributed generation is increasing the level of fault current to the protective equipment of the power system. In addition to increment of fault, it also alters the radial nature of the power distribution system and cause the power bidirectional rather than unidirectional. Integration of distributed generation to the distribution network causes increment of fault current effect, reliability drop, and affects security of protection system. The level of failure of protection be contingent on type, size, location and number of distributed generation. This fault current can cause a great damage to the electrical equipment with the miss operations of protective devices. The main aim of this paper is analysis of the fault current level to the protection of distribution network due to the integration of distributed generation which concerns on solar distributed generation, wind distributed generation and combination of solar and wind distributed generations at a time. This paper conducts the analysis for the increment of fault current by the integration of distributed generation and its impact on distribution network protection. integration of distributed generation to the distribution network have great value in terms of power system protection.


Introduction
Integration of distributed generation (DG) into distribution network (DN) in a radial distribution system (DS) has positive and adverse impacts to the power system [1]. Electrical power system needs plenty of protection mechanism to feed the requirement of the demand by having good reliability performance indices, best security level of supply, minimized harmonics effect, and better quality of power for power demand at any time. The power system can be classified as generation, transmission, and distribution and it needs warmly protection from any sorts of faults. The faults may come from the aging of equipment, outage of the use of the materials, human error in the system of operation, fault due to trees in transmission and distribution of power system, miss operation of protection device, overcurrent, overvoltage, and addition of DG to the DS without protection coordination of the system and from synchronization problem.
The art and science of protecting the components of power system and people during fault condition is called power system protection. In the absence of detecting fault there are conditions which can give hazard output to the power system and the users. Those situations are the over heating's of power system devices, higher or lower value of system voltages, instability of the power system and the conditions are risky. The operation of electrical apparatus with various levels of voltages are placed in open or enclosed for their safety of their health. Protection is needed for the abnormal conditions that can be necessary for electrical equipment safety and safety of humans [2]. The main objective or role of power system protection is detecting and isolating abnormal conditions. These abnormal conditions are happened in electrical power system that are caused by faults, equipment failure and overloads. In power system protection there are also roles that can be classified as secondary which is the result of primary roles and this can be minimizing equipment damages, personnel safety providing's and maximum continuity of services. Those roles listed above are fulfilled by the designing parameters or criteria of power system protections. Reliability, selectivity, speed of operation and economics are some of the design criterion [3].
Integration of DG to the DN causes increment of fault current effect, reliability drop, and affects security of protection system. The level of failure of protection be contingent on type, size, location and number of DG. This fault current can cause a great damage to the electrical equipment with the miss operations of protective devices. Faults that can be occurring by miss operation of protection device can be change from temporary fault to permanent fault. In addition to this, integration of DG complicates and disturbs the operation of protection arrangement.
The integration of DG may reason of the overloading of protective equipment from their ratings. Traditional DN does not consider the futurity of the protection system due to its cost nature.
The main aim of this paper is analysis of the fault current level to the protection of distribution network due to the integration of DG to the DN. This paper concerns only two types of DG and their impact on the protection of DN. The two DGs are wind and solar and their combinations (only solar DG, only wind DG and solar and wind DG at a time).

Fault analysis of distribution network with integrations of DG
DN is one of the main part of the electric power system which is characterized by design short circuit capacity with the relation of thermal and mechanical equipment competence and constructions [6].

3.1.Fault or short circuit
The event which results unwanted damage on a power system that need to be protected before causing broad damage is called short circuit current [7].The flow of current in an improper way or path of massive magnitude that can be cause damages of equipment which leads to interruption of power, death or injuries of personals is called fault or short circuit. It is also defined as the abnormal connection of relatively low impedance between two points of different potential.
The amount of equipment damage can be determined by the amount or magnitude of fault current with its duration or life span of fault. The level of voltage also varies and affects the insulation of equipment at the time or case of increase or equipment start-up failure if the level of voltage is minimum. As this case is happen, it results the neutral potential difference increment [3], [8]. Faults can be series fault or parallel fault types [9].

A. Series faults
Series faults are faults which can be happen by the causes of: LG: the contact of any of the three lines to the ground is called single line to ground fault.
Three-phase fault: the fault which is occurred by the contact of all the three lines at a time. This fault type may or may not include the ground by its naming. The contact of three lines including ground and without including ground is call by three phase faults.
Fault types are graphically presented in the figures below:  Line-Line (LL) 8

IEC 60909 standard overview
For each of specific location and time of short circuit perspectives in power system IEC 60909 standard derives the maximum and minimum short-circuit currents [7]. The analysis of short circuit current in 50Hz or 60Hz 3Φ ac system is also can be done by using IEC 60909 standard.
Nominal voltage of the system with rated values of the equipment is the only way to calculate fault current level [6].

Short circuit calculation overview
Accidental contacts in a minimal impedance value between an electrical circuit that may be two or more and operates with potentials differently are represented by short circuit. In the network of electric system short circuit can occur in four different types. These are 3Φ symmetrical (which have a characteristics of low frequency rate occurrence from 5% to 10% and used for as the basic element in the studies of electrical network operating and design), phase to phase, phase to phase to ground and phase to ground short circuits [12].

Components of short circuit
There are two types of components in a simple + inductive circuit. These components are decaying ac and dc component. By summing these two components the total current can be provided. The detail of the two components is discussed in [7].

3.2.Fault current calculation
Fault values of a circuit total resistance and reactance allows to determine fault current of installation to be calculated. Sum of respective network elements are obtained from the total fault reactance and resistance and the equivalent impedance is expressed as follows [13].
By using the equivalent impedance, voltage factor "c" and nominal voltage " ", a three-phase symmetrical fault current level can be determined. A fault which generates highest current is generally considered as the symmetrical fault level current and given by: In the absence of rotating machine or having low effect of it, a three-phase symmetrical fault level current value represents steady state current. The breaking capacity of the protection device is determining by this steady state current and it is taken as a reference to determine the breaking capacity of the protection device [13].

Fault level calculation in DN with DG
The level of fault current with the effect of fault is expressed in terms of power or current. This can indicate the increment of power and the fault level in PU is expressed as below. It is the collection of inner impedance of Thevenin equivalent circuit, z th and fault current related to the nominal current, i [14], [15].
The fault current in a faulted feeder is changed by the connection of DG to the distribution feeder and the fault current rate depends strongly on the DG ability to contribute the fault current [16]. Calculating the fault levels are performed on a 15KV DS. Since the algebraic sum of short circuit current is contributed from the network such as motor loads, generators and transformers, the initial short circuit, which is three phase symmetrical fault, is calculated by the following equations [17].

Fault contribution of the main grid and the DG
When the grid contribution is minimum to the short circuit, reversely the DG contribution to the short circuit is maximum [16].

3.3.1.1.Fault contribution of the main grid
Fault contribution of MG is the fault only without the integration of any type of power source and with fault happened to the system (with any type of fault). The fault is happened at bus A and the magnitude of the fault is expressed as equation (4) Figure 3 fault contribution of the main grid For the calculation of the main grid fault level contribution eq. (4) is used.
Where, : main grid impedance at the connection point Q, : transformer impedance, : series reactor (R) (if any), : correction factor, : line impedance (4) Quantities which are calculated above are obtained by using the following.
Where symbols , , and " used in the calculations are fault voltage of transformer, load loss at load current and initial symmetrical fault current at the high voltage connection point Q respectively.

3.3.1.2.Fault contribution of DG
After the integration of DG and fault happened at point a (fault location) near bus B4 the analysis becomes as figure (5). The fault contribution of the grid and the DG is calculated as shown in equation (9). U r = I nw * Z 23 + (I nw + I DG ) * Z 3a Where: Z r : apparent impedance; U r : voltage measured by the relay; I nw : grid current; Z 23 : line impedance from bus b2 to b3; I dg : DG current contribution; I f : fault current; Z 3a : impedance between z3 and fault location (point a)

3.4.DG Influence on Protection Scheme of Distribution Network
Integration of DG have influences on the protections of the DN schemes. These influences are protection blinding, untrue or false tripping, prevention or prohibition of automatic reclosing, reverse power flow and lack of synchronization of automatic reclosing [16], [18].

Protection blinding
This is one of the influences of the DG to the protection scheme of the DN. At the event of fault including DG short circuit current is contributed by both the MG and DG and it depends on the configuration of the network, size of the DG and the impedance of the grid. Therefor the influence DG on the protection in protection blinding is described as follows. Figure 5 Protection blinding on the DN Figure 6 Thevenins equivalent of Figure 6 The equivalent Thevenins impedance Z th is calculated based on figure 7 and it becomes: Where Z s − grid impedance including transformer, Z g − impedance of generator, Z l −impedance of the line and − relative length of the line which is between 0 & 1 Calculating the short circuit current in each phase is the next step by using the Thevenins equivalent impedance Z th and the Thevenins equivalent voltage U th .
The current which is contributed by the grid is also calculated by using grid impedance including transformer Z s , impedance of generator Z g , impedance of the line Z l and relative length of the line l. Therefore, the equation becomes: Z g (Z s + l * Z l ) + Z g * I K∞ = Z g (l * Z l ) + Z g * I K∞ = 1 l * Z l Z g + 1 * I K∞ (14) This indicates that the location and size of generator in the feeder the short circuit current contribution of the grid. When the value of the l * Z l Z g is small then I grid ≈ I K∞ this is because of smaller length or smaller capacity of the installed generator which have larger Z g . S k ′′ → ∞ is never happened at the real world and equation (14) no longer holds and specially S k ′′ in MV grid can have low value this leads the non-negligence of the impedance. Therefore, effect of DG on short circuit current contribution on the main grid can be explained in equation (12).
Equations (13) & (14) are not linear with location and size of DG and also I grid is not linear then the total short circuit current determined by equations (12) & (13). Due to the week grid Z s and Z g are large and short circuit contribution of the grid decreases and the short circuit stays with no detection by the low grid contribution of short circuit and it is unable to reach to the pickup current of the feeder relay. The above explanation is called blinding of protections [18].

False tripping
This is the process in which the disconnection of healthy feeder because of the influence of DG

4.1.Power system protection
In electrical power system there are three main processes. These processes are production, transmission and distribution of electrical power to the demand. Making these processes as safe as possible from the effect of failure and preventing the power system from risk is called power system protection. The power system protection goals are public safety, equipment protection and power system integrity [19].
The popularity of DG is increased due to increment of demand for energy and its emphasis of The criterion that the power system protection which are fundamentals of protection coordination must have satisfied the following five. These are reliability, selectivity, speed, simplicity and economics. The detail of these lists are as below [3], [20], [21].

A. Reliability
The level of dependability and security depends on the high reliability of protection system.
The reliability of the protective equipment (relays) have a percentage value above 99% and this indicates the device have good characteristics which is operate correctly with less malfunctioning.

B. Selectivity
As the name indicates the protective equipment must have the ability to select the faulty section to unfaulty for the purpose of disconnecting the system that have a problem that leads the overall system unhealthy. The protective devices must be protecting both the highest short circuit current Ifmax and the lowest short circuit current Ifmin.

C. Speed of operation
For the purpose of adequate protection isolation of the affected section of the power system is mandatory and if the protective devices are speedy to isolate the affected section it is used to minimize the short current magnitude and it leads less equipment damage. The rate of damage to electrical equipment with risk to personnel is depends on the flow of fault current durations.
Power system protection equipment must operate as fast as possible without stability compromising [21], [22].

D. Simplicity
In electrical power system there are different types of components inside the system. From those components protective device is the one. When the protective device is designed by engineers it must be in a minimum protective equipment and associate circuit to achieve the level of protection.

E. Economics
One of the constraints of designing equipment is its cost. The design is said to be good design when it fulfills the coordination of good protection and less cost.

4.2.Grid connected DG protection
Integration growing of DG into the DN requests the approach of traditional protection system and theories. When DG is integrated to the DN, it changes from passive network to active network since it gets generated power to it's near. This leads coordination problems of circuit breaker, fuses and reclozer and also changes the direction and magnitude of fault current that causes the coordination problem. The influence of DG depends on the penetration level, type of DG and interfacing device type (i.e. power electronics devices or synchronous machine) [23].

Result and Discussion
Integration of DG into existing DN have adverse impact on increasing level of fault current to the protective devices installed in the system. This increased level of fault current causes protective device blind, tripping without its scheduled time or coordinating time interval, and needs to be replace the protective device by another. In this paper section there are different sorts of scenarios to be studied. From those scenarios base case fault current level analysis result, impact on fault current level with two selected DG types are discussed.

Scenario I: Base-case fault current level
The base case fault current calculation is shown in figure 11 which is calculated without the integration of DG to the system and considering all the 62 buses as fault location turn by turn.

Scenario II: Fault current levels for the selected weak buses
By using loss sensitivity factor this research work identifies four weak buses. These buses are bus 31, bus 32, bus 33 and bus 62 as indicated in figure 12 of the voltage profile.

III. Solar and Wind DG
After the integration of solar and wind DG at the four weak buses which are bus 31, bus 32,

Scenario IV Optimal sized and placed DG impact on fault current level
The selected bus for applying the mitigation technique and fault level analysis with fault level is bus 62.

Conclusion
This  Ethics approval and consent to participate -Consent for publication I a need to the publication of these paper -Availability of supporting data Not applicable

-Competing interests
There is no opposing interest of these paper

-Funding
No funding -Authors' contributions I the author have done the paper which are the basic analysis, documenting, data collection, software analysis and submitting to the publisher.

-Acknowledgements
Thank you considering my paper to publications.