Figure 5.2 shows a section of a microgrid where DG1 and Relay 4 are separated by a distance of “x”. At point A, which is the connection point of the DG1, the entire fault current
contribution (IfaultDG1) of DG1 is experienced. . At a distance of x from DG1, at point B, this
current decreases to k14 * IfaultDG1.
In order to calculate the impact coefficient, consider the symmetrical representation of this system under fault conditions given in Figure 5.3. This representation is based on the symmetrical components calculations [177].
Figure 5.3. Symmetrical components scheme
Since these faults are steady faults, Rf is assumed to be zero [178]. From the electrical
equation for the direct loop voltage, the following equation can be written:
(5.6)
Where Vd is the direct voltage output of DG, Ztdis the output impedance, and Vdm is the rated
output voltage of DG which is regulated by the stability and safety code of the grid. Idm is the
fault current supplied, Zd is the impedance of the transmission/distribution line per length and
x is the distance between DG and the relay under consideration.
The fault current supplied is the maximum fault current available, i.e. IfaultDG; and Vd becomes
the rated voltage of the DG. Different fault current capacities of DGs are represented by this output impedance. Rearranging (5.6), the fault current equation can be written in terms of the impedances and the distance as:
(5.7)
There are two unknown parameters in this equation. In order to work them out, an additional equation is required. This is the voltage relation between Vd and Vdm, which is the voltage
that needs to be sustained according to the stability and safety code of the grid. It can be written as in (5.8):
(5.8)
Replacing Vd in (5.6), Idm can be expressed as:
(5.9)
In this fashion, a DG‟s fault current contribution at a distance x can be calculated. Note that
all the parameters on the right hand side of (5.9) are known before any fault occurs. This allows the MCPU to calculate all fault contributions and assign overcurrent thresholds to the relays for proper operation. Once the fault current contribution Idm is calculated, the impact
factor can be expressed as the ratio of Idmto IfaultDG:
(5.10) Where i and r denote the DG and the relay under consideration. The protection system will create a Kmatrix by calculating the impact factors of i-many DGs on r many relays.
(5.11)
From (5.4) and (5.5), the maximum fault currents of every DG can be calculated and the vector IfaultDG, having dimensions [1, i] can be represented as shown in (5.12).
The overcurrent thresholds of relays which represent the thresholds for relay operation can be calculated by the cross product of matrix KT and vector IfaultDG. The result is the relay
overcurrent threshold vector of dimension [r, 1], Irelay.
(5.13)
This equation can be written in compact form as;
(5.14)
MCPU will assign each row of the vector Irelayto the related relay in the network. With the
help of communication lines, the network will be monitored and these values will be constantly re-calculated and updated.
During the course of this work, only balanced three-phase to ground faults have been considered. It is assumed that the microgrids are considerably smaller than the conventional electrical networks. This means that the size of a microgrid is suitable for protection with delayed-type instantaneous relays which implement definite-time grading technique. In the case of a communication failure, the protection scheme can still provide reliable protection until the communication is restored. This is because the communication infrastructure is only critically needed for the update of protection settings. The latest fault current settings shall be kept in the relay until the link reconnects.
Thanks to the local decision making scheme of the proposed method, the relays will be able to operate based on these fault current settings. In this fashion, though it may not have the
most desirable settings, the protection system will be active in case of a communication failure and the microgrid will be protected from catastrophic conditions. The likelihood of a communication system failure right at the point when protection setting updates would be required is very low. It is possible to design a redundant communication network by having multiple meshed lines connect to various switches that relays connect to. Although this will add to the overall implementation cost, it is quite often realized in star topology communication networks especially when high reliability is required.
In this research, only the faults occurring in the microgrid have been considered. Faults in other feeders and/or grids are not discussed in this work. Since the microgrids under consideration are assumed to be sufficiently small, the relays are close to the fault locations in each branch. That is to say, the fault current magnitude does not differ significantly between the actual location of fault and the location of the relay for which the calculations are carried out. This protection scheme has been developed for radial electrical networks. Should an algorithm be developed to assign proper fault current settings and adjust relay hierarchy, the system can be extended to non-radial networks such as ring or meshed networks.