ABSTRACT
A smart grid is a technology that divides the electricity
grid into a two-way flow of data and electricity. The
technology includes energy measures ad operations
such as smart appliances, smart meters, energyefficient resources, and more. The technology of the
smart grid aims to support and integrate renewable
energy into the conventional energy source. Not only
does it use alternative energy sources, but the smart
grid technology also allows consumers to monitor
energy consumption.
DR / Abd El-Latif
Smart Grid
SMART GRID
Under Supervision: DR / Abd El-Latif
by: Eng / Abdallah Khaled
1 Distribution System and Relay Coordination
The problem of the relay coordination under different fault conditions has been solved for the three-phase
unbalanced IEEE 13-bus distribution test system shown in Fig. 1. This system contains a mixture of
single, double and three phase lines and loads. Note that the system lines 684-653 and 684-611 are single
phase; lines 671-684, 632-645, 645-646 are two phase and the remaining lines are three phase.
Directional over-current protection is used against fault current that could circulate in both directions
through the system. When DG is present, there are multiple power sources. Hence, the nature of
distribution network changes with multiple DG units. Consequently, directional relays are needed in the
network. Directional relays should be placed along the line that links the main grid and the DG. Relay
characteristic uses time delay to provide tripping. The relay, which is located at the furthest point from the
source, is tripped in the shortest time. Other relays are tripped in a sequence with longer time delays,
going back in the direction of the source. These relays are classified based on their characteristic curves.
IEC 60255 defines a number of standard characteristics as follows:
where t is the operating time of the relay, TMS is the time multiplier setting, Is is the pickup current of
relay, If is the relay current (overload and/or fault), and n (0.14, 13.5, 80 and 120) are the constants which
depend on the relay characteristics. Since relay operating time t is a nonlinear function of both TMS and
Is, the time characteristics of overcurrent relay are generally nonlinear. In general, the current setting is
selected to be above the maximum short time rated current of the circuit. The pick-up values of phase
over-current relays are normally set at 30% above the maximum load current.
3 Analysis of The Various Fault Scenarios
In this study, the IEEE-13 bus distribution system is chosen as test system to perform different fault
scenarios. Five different regions on the test system such as a, b, c1, c2 and c3 which is shown in Figure 2
are defined to observe the effects of fault currents from the fault location. Four different scenarios are
simulated using four DG connected to different regions mentioned before on the test system. Each DG
unit is chosen as wind turbine which has 1.5 MW capacity considering the total system load. In
simulations, three phase fault is applied to 675-bus for all scenarios and fault currents are observed for
every bus in test system. Thus, it can be analyzed the effect of faults on DGs connected to different
regions and the selectivity of relays.
In first scenario, four DGs are connected to the regions c1, c3, a, and b as shown in Fig 3. Three phase
fault is applied to node 675 and the fault currents are seen in red-dot-lines.
In second scenario, two DGs are connected to region a and others are connected to region b. This
configuration is given in Fig 4.
In third scenario, all four DGs are located to region c1. This location is the furthest point from the fault as
shown in the Fig 5.
Finally, four DGs are connected to region c3 as a nearest location of master source which is a diesel
generator. This configuration is seen from the Fig 6. The fault analyses are performed for all scenarios.
4 Test and Results
Four different scenarios are considered in an island-mode for the study. Line 675, which is an effective
node in distribution system, is selected as fault zone. Four wind turbines (WTs), each with a rating of 1.5
MW and one diesel generator with a rating of 3.125 MW are connected to the system for all scenarios.
The fault currents occurred in the various nodes are given Table 1.
The lines 632, 671, 692, 675 and 680 are investigated due to the fact that the fault substantially affects
these lines, as shown in Table 1. The fault currents change significantly for various scenarios in the same
node. For instance, considering the node 632, a difference of 900 A is observed comparing Scenario 1 and
Scenario 3 during the fault. Similarly, there is a difference of 500 A at the node 692 comparing the
Scenarios 2 and 3. The severe differences in the current values cause coordination problems, especially in
DOCRs. The operation times of each DOCR are calculated according to the Eq. 1 and given in Table 2
for IEEE 13-bus distribution test system without DGs.
In this study, the calculations of relay coordination are carried out separately for four scenarios and the
operating times are obtained which is shown in Table 3. It is to be noted that the fault currents in different
scenarios are the most influential factor on the operating times. For instance, an obvious time difference is
observed for R4-R5 coordination comparing Scenario 1 and Scenario 2. Likewise, R10-R11 coordination
has an important difference in Scenarios 2 and 3.
The time differences cause false operations while disturbing the relay coordination. Besides, unplanned
power outages can be realized in the distribution system and reliability of the protection system can be
decreased. Therefore, it is worthy to mention that the adaptive relay coordination should be implemented
in the system protection of smart grids.