also based on equations (5), (6), and (7). By application of autorecloser and overcurrent relays and creation
of three phase faults in the power network, it has been
shown that there was improvement and optimization of
voltages and currents waveforms in the transmission
line before, during, and after fault occurrences and the
outages were cleared (see Figure 12). As the power
outages occurred as shown in Figure 10, Relay 1 and
Relay 2 were coordinated and sent the tripping signals
to Backup CB and Main CB1, respectively, to open and
isolate the power system grid and trip the network; the
currents increased rapidly, waited for fault clearance
restoration signals from relays, and restored quickly.
The voltages decreased to zero volts instantly, as shown
also in Figure 12. This meant that during the outages, overcurrent and autorecloser relays for extremely
inverse tripped at a very short time, as indicated in the
Table 5 and Figure 17 normally. Whenever a transient
fault has occurred in the system, the autorecloser relay
checked the synchronism of the network and the outages were mitigated and prevented. From voltage parts,
at the 5
th sec, the grid worked normally, after 0.02 sec,
an outage occurred, was recorded, and remained in the
power system for a period of 0.064 secs, after which
the autorecloser with an overcurrent relay commanded
the main CB to trip and isolated the faulted part within
0.064 sec; at 5.084 sec it became normal, protected,
and optimized. Like the fault current’s part, at 5 sec,
the system normally worked within 0.01 sec, when the
network was faulted during 0.04 secs and restored at
5.05 secs. With reference to Figure 17 and Table 5, the
time of operation of overcurrent relays varied, with the
extremely inverse relay the smallest, followed by the
very inverse and standard inverse for both overcurrent
relay incoming and outgoing feeders, respectively. It
should be also observed that the three relay characteristics have been considered during the relay settings.
The standard inverse characteristic took care of faults
within the utility substation. The very inverse characteristic took care of faults at the midpoint of the feeder,
while the extremely inverse characteristic took care of
faults at the far end of the feeders.
4 CONCLUSIONS AND RECOMMENDATIONS
In this paper, we presented a frequency-based load
control scheme to balance demand with supply and
regulated frequency in a power system. We set up a
load control optimization problem with the objective
of minimizing the total frequency variation/error and
the constraint of demand–supply power balance. The
load frequency PID controllers using local frequency
measurements per unit to solve the load disturbances
and prove the convergence of the controllers have been
designed. Numerical experiments have demonstrated
that the proposed PID load control scheme was able
to relatively quickly balance the power demand with
supply and restore frequency under generation-loss.
It can be observed that under steady state condition,
incremental turbine power output of single and two
area in which disturbance were given, became equal
to the load disturbance and that of other area became
zero along with zero tie-line power deviation with
the controllers used in frequency load control of the
case studies. Further research should deal with more
than two frequency control areas in a microgrid to
ensure the reliability and the security of the generation,
transmission, and distribution systems. The overcurrent relay characteristics were developed and modeled
in MATLAB/SIMULINK. The performance characteristics of the overcurrent relay were evaluated at a
location with three-phase faults. The protection of a
110/15 kV substation with two bus systems and its
relay settings has been presented. The optimal coordination of overcurrent with extremely inverse time is
better than very inverse and standard inverse characteristics because overcurrent relay feeders as the main
protection and overcurrent incoming as backup protection were able to work faster to protect the distribution
system from phase-to-phase short circuit currents that
occur in the feeders. The information like fault data,
transmission line data, load data, power system data,
and different relay types, like Mho relay, differential
relay, and impedance relay need to be considered for
upcoming research. The model can be extended to
other categories of relay characteristics and fault types
to implement real-time relay modeling of the power
system.
ACKNOWLEDGMENT
This work was supported by Moi University, Eldoret,
Kenya under Africa Center of Excellence II for Phytochemical, Textile and Renewable Energy (ACE II
PTRE) through World Bank, Prof. Paul Wambua,
Prof. Simiyu Sitati and Prof. Jean Bosco Byiringiro.
REFERENCES
Aimable, N. (2020). Power blackout occured in Rwanda
Electricity Grid in 2019 and later 2020. Kigali: Aimable
Nsanzimana.
Akbar A., M. P. (2011). Optimal coordination of overcurrent
and distance relays by a new particle swarm optimization method. International Journal of Engineering and
Advanced Technology, 93–98.
Akbar, M. (2011). Optimal Coordination of Overcurrent and
Distance Relays by a New Particle Swarm Optimization Method. International Journal of Engineering and
Advanced Technology, 93–98.
Behera, N. (2019). Load Frequency Control of Power System.
Rourkela, India: P.K Ray.
Congzhi Huang, J. L. (2017). Linear active disturbance
rejection control approach for load frequency control of
two-area interconnected power system. Transactions of
the Institute of Measurement and Control, 1–9.
Dobson. (2007). Protection relay signals. . Mumbai: Dobson.
G. S. Thakur, A. P. (2014). ‘Load frequency in Single
area with tradition Ziegler-Nichols PID tuning controller. International Journal of Research in Advanced
Technology.
Gerard, E. A. (2020). Total Duration of Power blackouts in
Rwanda Electricity Grid Records. Kigali: Energy Utility
Corporation Limited.
60
of three phase faults in the power network, it has been
shown that there was improvement and optimization of
voltages and currents waveforms in the transmission
line before, during, and after fault occurrences and the
outages were cleared (see Figure 12). As the power
outages occurred as shown in Figure 10, Relay 1 and
Relay 2 were coordinated and sent the tripping signals
to Backup CB and Main CB1, respectively, to open and
isolate the power system grid and trip the network; the
currents increased rapidly, waited for fault clearance
restoration signals from relays, and restored quickly.
The voltages decreased to zero volts instantly, as shown
also in Figure 12. This meant that during the outages, overcurrent and autorecloser relays for extremely
inverse tripped at a very short time, as indicated in the
Table 5 and Figure 17 normally. Whenever a transient
fault has occurred in the system, the autorecloser relay
checked the synchronism of the network and the outages were mitigated and prevented. From voltage parts,
at the 5
th sec, the grid worked normally, after 0.02 sec,
an outage occurred, was recorded, and remained in the
power system for a period of 0.064 secs, after which
the autorecloser with an overcurrent relay commanded
the main CB to trip and isolated the faulted part within
0.064 sec; at 5.084 sec it became normal, protected,
and optimized. Like the fault current’s part, at 5 sec,
the system normally worked within 0.01 sec, when the
network was faulted during 0.04 secs and restored at
5.05 secs. With reference to Figure 17 and Table 5, the
time of operation of overcurrent relays varied, with the
extremely inverse relay the smallest, followed by the
very inverse and standard inverse for both overcurrent
relay incoming and outgoing feeders, respectively. It
should be also observed that the three relay characteristics have been considered during the relay settings.
The standard inverse characteristic took care of faults
within the utility substation. The very inverse characteristic took care of faults at the midpoint of the feeder,
while the extremely inverse characteristic took care of
faults at the far end of the feeders.
4 CONCLUSIONS AND RECOMMENDATIONS
In this paper, we presented a frequency-based load
control scheme to balance demand with supply and
regulated frequency in a power system. We set up a
load control optimization problem with the objective
of minimizing the total frequency variation/error and
the constraint of demand–supply power balance. The
load frequency PID controllers using local frequency
measurements per unit to solve the load disturbances
and prove the convergence of the controllers have been
designed. Numerical experiments have demonstrated
that the proposed PID load control scheme was able
to relatively quickly balance the power demand with
supply and restore frequency under generation-loss.
It can be observed that under steady state condition,
incremental turbine power output of single and two
area in which disturbance were given, became equal
to the load disturbance and that of other area became
zero along with zero tie-line power deviation with
the controllers used in frequency load control of the
case studies. Further research should deal with more
than two frequency control areas in a microgrid to
ensure the reliability and the security of the generation,
transmission, and distribution systems. The overcurrent relay characteristics were developed and modeled
in MATLAB/SIMULINK. The performance characteristics of the overcurrent relay were evaluated at a
location with three-phase faults. The protection of a
110/15 kV substation with two bus systems and its
relay settings has been presented. The optimal coordination of overcurrent with extremely inverse time is
better than very inverse and standard inverse characteristics because overcurrent relay feeders as the main
protection and overcurrent incoming as backup protection were able to work faster to protect the distribution
system from phase-to-phase short circuit currents that
occur in the feeders. The information like fault data,
transmission line data, load data, power system data,
and different relay types, like Mho relay, differential
relay, and impedance relay need to be considered for
upcoming research. The model can be extended to
other categories of relay characteristics and fault types
to implement real-time relay modeling of the power
system.
ACKNOWLEDGMENT
This work was supported by Moi University, Eldoret,
Kenya under Africa Center of Excellence II for Phytochemical, Textile and Renewable Energy (ACE II
PTRE) through World Bank, Prof. Paul Wambua,
Prof. Simiyu Sitati and Prof. Jean Bosco Byiringiro.
REFERENCES
Aimable, N. (2020). Power blackout occured in Rwanda
Electricity Grid in 2019 and later 2020. Kigali: Aimable
Nsanzimana.
Akbar A., M. P. (2011). Optimal coordination of overcurrent
and distance relays by a new particle swarm optimization method. International Journal of Engineering and
Advanced Technology, 93–98.
Akbar, M. (2011). Optimal Coordination of Overcurrent and
Distance Relays by a New Particle Swarm Optimization Method. International Journal of Engineering and
Advanced Technology, 93–98.
Behera, N. (2019). Load Frequency Control of Power System.
Rourkela, India: P.K Ray.
Congzhi Huang, J. L. (2017). Linear active disturbance
rejection control approach for load frequency control of
two-area interconnected power system. Transactions of
the Institute of Measurement and Control, 1–9.
Dobson. (2007). Protection relay signals. . Mumbai: Dobson.
G. S. Thakur, A. P. (2014). ‘Load frequency in Single
area with tradition Ziegler-Nichols PID tuning controller. International Journal of Research in Advanced
Technology.
Gerard, E. A. (2020). Total Duration of Power blackouts in
Rwanda Electricity Grid Records. Kigali: Energy Utility
Corporation Limited.
60