Frequency actuated load shedding programs have been implemented across power systems to safeguard systems during periods of declining frequency. These programs automatically disconnect a portion of load equal to or greater than a generation deficiency to maintain balance. Industrial facilities also use load shedding schemes, which have increasingly been integrated with computerized power management systems to provide automated load shedding. To maintain stability, automated load shedding systems are necessary as disturbances can plunge a system into a hazardous state faster than an operator can react. The proposed system implements an embedded system using a PIC microcontroller to constantly monitor frequency changes and disconnect loads according to priority to perform automated load shedding.

1. FREQUENCY ACTUATED LOAD
SHEDDING WITH AUTO TRIPPING
During the last few years, there has been expanded effort throughout the country
to install frequency actuated load shedding programs for the purpose of providing
means to safeguard a power system should a condition of declining frequency
occur. Hand in hand with a load shedding program a convenient means must be
provided for restoring load after the system returns to a normal state.
To ensure system stability and availability during disturbances, industrial facilities
equipped with on-site generation, generally utilize some type of load shedding
scheme. In recent years, conventional underfrequency and PLC-based load
shedding schemes have been integrated with computerized power
management systems to provide an “automated” load shedding system

2. It is an elementary case of ‘power economics’, load demand versus generation
supply. When a power system is in stable operation at normal frequency, the
total mechanical power input from the prime movers to the generators is equal to
the sum of all running loads, plus all real power losses in the system.
The frequency conditions of the overall system will directly depend on the
amount of active power that the generator prime movers could deliver to the
system. Also, the stored energy of the prime movers plays an important roll on
the system behavior. This stored energy varies drastically from gas, thermal, to
hydro units.
To halt the drop in frequency, it is necessary to intentionally, and automatically
disconnect a portion of the load equal to or greater than the generation deficiency
in order to achieve balanced power economics while maintaining system stability.
Automated load shedding systems are necessary for industrial power systems
since sudden disturbances can plunge a system into a hazardous state much
faster than an operator can react. These automated schemes must be designed
and implemented to possess in-depth knowledge of system operating
parameters and must rely on time sensitive monitoring and control
communication networks in order to achieve the desired outcome of fast and
optimal load shedding at the onset of a disturbance.

3. Proposed System:
To overcome the above mentioned scenario We have implemented a
Embedded system based frequency actuated load shedding scheme, which
constantly monitors the change in frequency in the substation, and disconnects
the loads according to the priority.
Hardware modules
1. PIC Microcontroller
2. PT and CT for Voltage and current sensing
3. PT for Frequecny sensing
4. Processing circuits
5. Relays for Tripping loads
6. RS232 interface
Software
1. MPLAB IDE
2. Hitech C
3. Visualbasic 6.0

4. Block Diagram:
RS232
Power
Supply
Unit
PIC
Microcontroller
PIC 16F877
Signal Conditioning
Unit (FWPR)
Alarm
Driver
POWER
Alarm
Relay Drivers
PT
CT
PT
Load 3
Voltage
Current
Frequency
Load 1
Load 2