Load Shedding based on Programmable Logic Controller

With a Programmable Logic Controller (PLC) scheme, load shedding is initiated based on the total load versus the number of generators online and/or detection of under-frequency conditions. Each substation PLC is programmed to initiate a trip signal to the appropriate feeder breakers to shed a preset sequence of loads. This static sequence is continued until the frequency returns to a normal, stable level.

EG Energy Controls offers a load shedding system that is based on programmable logic control. These systems have been used in industrial and commercial applications for over 15 years. With the onset of the internet, these systems have been upgraded to allow users to see online their demand usage and change load shedding priority with a click of a button.

How Does It Work?
The Demand Controller can receive the kW/KVA values from 2 different kinds of inputs:

• Pulse Input (utility meter)
• Analog/Digital Input
a) Meter
b) Current Transducers

The system is installed in the electrical room and is connected to sheddable loads like:
• Medium Temperature Compressors
• Low Temperature Compressors
• Electric Heaters
• Lighting


Old Demand 400 kW
New Demand 340 kW
Demand Reduction 60 kW

Real Demand Cost $10.31
Total Savings per Month $412.40
Yearly Savings $4,948.80*

*Savings will vary depending on demand cost and demand load.

A PLC-based load shedding scheme offers many advantages such as the use of a distributed network via the power management system, as well as an automated means of load relief. However, in such applications monitoring of the power system is limited to a portion of the network with the acquisition of scattered data. This drawback is further compounded by the implementation of pre-defined load priority tables at the PLC level that are executed sequentially to curtail blocks of load regardless of the dynamic changes in the system loading, generation, or operating configuration. The system-wide operating condition is often missing from the decision-making process resulting in insufficient or excessive load shedding. In addition, response time (time between the detection of the need for load shedding and action by the circuit breakers) during transient disturbances is often too long requiring even more load to be dropped.

Load shedding serves as the ultimate guard that protects the power system from a disturbance-induced collapse. Normally, this critical load preservation is done with the use of under-frequency relaying and PLC-based schemes.

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