Adding PI-Controller Functionality with SCADA Concentrator Logic
By J. Phillip Gray & Eric Mitchell
A proportional-integral (PI) controller provides process control loops in many industrial process applications. Power delivery can benefit from the use of the feedback loops that PI controllers can provide in applications such as voltage or power control. Unlike many industrial loops with one process variable (PV), power delivery loops can require multiple PVs and other external conditional state variables.
This article discusses a recent project that provides real power flow across an international border using a Phase Shifting Transformer (PST) located approximately 81 circuit miles from the controlled point. A PI controller is used to adjust the PST tap position based on a setpoint (SP) supplied by the Independent System Operator (ISO). The controller used can only calculate one PV in the controlled loop. However, the feedback loop for the project discussed also requires complex logic for automatic SP and bandwidth adjustments, conditional state changes and anti-hunting that most transformer tap changers cannot perform. In industrial applications external loop adjustments can be handled with a programmable logic controller (PLC), however a device already exists in many of today’s substations that can also perform these external calculations, the SCADA data concentrator.
There are many advantages to using the station data concentrator to add PI controller functionality. In this project, loop tuning was made simple by the use of the station data concentrator. Although PLC scan times are much quicker than the poll rates of the data concentrator, the slow reaction of tap changer mechanics make the polled data a viable option for collecting multiple PVs, evaluating/reacting to conditional state changes and adjusting provided SP. We will examine these and other factors for determining use of data concentration over PLC to monitor and react to external loop variables.
This paper was presented at the Power and Energy Automation Conference 2016 (PEAC).
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