PLC Basics and Voltage Sag Susceptibilities - Part 1
The electrical upgrade of the green carbon facility at the aluminum plant was complete. A myriad of programmable logic controllers (PLCs) and PC-based graphic user interface (GUI) systems replaced the hardwired system installed in the 1960s. The plant manager proudly watched the GUI screens as the fully automated system performed batching, mixing, and conveying operations faster than ever - with little operator intervention.
After spending $10 million on the cutover from hardwired relay control to the new PLC-based system, the change was finally complete. It was a new day for the plant. Then it happened. The lights blinked in the facility - an event that typically occurred at least twice a month. A minor voltage sag (to 75% of nominal), lasting only 5 cycles, occurred on the utility grid. Although it rarely affected the old hardwired system, the humming of the conveyors, crushers, mixers, and batching system stopped.
As the operators began to scramble to restart the automatic operations, confusion reigned as the batch-weighing system couldn't decipher how much of each ingredient was in the hopper and how much more needed to be added. Stopping at different points in its respective mix sequences, the PLC no longer knew how long the 10 batch mixers needed to continue operations to complete its cycles. The plant manager's face reddened with anger as he contemplated the cost of the new system and the reliability of the old scheme. The operators' confusion turned to frustration as they realized the batch in the weigh hopper would have to be scrapped because the system had lost track of the contents.
Events like this happen in many facilities that employ PLC systems in control schemes. Untold millions of dollars are lost when PLC-based control systems are upset by voltage sag events. However, with proper electrical and software design techniques, these systems can be made much less susceptible to voltage sag phenomenon.
The Voltage Sag To understand why PLCs are susceptible to voltage sags, it is important to understand the voltage sag itself. Industrial manufacturers almost always incorrectly assume all events that affect electrical equipment are "power surges," since the shutdown may have occurred during a lightning event. Although overvoltage conditions (known as voltage swells and surges) can occur, EPRI research has confirmed short duration reductions in voltage (voltage sags) lead to the most frequent complaints from industrial customers. These events typically occur when weather, trees, or animals instigate a line-to-ground fault on the utility grid.
The depth of the event seen by the industrial customer is determined by the magnitude of the fault current, stiffness of the grid, and the closeness of the customer's facility to the location of the fault. The duration of the event relates to the breaker-clearing time on the utility system. Typically described in terms of magnitude and duration (Fig. 1), voltage sag events can affect the operation of sensitive production equipment leading to shutdown, malfunction, lost product, and diminished revenue. When a voltage sag results in equipment shutdown or malfunction during normal power system operation, the equipment becomes incompatible with its electrical environment or develops poor system compatibility.
Typical voltage sag durations range from four to 30 cycles, depending on whether the facility is fed from the utility's transmission system (e.g., 69kV or 161kV), which is somewhat stiff, or a utility's distribution system (e.g., 13.8kV or 34.5kV), which typically cannot supply as much fault current.
PLC Basics and Voltage Sag Susceptibilities Fig. 2 shows a typical PLC I/O rack with a power supply, CPU, discrete input and output modules, and analog input and output modules. The following sections outline the function of each of these components and discuss related power quality considerations for each.
PLC I/O rack power supply. Utilizing the typical switch-mode power supply topology, the PLC power supply can be either a pillar of power quality robustness or an Achilles heel. Although available for both AC and DC input power sources, the most commonly procured units use an AC-input source of 120/230VAC.
Small in relative power output, the PLC power supply usually produces from 40W to 80W DC, for use across the I/O rack back plane. The purpose of the unit is to supply DC power to all devices physically mounted in the PLC rack. These devices may include the CPU and communications module(s), as well as discrete and analog I/O modules.
Typically, the PLC power supply does not provide power to field devices such as sensors, transmitters, motor starters, and solenoids. Some PLC manufacturers may use the power supply to provide an analog output signal for control valve and drive interfacing. Others require an additional external power supply for these functions.
Most PLC power supplies also perform continuous diagnostics for line voltages that are outside the tolerance envelope. If the power supply detects a serious problem, it will notify the CPU to halt program execution and shut down process operations. (Fig. 3 displays the general topology of a PLC power supply.)
PLCs monitor either the level of the incoming AC plant line voltage or the level of the power supply DC output to decide when to shutdown during voltage sag events. The leading U.S. PLC manufacturer's power supply literature states the following voltage sag shutdown philosophy.
"Each AC-input power supply generates a shutdown signal on the back plane whenever the AC line voltage drops below its lower voltage limit, and removes the shutdown signal when the line voltage comes back up to the lower voltage limit. This shutdown signal is necessary to ensure that only valid data is stored in memory."
With this philosophy, the manufacturer's most common PLC product will react to voltage sags as short as one cycle in duration. It is also interesting to note that other product lines from the same manufacturer base shut down on the PLC power supply's DC output. Since a DC power supply can inherently store energy in the power supply capacitors, sensing the DC level rather than the incoming AC line voltage can lead to improved system compatibility.
PLC I/O power quality issues. PLC inputs and outputs (I/O) can be grouped into four main categories: discrete inputs (DI), discrete outputs (DO), analog inputs (AI), and analog outputs (AO).
Discrete input (DI) modules are available for AC or DC sensor types. DIs include "on/off" status signals from push buttons, selector switches, motor starter auxiliary contacts, relay contacts, and process sensor inputs such as pressure, flow, proximity, or zero speed. Typical wiring examples for AC and DC discrete input modules as well as typical field devices such as a proximity switch and push-button station are shown in Fig. 4, on page 45.
The susceptibility of the DI module to voltage sags is only relevant if the PLC power supply has not already led to a system shutdown. Since DI modules are designed to react quickly to detect an input status change, they also can react quickly when a voltage sag event occurs. The common response time for AC inputs to detect a transition from "on" to "off" can be as short as 11 ms, which is less than one cycle. For DC inputs, response time for input status changes can be even shorter - 4 ms (1/4 cycle) is possible.
Once the DI module senses a real or perceived change in the status of the input, the PLC program will react. Since the effects of the voltage sag may translate directly to lower voltages at the input terminals of the module, the control system may misinterpret an "on" condition to actually be an "off" condition. Such false negative conditions from a process sensor can lead to the malfunction or immediate shutdown of the process.
In the case of AC input modules, the voltage sag immediately passes to the input terminals of the module. In the case of DC modules, the external DC power supply acts to filter a voltage sag so that the output power to the sensors may not be affected. The ability of the DC power supply to provide this "embedded" mitigation to voltage sags is dependent on the topology, sizing, load, and input voltage. If the DC power supply is unregulated, virtually no stored energy will be present to mitigate the voltage sag. However, a robust power supply means the input sensor signals also will remain robust to voltage sags.
Available in AC or DC types, discrete output (DO) modules act to switch the "on/off" voltage signal to field devices such as motor starters, relays, solenoids, and pilot lights. The susceptibility of the DO module is directly related to the PLC power supply shutdown signal as well as the susceptibility of the individual loads connected to the module. Since the module simply acts as a switch to the individual loads, it has little ability to affect the voltage sag response of the system. If the PLC decides to shut down as a result of a voltage sag, all DO signals typically will drop unless it is specially configured on the I/O rack. Most end users do not opt to allow the outputs to stay powered in this state because such a condition may lead to safety and machine damage issues. A typical AC module and motor starter is shown in Fig. 5.
In the case where the PLC power supply is robust to voltage sags, and field devices (such as motor starters and relays) are susceptible to voltage sags, the process may still malfunction or shut down. To ensure all outputs are robust, the most comprehensive approach is to ensure that the control power voltage source is robust. The system integrator can do this by conditioning the power source in an AC system or by using a robust DC power supply and DC output module, which, in turn, would require the use of DC- powered field devices such as motor starters, relays, and solenoids.
Utilizing DC signal ranges such as 4mA to 20mA, 1V to 5V, or 0V to 10V, analog input (AI) modules receive a continuous DC current or voltage signal from process transmitters. DC power supplies are required to source the voltage or current loops for the AI signals. Therefore, the voltage sag susceptibility of this module and the process transmitters is related to the ability of the external DC power supply to ride through the voltage sag.
Two basic configurations for process transmitters are known as "2-wire" and "4-wire," each of which leads to differing power quality considerations. An external DC power supply runs a 2-wire process transmitter (Fig. 6). This same supply may provide DC power for all transmitters in the system or control cabinet. With a single source of DC power, the AI signals can be made robust to voltage sags if the DC supply is robust.
With the 4-wire transmitter topology (Fig. 7), an external AC voltage source must power the transmitter. In this configuration, the transmitter provides the continuous DC signal to the individual channel on the AI card. The required DC power supply is located within the transmitter itself. For these reasons, you must consider the voltage sag robustness of the AC power source for each of the 4-wire transmitters in the process.
Analog output (AO) modules provide a continuous DC voltage or current signal to field devices. Examples of AO control loops include position control of a proportional valve or the speed control of a motor through an AC or DC adjustable-speed drive (ASD). Depending on the manufacturer and module type, these signals can be sourced by the PLC power supply through the I/O rack back plane or by an external DC supply. Therefore, the stability of the output signal to the field device is dependent on the robustness of the DC voltage source.
When sourced by the PLC power supply, tests indicate the PLC normally will shut down before the DC output voltage and integrity of the control signal is affected. When the PLC shutdown occurs, the analog signal is removed from the field device, which will directly affect the position of a valve, or the speed of a motor. In the case where an external DC power supply is required to source the AO current loop, the robustness of the power supply to voltage sag may directly affect the control of the process.
PLC CPU Module and Programming Considerations The Central Processing Unit (CPU) module. This device is the brains of the PLC. Usually occupying a single slot in the PLC rack, the CPU module (also referred to as the processor) holds the control program in random access memory (RAM). The CPU module receives operating power through the I/O rack back plane via the rack's power supply.
The I/O rack back plane also contains a data bus for communications between the PLC and rack I/O. A lithium battery and/or electrically erasable programmable read only memory (EEPROM) chip is typically used to maintain the PLC program in the event of lost power.
The CPU reads the input data table information, solves the control program, and updates the output data table. The PLC performs housekeeping to check it and other hardware components for faults and errors. You can use a secondary microprocessor to transfer data from the system inputs into the data table and from the data table to the system outputs.
The time required for the PLC to read the inputs, solve the control program, and update the output table is known as the "scan rate." This time varies greatly depending on the CPU model, size of the control program, and architecture of the system. A more definitive measurement of PLC response is known as "throughput."
Throughput refers to the amount of time required to detect an input from the field device, solve the control program, and manipulate an output field device. The throughput time includes the scan time plus the amount of time it takes for the actual PLC module's electronics to detect, input, and switch the state of an output.
Throughput measurements for PLCs can be as short as 17 ms (approximately one 60 Hz cycle) to several hundred milliseconds depending on the size of the control program and the number and speed of the I/O modules. With the ability to sense a state change and switch an output signal in such a short time, it is easy to understand why process upsets and shutdowns occur as a result of voltage sags.
Control programming techniques. The PLC control program may take various forms. The most basic and common control program format is ladder logic. This control program format was created to model hardwired electrical relay logic and is subsequently very user friendly for maintenance electricians. A ladder logic PLC program uses conventional seal-in techniques that have been used in past relays. Other program languages commonly used today include sequential function chart (SFC), BASIC, and C++.
The method or technique the PLC programmer uses to control process equipment is a potential cause for PLC system PQ immunity problems. For example, in process applications, the process step of a batch may be held in the PLC's memory by using a conventional seal-in technique in the ladder logic. If the PLC experiences a shutdown and restart as a result of a voltage sag, the process state of the batch will likely be cleared since the seal-in will be lost. As stated earlier, this may lead to the loss of the batch.
A better technique is to write process step information into nonvolatile memory areas that are not cleared when the PLC shuts down and restarts. By placing a process step number into a nonvolatile memory location, the PLC can then know where to resume process operations. This approach, which is known as the "state-machine method," can be a powerful ally in helping to restart a control system when a voltage sag or outage-related upset occurs in your facility.
Overview PLCs can react quickly when voltage sags occur, shutting automation systems down for events as short as one cycle. Susceptibility of the power supply or misinterpretation of I/O signals usually causes the shutdown. The use of power supplies that monitor the DC output rather than the AC input when deciding to halt operations can lead to improved ride-through.
In the PLC I/O, voltage sags can lead to the detection of false negative conditions from sensors. Such occurrences can bring the process to a halt or cause a malfunction. In instances were the PLC is upset as a result of a voltage sag or outage, the programming technique used by the system integrator can lead either to a slow and costly recovery or a speedy resumption of automated operations.
Note: In the second part of this article, we will introduce voltage sag tests and practical guidelines to help integrators and users of PLC-based systems make proactive design changes to improve voltage sag response.
Acknowledgments: This two-part series of articles is based on recent work conducted by EPRI and funded by the California Energy Commission. The information presented would not have been possible without the diligent efforts of Brian Laan and Promad Kulkarni of the commission. For more information about EPRI PEAC Corp., visit www.epri-peac.com.
