Today's engineers are designing increasingly sophisticated control systems that deliver higher productivity than earlier units, while keeping costs down. In order to achieve these results, they often are employing electronic equipment adapted from non-industrial applications—equipment that is almost always more sensitive to electrical disturbances than the equipment being replaced.
These new electronic systems, when mixed with the inherently poor power environment of an industrial operation and aging power-generation and distribution facilities—both inside and outside of the plant—can lead to a wide variety of power and electrical noise problems. Understanding these problems, along with their causes and solutions, is a must in helping to ensure the reliability and cost-effectiveness of mission critical electronic systems.
Where problems arise
Power problems—which can cause destruction, degradation or disruption of mission critical equipment—may originate either "inside" or "outside" an industrial facility. While "outside" events are typically the most obvious and spectacular, up to 80% of power problems in industrial operations originate on the customer's side of the meter.
"Inside" problems are caused by a variety of factors, including stopping and starting of motors, welding equipment, electronic motor speed controls and poor grounding, as well as some of the same problems facing the utility company: fault clearing and capacitor switching.
Types of problems
The most noticeable power quality problem is a power interruption. While interruptions are relatively infrequent, their effect can be dramatic and quite noticeable, as operations grind to a halt. Solutions to combat interruptions include alternate power feeds, local backup power generation (diesel- or gas-powered generators) and Uninterruptible Power Supplies (UPSs) on selected equipment.
Voltage sags are the most measured power line problem. The typical approach in protecting against sags is a voltage control device in the power path supplying the equipment. Choices include a transformer that stores energy (Constant Voltage Transformer), transformers with boost windings to raise voltages during sags (tap switching transformer) and UPSs that supply energy from batteries during sags.
In the past, the Constant Voltage Transformer (CVT) was a common method for sag control; however, today's control systems have changed. Typically, equipment now has a Switch Mode Power Supply (SMPS) and systems are no longer based on proprietary software that "crashes" well, but on commercially available operating systems that need to be properly shut down to retain status. Today, load requirements also change more often as control schemes are frequently updated with the latest technology.
While changes have been made in CVTs to adapt to new technology, the best solution is one that has been specifically designed to support SMPS and has energy to ride through severe sags—for example, a UPS with integral isolation transformer to provide regulation, isolation and backup. If a local isolation transformer already supplies the load, a UPS with double-conversion topology also serves effectively.
Transients on power lines usually occur randomly below a level that causes destruction and are generally not easy to measure. Among the most difficult to find are the high-speed transients that cause disruption of electronic equipment. Special power quality monitors are required to capture these high-speed impulse and oscillatory events that disrupt sensitive electronics. This "least measured" problem can be a major contributor to random errors and "lock-ups" that occur in a system.
Thankfully, most transient events are ignored by electronic equipment. If they were not, it would be impossible to run a computer. In mission critical applications, though, the goal is to push disruptions as close to zero as is possible. Thus, reducing the amplitude and edge speed of transients becomes paramount to achieve the goal of system reliability. To understand the methods that are used to control the amplitude and edge speed of transient voltages, it is useful to review how transient noise appears to electronic equipment.
Communication line issues
Control systems use a wide variety of communication lines for control busses, data lines to peripheral devices, such as Human Machine Interfaces (HMIs), and connections to plant-wide information systems. While not subject to all of the same problems as power lines, transients can damage communication lines and likely will cause system disruptions. In addition, grounded (non-isolated) communication schemes, such as RS232 and RS485, provide an additional path of disruption known as ground skew.
In communication lines, minimizing the chance of destruction or degradation is best addressed by using a Communication Line Protector (CLP). When choosing a CLP, the clamping voltage must be selected to be lower than the point at which damage to the communication channel will occur, but higher than the maximum voltage that can be applied to the line for normal communication. In addition, when using systems with the higher transmission speeds now available, it must be confirmed that the insertion loss due to the added capacitance and inductance of the protector will not cause unacceptable signal-level reductions.
External CLPs improve system reliability—even if a piece of equipment has internal protection for its communication ports. This results from the fact that a typical CLP will have a grounding lead that can be wired to direct transient noise away from the chassis ground of the control device, thus avoiding the introduction of potentially disruptive common mode noise into the equipment (a situation that can occur if the internal TVSS is triggered).
While CLPs provide protection against system destruction and degradation, they do little to assist in reducing disruptions from transient voltages that are below the level of component destruction, but above the level that interferes with routine communication. Protection against such disruption can be addressed in several ways.
Understanding these problems, along with their causes and solutions, is a must in helping to ensure the reliability and cost-effectiveness of mission critical electronic systems.First, the system grounding should follow good practice and meet the equipment manufacturers' guidelines. With grounded communication schemes in particular, a small grounding problem can lead to very inconsistent communication.
When working with grounded communication systems, there are other considerations, too—chiefly, "ground voltage skew." In systems with grounded communication schemes, while the primary ground connection between pieces of equipment is the power ground, there is a second ground connection: the shield or common lead in the communication cable. When ground currents flow in the power ground, they cause a voltage difference or "ground voltage skew" between two pieces of equipment. The voltage differential is then reflected in any grounded communication cable connected between the pieces of equipment. This differential, and the resultant current flow in the communication cable, can cause serious disruption of the communication path and may destroy devices not protected by a CLP.
The most cost-effective and reliable solution to ground skew-induced problems is a ground skew protective device in the power path. The type of device works on the principle of creating a high degree of impedance in the ground path at high frequencies, while still maintaining a "zero" or low impedance at power line frequencies.
By increasing the high-frequency impedance in the ground line, the resultant voltage produced by high-frequency ground currents is substantially reduced, thereby reducing the opportunity for disruption or destruction of the communication line. One ground skew device should be placed in the power path of each device containing a grounded communication port.
A final note of caution
Finally, it is important to note that once a system is properly installed and protected, vigilance is required to maintain the level of integrity that was originally designed in. One single "on-the-fly" addition or change can leave a system with an unprotected path and, unfortunately, subject to the disruptive effects of power and communication line anomalies. MT
Based in Libertyville, IL, Paul Haake is vice president of Engineering for Chloride North America, with responsibilities for all aspects of design and engineering of UPSs, power-conditioning and communication-line protection devices. He has more than 25 years experience in design and design management for power-conditioning products and equipment and instrumentation used in the process, utility, nuclear, HVAC, safety and assembly industries.