Today's engineers are designing more sophisticated control systems that bring higher productivity, while keeping system costs under control. In order to achieve these results, they are employing more electronic equipment, often adopted from non-industrial applications, and almost always more sensitive to electrical disturbances than the equipment being replaced.
These new systems, when mixed with the inherently poor power environment of an industrial facility and the aging power generation and distribution facilities — both inside and outside of the plant — lead to a wide variety of power and electrical noise problems.
Understanding these problems, along with their causes and solutions, can help ensure reliable, cost-effective design of mission-critical electronic systems.
Power Problem Sources
Power problems, which can cause destruction, degradation, or disruption of mission critical equipment, may originate either "inside" or "outside" the industrial facility. While "outside" events are typically the most obvious and spectacular, up to 80 percent of power problems in industrial facilities originate on the customer's side of the meter.
"Inside" problems are caused by a variety of factors including stopping and starting motors, welding equipment, electronic motor speed controls, poor grounding, as well as some of the same problems facing the utility company — fault clearing, and capacitor switching.
The most noticeable power quality problem is a power interruption. While interruptions are relatively infrequent, their effect can be dramatic and obvious as operations grind to a halt. Solutions to combat interruptions include alternate power feeds, local backup power generation (diesel or gas powered generators) and UPSs on selected equipment.
Voltage sags are the most measured power line problem. The typical approach to protect 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 Uninterruptible Power Supplies 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 specifically designed to support SMPS and has energy to ride through severe sags, such as 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 usually difficult 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.
Fortunately, most transient events are ignored by electronic equipment. If they were not, it would be impossible to run a computer. In mission critical applications, however, 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.
- Normal mode noise transients appear between the Line and Neutral conductors supplying the equipment. While somewhat troublesome, these can often be controlled by combining Transient Voltage Surge Suppressor (TVSS) devices and filters. Typically, equipment makers provide for controlling this noise mode internally.
- Common mode noise is far more difficult to control. This noise appears between the Neutral and Ground lines connected to the equipment. While the Neutral and Ground are bonded at the power transformer serving the system, this highly disruptive type of noise remains quite common. It typically occurs when current is "dumped" into the power system ground by TVSS's or noise suppression filters in other pieces of equipment.
Control of common mode noise requires a transformer-based conditioning device that provides a separately derived source of power in which the neutral and ground wires are locally rebonded. Almost all such commercial power conditioning devices also include components to control normal mode noise. Solutions are commercially available both as traditional power conditioners and as power conditioners with battery backup.
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.
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 also other considerations — chiefly, "ground voltage skew." In systems with grounded communication schemes, the primary ground connection between pieces of equipment is the power ground, but 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. This 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 device works on the principle of creating a high impedance in the ground path at high frequencies while 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.
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 therefore subject to the disruptive effects of power and communication line anomalies.
Contact: Emerson Network Power, 5810 Van Allen Way, Carlsbad, CA 92008 888-412-7832 or 760-930-4600 fax: 760-930-0698 Web: