Today’s engineers are designing more sophisticated control systems that bring higher productivity to meet ever-increasing expectations of performance in industrial settings-all while keeping system costs under control. In order to achieve these results, they are employing more electronics, much of it adapted from non-industrial applications and almost all more sensitive to electrical disturbances than the equipment being replaced.
Putting this sensitive equipment into the inherently poor power environment of an industrial facility and aging power generation and distribution facilities-both inside and outside of the plant-produces a wide variety of power and electrical noise problems.
Understanding these problems, along with some of their causes and solutions, can help ensure the design of mission-critical electronic systems that are both reliable and cost-effective.
The first task in protecting mission-critical elements is identification. While each system is unique, mission-critical components are usually easily recognizable. If they fail, customer displeasure, increased labor, or increased material cost result. A component is mission critical if its downtime causes lost profits.
Next comes determining the necessary level of protection.
- The first level of protection is a defense against instantaneous destruction of critical equipment. It’s really just enough protection to keep things from catastrophically failing.
- The second level provides additional protection against long-term degradation of equipment often seen in semiconductor devices.
- The third level adds defense against unexplained soft failures, system lock-ups, and resets for which no specific cause can be identified. Most industrial systems need this protection.
Power Line Issues
Events that can destroy, degrade, or disrupt mission-critical equipment include power line problems that can originate either inside or outside the industrial facility. Outside events are the most obvious and spectacular; however, in industrial facilities up to 80 percent of power problems originate on the customer’s side of the meter.
Factors such as stopping and starting motors, welding equipment, electronic motor speed controls, poor grounding, and some of the same problems facing the utility company-fault clearing and capacitor switching - can all cause inside power line problems.
Among the most noticeable power quality problems is a power interruption. Interruptions are relatively infrequent but also dramatic and obvious as operations grind to a halt. Alternate power feeds, local backup generating (diesel or gas-powered generators), and UPSs on selected equipment can prevent most work interruptions from power outages.
Voltage sags are the most measured power line problem. The practical approach for protecting controllers against sags is a voltage control device in the power path supplying the control system. Device choices include those that store energy in a transformer (constant voltage transformer), devices that use boost windings to raise voltages during sags (tap switching transformer), and devices that supply energy from batteries during sags (UPS). There are also devices that use combinations of these technologies.
In the past, a constant voltage transformer (CVT) controlled sags. Today, however, control systems have changed. Loads are typically switch-mode power supplies (SMPS), and sags, particularly with deregulation, are likely to become more severe. In addition, control systems are often no longer based on proprietary software that crashes well but on commercially available operating systems that need to be properly shut down to start-up smoothly. 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 more energy to ride through severe sags, such as a UPS with integral isolation transformer that provides robust regulation, isolation, and backup. If an isolation transformer exists in the power path near the load, a double-conversion UPS also serves effectively.
Transient voltages too weak to cause massive destruction are difficult to measure. These include high-speed transients that disrupt electronic equipment. To further complicate the situation, transients often occur randomly. Special power quality monitors are required to capture the high-speed impulse and oscillatory events that disrupt sensitive electronics. This “least measured” power quality event can be a major contributor to random errors and lock-ups that occur in a system.
Thankfully, electronic equipment ignores most transient events. If it did 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 possible. Reducing the amplitude and edge speed of transients becomes paramount in achieving this desired system reliability. In order to understand the methods that are used to control amplitude and edge speed of transient voltages, it is useful to know how transient noise appears to electronic equipment.
Normal-mode noise transients appear between the line (hot or phase) and neutral conductors supplying the equipment. While somewhat troublesome, this noise can often be controlled by combining transient voltage surge suppressor (TVSS) devices and filters. Typically, individual equipment includes provisions for controlling this noise mode.
Common-mode noise is far more difficult to control. This noise appears between the neutral line and the ground line connected to the equipment. While the neutral and common are bonded either at the service entrance or at an intermediate transformer, noise in this mode is quite common and very disruptive. It typically occurs when current is dumped into the ground lead by other equipment-input and output filters to suppress high frequency line noise are a typical cause - as are protective devices such as TVSSs.
Control of common-mode noise requires a transformer-based conditioning device. This 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. These devices are typically available as traditional power conditioners or as power conditioners with battery backup.
Communication Line IssuesControl systems use communication lines for several purposes. These include control busses (DeviceNet or Profibus), 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 problems of power lines, communication lines are more likely to cause system disruption due to transients. 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, a communication line protector (CLP) best minimizes the chance of destruction or degradation. The clamping voltage must be lower than the point at which damage will occur, but higher than the maximum voltage that can be applied to the line for normal communication. In addition, for systems with higher transmission speeds, the insertion loss due to the added capacitance and inductance should not cause unacceptable signal level reductions.
External CLPs are often suggested to improve system reliability, even if a communication port is internally protected by a TVSS against overvoltage. This can lead to improved reliability since 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. This approach avoids introducing potentially disruptive common-mode noise into the equipment, a situation that can occur if the internal TVSS is triggered.
While CLPs can 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 disruptive level that interferes with routine communication. Protection against such disruption can be addressed in several ways.
It is critical that system grounding follows good practice and meets the equipment manufacturers’ guidelines. With grounded communication schemes in particular, a small grounding problem can lead to very inconsistent communication.
In grounded communication systems, the primary connection is the power ground, while the second ground lead is the shield or common lead in the communication cable. When ground currents flow in the power ground, they cause a voltage difference known or “ground voltage skew” between the two locations, thus causing a voltage differential to be reflected in the communication cable. 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 high impedance in the ground path at high frequencies while maintaining a zero or low impedance at power-line frequencies.
Increasing the high-frequency impedance in the ground line substantially reduces the resultant voltage produced by high-frequency ground currents, 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.
ConclusionProviding the highest level of reliability in a mission-critical industrial system requires two steps. First, designers should employ robust equipment made for use in industrial environments. And they should provide total protection on power and communication ports.
Total protection means that every power and communication port in a system be protected and that its grounding scheme is in accordance with NEC and manufacturers’ guidelines. Power ports should be protected with low-impedance transformer-based power conditioners to control both common and normal-mode noise. On some power ports a low-impedance transformer-based power conditioner with batteries (UPS) will provide protection against extended sags and outages when controllers must be shut down in an orderly fashion.
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 subject to the disruptive effects of power and communication line anomalies.