Filling the Gaps in Surge Protection

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Major equipment manufacturers, in fields as diverse as industrial automation, medical electronics and consumer electronics, are seeing 30% to 50% of their field failures resulting from a single root cause. Such field failures typically occur in the input power-supply stage, and have routinely been attributed to “power surges.” The primary form of surge protection today is transient voltage surge suppression (TVSS) to protect against voltage surges that occur as a result of direct lightning strikes.

In spite of the ubiquity of such protection, equipment failures continue, which suggests the root cause has not been identified and addressed. Billions of units of electrical equipment are operating and continue to be deployed worldwide and remain vulnerable to premature failure, resulting in unscheduled downtime and excessive warranty and replacement costs. This points to a possible “gap” in equipment protection, and a need to better understand the root causes for these failures.

This article will analyze the issues related to premature equipment failure and propose a new type of surge-protection device, a current-inrush voltage surge suppressor (CVSS), to provide complete protection for susceptible equipment.

It is commonly believed that equipment damage occurs when a lightning strike on the utility system causes a voltage surge. The standard UL 1449 specifies a 6-kV, 20-µs voltage impulse, as representative of a voltage that can be impressed by a direct lightning strike, and specifies a test protocol.

While lightning strikes on the exposed high-voltage power grid are common, there is little evidence to show that these events frequently translate into destructive voltage surges at the point of use for the vast majority of utility customers — all typically connected at low voltage. Rather, given the high-frequency content of the lightning surge, it is clear that damage can only occur if lightning strikes the power line in the immediate vicinity (approximately 200 m) of a customer's premises. This can have severe consequences, but is very rare and does not account for the much higher incidence of equipment failure that is observed.

Furthermore, deployment of a layered-voltage surge-protection philosophy, including service-entrance protection, theoretically should eliminate the potential for equipment damage, but in practice fails to do so. This may also point to a puzzling phenomenon frequently encountered where equipment connected in close proximity to the damaged unit does not suffer any damage whatsoever. Trying to explain such occurrences has been one of the “mysteries” in the field of surge protection.

The leading cause of equipment failure may not be the voltage surge after all, but rather a current-inrush surge that occurs at the end of frequent voltage-sag events. Let's look at the analysis and evidence supporting this premise, as well as the behavior of typical electronics equipment under voltage-sag conditions. We will also look at how to assess susceptibility to voltage and current-inrush surges.

Root Causes

For almost any piece of electronics equipment, the input rectifier stage consists of a rectifier and a capacitor. This is true for most electronics equipment, including PLCs, robots, medical equipment, drives and consumer electronics equipment. It is well known that startup conditions can cause a current surge that can damage the rectifier, capacitor or fuses. An inrush current limiter, using either a negative-temperature-coefficient (NTC) resistor or a relay-resistor series combination, is commonly used to limit the current drawn at startup and is bypassed for normal operation (Fig. 1).

Any major fault on the power system normally results in a voltage sag that can be measured more than 100 miles from the fault location. When a voltage sag occurs on a utility bus connected to electronics equipment, the line current drawn by the equipment goes to zero as the diode bridge is reverse biased, and the dc bus capacitor begins to discharge slowly. When the fault is cleared, the line voltage abruptly returns to normal, and begins to recharge the dc-bus capacitance with the inrush current limiter bypassed. This then results in a large unprotected current surge that flows into the equipment as shown in (Fig. 2).

Clearly, equipment designers would consider such current surges to be potentially damaging, because they use an inrush current limiter at startup. However, the input power-supply design specification does not typically specify behavior under short-duration voltage sags, almost universally assuming that the voltage will remain within a band of say ±10% of nominal value, or will go to zero and eventually return to a normal startup condition.

Analysis shows that the current-inrush surge can put stress on several power-supply components. While the diode rectifiers can handle large current peak currents, it can be shown that the thermal shock or I²T rating of the diode can easily be exceeded for typical voltage-sag conditions. This I²T limitation can also cause fuses to blow and printed circuit board tracks to vaporize, causing downtime and warranty costs.

Finally, a resonant charging between the line inductance and the capacitor can cause the surge voltage rating of the capacitor to be exceeded (Fig. 2). Most capacitor vendors specify that a capacitor surge voltage rating can only be exceeded a few times before damaging the capacitor.

Susceptibility of Equipment

The premise that the vast majority of electronics equipment sold today may be unprotected and susceptible to damage from frequently occurring power disturbances might seem unjustifiable. To validate the concept, a cross-section of commercially available equipment and appliances were selected and tested for current-inrush susceptibility.

Fig. 3 shows typical data for current inrush and I²T measured for a Dell 15-in. LCD monitor operated only with TVSS protection. The voltage was interrupted for a duration that varied in 0.1-cycle increments. The current-inrush surge amplitude and I²T of the resulting current pulse were measured, and are plotted as a function of the sag duration. The monitor consumes 0.38 A_RMS in the steady-state, but is subject to as much as 40 A_PK at the end of a voltage-sag event. The I²T stress also is seen to reach 50 times the normal level. The measurements were done in a laboratory setting where the source impedance was measured as having an inductance of 100 µH and a resistance of 0.3 Ω to 0.4 Ω.

Similar measurements have been done for a wide variety of equipment, including TVs, PLCs and printers, with almost all types of equipment demonstrating significantly higher-than-normal (20 to 50 times greater) current-inrush levels. Calculations also show that the dissipated energy associated with a current surge can exceed that of a voltage surge associated with a lightning strike, showing the potential for significant damage.

Fig. 4 shows a computation of the I²T stresses for different sag duration and source impedance levels. Note how the peak current is a periodic function of the sag duration with respect to the cycle times of the ac input voltage. This is because the ac voltage will cross zero twice per cycle, and the time when voltage recovery occurs in relation to these crossings determines the magnitude of the voltage step.

The characteristics in Fig. 4 are similar to the measured data in Fig. 3, and validate the physics behind the underlying phenomenon. It also shows that the I²T levels can, under the right conditions, dramatically exceed the diode ratings. This can cause the weakening and eventual destruction of the device.

Similarly, failure modes for the capacitor are seen to depend on the reactance-to-resistance ratio of the ac-line source impedance, and repeated exposure to the rated surge voltage can result in early failure of the device. As the dc power supply is a universal part of all equipment today, and because there are no governing specifications or standards regarding equipment behavior under short-duration voltage sags, it is not surprising that the susceptibility to voltage sags is to be found across all categories and classes of equipment — industrial, medical, commercial and consumer — pointing to the ubiquity of the problem.

If voltage sags rarely occurred, the previous discussion would not be of great significance. On the other hand, if deep and potentially damaging voltage sags and their associated current surges occur frequently, then it is important to find suitable protection. Data from years of power-quality monitoring show that voltage sags are caused by all types of faults, including lightning strikes on the power grid, and occur 100 times more frequently than voltage surges.

Fig. 5 shows a typical recorded voltage profile for a power disturbance measured at a large industrial facility. The voltage sag is clearly seen with a rapid return to normal voltage occurring once the fault is cleared. Extensive data confirms that voltage sags of this type occur 30 to 100 times per annum for all categories of customers connected to the U.S. power grid.

Further analysis shows that 80% of all voltage sags are seen to have at least a 20% jump in voltage as the voltage-sag event ends. This is sufficient to cause a significant current-inrush surge. Larger plants and buildings with connected load of 1 MW to 30 MW tend to have stiffer buses and lower source impedance, resulting in even higher current surges and trapped energy. It should be noted that most light commercial, office and residential customers will see an even higher incidence of voltage sags.

Given the susceptibility of most electronics equipment to current inrush occurring at the end of voltage sags, the relatively high frequency of voltage sags and the inability of existing TVSS devices to protect against such current surges, it is clear there is a need for CVSS, which is cost-competitive with TVSS devices.

Innovolt (Atlanta) has recently introduced the first patent-pending CVSS product in the market. The CVSS protects against both voltage surges and current-inrush surges caused by voltage sags. The current surge limiting function relies on sensing when the ac voltage conditions are likely to result in a current-inrush surge. At such time, the CVSS limits the maximum current allowed until the voltage has returned to nominal conditions. Voltage-surge protection is provided as per UL 1449 requirements.

The product is available in both plug-in as well as hard-wired (with DIN-rail option) formats, presently at current ratings of up to 15 A at 120 Vac, with additional ratings due for release later in 2006. The CVSS also has a built-in data logger to provide a history of the actual power disturbances protected against. Fig. 6 shows a rendering of the plug-in CVSS unit.

The CVSS units have been tested with different loads under normal and abnormal voltage conditions. Fig. 7 shows the current inrush and I²T levels for the Dell 15-in. LCD monitor, when operated with conventional TVSS (upper curve) and CVSS (lower curve) protection. The peak stresses are seen to be reduced to safe levels, by four times for the current and 12 times for the I²T. A similar curve showing only the peak current envelope for a Sony 32-in. TV is shown in Fig. 8 for measured current inrush when protected by TVSS (upper) and CVSS (lower) devices, showing similar levels of improvement.

Additional testing of the CVSS with a variety of office, consumer and industrial equipment has confirmed a dramatic reduction in peak currents and I²T stresses. There is virtually no impact on the operation of the equipment itself. However, it should be noted that this is not a ride-through device, such as a UPS, and that its function is limited to equipment protection. CVSS protection can provide value in industrial, commercial, medical, office and consumer applications.

The solutions are simple to use with no user training required. By providing complete protection in one package, it circumvents the question of whether voltage or current-inrush surges represent a bigger problem for a specific consumer.

While the analysis in this article focused on single-phase input systems, the conclusions generally apply equally to three-phase input systems and systems with active front-end input stages. It has also been shown that current surges at the end of frequently occurring voltage-sag events can result in stresses on the input-stage components that can dramatically exceed their ratings.