Why inverters and UPS fail

All electrical and electronic equipment users in countries like India encoun­ter unusual difficulties in trying to cope with AC power problems. Among these problems are frequent blackouts due to in­adequate generation capacity and wide AC mains voltage variation due to inadequate distribution infrastructure. Also, poor quality of power results in distortion, spike/noise, frequency, sags/surges and dropouts, espe­cially when diesel generators are used to supplement utility power.

By Venkat Rajaraman (The author is CEO, Su-Kam.)

Friday, October 16, 2009: Due to these common power quality problems, creating a reliable inverter and UPS system poses a special challenge to manufacturers. If inverters and UPS systems are not specifically designed to address the power conditions mentioned above, then they may meet with a high failure rate.

Power quality varies significantly from one area to another and is typically worse in rural India than in urban India. Power disturbances are caused by the generation, distribution and use of power and lightning, and occur when a power station breaks down or if power switching takes place on the dis­tribution lines, transformers or high voltage lines. Lightning, too, generates vast amounts of excess energy, which manifests as a power line disturbance. A power disturbance can be defined as unwanted excess energy that is presented to the load.

Causes of power disturbances

• Power disturbance originates both out­side and inside customer facilities
• Load switching causes surges due to col­lapsing fields
• Over loaded power distribution systems can cause significant voltage variations between peak and off-peak hours
• Large diesel generators and variable speed drives can cause huge surge volt­ages inside buildings
• Significant momentary load changes, such as heavy inrush currents, can cause severe voltage variations
• Blackouts can cause severe voltage surges both on loss and return of power
• Circuit breaker tripping and fuse blowing can cause severe surge voltages

Impact of power disturbances

Sags, under voltages, surges and over volt­ages can cause component overheating or destruction, or can trigger other electronic components such as SCRs. Component over­heating reduces the life and deteriorates the real reliability of the product as opposed to the estimated reliability, based on steady state conditions. False triggering of other compo­nents can lead to nuisance alarm tripping, or worse, can cause overheating or destruction of other electronic components.

Why semiconductors fail

Most semiconductor devices are intolerant to surge voltages in excess of their voltage ratings. Even a rapid surge of a few micro­seconds can cause a semiconductor to fail catastrophically, or may degrade it so as to shorten its useful life. Damage occurs when a high reverse voltage is applied to a non-conducting P N junction. The junction may avalanche at a small point due to the non-uniformity of the electric field. In this case, thermal runaway can occur because of localised heat build up and cause a melt-through, which will destroy the junction.

Problems with current solutions

• Typical EMI filters used in most systems are not well damped, which produces dramatic effects in case of voltage dis­turbances, resulting in oscillations inside the EMI filter under transitional condi­tions. Severe voltage surges may result from fly-back from saturated inductors, looking for a path to release energy

• Boost converters can be destroyed by surges, causing increased energy storage in input filters. The output capacitor (c) is charged to an unsafe level depending on capacitance value and load levels for the DC/DC converter connected to the output of the boost

Installation and wiring practices

• Installation and wiring practices are very important to ensure a reliable installa­tion. Some common problems are:
• Inadequate wire size or type
• Long overloaded wire runs
• Too many loads on a branch circuit
• Poor connections
• Ground currents due to incorrect wiring
• Systems that do not have a clear path to ground for fault currents have to rely on ground fault detection that sometimes malfunction
• Mixing electrical equipment that is de­signed for different standards
• Overloaded utility transformers
• Incorrectly applied surge protection devices

Limitations of inverter and UPS equipment

Due to the frequency of power disturbances, the power electronic circuitry in the in­verter/UPS must work under greater stress. This especially holds true for inverters and line interactive UPS, which must make the transfer to battery backup every time there is a power disturbance. Also, power transis­tors and transformers must operate at higher currents and voltage levels during brownout and over voltage conditions, which result in a shorter-than-normal operating lifetime for the inverter or UPS system. Batteries cannot be charged fully when power disturbances occur so frequently. Consequently, inverters/UPS cannot render the rated backup time during each power disturbance.

Standby power generators such as DG sets can cause special problems when they are used to power inverters/UPS systems. They are known to release wide frequency/ voltage fluctuations, surges/spikes and high­ly distorted (often non sinusoidal) outputs, which can cause continuous mains/inverter transfer operations, shutdowns and even catastrophic failure.

If the ambient temperature is outside of the optimum 10-25°C range, standard flooded lead acid batteries cannot be used. Standard flooded batteries are rated to work only when the temperature is above 0°C and sudden battery failure can occur when the operating temperature goes above 40°C. Even at 30°C, expected battery lifetime is reduced by 50 per cent, compared to the expected battery lifetime at 20°C. Also, the industry is driven by low cost and users don’t care much about power quality. Most inverters/UPS systems do not possess quality battery chargers. If they do, the chargers are not designed for long-term backup. Most inverters and UPS do not provide real power conditioning (there are exceptions). Some UPS equipment are poorly protected against surge voltages.

Providing the ideal solution to power problems in developing countries requires an intelligent usage of technology, coupled with an intimate familiarity with the nature of the problems.

Electronics Bazaar, South Asia’s No.1 Electronics B2B magazine