Eur Ing Dr Robert E Brown BEng PhD CEng MIET IntPE (UK)
Chartered Electrical, Electroncic and Control Engineer
Expert Witness
In the past week Dr Brown has been reviewing circumstances surrounding an incident which occurred within the operational system of Heat Recovery Steam Generator located on a construction site in Uruguay South America.
The incident focused on two (2) uninterruptable power supply (UPS) invertor panels and seventy four (74) electrical relays, which were severely damaged by some phenomena thought to be accountable to a ‘short circuit’ within a UPS, which provoked further damage to electronic switching units (IGBT’s) of the UPS and electrical relays configured downstream of the UPS.
An uninterruptible power supply or uninterruptible power source (UPS) is an electrical apparatus that provides emergency power to a load when the input power source or mains power fails. A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide near-instantaneous protection from input power interruptions, by supplying energy stored in batteries, supercapacitors, or flywheels. The on-battery run-time of most uninterruptible power sources is relatively short (only a few minutes) but sufficient to start a standby power source or properly shut down the protected equipment. It is a type of continual power system.
An electrical power inverter, or inverter, is an electronic device or circuitry that changes direct current (DC) to alternating current (AC). The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry. The inverter does not produce any power; the power is provided by the DC source, i.e. a battery/ies. A power inverter can be entirely electronic or may be a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry.
An electrical relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.
With reference to a service report generated by the manufacturers of the UPS, they consider that the most likely phenomena to bring about the damage was due either to a smashed temperature sensor cable inside the UPS or that the UPS was heavily laden with dust, probably due to the location of the UPS when in operation i.e. a construction site. So to give insight to the most probable cause of failure of the UPS Dr Brown was asked to opine in respect of either scenario.
Dr Brown considered that, …
In terms of the damaged temperature cable, in an electrical sense the electrical signals conveyed by the cable would be of a very small magnitude and hence ‘electrically’ the sensor could not have provoked the conduction of a significant magnitude of electrical energy to cause the IGBT’s to ‘explode’ as observed.
The location of the temperature sensor was unclear within the UPS, yet it was expected that the function of the sensor was to monitor for overheating of the UPS core. In simple terms the temperature sensor would control the operation of a fan(s) for cooling purposes, i.e. when the temperature sensor measures a high temperature this is used to turn on a cooling fan. So in the case of the failure of the temperature sensor, it would be normal design practice that the system would be configured in a ‘fail safe manner’, i.e. if the temperature sensor fails the fan would continue to run.
However, it could be possible that if the temperature sensor had failed in a mode to switch the fan ‘off’ then if the IGBT’s within the UPS core could overheat with ensuing catastrophic destruction.
In the course of manufacture and design of the UPS, the IGBT’s would have been mounted on aluminium ‘finned’ heat sinks, to provide very effective cooling for the IGBT’s. If these heatsink’s were ‘clogged’ with dust and debris the efficacy of heat dissipation from the IGBT’s would be severely reduced and so the IGBT’s could overheat and fail in this scenario also,...
Of the two scenarios (being the damaged sensor or excessive dust), Dr. Brown considered it more likely that dust caused the fundamental malfunction of electronic controller boards which in turn provoked the controller to ‘drive’ the electronic switching elements IGBT’s to ‘over drive/stress’ causing eventual catastrophic failure.
So given the request for opinion of Dr. Brown was driven by an insurance company hoping fro guidance for culpability of damage given due consideration to all factors, Dr. Brown considered that the failure most likely point to poor maintenance of the UPS on site. In other words excessive levels of dust were allowed to build up within the UPS.
For further information or help on any electrical related matter, please contact Dr Brown, ...
Email: robert.brown@robertbrown.uk.com Web: www. robertbrown.uk.com Tel: +44 (0) 1709 739228