Case Study: Voltage Instability Ruins Valuable Research Work
A number of incidents of significant voltage dips at a scientific laboratory conducting research into material properties resulted in failed experiments and the loss of sometimes many weeks of research time.
In order to determine the nature and frequency of such voltage events, a week-long power quality analysis was carried out, the results of which were used to propose a solution to this very time consuming and costly problem.
An economic solution was designed and implemented that prevented further loss of valuable research time, the details of which are presented below.
During the analysis period there were five voltage dips were recorded, a typical example of which is shown in Figure 1. It can be seen that a voltage dip of a magnitude of nearly 50 Volts, lasting for approximately 140 ms occurred late at night.
This voltage event is typical of those responsible for the failure of the experiments where sensitive electronic controllers either mal-operated or reset causing the loss of many days’ worth of valuable data.
From site event logs, there was no real pattern to the voltage dip events; no time of day or day of week being statistically more likely to experience such events.
Figure 2 shows the current transient associated with the voltage event shown in Figure 1. It can be seen that at the time that during the voltage dip on L3, the L3 phase current dipped accordingly. This would suggest that the voltage dip was externally derived. The increase in L1 current during the voltage event was likely to be associated with some internal load compensating for the drop in voltage by increasing the current to maintain power output.
Figure 1 – Voltage waveforms and RMS profile during typical voltage event
Figure 2 – Current waveforms and RMS profile during typical voltage event
Clearly, what was required for this supply was some voltage conditioning between the raw mains and the load that would allow the laboratory equipment to ride through the external voltage dips without interruption. There are a number of devices available that could potentially provide this functionality e.g.
- Voltage stabiliser
- Voltage conditioner (includes surge protection and filtration as well as well as maintaining the RMS voltage at all times)
- CVT (constant voltage transformer)
Due to the prohibitive cost and complexity of the first two items for what was a basic requirement, these were discounted. A CVT is very good at riding through voltage dips and is essentially maintenance free as it is basically an inductor / capacitor combination. Unfortunately, a CVT would not have been suitable for this application as they are only commonly available in ratings of up to 30 kVA and the supply requiring protection was rated at 300 Amps.
Having considered all of the above, it was decided to recommend a 200 kVA double conversion, online UPS with bypass that would effectively decouple the load voltage from the incoming mains voltage.
In order to minimise the maintenance requirements of this device, it was decided to offer a UPS with no batteries as there was no requirement for prolonged backup of the supply, just transient ride through capabilities which would be achieved by the storage offered by the DC capacitor.
Having presented the solution to the client, the order was placed for this mitigation work, with power quality analysis of the supply being commissioned for a week after the completion of the project.
The power quality analysis carried out on the newly conditioned supply showed no transient behaviour at all and in the subsequent year of operation, no transient voltage events have resulted in the failure of testing and the associated lost time and research data.