On Friday the residents of Taipei were left stranded when their newly built $1.6 billion transit system shut down. It seems that the cause of this, was due to the failure of the Uninterruptible Power Supply (UPS System) at one of the stations. This UPS System controlled the signalling and communication functions and without power to these systems the trains could not operate.
Now UPS Systems are designed to increase the reliability of connected systems, by providing power protection and back up power. If the UPS System should fail then it should revert to bypass and allow utility mains power through (this is for online double conversion Uninterruptible Power Supplies and particularly three phase systems). It’s unclear what happened in this case, but whatever the outcome the critical systems were left without power and the UPS System is perceived not to have done its job.
Here’s where redundancy is required. Basically, you have one more system than you actually need, therefore if one system should fail, the other can take over without any loss of UPS System support. Should the utility fail for an extended period, then you either need batteries to keep you going (or shut down gracefully) or an external generator to kick in and simulate utility power.
Redundancy is usually expressed as ‘n+1′, which means that if you need ‘n’ UPS Systems to power your load, then you install ‘n+1′. For example, if you have a 100KVA load, you can achieve this with 1x100KVA UPS System. If you want redundancy you will need to use n+1 = 1+1 =2 UPS Systems, i.e. 2x 100KVA UPS Systems. Alternatively, you may have achieved your load by using 2x50KVA UPS Systems in parallel, and redundancy can be achieved by installing an additional 50KVA UPS System i.e. 3x50KVA UPS Systems, which may be more cost effective.
I’ll blog more about this later, as the citizens of Taipei walk to work.
You can read about the Taipei metro power cut here