Low Voltage problem What type PLC use?

is it a stable 208VAC? or does if fluctuate between 220VAC and 208VAC? This is important. First, call the local utility and complain loudly so that they will do something about the low voltage problem. Most utilities will fix the problem if you complain both vocally and more importantly, in writing. You apparently are at the end of the line or transformer drop for your neighborhood and have a voltage support problem. They need to either add another line transformer for you or add capacitors to boost the voltage support. But, if you don't complain in writing, nothing will happen. Second, PS audio makes power conditioners/generators where you can actually increase the output voltage to your system. Look on PS Audio's web site. But answer the first questions first. does your voltage fluctuate? if so, you must have the local utility fix the problem. You need voltage support for your area. They will fix this.

208V doesn't seem to be an issue.

If the Hong Kong electrical system standard is anything like that of the UK, with a lower limit of -6%, then 208V is within normal voltage tolerance range.

Are you sure what you are describing is due to low voltage?

I don't know, the OP did state that the voltage standard is 220VAC. This to me means that if your equipment was designed to operate at 220VAC, the 208VAC may trigger problems. I know that for my Audio Research REF 3, if the voltage dropped by the same percentage here, it would go into low voltage shut down. Turn Table motors won't run correctly at such a low voltage. It sounds like a pretty large issue to me.

enjoy