HEC provides a very reliable power supply to our customers with a pledged reliability of over 99.998%. However, as public electricity supply systems, they are liable to external influences. Therefore, despite the fact that we have made every endeavor to ensure reliability of electricity supply and good power quality, power quality problems may still occasionally occur due to various causes.
Besides, loading conditions, defects or faults of the equipment / installations of any one customer may also affect other customers who share the same customer communal electrical installations and / or supply source. In other words, power quality problems may also originate from the customer side.
Let us have a closer look at the common causes of power quality problems.
Supply Interruptions
The most common cause of power interruption is due to damage or fault of underground cable often arising from disturbance by third parties' excavation works. Customers who depend on the supply through the faulty or damaged cable would experience supply interruption until backup supply is connected or the cable is repaired.
When the normal 11kV supply cable is faulty or damaged, our System Control Engineer will remotely isolate the faulty or damaged cable and switch-in the backup cable through the System Computer to restore supply. Thus the duration of supply interruption is usually very short and only a few minutes is needed to restore supply. However, in exceptional circumstances like simultaneous cable faults or transformer or switchgear fault, supply interruption of longer duration may occur.
For low voltage (LV) network faults, supply can only be restored after the fault has been repaired or the backup supply has been connected by the emergency crew. Thus, it will take longer time for the supply to restore. "The Average Time for Supply Restoration After Interruption of Supply" is shown in our "Customer Service Standards".
Other than cable faults and damage incidents, power interruptions may also be due to equipment failures in suppliers' system as well as in customers' installation.
Voltage Dips
When there are faults on a supply cable or equipment, voltage dip will occur in the part of the power supply system affected by the fault until the protective devices are initiated to clear the fault. Thus, apart from supply interruption to those customers who are supplied directly by the faulty cable or equipment, other customers may also experience voltage dip. In case of an 11kV fault, the voltage dip is localized in the affected geographical area but in case of a transmission fault, the voltage dip will be territory-wide and affect more customers.
Since our supply network is interconnected with CLP Power, which is interconnected with Mainland China, customers on Hong Kong Island might experience momentary voltage dips as a result of faults in the transmission power systems outside Hong Kong Island.
Harmonics
Harmonics are solely generated from non-linear devices at customer side.