Voltage Protection: Set to Match Eng’r. Dean Arnold S. Sempio MERALCO Introduction In the past, not too much attention is paid to voltage variations. Many electrical appliances at that time are robust enough to tolerate big differences between their rated voltage and the actual voltage supply, e.g. incandescent bulbs. Today, computers and other electronic based equipment rely on a stricter voltage supply variations in order to operate reliably. However, the utilities supplying the voltage pointed out that providing the rigid voltage regulations that customers want need substantial amounts of investment which neither the utilities nor their customers can afford. A compromise has struck after considering the sides of the industry/users, the designers/operations/maintenance personnel and the manufacturers. Voltage supplied by the utilities would be limited to certain economical levels (generally plus/minus10%) and that equipment manufacturers can make equipment that will operate on these “standard” voltage swings without imparting substantial additional costs. The designers and operations personnel should also ensure that equipment be fed with voltages they can handle. Case 1 A large commercial center is being plagued by constant power interruptions. The mall operators have to put up with the inconvenience of re-starting their loads as well as the inconvenience to their customers and tenants. The mall administrators are puzzled since the utility has told them that there were no disturbances on the distribution circuit serving the mall yet they still suffer from power interruptions. Power Quality Study A monitoring device was installed at the mall’s main panel as well as in the utility substation serving the mall. In the week of monitoring, there was an incident of power interruption at the mall; however, no disturbance was recorded at the substation. It must be noted that the substation was less than 1,000 meters away from the mall. The data from the monitoring device at the mall showed the following: Customer Panel (34.5kV) Maximum 37913.46V (+9.89%)
Minimum 32415.87V (-6.04%)
Substation Panel (34.5kV) Maximum 37090.85V (+7.51%)
Minimum 32281.18V (-6.43%)
The power interruption at the customer occurred when the voltage was only minus 6.04% from the supply voltage. The suspected culprit is the under voltage relay that the mall has installed in their main panel.
Subsequent tests on the relay revealed that the relay activates at 6% below the customer’s contracted service voltage. This is relatively sensitive for the utility supply of plus/minus 10% of the service voltage; the utility‘s voltage variation limits being mandated by government regulators. Solution The relay was calibrated to match the voltage variation limits of the utility, in which equipment should operate without any foreseen problems. The number of power interruptions of to the mall was significantly reduced; to the satisfaction of the mall administrators and their customers. Case 2 A BPO (call center) has installed UPS (uninterruptible power supply) units for their computer equipment.
However, the UPS units kept activating and would run on battery power until the battery runs out. The UPS units would have to be removed from duty in order to have their batteries recharged. The operation of their UPS units is relayed to their principal company located outside the country; the engineers there would then ask what power disturbance had activated the UPS units. The local BPO administrator cannot answer since no power disturbance was felt by their office. PQ Study Voltage monitoring was done on to determine the characteristics of the voltage supply for the UPS units. The result of the monitoring was: Substation Panel (34.5kV) Maximum 37090.85V (+7.51%)
Minimum 32281.18V (-6.43%)
It was found out that the UPS units activate when the voltage goes beyond plus/minus 5% of the contracted service voltage. This is a mismatch to the actual voltage variations and mandated limit of plus/minus 10%, the voltage variations allowed for services and the level that can be handled by most equipment. This have made the UPS too sensitive to voltage variations, which resulted to the frequent “cutting-in” of the UPS and drained the batteries. This is also the reason why the UPS did not return to “standby mode” to re-charge – the voltage was still beyond the 5% limit of the voltage setting before the UPS can re-charge. Solution: Re-adjusting the voltage settings of the UPS to match the allowed voltage variations have enabled the UPS units to activate as intended – when there is a power disturbance or abnormal voltage variations. Conclusion: Voltage Protection is employed to protect equipment from destructive voltages or those which can result to detrimental over-currents. Most equipment are designed to tolerate some “stresses” that are inherent in any electric power system. While we value our equipment, we also want a system that is continuous and reliable. We should aim for the best compromise - compatibility in our system’s components.