Power Quality

HK Electric is committed to providing adequate and reliable supply of electricity to support the business growth in Hong Kong. With proliferation of sensitive equipment and electronic products, there is a growing concern among customers about the power quality of electricity supply.

What is Power Quality? Power quality is about the compatibility between the power supply source (voltage, current and frequency) and the equipment served. In case of electrical disturbances, the equipment might not be able to operate normally if the equipment has not been specified to cater for such supply conditions.

The following information may help you understand more about power quality and how to safeguard the power supply for your important services.

Typical Power Quality Problems

Generally, supply interruptions, voltage dips and harmonics are the most common power quality problems. Let us first have a closer look at these power quality problems and their definitions based on the European Standard EN 50160.

Supply Interruptions

Supply interruption is defined as a condition that the voltage at the supply-terminals is less than 1% of the declared voltage.

Possible impacts:

System suspension
Loss of computer / controller memory and information stored
Production loss
Safety and security concerns arising from the suspension of critical systems

Voltage Dips

A voltage dip is defined as a sudden reduction of the voltage to a value between 90% and 1% of the declared voltage with a voltage recovery after a short period of time. The duration of a voltage dip may last between 10ms to 1 minute. For HK Electric, the duration of voltage dips is usually less than 1 second.

Possible impacts:

Motor stalling / stopping
Memory loss
Data errors
Equipment shut-down
Light flickering

Harmonics

Harmonic voltage is defined as a sinusoidal voltage having a frequency equal to an integer multiple of the fundamental frequency (i.e. 50Hz) of the supply voltage. Harmonics of the supply voltage are mainly produced by customers' non-linear loads connected to the supply system and harmonic currents flowing through the system impedance give rise to harmonic voltages.

Possible impacts:

Overheating of motors / transformers
Overheating of electrical wiring
De-rating of electrical wiring and devices
Decreased motor performance
Abnormal operation of protection devices like breakers, relays or fuses
Telecommunications interference
Abnormal operation of sensitive electronic devices like motor control circuits

Common Causes of Power Quality Problems

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 Side

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.

Customer Side

Supply Interruptions

Customers' electrical installations are usually connected in a radial network. A fault occurring in any part of customers' electrical installations will trigger the protective device to isolate the faulty part. Supply interruption will then occur to downstream feeders. Common faults in the customers' electrical installations that cause supply interruptions include:

Overload, loading imbalance and under-rated equipment
Improper connections
Deteriorated wires or insulation or faulty switchgear
Faulty electrical appliances
Improper setting of protection devices

Voltage Dips

Voltage dips may be caused by customers' electrical installation, such as:

Excessive starting current of large motors of refrigerating and air-conditioning plants
Large fault current causing voltage dips in adjacent circuits before the protective device isolates the fault

Harmonics

Harmonics are generated by any load which draws current that is not proportional to the voltage applied. In other words, harmonics are generated by non-linear loads that draw non-sinusoidal current. Harmonic currents flowing through the impedances of the electrical system give rise to harmonic voltages. Thus, harmonic currents and harmonic voltages both vary with time. Such non-linear loads include but are not limited to the following:

Fluorescent lamps
Electronic machines (copiers and faxes)
Power electronics controlling device of motors
Battery chargers

Mitigation Measures against Power Quality Problems

Power quality problems may originate from either the supply or customer side, efforts from both parties are essential to achieve a high level of power quality.

HK Electric's Input in Power Quality

HK Electric is committed to supplying quality & reliable power supply to our customers. As we have observed, most of the supply interruptions and voltage dips are caused by third parties' excavation works. Thus prevention of cable damage is crucial in the improvement of power quality. The following paragraphs outline some of the actions we have taken to prevent cable damage as well as the services that we provide to ensure power quality.

Liaison with Road Works Contractors

In order to prevent cable damage incidents, HK Electric has arranged seminars to educate the contractors and to provide them with cable map records before they start their excavation works. There are also regular meetings held between our engineers and major contractors about cable damage prevention.

Law Enforcement

The "Electricity Supply Lines (Protection) Regulation" has been enacted by the Government to govern all works in vicinity of electricity supply lines to ensure that all reasonable measures are taken to prevent cable damages causing supply interruption.

HK Electric's Services on Power Quality

Relevant technical information on power quality matters is available to the customer upon request. HK Electric may also arrange site measurements on the supply sources to identify possible types and sources of power quality problems.

What Customers Can Do to Safeguard Power Supplies for Their Important Services

HK Electric has been constantly reviewing and upgrading the reliability of the supply of our system. However, an effective immunity to power quality problems can only be achieved with the joint effort from our customers. Additional information on this topic is available in our "Guide to Connection of Supply" and the leaflet "How to Safeguard Power Supplies for Important Services".

The following suggestions illustrate some effective means that customers can implement for minimizing the impact of power quality problems:

Against Supply Interruptions

Backup Supply

Uninterruptible Power Supply (UPS) can be divided into static type and rotary type.

Static Type UPS System

Static type UPS system consists of a rectifier / battery charger, a battery, an inverter and a system controller. During normal operation, the rectifier / battery charger converts the input AC power to DC which supplies the inverter and maintains the battery in a fully charged condition. The inverter converts DC to regulated clean AC power for use by the critical load. The power conversion process isolates the critical load from any disturbances present in the power network and, if the power source fails, the inverter draws its power from the storage battery. When power returns to normal, the rectifier resumes its recharging role automatically.

Rotary Type UPS System

Rotary type UPS system is composed of a static type UPS and a Motor / Generator set. During normal operation, power flows from the main supply to the static type UPS, which in turn powers the Motor / Generator set. The Generator converts the mechanical power back to clean AC power and feeds the critical load. In addition to a clean and non-stop power supply that can be provided by a static UPS system, a rotary type UPS system offers also absolute electrical isolation between the supply input and output to critical load provided by the Motor/Generator set.

Automatic Backup Supply (ABS) Video - Automatic Backup Supply (ABS)

For services that can tolerate a short delay in power resumption after power failure, a standby generator or alternative supply with automatic change-over device can be a possible solution. For services where a continuous electricity supply is required, a UPS system together with ABS is preferred and should be installed to prevent and minimize any damage, loss or inconvenience caused directly or indirectly by voltage fluctuation, interruption or failure of supply.

Bus Section Switch Video - Bus Section Switch

If customer installations receive supply from two or more HK Electric transformers or 11kV supplies, customers are strongly recommended to install bus section switch(es) so that a backfeed source is available through the other transformer or 11kV supply in the event of loss of supply from one of the sources.

Discrimination of Protective Devices

Any improper settings of protective devices will result in unnecessary interruption of supply. In the event of a fault, protective devices should be discriminated, so that only the minimum necessary portion of the electrical installations would be isolated. Therefore, customers are recommended to regularly check and review the protection settings in their installations.

Inspection and Maintenance of Customer Installations

Regular inspection and preventive maintenance of customer installations should be carried out to improve the reliability of their electrical installations. Inspection and maintenance for installations that supply important services should be more frequent. More information can be obtained in our leaflet "Guide to the Maintenance of Communal Electrical Installations In Multi-Tenant Building".

Against Voltage Dips

Power Conditioning Devices

Uninterruptible Power Supply (UPS)

Can provide complete security of supply for important services but it is more expensive.

Voltage Stabilizer

Can maintain the output voltage at a range within 2% of the normal value even when the mains supply voltage varies up to 20%. The device would also prevent voltage spikes and other transient voltages from entering into the circuits of the important services. However, voltage stabilizer does not compensate for a complete interruption of main electricity supply.

Line Conditioner

Can prevent voltage transients and voltage spikes from entering into the circuits of important services. Similar to voltage stabilizers, it cannot provide complete compensation in some cases.

More information can be obtained in our leaflet "How to Safeguard Power Supplies for Important Services".

Handling Large Motors of Air-Conditioning Systems

The starting of large motors of air-conditioning systems usually draws a large current and may result in voltage dips. Customers should therefore specify their motors in such a way that the starting characteristics of the motors will not cause interference with the sensitive equipment in their installation. In addition, the starting current shall comply with the requirements as stipulated in the Supply Rules. Some plants and machines are sensitive to voltage dips which may cause stalling of the motors and tripping of the control circuits. To minimize inconvenience, plants and machines should be specified to be capable of running and withstanding continuously the momentary voltage dip without producing injurious effects. If there are two or more supply sources, customers should separate the supply sources to motors from those to the sensitive equipment.
For existing installations, if it is confirmed that the machines possess the capability of running continuously and withstanding the momentary voltage dip without producing injurious effects, a time delay of suitable duration may be introduced, and / or the voltage protection setting should be adjusted to prevent nuisance tripping.

Handling High Pressure Discharge Lamps (HPDL)

HPDL is sensitive to voltage dips and cannot be re-ignited in a short time even when the supply has returned to the normal voltage. This may cause interruption to important events and safety problem. Here are several suggestions to relieve this problem:

Employ Constant Wattage Auto-Transformer with higher voltage dip ride-through capability. Video - Employ Constant Wattage Auto-Transformer
Employ lamps with double-tube feature, which the side tube will ignite once the main tube extinguishes. Video - Employ lamps with double-tube feature
Employ hot re-strike igniter to produce higher re-strike voltage after voltage dips while the lamp is still hot. Video - Employ hot re-strike igniter
For critical lighting applications, install non-HPDL such as tungsten-halogen or fluorescent lamps, or backup the emergency HPDL lighting with UPS.

Against Harmonics

Specifying Non-linear Loads

It is always more economical to eliminate problems in the design stage rather than to carry out remedial actions afterwards. Customers should specify to the equipment suppliers that the equipment of non-linear load must comply with their relevant international standards for the harmonics performance. Although it may involve higher initial cost, it is a worthwhile investment since the cost of any subsequent remedial work will be more expensive, not to mention the possible inconvenience arising from harmonic interferences caused to business.

Isolation of Harmonics Source

Customers should relocate essential and sensitive equipment to "clean" circuits and separate them from non-linear load as far as possible.

Harmonic Filter

There are passive and active harmonic filters. Passive harmonic filter is capable of minimizing the several pre-determined orders of harmonics only on specified load conditions. Active harmonic filter is more sophisticated and can minimize more orders of harmonics under varying harmonic load conditions.

Disclaimer

The materials in these web pages are for general guidance only. In case of doubt, you should seek advice from an expert or a professional engineer. We shall not be liable for any loss and damage suffered by any persons howsoever arising from or in reliance upon the whole or part of the content of these materials.