Voltage Transients & Power Quality

How Do Voltage Transients Affect Power Quality?

Voltage Transients, which are often called spikes, can have an effect on equipment and installations that ranges from mildly irritating to extremely damaging and costly. An electrical transient is a very fast, short duration spike in voltage that can be several kV in magnitude. It may be a single event, but transients can also come in bursts. The voltage spike produces an increase in current (in the load) seen as a current spike, which results in a momentary increase in the energy transferred from the supply to the load. Depending on the magnitude and duration of the transient, the amount of extra energy transferred in this way may be of little or no consequence, or it may be enough to cause serious damage.

It is often assumed that most transients are generated by events external to the installation that’s affected, such as lightning strikes, load switching and fault clearance in the utility company’s supply equipment. It is true that because of their high voltages and energy content, transients produced by lightning pose the highest risk of equipment damage and failure, but most transients – more than 80%, studies have shown – are in fact generated within the installation itself.

Lightning induced transients are rare, so why are they potentially so damaging? The current in a typical lightning strike rises quickly to its maximum level within 1 to 10 microseconds, then it decays more slowly in around 50 to 200 microseconds. This rapidly changing current creates electromagnetic radiation (radio waves) that travels outward from the location of the strike. If this radiation encounters an electrical conductor, such as a power line, a communication line or a metallic pipe, the conductor acts like an aerial and a high voltage – the transient – is induced into it.

Note that the conductor doesn’t have to be struck directly by the lightning; even a strike to the ground near to the conductor can induce large transients. 

Other external factors like load switching and fault clearance within the utility supply can generate transients. Load switching transients result from the sudden release of electrical, magnetic, or in the case of rotating machines, mechanical energy stored in a device at the instant it is switched on or off. Transients produced by fault clearance are produced by a similar sudden release of energy at the instant the fault current is interrupted. Transients from these external sources are comparatively rare and are almost always much smaller than those produced by lightning.

A much more frequent source of transients is load switching within an installation. The event that gives rise to the transient could be something like bus transfer switching, but it is even more likely to be something simple like a circuit breaker or a contactor opening or closing. Even operating a light switch can create transients and, in every case, the level of transients will be increased if the switching device has faulty or corroded contacts. Office equipment, such as photocopiers and laser printers, is notorious for generating transients, as are HVAC systems. In fact, whenever an inductive or capacitive load is switched on or off, it will almost certainly produce a transient – albeit a small one; it is this example of a transient which will propagate through the electrical installation most frequently.

Considering the effects of transient spikes... on an electrical installation and the equipment connected to it

• Those which are usually small - it is generally the case that these transients are generated internally but they are likely to cause slow degradation over time.
• Those which are much larger produced by lightning and  the switching of large inductive loads can cause immediate insulation breakdown and subsequently deliver large amounts of energy into the equipment, resulting in failure and, in the worst cases, fire or even an explosion.

So what is the mechanism of these dramatic failures?

When equipment is subjected to a transient that has a voltage higher than the 'breakdown voltage' of the equipment’s insulation... a flashover is likely to occur. This flashover is a low impedance electrical arc through which current from the supply system can flow. With all of the energy of the mains supply behind it, the strength of the arc, and the heat it produces, increases almost without limit - creating the risk of fire, explosion and danger to life!

Even when transients do not lead to equipment failure, they can still be disruptive causing computers to crash and lose data, for example, process control systems to shut down unexpectedly and even residual current devices (RCDs) to trip for no obvious reason.

A wide range of measures is available for providing protection against transients, and selection of the most appropriate type must take into account:

• Voltage
• Duration
• Power level
• Type of equipment that is to be protected

Some types of equipment, such as motors may be designed to withstand transients on a typical supply system without further protection, but this should never be taken for granted. Electronic equipment may also feature integral protection, but this is unlikely to be adequate on its own if no other form of protection is fitted to the distribution system of the premises in which it is being used.

It may at first seem unnecessary to provide transient protection for test equipment like multimeters and multifunction installation testers but, in reality, such protection is essential. An electrical installation is just as likely to experience transients while tests are being carried out as at any other time and if the energy released as a result of the transient is enough to destroy an unprotected instrument, the user, who is likely to be close to it or even holding it, may well be injured or worse. The need for transient protection in instruments is reflected in BS EN 61010, "Safety requirements for electrical equipment for measurement, control, and laboratory use".

This requires test equipment to be able to withstand levels of transients appropriate to the point in the installation where the instrument will be used (see table).

BSEN61010-1 Transient Overvoltage Tests

Transient Overvoltage

A journey through the electrical installation

BS EN 61010 recognises that externally generated transients will be at their most severe at the point where the supply enters the building and will gradually reduce in magnitude as they travel through the electrical installation, because of the inductance, capacitance and resistance of the wiring and other equipment.

Put simply, this means that instruments connected at the point of supply need to be able to withstand transient voltages higher than those instruments connected to fixed wiring (within the installation) which in turn need to withstand higher transient voltages than those instruments used solely on equipment plugged into socket outlets. This is summarised in the category (CAT) ratings shown in the table above.

CAT I rated instruments can be used for measurements performed on secondary circuits not directly connected to mains. CAT II instruments are suitable for measurements performed on items connected to a standard 230v mains socket. CAT III instruments suitable for measurements performed on the fixed wiring within a building installation, which includes, for example, distribution boards, circuit breakers, bus-bars, junction boxes and industrial equipment.

CAT IV instruments can be used for measurements performed at the source of the low voltage installation.

Since instruments with a particular category rating can also be used in lower category applications – a CAT IV instrument can be used in any location within a low-voltage installation – it is often worthwhile investing in instruments with a high CAT rating since this will reduce the risk of an unsuitable instrument being used to carry out a particular task.

Transients can be mitigated using surge protection devices (SPDs), which are designed to prevent voltage spikes and surges damaging the installation wiring, infrastructure and equipment. If an overvoltage occurs, the SPD diverts the resulting excess current flow to earth and limits the voltage to a predetermined maximum value. Depending on circumstances, SPDs can be installed close to the internal source of the transients, close to the loads that need protection, or both.

Three types of surge protection devices are currently available:

1. SPDs can discharge partial lightning currents and are used in buildings that are supplied via overhead lines or that have a roof-mounted lightning protection system in line with BS EN 62305.
2. SPDs are suitable for use in all other types of installation and are often installed at the incoming supply point and/or in sub-distribution boards.
3. SPDs have a low discharge capacity and are used to provide localised protection for sensitive equipment. In most instances, they should only be used to supplement protection provided by Type 2 devices. Much more detailed information on the selection and application of SPDs is available on the manufacturers’ websites and reference should also be made to the latest edition of the IET Wiring Regulations (BS 7671) which now includes a section devoted to surge protection and SPDs.

To decide whether your installation is experiencing problems created by transients, the first action is to use a power quality analyser and, since transients are almost always intermittent, this needs to be equipped with data logging functionality so that recordings can be made over appropriately long periods of time. A good analyser will allow limits and alarms to be set to alert you when a significant transient has been detected, and you can then examine the data stored by the instrument to gain further information about the form and duration of the transient. This information is invaluable in determining the source.

 

The Power Quality Blog Post Series by Chauvin Arnoux

In practice, almost all power quality issues can be divided into 6 main areas and our Power Quality Series ('Understanding The Basic Issues of Power Quality and How to Address Them!') covers these 6 main areas:

1. Harmonics
2. Dips and swells
3. Transients (spikes)
4. Electrical interference
5. Voltage imbalance
6. Poor power factor

VIEW POWER QUALITY LOGGERS