Voltage dip profile

Figure  shows the profile of a voltage dip, together with  the associated definitions.  The major cause of voltage dips on  a supply system is a fault on the system, that is sufficiently  remote electrically that a voltage interruption does not occur. Other sources are the starting of large loads (especially  common in industrial systems), and, occasionally, the supply  of large inductive loads. Voltage dips due to the latter are usually due to poor design of  the network feeding the consumer.  A voltage dip is the most  common supply disturbance causing interruption of production  in an industrial plant.  Faults on a supply network will always  occur, and in industrial systems, it is often practice to specify  equipment to ride-through voltage dips of up to 0.2s.  The  most common exception is contactors, which may well drop  out if the voltage dips below 80% of rated voltage for more  than 50-100ms.  Motor protection relays that have an  undervoltage element setting that is too sensitive is another  cause.  Since contactors are commonly used in circuits  supplying motors, the impact of voltage dips on motor drives,  and hence the process concerned, requires consideration.

Recording of a voltage dip

Other network-related fault causes are weather–related (such  as snow, ice, wind, salt spray, dust) causing insulator  flashover, collisions due to birds, and excavations damaging  cables.  Multiple voltage dips, as illustrated in Figure,  cause more problems for equipment than a single isolated dip. The impact on consumers may range from the annoying (nonperiodic light flicker) to the serious (tripping of sensitive loads and stalling of motors).  Where repeated dips occur over a period of several hours, the repeated shutdowns of equipment can give rise to serious production problems.  Figure
shows an actual voltage dip, as captured by a Power Quality recorder. Typical data for undervoltage disturbances on power systems during evolving faults are shown in Figure  Disturbances
that lie in the front right-hand portion of the histogram are the
ones that cause most problems, but fortunately these are quite
rare.

Voltage surge profile

Voltage surges/spikes are the opposite of dips – a rise that  may be nearly instantaneous (spike) or takes place over a  longer duration (surge).  These are most often caused by  lightning strikes and arcing during switching operations on  circuit breakers/contactors (fault clearance, circuit switching,  especially switch-off of inductive loads).  Figure 23.6 shows  the profile of a voltage surge. Equipment may suffer serious damage from these causes,  ranging from insulation damage to destruction of sensitive  electronic devices.  The damage may be immediate and  obvious by the fact that equipment stops working, through to  failure at a much later date from deterioration initiated from a  surge or spike of voltage.  These latter failures are very difficult  to distinguish from random failures due to age, minor  manufacturing defects, etc.

Sustained overvoltages are not common.  The most likely  causes are maladjusted voltage regulators on generators or onload tap changers, or incorrectly set taps on fixed-tap  transformers.  Equipment failures may immediately result in  the case of severe overvoltages, but more likely is accelerated  degradation leading to premature failure without obvious  cause.  Some equipment that is particularly sensitive to  overvoltages may have to be shut down by protective devices.

 Read: What is harmonics in electrical system and effect of harmonics

Supply waveform distorted due to the presence of harmonics

This is a very common problem in the field of Power Quality.The main causes are Power Electronic Devices, such as  rectifiers, inverters, UPS systems, static var compensators, etc.   Other sources are electric discharge lamps, arc furnaces and  arc welders.  In fact, any non-linear load will be a source of  harmonics.  Figure illustrates a supply waveform that is  distorted due to the presence of harmonics. Harmonics usually lead to heating in rotating equipment  (generators and motors), and transformers, leading to possible  shutdown.  Capacitors may be similarly affected.  If harmonic  levels are sufficiently high enough, protective devices may shut  the equipment down to avoid damage.  Some equipment, such  as certain protection devices, may maloperate and cause  unnecessary shutdowns.  Special provision may have to be  made to filter harmonics from the measured signals in these  circumstances.  Interference may be caused to communication  systems.  Overloading of neutral conductors in LV systems has  also occurred (the harmonics in each phase summing in the  neutral conductor, not cancelling) leading to failure due to  overheating.  This is a particular risk in buildings that have a  large number of PCs etc. In such cases a neutral conductor  rated at up to 150% of the phase conductors has been known  to be required.  Busbar risers in buildings are also at risk, due  to harmonic-induced vibration causing joint securing bolts, etc.  to work loose.

Read: What is VFD ( veriable frequency drive) and how it works?

Frequency variations that are large enough to cause problems  are most often encountered in small isolated networks, due to  faulty or maladjusted governors.  Other causes are serious  overloads on a network, or governor failures, though on an  interconnected network, a single governor failure will not  cause widespread disturbances of this nature.  Network  overloads are most common in areas with a developing  electrical infrastructure, where a reduction in frequency may be  a deliberate policy to alleviate overloading.  Serious network  faults leading to islanding of part of an interconnected network  can also lead to frequency problems.
Few problems are normally caused by this problem.  Processes  where product quality depends on motor speed control may be  at risk but such processes will normally have closed-loop speed  controllers.  Motor drives will suffer output changes, but  process control mechanisms will normally take care of this.   Extreme under- or overfrequency may require the tripping of  generators, leading to the possibility of progressive network  collapse through network overloading/underfrequency causes.

What is voltage fluctuation?

These are mainly caused by load variations, especially large  rapid ones such as are likely to occur in arc and induction  heating furnaces, rolling mills, mine winders, and resistance  welders.
Flicker in incandescent lamps is the most usual effect of  voltage fluctuations.  It is a serious problem, with the human  eye being particularly sensitive to light flicker in the frequency  range of 5-15Hz.  Because of the wide use of such lamps, the  effects are widespread and inevitably give rise to a large  number of complaints.  Fluorescent lamps are also affected,  though to a lesser extent.

Unbalanced loading of the network normally causes voltage  unbalance.  However, parts of the supply network with  unbalanced impedances (such as untransposed overhead  transmission lines) will also cause voltage unbalance, though  the effect of this is normally small.
Overheating of rotating equipment results from voltage  imbalance.  In serious cases, tripping of the equipment occurs  to protect it from damage, leading to generation/load  imbalance or loss of production.

Faults on the power system are the most common cause,  irrespective of duration. Other causes are failures in  equipment, and control and protection malfunctions. Electrical equipment ceases to function under such conditions,  with undervoltage protection devices leading to tripping of  some loads.  Short interruptions may be no more than an  inconvenience to some consumers (e.g. domestic consumers),  but for commercial and industrial consumers (e.g.  semiconductor manufacture) may lead to lengthy serious  production losses with large financial impact.  Longer  interruptions will cause production loss in most industries, as  induction and synchronous motors cannot tolerate more than  1-2 seconds interruption without having to be tripped, if only  to prevent excessive current surges and resulting large voltage  dips on supply restoration.  On the other hand, vital computer  systems are often fed via a UPS supply that may be capable of  supplying power from batteries for several hours in the event of  a mains supply failure.  More modern devices such as Dynamic  Voltage Restorers can also be used to provide continuity of  supply due to a supply interruption.  For interruptions lasting  some time, a standby generator can be provide a limited  supply to essential loads, but cannot be started in time to  prevent an interruption occurring.

Excessive network loading, loss of generation, incorrectly set transformer taps and voltage regulator malfunctions cause undervoltage.  Loads with a poor power factor (see  for Power Factor Correction) or a general lack of reactive power support on a network also contribute.  The location of power factor correction devices is often important, incorrect location resulting in little or no improvement.
The symptoms of undervoltage problems are tripping of equipment through undervoltage trips.  Lighting will run at reduced output.  Undervoltage can also indirectly lead to overloading problems as equipment takes an increased current to maintain power output (e.g. motor loads).  Such loads may
then trip on overcurrent or thermal protection.

Transients on the supply network are due to faults, control and protection malfunctions, lightning strikes, etc. Voltage-sensitive devices and insulation of electrical equipment may be damaged, as noted above for voltage surges/spikes. Control systems may reset.  Semiconductor manufacture can
be seriously affected unless the supplies to critical process plant are suitably protected.