Power Quality

Power Quality, Harmonics & Transients By P.K.Agarwal DGM, NRLDC

Agenda
Peculiarity of power
What is quality power.
Power quality defects.
Who is responsible?.
Reasons of bad power quality.
Cost of bad power quality.
Typical problems.
Solutions.

Peculiarity of Electric Power
Electric power is the most essential raw material.
It is unusual commodity as it requires continuous flow and can not be conveniently stored.
It is a typical example of “Just in Time” philosophy – Components are delivered to the production line at the point and time of use by a trusted supplier with no requirement of “goods in” inspection.

Peculiarity of Electric Power (contd..)
In reality electricity is very different from any other product.
It is generated far from the point of consumption.
Is fed to the grid together with output of many other generators.
And arrives at the point of use via several transformers and many kilometers of overhead or possibly underground cabling.
The network is owned and maintained by different owners

Peculiarity of Electric Power (contd..)
Assuring the quality of power is not a easy task.
These is no way that sub-standard electricity can be withdrawn from the supply chain or rejected by the consumer.
From the consumer point of view situation is even more difficult.
Acceptable quality level as perceived by the supplier is different then desired by the consumer.

Quality of power
The perfect power supply will be one which is
Always available
Always with voltage and frequency within tolerances
Has pure noise free sinusoidal wave shape.

Defects in Power
The most obvious defect in power is complete interruption for few seconds to several hours.
Voltage dips or sag where voltage drops to lower value for a short duration.
Voltage swells or surges where voltage raises to higher value for a short duration.
Long power interruption are problems to all users.
Many process are very sensitive to even a short duration of interruption.

Defects in Power (contd…/-)
Continuous process industry where short outage of power may loss synchronizing of machinery and may result in large quantity of semi processed product – Paper making where clean up process is long and expensive
Outage in multistage batch process may destroy the value of previous process – Semiconductor industry – Production of wafer requires few dozen process over several days.

Categories of Defects in Power
Power quality defects i.e. deviations from perfection falls in five categories:-
Under Voltage or Over Voltage
Dips (or sags) and surges (or swells).
Blackouts.
Harmonic distortion.
Transients.

Under voltage or Over Voltage
Over voltage or under voltages means, supply voltage is more or less then the allowable tolerances.
These are the causes of unbalance of supply and consumption of reactive energy in network.
If generation of reactive power is more then the consumption supply voltage will be high.
If generation of reactive power is less then the consumption, voltage will be less.

Under voltage or Over Voltage (contd..)
Sustained over voltage or under voltage stress the equipment.
It weakens the insulation of system and equipments.
By proper reactive power management the supply voltage can be maintained within the tolerance.
As far as possible reactive power compensation should be done locally by consumer.

Voltage Dips (or Sags) and Surges
Voltage dips or sags are caused by abrupt increases in loads such as short circuits or faults, motors starting, or electric heaters turning on, or they are caused by abrupt increases in source impedance, typically caused by a loose connection.

Voltage Dips

Relative Cost of Voltage Dips Elimination

Voltage Dips (or Sags) and Surges (contd..)
Voltage swells are almost always caused by an abrupt reduction in load on a circuit with a poor or damaged voltage regulator, although they can also be caused by a damaged or loose neutral connection.

Blackouts (Power Outage)
Power outage disrupts more business that any other factor.
Power outage or blackout means energy is not available for work.
Power outages may be caused by generator maintenance outage, failure of equipment of any critical element.
The quality of sustained supply of power is known as reliability.
Reliability indices is used to track the performance of a utility.

Harmonic Distortions
Electricity generation is normally produced at constant frequency of 50Hz and generated voltage can be considered practically sinusoidal.
However, when a source of sinusoidal voltage is applied to a nonlinear device or load the resulting current is not perfectly sinusoidal.
In the presence of system impendence of this causes a non-sinusoidal voltage drop causing voltage distortion at the load terminals known as Harmonic Distortion.

Harmonic Distortions (contd…) Circuits containing thyristor is when supplied a sinusoidal voltage produces the distortion.

Harmonic Distortions (contd…)
Harmonics generated by microwave oven

Harmonics generated by SMPS of computer

Harmonics generated by variable speed drive

Transients
Transients disturbances are high frequency events with durations much less than one cycle of supply.
Causes are switching, lightening strikes on the network, switching of reactive loads on consumer sites.
Transients can have magnitudes of several thousands volts and so can cause serious damage to both the installation and the equipment connected to it.
A good earthing greatly reduces the transients.

Transients (contd…/-)
Majority of power system transients are due to switching action.

Transients (contd…/-) Voltage across the capacitor while Switching a RC circuit

Transients (contd…/-)
Transients are triggered from capacitor switching.
Capacitor transients normally causes the highest peak magnitudes. If the capacitor is switches when the system voltage near its peak, the capacitor pulls the system voltage down (as the current ruches to capacitor to charges it). The system rebounds, the voltage overshoots, and oscillates about the system fundamental frequency wave forms. This transients normally decays quickly.
Capacitor Switching Transients

Switching Transients
A restrike can occur when switching a capacitor off: the switch opens at current zero, trapping the peak voltage on capacitor. As the system voltage decreases from the peak, the voltage across the switch rapidly increases. By the time system voltage reaches opposite peak, the voltage across the switch is double. Switch may restrikes.
Restrike of a capacitor Bank

Who is Responsible for Quality
Suppliers argue – Critical users must bear the cost of ensuring quality of supply.
Rather than expecting the supply industry to provide very high reliability to every customer everywhere on the network.

Who is Responsible for Quality (Cond..)
Some quality problems are the result of shared infrastructure.
Fault on a one part of network may cause dip that will affect some customers.

Who is Responsible for Quality (Cond..)
Higher the level of the fault, the greater will be the number of customers affected.
Problem on one customer’s site may cause a transient that affects all the customers on the same subsystem.
Harmonics, arise within the customer’s won installation may propagate onto the network and affect other customers.

Reasons
Such a guaranteed quality supply would require a very substantial investment for the benefits of relatively few customers (in count, not in consumption) and would be uneconomic.
It may not be technical feasible with in the current social and legal frame work.
Any customer is normally entitled to be connected to the supply.
Utility providers has the right to excavate roadways with the risk of cable damage.

Reasons (contd…)
Weather conditions, such as high winds and rains, snow fall, frequently cause damages overhead lines.
Repair of lines under the same conditions, are difficult and time consuming.
It is therefore the consumers responsibility to take steps to ensure that the quality of power delivered to his process is good enough.
This quality level may well be higher than that delivered to the plant by the supplier.

Cost of Power Quality defects
It is difficult to attribute a business loss to a particular event.
The electricity supply industry tends to value an interruption in terms of the cost of the electricity that was not supplied as a result.
Consumer values it in terms of the revenue lost as a consequence of the break in production.
Electricity is relatively cheap and the supply interruption is relatively short.

How much good is good enough
This question is impossible to answer.
While it is relatively simple to quantify the behavior of a particular piece of equipment to voltage dips.
Determining the likely incidence of voltage dips at a particular on supply system is difficult.
It will change over time as new consumers are added and assets replaced.

How much good is good enough
There are some international standards available that set limit of voltage variation and harmonic voltage distortion below which the equipment should function without error.
Similarly, there are standard limits for voltage deviation and harmonic voltage distortion of the supply.

Typical Problems
A study was performed by European Copper Institute in 2001, covering 1400 sites in 8 countries.
Any given site in Europe has 5-20% probability of suffering from one or more of the problem listed.
Computer Lockups
Flickering screens
Flickering lights.
Overheating of transformers at moderate load.

Typical Problems (contd…)
Induction Motors
Heating of conductors due to skin effects.
Correct functioning of process control equipments
Data network congestion
Problem with power factor correction equipment.
Problem with specific (long) lines or when switching heavy loads.
Overloaded neutrals.

Nuisance tripping of protective devices.
Utility claims resulting from harmonics affecting supply.

Solutions
The list of possible solutions for power quality problems is long and non-complete. Solutions listed below are adopted according to a the survey:-
Surge Protection
UPS
Back-up generator
True RMS measurement
Transformer derating
Motor derating

Solutions (contd…)
Dedicated circuit
Multiple cables for harmonic loads.
Complete rewiring of the installation.
Zoning of electrical loads.
Meshed Earth.
Passive filters
Active conditioners
TN-S wiring
Natural Upsizing

Computer Lockups:

Solutions (contd…)

Thank You

Power Quality

by P K Agarwal on Jan 02, 2010

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