POWER QUALITY:

Power Quality is defined in the Institute of Electrical and Electronics Engineers (IEEE) 100 Authoritative Dictionary of IEEE Standard Terms as “the concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment.” Utilities may want to define power quality as reliability.

IMPACT OF HARMONICS IN ELECTRICAL SYSTEMS:

When there is a power problem with a piece of equipment, end users may be quick to complain to the utility of an “outage” or “glitch” that has caused the problem. However, the utility records may indicate no abnormal events on the feed to the customer. One example is capacitor switching, which is quite common and normal on the utility system, but can cause transient over-voltages that disrupt manufacturing machinery. Another example is a momentary fault elsewhere in the system that causes the voltage to sag briefly at the location of the customer in question. This might cause an adjustable speed drive or a distributed generator to trip off, but the utility will have no indication that anything was amiss on the feeder unless it has a power quality monitor installed.

All harmonics tend to distort the original fundamental 50 Hz sine wave creating operational problems with digitally controlled equipment such as, PLC’s, automation equipment, machine tools, and computers.

Odd triplen harmonics, (3rd, 9th, 15th, etc.), do not cancel each other, but add together in neutral conductors of 3-phase, 4-wire systems to cause overheating in panels, neutral conductors, terminations and transformers.

3rd harmonics generated by the transformer secondary loads in the 3-phase delta-wye distribution transformer are reflected back into the delta primary resulting in circulating primary currents and overheating.

5th harmonics of sufficient magnitude result in motor inefficiencies and overheating. The negative sequence may produce sufficient counter-torque to cause excessive motor vibration.

Generalized harmonic effects include: unexplained operation of protective devices, audible noise interference on telephone circuits, blown fuses on power factor correction capacitors and, erratic operation of generators with solid-state controls.

WHAT GENERATES HARMONICS:

Harmonics are produced by not-linear loads that absorb non-sinusoidal current. The most common loads, both in industrial surroundings and domestic ones, are the following ones:

•  Frequency / Variable speed drives.

•  Discharge lamps (high pressure sodium vapour lamp, mercury vapour lamp, low consumption, fluorescent, etc.).

•  Rectifiers.

•  AC/DC Converters.

•  Arc welding.

•  Induction ovens.

•  UPS.

•  Computers and laptops.

•  Etc.

PQ AUDIT:

PQ Audit focuses on common issues such as harmonic distortion, low voltage, voltage sags, unbalanced loads, wiring and grounding, energy wastage on unoccupied space and poor power factor. The audit also helps in identifying and assessing major problems causing PQ issues. Hence, it distinctively helps in understanding what type of PQ issues are going on at various locations within the system and how are they affecting the electrical.

WHY PQ AUDIT?

PQ Audit provides a technical and economic basis to help, make informed decisions in solving power related problem and enhancing the electrical system. The audit provides a structured hierarchical step-by-step approach to tackling utility and end user power quality challenges. It is analogous to a body check-up by a doctor.

HOW VULNERABLE IS AUTOMATION TO POOR QUALITY?

No automation can be successful without ensuring good Power quality. However Poor quality is inherent in the electrical system, and are also not in our control, and hence there is an absolute necessity to protect you valuable automated machines from the power disturbances, which sometimes could be catastrophic. With automation like CNC machines, Robotics and Internet of things is the norm in today’s competitive Industrial environment. Industries are going all out to gain competitive advantages, just to stay in business. Automation guarantees more than just that. However, the flipside of it is the poor power quality generated by the power supplies of automated machines, and its deleterious effect on the digital circuits, like mis-operation and break downs.

To achieve and implement an automation program, industries need to make sure that the Power quality is to IEEE-519 standards, it like putting the horse before the cart. Industries ignoring this critical fact would imperil the entire Automation Process. Good power Quality means, energy efficiency and reliability of operations, savings on maintenance and improving the life of the automated equipments. One should remember that the cost of breakdowns can be very expensive and affect the smooth production.

For any IOT (Internet Of Things) or digital device to operate reliably, signal integrity and power integrity must be high. This is particularly important in low voltage, high frequency circuits, which are less tolerant of just 1 %. These incredibly tight bands mean, data and clock signals would be impacted by any transients, ripple and noises on the supply rails. Hence the challenge is to ensure excellent power Quality.

Given below is a typical example of the kind of problems automobile Robotic welding line can create. In this case the harmonics generated by the welding robots were creating harmonic frequencies and causing breakdowns, capacitor damages and poor Power factor.

SOLUTION:

a. Is the solution cost effective?

b. Is the solution robust and durable?

c. Is the mitigation closer to harmonic loads?

d. which of the solution will be efficient?

Some of the probable solution are listed below.

• Neutral current eliminators and phase shift systems (for four-wire systems).

• Standard AC line and DC bus reactors.

• Wide spectrum (reactor/capacitor) filters.

• Duplex reactors.

• Passive L-C (inductance/capacitance) filters.

• Multi-pulse (phase shifting).

• Quasi-multi-pulse (phase staggering).

• Active filters.

• Active front ends (sinusoidal input rectifiers).

Article by: J A SIMON & J K VENKATESH

waveformspower@gmail.com