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CAUSES OF POOR POWER QUALITY Projects

VIEWS: 13 PAGES: 10

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                                 POWER QUALITY




                                        ABSTRACT

The power quality is a term used to broadly encompass the entire scope of interaction among
electrical suppliers, the environment, the system and products energized, and the uses of those
systems and products. It is more than the delivery of “clean” electric power that compile with
industry standards. It involves the maintainability of that power, the design, the selection, and
the installation every piece of hardware and software in the electrical energy system.
Stretching from the generation plant to the utility customer, power quality is a measure of
how the elements affect system as a whole.
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This paper presents an overview of electric power quality with special emphasis on power
quality problems, its adverse impacts on utilities and customers and the mitigation techniques.
The wide spread usage of power electronic loads aimed at enhancement of energy efficiency
and productivity has resulted in serious power quality problems such as voltage distortion due
to current harmonics, flicker, voltage sag, voltage surges etc., which call for assessment and
solution techniques. Here, we also discuss about two major power quality issues –grounding
and harmonics and some power system components, which correct the harmonics problems.
This paper broadly describes the above features along with the means for improvement of
power quality.




INTRODUCTION TO POWER QUALITY

The term ‘power quality’ means different things to different people. One definition is the
relative frequency and severity of deviations in the incoming power supplied to electrical
equipment from the customary, steady, 50Hz sinusoidal waveform of voltage or current.
These deviations may affect the safe or reliable operation of equipment such as computers and
electronic instruments. It also refers to the delivery of high grade of electric service
maintaining a sinusoidal load, bus voltage and current at stipulated magnitude and frequency.
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As we connect electronic devices to our power system, the “quality” after power is more
important. Quality can be defined in many ways. Stable voltages and undistorted waveforms
are two characteristics, which are very desirable in power systems. Thus while not having a
strict basis of measurement, terms like” Poor Power Quality” generally mean there is
sufficient deviation from norms in the power supply to cause equipment mis-operation or
pre-mature failure.


CAUSES OF POOR POWER QUALITY
The causes of poor quality can be attributed to :
    Variations in voltage, magnitude and frequency
    Variations in magnitude can be due to sudden rise or fall of load, outages, repetitive
       varying loading pattern in rolling mills, power electronic converters, lightning etc.
    Variations in frequency can rise of out of system dynamics or harmonics injection.


Consequently the voltage or current waveforms of a power system ceases to be purely
sinusoidal in nature but consist of harmonics and other noises.


IMPACT OF POOR POWER QUALITY:
The effect of these aforesaid poor power quality problems has serious implication on the
utilities and customers. Utility side impacts higher losses in transformers, cables etc. In
conductors the neutral wires can burn due to the presence of third harmonics generated by
non-linear loads. The power factor correction capacitors may puncture due to resonant
conditions at resonant frequencies near lower order harmonics. The energy-meters, which are
calibrated to operate under pure sinusoidal conditions, may give erroneous readings. The
solid-state protective relays can maloperate due to poor power quality. There can be increased
losses in cables, transformers and conductors.
The customer side of the power network also experience adverse effects of poor power
quality. The automatic processes employing adjustable speed drives may shut down because
of nuisance tripping due to even short voltage sags.. The induction synchronous motors can
have increased copper and core loses, pulsating torques and overheating with derating effect.


The non-sinusoidal power supply thus reduces torque and efficiency of the motors. The
computers and telecommunication equipment encounter loss of data and maloperation due to
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poor power supply quality. The domestic electronic gadgets such as digital clocks, VCRs and
TVs are also affected by voltage distortions.


SYMPTOMS OF POWER QUALITY PROBLEMS
    Electronic controlled systems that stop unexpectedly.
    Many systems reboots required.
    Abnormal failure rate of electronic systems.
    Transformers over heating.
    Motors failing.
    PF capacitors failing.
    Test results unreliable.


TYPICAL POWER QUALITY PROBLEMS & THEIR SOURCES
1. Sags: A reduction in r.m.s voltage or current at the power frequency for duration of 0.5
cycles to 1 minute. Also called voltage dip. Events below the equipment: ride through
capability cause load dropout. Voltage sags are originated in lighting strikes, short circuits
and sudden overloads.


Sags are under voltages on the power system and commonly caused by power failures, down
lines, utility recloser operations and storms. They can be corrected by using backup power
source such as UPSs, generators or similar voltage restoration technologies.


2. Surges: Voltage variations are another common source of problems to home computers
and other sensitive electronic equipment. Voltage variations can be positive (higher than
normal) or negative (lower).


Positive voltage variations can be even more troubling than negative ones. If powerful enough
they can destroy components in sensitive electronic equipment. Lighting striking power lines
is a frequent cause, as is load switching (re-routing power around the grid), by utility. Voltage
surges can also be caused by equipment in our home Refrigerator motors, air conditioners,
vacuum cleaners and other electrical loads can generate voltage surges and electrical noise.
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                 Voltage Sag                             Voltage Swell
3. Transients: The main difficulty with transients is in detection, since they manifest only as
a short duration change in voltage. The switching on and off of the electric motors that power
air conditioners, power tools, furnace ignitions, electrostatic copiers, arc welders and elevators
causes low energy swells. Lighting usually causes larger swells. Electrical noise is another,
milder transient power irregularity that often manifests as a computer glitch rather than an
equipment failure. Essentially, electrical noise is created when one piece of equipment
interacts negative with another, or with building grounding or wiring. Loose connections or
the equipment itself can be responsible for noise. Known noise-generating equipment includes
everything from computers, radios and fluorescent lights to fax machines, welders and light
sockets.

4. Voltage Fluctuations: Flickering lights can be an indication of voltage fluctuations in your
building’s or facilities electrical system. Left unchecked, high and low-voltage conditions can
result in equipment damage, data loss and erroneous readings on monitoring systems.
Overloaded power circuits are typically the cause behind under voltage conditions. Heavily
loaded motors such as air conditioners can result in intermittent low voltages. Less common
but more damaging, facilities with rapidly varying loads can cause over voltage conditions.




5. Two modern power quality issues – Harmonics & Grounding
          Voltage Surge                            Harmonic Penetration
Harmonics: A sinusoidal component of a periodic wave of quality having a frequency that is
an integral multiple of the fundamental frequency. It is a mathematical model, which is used
to analyse distorted waveforms and the current drawn by computers, electronic ballasts;
variable frequency drives and other equipment, which have modern “transformer-less” power
supplies.
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The dynamic power system loads produce a time varying amplitude in current waveforms
depending on the load characteristics which consists of the fundamental and harmonics
components. These harmonic components distort the voltage or current waveforms thereby
deteriorating the power quality. The non-linear loads such as inverter fed adjustable speed
drives. UPS (uninterrupted power supply system), rectifiers and furnaces, cyclo-converters
etc., which form the major chunk of industrial loads, contribute to the severe fluctuations in
power quality

The industrial load also consist of large percentage of power factor improvement capacitors
which often create resonance conditions at particular harmonic frequencies generated by non-
linear loads fed from the load bus, producing high oscillating currents at resonant frequency
and there by induces harmonic voltages distorting the pure sinusoidal voltage waveform..

For assessing power quality it is important to know the total harmonic distortion i.e. the
voltage and current distortion factors

                   V THD =    VK        & I THD =    IK
                                    V1                      I1

Vk = Voltage of Kth harmonic, Ik = Current of Kth harmonic
Where V1 and I1 are the r.m.s values of fundamental components of voltage and current
waveforms. The power quality deteriorates if the source has significant impedance causing the
distortion of voltage of the load bus supplying combination of linear and non-linear loads.

Harmonics problems often can be corrected by filtering or resizing power system components
like:


       Harmonic Filters
        Filters are sometimes most cost effective in an existing structure where rewiring is
        difficult or costly. The filters are used to block or trap the offending currents,
        lessening the harmonic loads on the wiring. But the filter design is dependent on the
        equipment on which it is installed, and may be ineffective if the particular piece of
        equipment is changed. Filtering characteristics need to be carefully designed for a
        given installation, and seeking professional design advice is recommended. Filters are
        also fairly expensive on a per-kVA basis.

     Shielded Isolation Transformers
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        Shielded isolation transformers are filtering devices that lessen feed-through of
        harmonic frequencies from the source or the load. They are a plausible retrofit
        technique where power problems have already been encountered, but are also quite
        expensive per-kVA.

     K-Rated Transformers
        K-rated transformers have beefed-up conductors and sometimes cooling to safely
        handle harmonic loads. Alternatively, standard transformers are sometimes de-rated to
        allow for the extra heating due to harmonics. Depending on the conditions
        encountered, a load limit of as little as 50% of the nameplate rating is observed. This
        may be adequate to handle harmonics, but lowers effective transformer efficiency. A
        careful comparison of the relative costs of K-rated vs. de-rated standard transformers
        should be made.

     Harmonic-Rated Circuit Breakers and Panels Overheating due to harmonics is the
        danger here, and beefed-up components used in these elements offer protection.
        Neutral buses should be rated for double the phase current.

Grounding:
The primary purpose of grounding electrical systems is to protect personnel and property if a
fault (short circuit) were to occur.

Grounding conductors connect all of the non-current carrying parts of the electrical system, or
any metallic parts in the vicinity of the electrical system together. This part includes conduits,
enclosures, supports and other metallic objects. This grounding system has two purposes:

    1. Safety. The grounding conductor system provides a low impedance path for fault
        currents to flow. This allows the full current to be detected by over current protective
        devices (fuses and circuit breakers), safely clearing the fault quickly.
    2. Power quality. The grounding system allows all equipment to have the same
        reference voltage. This helps the facility electronic equipments operation and helps
        prevent the flowing of objectionable currents on communication lines, seals and other
        connections.

6. Wiring:
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Generally, wiring and grounding problems come in the form of intermittent network failures,
buzzing sounds (corona effect), scorched insulation, intermittent voltages at equipment, and
burned panel or junction boxes. The table below illustrates some of the new wiring practices
recommended to achieve a high level of power quality. Many of the "before" practices are
still reflected in building codes today.

Receptacle           Before: 13                             Recommended: 3 to 6
Outlets per
20 Amp Circuit
Neutrals             Before: Full size or downsized         Recommended: Use double size
                     neutral (on 3-phase systems)           neutral (CBEMA) or larger
                     Before: One neutral shared among       Recommended: Separate neutrals
                     equipment (on 1-phase branch)          or upsized neutral back to panel
Phase                Before: Standard phase conductor       Recommended: Use upsized phase
Conductors           sizing per code                        conductors to minimize heating for
                                                            harmonics
Circuits             Before: Can shared among many          Recommended: Use separate
                     outlets and uses                       circuits for harmonic-sensitive
                                                            loads
Grounding            Before: Can use metal conduit as       Recommended: Use separate
                     grounding conductor                    insulated wire as grounding
                                                            conductor
                 Before: Downsized grounding                Recommended: Use full size or
                 conductor                                  over size grounding conductor
                 Before: (Commercial/Industrial)            Recommended: Use a copper
                 Must use metal water pipe and a            ground ring and multiple
                 concrete-encased electrode (if             interconnected ground rods
                 available)
                 Before: Use a second ground rod if         Recommended: Use multiple rods
                 first is over 25 ohms (no resistance       or ring and measure to ensure very
                 measurement required)                      low resistance to ground
                 Before: Access floor for                   Recommended: Use copper system
                 equipotential grid in computer             for equipotential grid
                 mainframe room
                 Before: No lightning or surge              Recommended: Use lightning and
                 protection                                 surge protection
Courtesy: Copper org.

7. Lightning:

Lightning Protection Systems
In simple terms, if part of the "path of least resistance" to ground the lightning sees is through
your wiring or equipment that is where it will flow. Lightning produces very high currents,
for a short time interval, but enough to cause fires or to destroy microcircuits even miles
away. The idea of air terminals, or lightning rods as commonly known, goes back to
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Benjamin Franklin. The purpose is to provide a convenient, controlled point for lightning to
strike, and then be safely conducted to ground. To provide the least resistive path, heavy-gage
copper wire should be employed in the leaders and down conductors.


Grounding of Lightning Systems
The down conductors tie directly to the ring ground described above, or other grounding
electrode system, along with all building steel and electric service grounds. Use heavy-gage
copper conductors to minimize impedance.

Techniques to mitigate power quality problems:

The increasing application of sensitive loads in the power networks has necessitated the need
to mitigate the serious power quality problems. The compensation techniques can be broadly
classified into two main categories viz.: passive and active techniques.

      Passive techniques: these techniques employ following devices:
      Passive shunt L-C Filters
      Power Factor Correction Capacitors.
      Active techniques: these techniques employ the following devices:
      PWM (VSI/CSI ) Active Filters.

Thus, finally the following steps may prevent most of the power quality problems from
occurring:

    Use double-size neutral conductors or separate neutrals for each phase.

    Specify a separate, insulated full-size grounding conductor, rather than relying on the
       conduit alone.

    Use an isolated grounding conductor for sensitive equipment.

    Segregate sensitive loads on separate branch circuits, fed from a separate panelboard,
       fed from separate feeders (and even separate transformers if possible).

    Run a separate branch circuit for every 4 to 6 duplex outlets.
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    Use an outside copper ground ring and multiple ground rods as part of the grounding
       electrode to achieve lowest practical resistance to ground. Measure ground resistance.

    Use harmonic-rated circuit breakers, panelboards, and transformers.

    Use surge and lightning protection.

    Oversize phase conductors to minimize voltage drop. (This will save energy too, and
       may even pay for itself through lower I2R losses.)

    Choose materials based on superior connect ability. Poor quality connections are a
       major consideration. This is where all-copper wiring excels over other materials.

Design these features into new construction or renovation work. Reduced downtime or
data loss will more than pay for these measures.

CONCLUSION:

The wide spread applications of non-linear power electronics loads nave brought but
degradation of power quality in the electric network. This paper has focused broadly on the
power quality issues, the implications on the utilities and customers in the power system. At
the same time, the paper has discussed in brief the assessment of power quality. The effective
means of compensation through various techniques have also been highlighted.




REFERENCES:

   1. Power quality and Harmonics: JOHN H. WAGGONER.
   2. Inside PQ: MAX McGRANAGHAN.
   3. Grounding and lightening protection: ROBISON, M.D.
   4. Electric Power Quality: problems and means to improve them: S.MOHANTY,
       B.R.MISHRA, Dr. D.S. CHAUHAN.

								
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