Voltage Sags

Document Sample
Voltage Sags Powered By Docstoc
					                                   POWER QUALITY IN ENERGY
                                     CONVERSION SYSTEMS.

                   ABSTRACT                                      The term “power quality “means
           Power electronics and power quality        different things to different people. To utility
are irrevocably linked together as we strive to       suppliers, power quality initially refers to quality
advance both board areas. With the dramatic           of service delivered as measured by the
increases over the last20 years in energy             consumer’s ability to use energy delivered
conversion Systems utilizing power electronic         desired manner. This conceptual definition
devices, we have seen the emergence of “power         included such conventional utility planning
quality” as a major field of power engineering.       topics as voltage and frequency regulation and
From a Power quality impact view point,               reliability.
applications such as Switched Mode Power
Supplies, adjustable speed drives, and FACTS                    The end-user’s definitions of      Power
are often cause for concern. From utility supply      quality also centers around their ability to use the
system     view   point,   these   converter-based    delivered energy in the desired manner, but the
systems can lead to Operational and life              topics considered can be much more specific
expectancy problems for other equipment ,             and include magnitude and duration of different
possibly not owned or operated by the same            events as well as wave shape concerns.
party. It was from his initial perspective that the
field of power quality emerged.                       I. Categories of power quality:-
                                                           1.    Steady state voltage magnitude and
           In most cases, the same devices and        frequency,
systems that create Power quality problems can             2. Voltage sag’s,
also be used to solve power quality problems."             3. Grounding,
Problems solving” applications such as active              4. Harmonics,
harmonic filters, Uninterruptible power supplies           5. Voltage fluctuations and flicker,
all utilize the same switching device technology           6. Transients and
as the “problem causing” applications.                     7. Monitoring and measurement.

                                                      II.Reactive power and harmonic compensation:
         In this paper we first discuss what power              1. Typical harmonics produced by equipments
quality is and then various Categories of power                 2. Harmonic filters
quality .Harmonics is identified as potential
power quality problem. We discuss effects of          1. STEADY STATE VOLTAGE FREQUENCY AND
harmonics and methods to minimize harmonics.          MAGNITUDE:-
Finally we discuss various IEEE Standards in
case of power quality problems.                                             Frequency deviations can
                                                      affect power electronic equipment
          Introduction                                that use controlled switching devices unless
                                                      the controlled signals are derived from a signal
that is phase-locked with the applied voltage.      areas. Sags can originate any where in a
In most cases, phase-locks are used, or the         system, but are more pronounced in utility
converters consist of uncontrolled rectifiers. In   distribution   systems   because   of   greater
either case, frequency deviations are not a         exposure of low voltage systems to the causes
measure cause of problems. Steady state             of short circuits.
voltage regulation is a much more pronounced
issue that can impact a wide range of endues
equipment in most cases, utility supply
companies do a very effective job of providing
carefully regulated voltage with in permissible
ranges. Equipment manufactures typically
offer equipment that is tolerant of steady state
voltage    deviations     in    the   range   of
+10%.virtually     all   equipment,   especially             Most utility companies implement
sensitive electronic equipment ,can be effected     distribution system protection in what is
by voltage deviating out side the + 10% range       known as a “fuse saving” methodology.
in most cases over voltages above +10% lead         Following figure shows a typical over head
to loss of life, usually over time ;excessive       distribution system with two feeders being
over voltages can immediately fail equipment.       supplied from the same substation transformer.
Under voltages below -10% usually lead to           Each primary circuit has its own automatic
excessive current demands, especially for           circuit recluse (ACR) and shows one fused tap.
equipment that as controlled out put like an        Voltage sags are probably the most common
adjustable speed drive controlling a motor to a     power quality problem that is “given” to the
constant speed /torque point.                       end user by the supplying utility. However,
                                                    improper equipment grounding is responsible
2 Voltage Sags:-                                    for the vast majority of power quality
                    Voltage sags are probably       problems on the customer’s side of the meter.
the most problematic of all power quality
problems. At this time, a number of standards
making bodies, including IEEE ANSI and
ICE, are working on standards related to sag.
In most cases, sags are generally agreed to be
more serve and outside of scope of ANSI
C84.1 and they are temporary in nature due to
the operation of system protection elements.
Because the electrical system is a continuous
electrical circuit, faults in any location will     3 Grounding:-

have some impact on voltages throughout the
network of course, areas closer to the faulted                 Grounding     of   equipment      was

area will sea greater voltage sag due to the        originally conceived as a personal safety

fault than other, more (electrically) remote        issue.However, the presence of an electrical
conductor that is zero potential has been widely     are also produced by               a large variety of
used    in   many     power     electronic    and    “conventional” equipment including:
microprocessors-controlled loads.
          From a power quality perspective,               power       generation        equipment          (slot
improper grounding can be considered into three            harmonics),
broad categories:                                         induction motors (saturated magnetics),
                                                          transformer         (overexitation     leading     to
1.   ground loops,                                         saturation),
2.   improper neural-to-ground connections, and           magnetic-ballast           fluorescent        lamps
3.   Excessive neural-to-ground voltage.                   arcing),and
                                                          Ac electric arc furnaces (arcing).
                                                     All these devices will cause harmonic currents to
                                                     flow, and some devices actually directly produce
                                                     voltage harmonics.

                                                                           Any ac current flow trough
                                                     any circuit at any frequency will produces a
                                                     voltage drop at that same frequency. Harmonic
                                                     currents, which are produced by power electronic
     The ground loop problem is a significant
                                                     loads, will produce voltage drop in the power
issue when power, communications, and control
                                                     supply impedance at those same
signals all originates in different locations, but
                                                     Harmonic          frequencies.Besause          of      this
come together at a common electrical point.
                                                     interrelationship between current flow and
Transient include in one location can travel
                                                     voltage drop, harmonic currents created at any
through the created ground loop, damaging
                                                     location will distort the voltage in the entire
equipment along the way. Improper neural-to-
                                                     supply circuit.
ground connections will create a “noisy” ground
reference that may interfere with low-voltage
                                                     5. Voltage Fluctuations and Flicker:-
communications and control devices. Excessive
neural-to-ground voltage may damage equipment
                                                                                 Voltage flickers is not
that is not properly insulated or that has an
                                                     directly caused by electronic loads except in
inexpensive power supply.
                                                     the     largest      of      applications.     Voltage
                                                     fluctuations, and the corresponding light
4. Harmonics:-
                                                     flicker due to them, are usually created by
                                                     large power fluctuations at frequencies less
              In most cases power electronic
                                                     than about 30 Hz.
equipment is considered to be the “cause” of
                                                     In most applications, only
harmonics. While switching converter of all
                                                               1. Large dc arc furnaces and welders,
types produces harmonics because of the non
                                                               2. Reactive power compensators, and
linear relationship between the voltage and
                                                               3. Cycloconverters.
current across the switching devices, harmonics
     6. Transients:
              Transients, especially in the voltage                        The         previous       section
     supply, can create numerous power quality           specifically identified harmonics as a potential
     problems. The major source of transients is         power quality problem. In that discussion it was
                                                         pointed out that nonlinear loads such as
1.     lightning,                                        adjustable speed drives create harmonic currents,
2.     utility circuit switching and fault clearing,     and when these supply systems, harmonic
3.     capacitor switching, and                          voltages are produced.
4.     Load switching.
      Lightning events can create the most severe
overvoltages, but these transients decay
Rapidly, a typical lightning transient has decayed
to zero in a few hundred microseconds,                   1.   Typical      Harmonics       produced        By
        But it can reach a peak magnitude of several     Equipment:-
hundred percent if not controlled with surge
suppression devices.                                                  In theory, most harmonic currents
                                                         follow the “1/n” rule where n is the harmonic
                                                         currents in three-phase systems are not integer
                                                         multiples of three. Finally
                                                         , in theory, harmonic currents are not usually
                                                         even-order integer multiples of the fundamental.
                                                         In practice, none of these statements are
                                                         completely true and using any of them “exactly”
                                                         could lead to either over- or under-conservatism
                                                         depending on many factors. Consider the
                                                         following examples:
7. Monitoring and Measurements:-
                                                                     1.   switching-mode power supplies,
                      The most basic category of                          such as found in televisions,
power quality measurement tool is the hand-held                           personal computers, etc., often
voltmeter. It is important that the voltmeter be a                        produce a third harmonic current
true-rms meter, or erroneous readings will be                             that is nearly as large (80 -90 %)
obtained that incorrectly suggest low or high                             as the fundamental frequency
voltages when harmonics are present in the                                component.
signal. It is especially important to have true rms                  2.   Unbalance in voltage supplied to
capability when measuring currents; voltage                               the production of even-order
distortion is not typically severe enough to create                       harmonics and, in some extreme
large errors in the readings of non-true-rms                              cases,   establish      a   positive
meters.                                                                   feedback situation leading to
       II.REACTIVE              POWER              and                    stability problems.
            3.    Arcing loads, particularly in the              Shunt filters are designed in three basic
                  steel        industry,          generate       categories as follows:
                  significant harmonics of             all
                  orders, including harmonics that                             1.   single-tuned filters,
                  are not integer multiples of the                             2.   multiple-tuned filters, and
                  power frequency.                                             3.   Damped filter.
            4.    Cycloconverters                 produce
                  dominant harmonics that are
                  integer multiples of the power
                  frequencies that are not integer
                  multiples       of       the      power
                  frequency.      In     some      control
                  schemes, the amplitudes of the
                  sideband components can reach
                  damaging levels.

2 Harmonic Filters:-

            Harmonic filters come in many
“shape and size”. In general, harmonic filters are
“shunt” filters because they are connected in
parallel with power system and provide low
impedance paths to ground for currents at one or
more    harmonic          frequencies.     For      power
applications, shunt filters are almost always
more economical than series filters for the                            III.IEEE STANDARDS:-
following reasons:

                                                                                IEEE 519-1992 establish the” point
          1.     Series components must be rated             of common coupling”, or PCC, as the point at which
                 for the full current, including the         harmonic limits shall be evaluated. In most cases
                 power      frequency      components.       (recall    that   IEEE519-1992       is   a    “recommend
                 Such a requirement leads to large           practice”), this point will be:
                 components sizes and therefore
          2.     Shunt filter components generally
                 must be rated for only part of the
                 system           voltage.           Such
                 requirements      lead      to    smaller
                 component size and therefore
                                                                                               demand,                 usually
                                                                                               history, or     considering
        Current Distortion Limits For General Transmission Systems > 160kV
                                                                                               the previous 12 month’s
                                                                                              .The connected KVA or
         Maximum Harmonic Current Distortion Percent OF IL
                                                                                               horse       power,     perhaps
         Individual Harmonic Order h (odd harmonics)                                           multiplied by a diversity
                                                                                               factor        with      electric
         Isc/Il      < 11          11 < h < 17       17 < h < 23      23 < h < 35             h > 35           TDD

         < 50        2.0                 1.0               0.75            0.3                  0.15            2.5

         > 50        3.0                 1.5               1.15            0.45                  0.22           3.75

              In supply system owned by the utility
                            The closest electrical point
                             to the end-user’s premises,
                                                                                 In this paper, various power quality
                            As    in   (2),   but   further
                                                                   phenomena have been described, with particular
                             restricted to points where
                                                                   focus on the implications on power electronic
                             other customers are
                                                                   converters and equipment. While one popular
                            (or could be in the future)
                                                                   opinion “blames” power electronic equipment
                                                                   for “causing” most power quality problems, it is
                                                                   quite clear that power electronic converter
           In this context, IEEE 519-1992 harmonic
                                                                   systems can play an equally important role in
limits are designed for an entire facility and should
                                                                   reducing the impact of power problems. While it
not be applied to individual pieces of equipment
                                                                   trues that power electronic converter s and
without great care                              because the
                                                                   systems are the major cause of harmonic related
PCC is used to evaluate harmonic limit compliance,
                                                                   problems, the application (in general terms) of
system strength(impendence) is measured at this
                                                                   IEEE 519-1992 limits for current and voltage
point and is described in terms of available(three-
                                                                   harmonics has led to the reduction, elimination
phase) short circuit current. Also the end user’s
                                                                   and prevention of most harmonic problems.
maximum average demand current is evaluated at this
                                                                   Other     power          quality       phenomena,      like
                                                                   grounding’s, sag’s, and voltage flicker, are most
.Maximum demand is evaluated based on one of
                                                                   often completely unrelated to power electronic
the following:
                                                                   systems. In reality, advances in power electronic
                              The maximum value of
                                                                   circuits and control algorithms are making it
                               the 15or30 minute average
                                                                   more possible to control these events and
minimize the financial impacts of the majority of
power quality problems.


                          1.   ANSI STD C84.1 –
                               1995,         Electric
                               Power System and
                               Rating (60 Hz).
                          2.   IEEE STD 493 -
                               1997,              IEEE
                               Practice     for    the
                               Design of Reliable
                               Industrial          and
                               Commercial power
                          3.   IEEE
                               Practice            for
                               Grounding            of
                               Industrial          and
                               commercial Power

Shared By: