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PQ _ HARMONIC AT SUPPLY NETWRK

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PQ _ HARMONIC AT SUPPLY NETWRK Powered By Docstoc
					Power Quality and harmonics in the supply network :
    a look at common practices and standards .
                       Avishai Rash BScEE & PE
                Consulting Engineer Power & Control Systems
                                    a-rash@nter.net .il



Power Quality and harmonics in the             13 Transformer Failures and Over
supply network is one of today's most          Heating .
discussed      topic     in    Electrical
Engineering but also one of the most            Less often , articles discuss the
problematic subject as it is only quite        monitoring process of power quality and
recently that it became one of the main        harmonics as well as possible solutions to
devastating problem in the supply              these problems based on known
network . It is also considered as one of      standards. In my opinion , there are two
the most expensive and wasteful                main reasons for fewer papers published
parameter in the supply network .              on these subjects :
Since the beginning of the use of
electricity Power Quality has always           1- Standards are quite new and
been one of the major points of concern        complicated . Only some professionals
for all professionals involved in              are familiar with them and usually it is
Electrical Engineering .                       more significant to explain a specific
When reviewing a journal concerning             solution than a general concept ,
electric power and its technical               2-Monitoring      power      quality and
applications in industry , it is quite         harmonics is not as simple as measuring
common to find at least one article           power quality variables . There is a need
discussing problems caused by Power           to      understand      power        network
Quality and harmonics in the supply            behaviour before being able to monitor it ,
network . Here under is a collection of        and this is not a           simple task ,
examples I collected lately from              Moreover, it is more significant to discuss
various articles and seminars :               specific      problems        related     to
1 Blinking of Incandescent Lights .           results from measurement and then derive
2 Power Factor correction Capacitor           a general concept .
Failure .
3 Circuit Breakers Tripping for no             1 - Standards
visible reason .                              There are three main globally recognized
4 Computer Malfunction or Lockup or           standards dealing with harmonics as their
Communication failure  ,                      main goal :
5 Conductor Failure or Heating  ,             - IEEE 1159-1995 : IEEE Recommended
6 Electronic Equipment Shutting down.         Practices for Monitoring Electrical Power
7 Flickering of Fluorescent Lights .          Quality     ,

8 Fuses Blowing for No Apparent               - IEEE 519-1992 : IEEE Recommended
Reason.                                       Practices and Requirements for Harmonic
9 Motor Failures and Overheating .            Control in Electrical Power Systems .
10 Neutral Conductor and Terminal             - TEC          1000          (1991-1995) :
Failures.                                        Electromagnetic compatibility (EMC) .
11 Overheating of Metal Enclosures .          - NFPA 70 ( 1996 NEC )
12 Power Interference on Voice                These three standards deal separately with
Communication added Noise .            - 1219
                                              two aspects of the problem :
0-7803-3879-0I 9a I $10.00
1 - Quantity or level of harmonics or             leads on the location in the flowchart of
other Power Quality deviations tolerated          the equipment category according to IEC.
at different points in the network.               Table #1 gives the general level of tension
2 - Ways to measure or monitor Power              harmonics in the supply network at the
Quality deviations and harmonics in a             PCC as a percent of the rated voltage
network.                                          according to IEEE .
                                                  Table #2 gives the individual value for
First let us consider the differences             each current harmonic in the Network
between the two standards , IEEE                  according to the classification of the
1159-1995 and IEC 1000 (1991-1995)                network based on its short circuit ratio at
and krther NFPA 70, when dealing with             the PCC according to IEEE .
Power Quality deviations and harmonic             Graph # 1 gives the flowchart of the
levels in a network (firther more when            equipment classification according to
relating to Power Quality deviations and          IEC .
harmonics I am writing only Harmonics
for convenience ) :
Both standards divide the levels of
allowed harmonics           into    different
categories :                                          U5 k V b 161 kV

- The IEEE standard divides the levels
according to the level of the short circuit
value at the PCC of the network , so that         Table 1 : Recommended Voltage
the more robust the network is the more           Distortion Limits for General Systems
it will be able to handle higher levels of        IEEE 5 19 .
harmonics.
- The IEC standard divides the levels
according to the type of equipment                               HARMONC CUWIENT DISTORTIONLHAlTS IN% OF 4
                                                                                     v I65kV
connected to the network , so that the
network will be sufficiently robust to
handle the level of harmonic disturbances
that occur when any given equipment is                                            69kV < v i 1 6 W
connected to the network at any given                  vo*         20       I O        075           03    015   25
                                                      2050         35       175        125           OS    025   40
point in the network . In fact this                   50100        50       225        20            125   035   60
                                                      100 loo0     60                  25            10    05    75
approach gives the possibility to analyze              ~l000       75
                                                                        '
                                                                                        30           125   07    100

any branch in the network or to isolate the                                 10
                                                                                     V>16lkV
                                                                                       075           03    015   25
requested parameters from any device                                        175         125
                                                                                              - -
                                                                                                     05    025   40


connected to any point in it .                    Table 2 : Harmonic Current Limits for
In fact when evaluating a specific case ,         Individual End Users IEEE 5 19
no matter how one evaluates the level of
harmonics allowed by those standards ,
one will find them to be similar , so close
one to the other , that the differences are
negligible . Therefore , it is good practice
to use either one of these standards
equally, guided by requirements of the
case , and perhaps even check results
reached when comparing the specific
circumstances and calculations based on
 one standard to those based on the other .
 In the following two tables , it is possible
to see the levels into which IEEE divides
the network , compared to graph 1 that
                                          -1220   -
        NT
         O                                  I



                                                         :*


                                                          1


                                                                          ttME
                                                        CBEMA Curve. A set of curves
                                                       representing the withstand capabilities
                                   U
                    IDE 1000-3-2
                                                       of computers in terms of the magnitude
                                                       and duration of the voltage disturbance.
Graph 1     : Classification of the                    Developed by the Computer Business
Network Users according to IEC 1000                    Equipment Manufacturers Association
                                                       (CBEMA), the CBEMA Curve has
One of the results caused by the                       become a de facto standard for
differences between the standards and                  measuring the performance of all types
the variety of interpretations given to                of equipment and power systems, and is
them , and also the amount of                          commonly referred to by this name.
phenomena in the network that are not
cleared in a straight forward matter , is
that the requirements for Power Quality                2- MonitorinP Power Quality and
deviations and harmonics allowed or                    Harmonics
tolerated in a network are often                       One      of      the     most      common
unclear.                                               misconceptions when dealing with
More to all this is that when an                       harmonics is to ignore the differences
electrician is coming to monitor a                     between monitoring a network and
network it is difficult for the customer               measuring power quality variables .
to understand how the monitoring has                   The main difficulty in explaining the
to be executed to fit the requirement of               difference between monitoring a
the standards and one of the standards                 network and measuring power quality
requirements is to work in common                      variables is that it is crucial to be aware
with the customer to be clear and sure                 of the fact that a network changes
about the overall goal of the survey .                 continuously The network impedance
                                                                     ,
After receiving the results of the Power               varies continuously and one must
Quality deviations and harmonics                       monitor the network using models that
surveys , the difficulty is how to                     take this into consideration . Without a
interpret those results and how to solve               relevant      representation of         the
the problems .                                         impedance , the results of the
One useh1 tool is the CBEMA set of                     monitoring will be worthless .
curves that indicates in a clear way how               The specific subject of modeling was
to interpret the meaning of           the              discussed       in     IEEE       Computer
obtained result from the survey .                      Applications in Power 04-96 .
                                                       IEEE 1159-1992 provides us the steps
                                                       for monitoring a network and the
                                                       guidelines for understanding properly
                                                       the results obtained .


                                                1221
As we can see , in order to design a        during the monitoring stage and after
solution for an harmonics problem , we      adding new elements to the network .
have to execute the following steps :       In my opinion , the modeling process
1- Monitor the network according to         still remains the more problematic for
the standard .                              two reasons :
2- Measure the specific branch that is      - After buying a standard , it is just a
suspected to be the cause of the            question of a proper learning process
    problem or the specific branch that     for a professional to master the
is suffering from problems .                understanding of this standard .
3- Analyze the results of both steps        - For being able to model properly a
 1 & 2 to achieve a unique conclusion       network there , great knowledge is
with a clear recommendation for a           needed of the behaviour of the elements
solution .                                  composing the networks at different
                                                        -
                                            frequencies , different levels of use and
                                            many different variations of inter
                                            connections .
                                            The greatest difficulty when anticipating
                                            a network model is that there are neither
                                            two identical networks nor a manual
                                            describing how to execute and achieve
                                            this task .
                                            Why should one be so preoccupied with
                                            this kind of problem ?
                                            Is there not an exaggeration in the fear
Su-ggested monitoring locations on a        Erom the results of these problems ?
typical low voltage system (Based on        I have collected a partial list of possible
IEEE 1159 -1995 p 39 figure 15              problems generated by Power Quality
                                            and harmonics phenomena :
4- According to the Model chosen to         1 Over heating of transformers and
represent the network , make the              other electromagnetic devices such as
proper calculation for obtaining a            motors, relays, and coils (due to the
realistic representation of the ohmic         inductive heating effects of eddy
resistance values of the network              currents, skin effect, and hysteresis).
components or branches to avoid              2 Over heating of conductors,
unwanted resonance effects ( that can         breakers, hses, and all other devices
be disastrous ).                              that carry current (because of eddy
5- Following the implementation of the        currents, skin effect, and hysteresis).
solution, monitor again the network to       3 Inductive heating of metal parts
compare the expected result to the real       such as raceways, metal enclosures,
one and adjust the elements that              and other ferrous (iron or steel) metal
require adjustments .                         parts (because of eddy currents and
                                              hysteresis ).
                                            4 Voltage distortion resulting in
As described above and according to
many articles and books related to all        unpredictable equipment operation
the aspects of the Harmonics subject ,        because of harmonics.
                                            5 Excessive neutral current resulting in
there are two main obstacles while
dealing with Harmonics problems :             equipment overheating or failure
-Know and be able to use existing             because of additive harmonic
standards and rules .                         currents, excessive voltage drop, and
-Be able to create a relevant model           distortion.
representing the monitored network          6 Malhnction of generators and UPS
                                     - 1m     systems due to voltage distortion
 resulting in unpredictable behaviour of   electrical network and measuring the
 electric variable .                       temperature of somebody by just
                                           touching the front , After measuring the
The main point with which all the          fever there supposed to be more testing
professionals involved in Power Quality    or understanding and according to the
and harmonics in the supply network        combine results of all the tests only
deal is that the problem is here to stay : prescribe a list of treatments .
the Electric Power Research Institute      After monitoring an Electrical Network
(EPRT) estimates that in 1992 , 15 to      a professional has to be able to interpret
20% of the total load has been nonlinear,  the results and be accredited to design
while by the year 2,000 50 to 70% of all   the proper solution .
loads are expected to be nonlinear .
There is at least one concept that must
change fast , .i.e. traditional measuring
equipment is not usefbl in monitoring
Electric Network Variables in networks
that contains nonlinear loads but only
equipment that is able to measure true
RMS parameters ( it is necessary for the
user to check up to which level of
Harmonics it can measure to be sure that
it covers the Power Quality and
harmonic deviations present in the local
supply network to be monitored ) .
One of the greatest difficulty of all the
organizations involved today            in
Monitoring and Measuring networks is
that there must be a sort of
recommendation adequate to describe
the professionals who are able to
perform this kind of very focused
monitoring and also be able to analyze
the results obtained and lead to a
practical and economical solution fitted
to the network . Especially they have to
be aware of the fact that a wrong
solution could be disastrous .
Some organizations require today that
performing electricians have a certificate
that accredit them for being able to
monitor and understand network
variables , Power Quality and harmonics
according to accepted valid standards .
Some organizations require today that
performing electricians have a certificate
that accredit them for being able to
monitor and understand network
variables , Power Quality and harmonics
according to accepted and valid
standards.
It is possible to make a general
comparison between monitoring of an - 1223 -

				
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