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Custom power devices for voltage sags mitigation: a techno-economic analysis


									                                             Mahmoud EL-GAMMAL1, Amr ABOU-GHAZALA1, Tarek EL-SHENNAWY3
                                  University of Alexandria, Egypt (1), Alexandria National Refining & Petrochemicals Co. (ANRPC), Egypt (2)

                               Custom power devices for voltage sags mitigation:
                                                   a techno-economic analysis
Abstract. The increased concern about the financial losses due to voltage sags and interruptions plus the advancements in power electronics
technology have led to the innovation of very fast acting intervening equipment that can mitigate such disturbances. However, the high cost of these
custom power devices can offset their technical benefits. In this paper, a procedure is proposed to find a compromised solution that makes technical
and financial sense. A case study of an oil refinery in Alexandria, Egypt is considered to illustrate the proposed procedure.

Abstract. Wzrastające znaczenie ekonomiczne strat powodowanych przez zapady napięcia i przerwy prowadzą do opracowania urządzeń
pozwalających na szybką interwencję. Niestety takie systemy są dość kosztowne. W artykule zaproponowano rozwiązanie kompromisowe aspektu
technicznego i ekonomicznego. Analizę wykonano na przykładzie rafinerii w Alaksandrii w Egipcie. (Specjalistyczne układy zasilania ze
złagodzeniem zaników napięcia - analiza techno-ekonomiczna)

Keywords: Voltage Sags, Short Interruptions, Power Quality, Custom Power Devices, Techno-economic Analysis.
Słowa kluczowe: jakość energii, zapady napięcia.

Introduction                                                                Custom Power Devices
    Voltage sags and short interruptions are brief voltage                      The concept of custom power is the employment of
reduction events, followed by restoration of the normal                     power electronic or static controllers in medium voltage
supply conditions. Voltage sags and short interruptions are                 distribution systems for the purpose of supplying a level of
frequent causes of tripping of electrical equipment in                      power quality that is needed by customers sensitive to
industrial installations, leading to costly shutdowns [1].                  power quality disturbances. Custom power controllers may
    Custom power devices are power electronic based                         include static switches, inverters, converters, injection
devices used to protect the entire facility from such voltage               transformers, master control modules, and energy storage
disturbances. Custom power devices have to work within                      modules [5].
parts of a cycle, such that the load bus will not be affected                   Existing Custom Power Devices include the solid state
by the supply disturbance [2].                                              or the Static Transfer Switch (STS), working by transferring
    This paper proposes a methodology for the decision                      the load bus to another healthy feeder in sub-cycles, the
makers, to compare the technical merits and limitations of                  Dynamic Voltage Restorer (DVR), working by boosting the
these devices, along with an economical evaluation of their                 voltage at the load bus during voltage sags, utilizing an
costs against the financial losses of sags and interruptions.               energy storage unit and a voltage source converter, and the
                                                                            Backup Storage Energy Systems (BSES), working by
Voltage Sags and Short Interruptions                                        isolating the supply once a disturbance is detected and
    The IEEE defines voltage sag as: A decrease to                          feeding the load from an energy storage unit (batteries or
between 0.1 and 0.9 pu in rms voltage or current at the                     flywheel) and an inverter [6].
power frequency for durations of 0.5 cycle to 1 min. The
amplitude of voltage sag is the value of the remaining                      A. Dynamic Voltage Restorer (DVR)
voltage during the sag. A momentary interruption is a                           The DVR, sometimes called the Static Series
complete loss of voltage on one or more phases for a time                   Compensator (SSC), provides a controllable voltage, whose
period between 0.5 cycles and 3 s [3].                                      phasor adds to the source voltage to obtain the desired load
    The IEC terminology for voltage sag is dip. A dip is a                  voltage. In its simplest configuration, shown in Fig. 2, the
sudden reduction of the voltage at a point in the electrical                DVR consists of the following components [7]:
system, followed by voltage recovery after a short period of
time, from half a cycle to a few seconds. The amplitude of a
voltage dip is defined as the difference between the voltage
during the voltage dip and the nominal voltage of the
system expressed as a percentage of the nominal voltage.
A short supply interruption is a voltage dip with 100%
amplitude [4]. Fig. 1 shows a representation of voltage sag.

                                                                            Fig. 2. Dynamic Voltage Restorer (DVR)

                                                                            1.   Energy storage unit, which is used to provide the
                                                                                 missing energy during the sag condition. DVRs can be
                                                                                 configured to use line energy supply; that is, they
                                                                                 absorb the energy that is to be injected into the
Fig. 1. Voltage sag
                                                                                 distribution circuit from the utility feeder itself.

324                                       PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 8/2010
2.   Voltage Source Inverter (VSI), or Voltage Source           must be available. Therefore, this solution is particularly
     Converter (VSC), which converts the dc voltage from        attractive for installations that already have mechanical
     the energy storage unit to a controllable ac voltage to    transfer systems, where upgrading to a static system does
     be inserted with the line voltage.                         not require major changes in the layout of the distribution
3. Injecting transformer, connected in series between           system. Note that the STS cannot protect against sags
     the source and the load, and which injects the DVR         originating in the transmission system, which will also affect
     voltage at the load bus.                                   the alternative supply [9].
4. Filter circuit, normally, a second-order LC filter is            The main problem with the STS comes from plants with
     inserted between the inverter and the transformer to       a high percentage of motor loads, a “voltage collapse”
     cancel high frequency harmonic components in the           problem may occur due to motors drawing much current to
     inverter output voltage.                                   re-accelerate, and may be tripped by protection devices due
5. Bypass switches and Control circuits, through which          to the high current drawn [10].
     the DVR may be configured to operate as a standby
     compensator where the inverter is not actively in the      C. Backup Storage Energy System (BSES)
     circuit until triggered by a voltage sag event.                A backup stored energy system (BSES) device
     Alternatively, the DVR may be working continuously         disconnects a protected load from the utility supply within
     during normal and abnormal conditions.                     milliseconds of the detection of a disturbance and supplies
    The major disadvantage of the DVR is that it does not       the entire load using stored energy. Typical sources for the
protect a load against an interruption [8].                     stored energy are batteries, flywheels, or superconducting
                                                                magnetic coils. A BSES unit typically consists of a static
B. Static Transfer Switch (STS)                                 source transfer switch (isolation switch), energy storage
    The Static Transfer Switch (STS) allows fast transfer of    system, voltage source converter, and isolation transformer,
sensitive loads from a primary source affected by a             as in Fig. 5 [11].
disturbance (sag or interruption) to an available alternative
healthy feeder. The STS system consists of three main
components, as shown in Fig. 4 [9]:

                                                                Fig. 4. Backup Energy Storage System (BESS)

                                                                    A BSES can be seen as an alternative to an
                                                                uninterruptible power supply (UPS) when the load power
Fig. 3. Static Transfer Switch (STS)
                                                                increases. During normal operation, power coming from the
                                                                ac supply is rectified and then inverted, and the batteries
1. The static transfer switch STS, consists of two three-
                                                                only serve to keep the dc bus voltage constant to avoid high
phase ac thyristor switches connected back to back (anti-
                                                                steady-state losses. During a voltage sag or interruption,
parallel), directing power from two independent feeders to
                                                                the battery block releases energy to supply the load. One of
the load.
                                                                the main advantages to the BSES compared to the DVR is
2. The mechanical bypass switch MTS, operates as a
                                                                that it is able to carry a load through a voltage interruption.
standard mechanical transfer switch when the static transfer
                                                                Because of this, however, its storage requirements are
switch is out of service.
                                                                higher. On the other hand, since a BSES unit disconnects
3. Isolating switches and Control, during normal
                                                                the protected load from the utility system, its converter must
condition, the switch connected to the primary feeder is kept
                                                                be rated to carry the entire rating of the protected load.
closed and the switch on the secondary feeder is kept
                                                                    For storing the necessary energy, batteries or flywheels
opened. On the detection of a sag or an interruption on the
                                                                (rotary UPS) or Super Magnetic Energy Systems (SMES)
primary feeder, the switches on the secondary feeder turn
                                                                can be used. The main advantages of SMES as compared
on immediately and that on the primary feeder turn off at the
                                                                to the batteries are the reduced size and lower maintenance
first natural current zero, hence transferring loads to the
                                                                requirements, but a cryostat and refrigeration system is also
healthy feeder, thus providing a seamless transfer of
                                                                needed negating some of the size advantage [12].
electrical energy between the two feeders.
     A requirement is that a secondary feeder, independent
from the main source (e.g. a feeder to another substation),

PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 8/2010                                            325
Economic Evaluation                                                   Where net investment is the initial cost (mitigation
A. Losses due to sags and interruptions                           equipment cost + installation cost) and net annual return is
    A common misconception is that power quality issues           the annual expenses (operation + maintenance) subtracted
are technical problems, but in fact, power quality is a           from the annual benefits.
techno-economic problem. Reference [13] estimated the                 Although widely used, payback time suffers from several
average costs of disturbances as shown in Table 1:                drawbacks. First, PBT does not consider the time-value of
                                                                  money. The second flaw is that payback does not consider
Table 1. Losses due to sags and interruptions                     the effects of different life-spans of the alternatives, thus
                                  Cost of voltage sags and        penalizing projects that have long potential life-spans. The
                                interruption / event (in US $)    third drawback is that the accept/reject criterion is often
       Semiconductor                     2,500,000                short. For example, many organizations require a 1 to 3
        Credit Ccard                      250,000                 year payback period to consider a cost-saving project and
      Equipment Manuf.                    100,000                 place a higher priority on projects with a shorter payback
         Automobile                        75,000                 time.
          Chemical                         50,000
           Paper                           30,000                 ii) Net present value (NPV)
                                                                       The net present value (NPV) of a project indicates the
    It should be noted that not all voltage sags lead to          expected impact of the project on the value of the company.
process shutdown. Costs will typically vary with the severity     Projects with a positive NPV are expected to increase the
of the sag. This relationship can often be defined by a           value of the company. Thus, the NPV decision rule
matrix of weighting factors. The weighting factors are            specifies that all independent projects with a positive NPV
developed using the cost of a momentary interruption as the       should be accepted. If NPV is greater than zero the project
base. Usually, a momentary interruption will cause a              is valid, since the revenues are enough to pay the interest
disruption to any load or process. If a voltage sag to 50%        and recover the initial capital cost before the end of the life
causes 80% of the economic impact that an interruption            of investment. When NPV equals zero, the balance occurs
causes, then the weighting factor for a 50% sag would be          at the end of the life, and the investment is scarcely
0.8 [14].                                                         attractive. When selecting from mutually exclusive projects,
                                                                  the project with the largest positive NPV should be chosen.
B. Custom Power Devices Costs                                          The NPV can be calculated from eq. (2):
    Each solution technology needs to be characterized in
terms of cost and effectiveness. In broad terms, the solution                        n
                                                                                          (solution net saving) t
cost should include [15]:                                         (2)       NPV                                  C0
1) Fixed Costs: The fixed costs mainly consist of the                                t 0         (1  r) t
mitigation device cost and the cost to install it including
labor hours, footprint of the device, time and so forth.              Where r is the discount rate, C0 is the initial investment,
2) Operating Costs: The operating or variable costs are           t is the number of years, and n is the lifetime of the
those which allow the mitigation device to work. These            investment.
operating costs consist of heating losses, maintenance and
additional costs such as replacement of batteries at the end      Proposed Methodology
of their life, air conditioning to cool the battery room.         1. A good estimate of the number of short interruptions and
    The investment costs for the used mitigation methods          voltage sags with different severities is the first step in any
are given in Table 2 [14].                                        financial procedure. Historical survey data, similar available
                                                                  information or case studies (see Table 1), data from the
Table 2. Example costs for PQ mitigation techniques               utility electrical supplier, will be a good start. In case of lack
                                     Operation & maintenance      of data, probabilistic methods may be used.
      Alternative      Cost ($)
                                         annual costs ( % )       2. Convert the different sags to a per unit interruption base
   BSES (Battery        500 $ /                                   value, cumulate the summated events in one variable;
                                               15 %
    ride through)        kVA
                                                                  equivalent number of shutdowns / year.
      DVR (50%          300 $ /
                                                5%                3. Calculate the average total cost of one shutdown.
   voltage boost)        kVA
   STS (10 MVA)                                                   4. Multiply the result of step (2) by that of step (3) to
                      600,000 $                 5%                calculate the annual cost of shutdown.
  ¼ cycle transfer
                                                                  5. Investigate the available custom power devices. Get the
C. Financial Analysis                                             cost of installation, operation and maintenance costs, and
    Several evaluation methods can be used, according to          any available technical and economical data. Tables II may
the company’s internal evaluation criteria for investment.        be helpful.
The most familiar methods are [16]:                               6. Instead of investing a small fortune in purchasing the
i) Payback time (PBT)                                             required solution, a bank loan could be more attractive. If
    The payback time represents the amount of time that it        the required figure is C, then the annual share for the
takes for a project to recover its initial cost. The use of the   lifespan of the equipment (n years) with a discount rate of r
PBT as a capital budgeting decision rule specifies that all       can be calculated from eq. (3):
independent projects with a PBT less than a specified
number of years should be accepted. When selecting from                                      C 1  rn 
mutually exclusive projects, the project with the shortest        (3)       Yearly share 
payback is to be preferred.
                                                                  The annualized costs can be calculated based on a 15-year
    The PBT can be calculated from eq. (1):
                                                                  life and an interest rate of 10%.
                                                                  7. Determine the total annual cost for each alternative,
                      Net Investment                              including both the operations and maintenance costs and
(1)       PTB 
                    Net Annual Re turn

326                                    PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 8/2010
costs associated with the residual shutdowns (remember                 Note that the DVR cannot mitigate for interruptions. The
that the solutions do not completely eliminate these costs).       STS will transfer the load to another feeder, and the high
8. For solution alternatives, make a comparison with the           percentage of its capability depends on the situation of the
“No-Mitigation” or “Do-Nothing” case. This is the figure           other feeder at the sag instance. Interruptions and severe
calculated in step (4).                                            sags are likely to occur due to faults near the substation.
9. The optimum solution will be the device which gives a           Costs of the alternatives are calculated from Table II.
total annual cost lower than the annual costs associated           6. To calculate the annual cost of solution, apply eq. (3) for
with the shutdown. Note that if none of the alternatives is        n=15 and r = 10% to get the first row of table V.
lower than the threshold, the “No-Mitigation” will be the          7. For each alternative, add to Table V a second row for
most economical action.                                            the operation and maintenance costs, and a third row for
10. Discuss and comment on the results.                            the cost of unmitigated events.
Case Study                                                         Table 5. Total cost for different alternatives
    Alexandria National Refining and Petrochemicals Co.                                                         STS
(ANRPC) is a refinery based in Alexandria, Egypt, with an
average load of 10 MW, of which 80% are directly                                                                  DVR                                  (another   BSES
connected induction motors at various voltage levels (11,                                                                                               feeder
6.3, 0.4 kV). As the motor torque is directly proportional to                                                                                          needed)
the square of the supply voltage, a decrease (sag) to 70%           Annual solution
of the rated voltage will cause the motor torque to decrease                                                  125,000           100,000                166,666    250,000
                                                                     cost In US $
to 49%, which may not be sufficient for driving the load. In
addition, applying out of phase voltages whenever the               Annual op. and
                                                                                                                  37,500                           30,000         225,000
                                                                     maint. costs
voltage is restored may result in transient currents and
                                                                    Annual costs of
torques of excessive values. To avoid the risk of damage of          unmitigated                                  75,000                           62,500           0
the motors’ shafts, strict protection settings are applied,             events
causing induction motors to trip, leading to numerous                 Total costs                             237,500           192,500                259,166    475,000
                                                                   8. Comparing the results of Table 5 with that of the annual
                                                                   costs of shutdown, it can be better done using the
1. A 24 months survey is conducted to investigate the
                                                                   comparison chart of Fig. 5
number (frequency) of interruptions and sags. The results of
the survey are summarized in Table 3:                                                                      annual O&M costs
                                                                                                           annual solution costs
Table 3. Site Survey Results
             Event             No. of events (2 years)                                                     annual event costs
          Interruption                   2                                                   500
                                                                      Cost (Thousand US $)

  Sag magnitude < 75% pu &                                                                   450
     Sag duration > 1.5 s                                                                    400
       Other minor sags                  25                                                  350
              Total                      31                                                  300
2. To calculate the annual no. of equivalent events, the                                     150
settings of the undervoltage protection relay were adjusted                                  100
to trip for events below 75% of the nominal voltage for                                       50
durations of 1.5 seconds. Sags with magnitude and duration
                                                                                                                           STS (1 feeder)




more than these settings cause the protection relay to trip
leading to the process shutdown. These sags will be
weighted the same as the interruptions. Sags resulting in                                          Mitigation Method
minor or partial effects are weighted 20% of the base event.
            Equivalent number of shutdowns / year =
                 (1 * 2 + 1 * 4 + 0.2 * 25)/2 = 5.5                Fig. 5. Comparison chart
3. The cost of one shutdown is estimated by the company
financials to be 50,000 US $ per event.                            9. From the comparison results, we can conclude that the
4. The annual cost of shutdowns = 5.5 * 50,000 = 275,000           best solution for this particular case study is the
US $. This is the value to be compared with the annual             implementation of the STS, provided that another feeder
costs of solution alternatives.                                    already exists. If there is only one feeder available, then the
5. In this study, the following custom power devices will be       best solution in this case is the DVR.
investigated: the DVR with 50% boosting capability, the            10. Discussions and comments are presented in the next
STS with 10 MVA rating, and the BSES with batteries. Their         section.
techno-economicl data are summarized in Table 4.
                                                                   Discussions and comments
Table 4. Custom Power Devices Data
        Feature        DVR             STS               BSES
                                                                   - Each custom power device has its merits and limitations.
      Interruption                                                 For example, the STS requires the presence of another
                         0             90%                100%     feeder, and cannot protect against transmission system
 Sag voltage > 75%     100%            70%                100%     sags of common transmission circuit. The DVR can protect
 Sag voltage < 75%     75%             80%                100%     for a 50% sag for a couple of seconds but cannot protect
    Requirements         -       Another feeder             -      against complete interruptions. The BSES requires a huge
                                     600,000                       energy storage unit (either batteries or flywheel) which
  Initial cost (US $) 750,000     + 400,000 for      1,500,000     requires large maintenance and operating costs.
                                additional feeder
    Annual costs      37,500         30,000              225,000

PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 8/2010                                                                                      327
- The STS and the DVR show justified economical value,                                      REFERENCES
however the plant in this case will suffer from a fewer           [1]    Bollen, M., Understanding Power Quality Problems: Voltage
number of voltage sags and interruptions.                                Sags and Interruptions, Wiley-IEEE Press, New York, 1999.
- In the case of requiring another feeder for applying the        [2]    Ghosh, A. and Ledwich, G., Power Quality Enhancement
                                                                         Using Custom Power Devices, Springer, 2002.
STS, the estimated figure is based on personal experience
                                                                  [3]    IEEE Std 1159 – 2009, IEEE Recommended Practice for
and may vary according to the costs of cables, digging and               Monitoring Electric Power Quality.
laying, road rites, switchgear modification, and re-              [4]    IEC 61000-2-1, Electromagnetic Compatibility (EMC) – Part 2:
engineering the system.                                                  Environment – Section 1: Description of the Environment –
- The estimated financial losses due to the process                      Electromagnetic Environment for Low-Frequency Conducted
shutdown are based on the expected number of events                      Disturbances and Signalling in Public Power Supply Systems,
through the next 10-15 years, and which is assumed to be                 1990.
constant. The costs of the solution alternatives are also         [5]    Hingorani, N., Introducing Custom Power, IEEE Spectrum,
                                                                         June 1995, pp. 41-48.
changing. Advancements in solid state technology are
                                                                  [6]    Chang, C., Ho, Y. and Loh, P., Voltage Quality Enhancement
making these devices more feasible. probabilistic methods                with Power Electronics Based Devices, in Proc. IEEE Power
might be necessary in similar studies, at least as a means               Engineering Society Winter Meeting, 23-27 Jan. 2000, pp.
to validate the results. This topic would be addressed in a              2937-2942.
future work.                                                      [7]    Benachaiba, C. and Ferdi, B., Voltage quality improvement
- Managers and decision makers often want some indices,                  using dynamic voltage restorer, Electrical Power Quality and
payback rates, added values of the investment, etc. In this              Utilization Journal, Vol. 14, No. 1, 2008, pp. 39-46.
sight some simple calculations using eq. (1) & (2) may be         [8]    Middlekauff, S. and Collins, E., System and Customer Impact:
                                                                         Considerations for Series Custom Power Devices, IEEE
helpful yielding the results of Table 6:
                                                                         Trans. Power Delivery, Vol. 13, No. 1, January 1998, pp. 278-
Table 6. Economical measures for solution alternatives            [9]    Jipping, J. and Carter, W., Application and Experience with a
                                         STS                             15 kV Static Transfer Switch, IEEE Trans. Power Delivery,
                                      requiring                          Vol. 14, No. 4, Oct. 1999, pp. 1477-1481.
              DVR         STS                           BSES
                                       another                    [10]   Sannino, A., STS and Induction Motors, IEEE Industry
                                        feeder                           Applications Magazine, July 2003, pp. 50-57.
   PBT                                                            [11]   Sannino, A., Svensson, J. and Larsson, T., Power Electronic
              4.6         3.3             5.5            30
 (years)                                                                 Solutions to Power Quality Problems, Electric Power Systems
   NPV                                                                   Research, Vol. 66, 2003, pp. 71-82.
            485,975     788,095        388,095       -1,119,700
  (US $)                                                          [12]   Sabin, D. and Sannino, A., A Summary of the Draft IEEE
                                                                         P1409 Custom Power Application Guide, in Proc. IEEE Power
It can be easily shown that these results enhance the                    Engineering Society Transmission and Distribution Conf. and
results of the proposed procedure. The best solution for this            Exposition, 7-12 Sep. 2003, Dallas, USA, pp. 1-6.
case is the STS (if two feeders are available) since it results   [13]   Chowdhury, B., Power Quality, IEEE Potentials, Apr. 2001, pp.
in the minimum PBT and the maximum positive NPV. If only
                                                                  [14]   McGranaghan, M. and Roettger, B., Economic Evaluation of
one feeder is available, the DVR will be the best solution.              Power Quality, IEEE Power Engineering Review, Feb. 2002,
The BSES needs 30 years to break positive (remember that                 pp. 8-12.
the lifetime of the equipment is estimated by 15 years only)      [15]   Hertem, D., Didden, M., Driesen, J. and Belmans, R. ,
and its NPV is negative.                                                 Choosing the Correct Mitigation Method Against Voltage Dips
                                                                         and Interruptions: A Customer-Based Approach, IEEE Trans.
Conclusions                                                              Power Delivery, Vol. 22, No. 1, January 2007, pp. 331-391.
    The increasing interest in the Power Quality studies in       [16]   Baggini, A. and Bua, F., Investment Analysis for Power Quality
                                                                         Solutions, Leonardo Power Quality Initiative (LPQI): Power
recent years is attributed not only to technical issues, but
                                                                         Quality Application Guide, Chapter 2.5, June 2004.
also to the huge financial losses associated with poor
quality of the delivered power. Industrial customers seeking
for a cost-effective mitigation solution are faced by several     Authors
                                                                  Prof. Dr. Mahmoud El-Gammal,
custom power devices, all requiring large investments, and        Electrical Engineering Dept., Faculty of Engineering, Alexandria
probably will not stop all the process shutdowns due to           University, Egypt, Alexandria 21544 – Egypt,
these events.                                           
    In this study, a methodology was proposed to help the         A. Prof. Dr. Amr Abou-Ghazala
decision maker to compare the solution alternatives from a        Electrical Engineering Dept., Faculty of Engineering, Alexandria
techno-economic perspective. The annual cost of voltage           University, Egypt, Alexandria 21544 – Egypt,
sags and interruptions is calculated first, then compared
                                                                  Eng. Tarek El-Shennawy (corresponding author)
with the annualized solution alternatives costs to select the
                                                                  Alexandria National Refining and Petrochemicals Co. (ANRPC),
best solution for each case study.                                Wady El-Kamar, El-Max, Alexandria 23111 – Egypt,
   The authors would like to thank Prof. Dr. Abdel-Mon’em
Moussa, Prof. Emeritus at the Faculty of Engineering,
Alexandria University, and Vice President of Pharos
University in Alexandria, for conducting and supervising the
preliminary stages of this study.

328                                   PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 8/2010

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