Bai bao phan tich etap

019-026, June 2013 Available online at http://academeresearchjournals.org/journal/ijps ISSN 2331-1827 ©2013 Academe Research JournalsISSN 2331-1827 Full Length Research Paper Power flow

International Journal of Physical Sciences Vol 1(2), pp 019-026, June 2013 Available online at http://academeresearchjournals.org/journal/ijps ISSN 2331-1827 ©2013 Academe Research JournalsISSN 2331-1827

Full Length Research Paper

Power flow analysis of Rafah governorate distribution

network using ETAP software

Nadia M Mahdi

Electrical Engineering, Technical Administration, GEDCO Company, Gaza, Palestine

E-mail: engineer_nadia@yahoo.com

Accepted 29 May, 2013

The results of a load flow analysis can be used for operational purposes to evaluate various operating states of an existing system They can also be used in the planning stages to evaluate possible future extension projects This paper discusses the analysis of the power distribution network in Rafah governorate by using ETAP software The aim is to evaluate the technical status of the present medium voltage network (22-kV) The problems and challenges faced by the existing network are analyzed Those problems include the power deficit, high power losses, poor voltage levels and feeders’ overloading Then some solution techniques are suggested considering the system current state and future growth for different scenarios for each problem to obtain a full understanding of the system problems and solutions

Key words: Distribution network, power flow analysis, ETAP

INTRODUCTION

The power-flow analysis of a distribution feeder is similar

to that of an interconnected transmission system

Typically, before starting the power-flow analysis, the

three-phase voltages at the substation and the complex

power of all of the loads and the load model must be

known A load flow study determines the voltage, current,

power and reactive power in various points and branches

of the system under simulated conditions of normal

operation (William, 2002) Load flow studies are essential

in optimizing existing networks, ensuring an economical

and efficient distribution of loads, and plan future

networks (Jan de Kock and Kobus, 2004) The analysis

of a distribution feeder will typically consist of a study of

the feeder under normal steady-state operating

conditions All of the approximate methods of modeling

assume perfectly balanced three-phase systems,

balanced three-phase loads, and perfectly transposed

three-phase line segments (William, 2002)

There are wide ranges of power system analysis

programs available in the world market that perform all

sorts of electrical analyses They range from basic,

commercially packages to large, complex programs

developed for a specific customer (Jan de Kock and

Kobus, 2004) The software used in this paper is ETAP It

is for the design, simulation, and analysis of generation, transmission, and distribution power systems For a power distribution system, it is capable of calculating balanced and unbalanced load-flow This analysis produces detailed reports of system losses, line flows, and voltage at every node The software is capable of recommending optimum capacitor placement, and wire size upgrades (Barn and Jewell, 2005) ETAP Load Flow Analysis module calculates the bus voltages, branch power factors, currents, and power flows throughout the electrical system on both radial and loop systems (ETAP 7.0.0 User Manual, 2009)

As a power distribution system load grows, the system power factor usually declines Load growth and a decrease in power factor leads to a number of challenging problems such as: voltage regulation problems, increased system losses, power factor penalties and reduced system capacity (Holm et al., 2010) The distribution network in Rafah experiences many technical problems which need quick solutions The most pressing problems are high power losses and poor voltage especially at the feeder ends To face these challenges, a power flow study for the distribution network must be carried out to find the suitable technical

Figure 1 The main feeders of Rafah Governorate

solutions The main contribution of this paper is that it

evaluated the 22-kV network in Rafah and suggests the

suitable available technical solutions

METHODOLOGY

First of all, the MV grid was analyzed using ETAP

software in a way such that all problems and deficiencies

are investigated clearly and then suitable solutions are

tested by simulation

Description of the existing network

Rafah is a Palestinian city that is located in the south of

Gaza Strip on the border between Egypt and Palestine

Rafah receives power through four feeders operating at

22 kV as shown in Figure 1 (Gaza Electricity Distribution

Corporation [GEDCO], 2010) Three of the feeders are

from Egypt and they enter Rafah from the southern side

The first feeder has a maximum capacity of 5 MW The

second feeder has a maximum capacity of 12 MW, and

it’s divided into two feeders The fourth feeder enters

Rafah from its eastern side and is owned by the Israeli

Electric Company (IEC) All those feeders are governed

by Gaza Electric Distribution Corporation (GEDCO), the

unique electric distribution company in Gaza Strip MV

distribution system is stepped down to 400-V at

distribution transformers Then the power is distributed to

individual consumers via low voltage distribution

networks at 400-V This network serves 22,038

customers which represent 12% of the total customers of

GEDCO in year 2011 The MV network falls into two

categories: overhead lines and underground cables

It is fairly wide spread in Rafah that a total length of

about 68800 m feeding 133 of distribution transformers Moreover, the overhead lines cover about 93.8% of the overall length of the network This is because the overhead network is much cheaper than the underground network Most of the 22-kV lines are constructed using ACSR 150/25, ACSR 50/8 and ACSR 95/15 conductors

It is important to know that all the previous mentioned data of Rafah network and all work done in this paper represent the network status till the end of 2011

SIMULATION RESULTS AND ANALYSIS

Since all feeders in Rafah city are in radial configuration, then each feeder can be analyzed separately The four feeders are simulated in separate projects Their one-line diagrams are drawn and their parameters are entered into ETAP software Then they are exploited in different study cases to evaluate the performance of the grid The feeders are studied assuming balanced load-flow For each study case, the program produces detailed accounts of system losses, line flows, and voltage at every node or bus These detailed reports are used to investigate all problems and deficiencies of the grid Figure 2 depicts a part of the one-line diagram of the IEC

feeder

The results of load flow module for all feeders are divided into three parts: the first part of the results is concerned with power demand and capacity issues, the second is related to the system losses, while the third part is for voltage magnitude and voltage drop at each node on the feeders

It is found that there is a deficit in the available power

by about 35% Also the grid suffers from phase load imbalance in the LV networks The line current unbalance

Int J Phy Sci 021

Figure 2 One-line diagram of the 1st Egyptian Feeder

rate is 7.4% in average to all feeders Moreover, the

percentage voltage drop obtained by power flow solution

drops below 90% of the nominal voltage in the first one

third of the feeders’ length except the second Egyptian

feeder The estimated cost of the resulted average

energy losses reaches 3.3 million NIS for MWH and 5

million NIS for MVAR in year 2011 It is observable that

the percentage of reactive power loss is approximately

about 1.6 times more than the active power loss and this

is due to the nature of the lines which normally have the

value of X/R larger than unity In addition to these

problems, the existing grid has a poor lagging power

factor in all 22-kV feeders which stay in the range of 81 -

84%

SOLUTION TECHNIQUES

After the presentation of all problems and deficiencies of

network, the study addresses and suggests solution

techniques for those problems

Managing the growing power demand

In order to satisfy the ever increasing energy demand,

several actions have to be implemented These actions

have to be carried out in parallel Those actions include

the load balance and upgrading of the supplied power

according to the actual power demand Also it is important to predict the future growth of the power demand to be taken into account in the upgrade and planning of new projects So the forecasted growth in the power demand was evaluated for the coming ten years from 2012 to 2022 The predicted data were evaluated by excel “FORCAST” function which predicts the future values along a linear trend by using the existing values and they are shown in Figure 3 It is perfectly suitable since the existing data have a linear behavior

According to the power demand requirements, it is found that the preferred solution to support the existing load and the future demand growth is to install a new substation in Rafah as an extension to the Egyptian high voltage network The Palestinian Authority of Energy and Natural Resources in Gaza proposed a complete study of

a project to install two new substations in Gaza Strip, one

of them is intended to extend the Egyptian system and rated at 220/22 kV It will have two of three winding

power transformers rated at 60/75 MVA This substation

will feed the southern part of Gaza Strip with the required power demand

Load balancing

A three-phase, four-wire distribution system has been widely used to facilitate low voltage supply to single-

Figure 3 Growth of peak power demand in summer

Figure 4 Power demand in balanced and unbalanced loads

phase and three-phase loads This mixed loading in the

secondary distribution system may result in serious

phase unbalance (Nikhil et al., 2011) By calculations, it is

found that the line current unbalance rate in average is

6.37% for the 1st Egyptian feeder, 9.99% for the 2nd

Egyptian feeder, 7.21% for the 3rd Egyptian feeder, and

5.76% for the IEC feeder The line current unbalance rate

(LCUR) is calculated by equation (1) as shown thus:

Max line current deviation from average

Average line currents

(1)

It is noted that the current imbalance exceeds the

standard limit of LCUR which equals 3% at maximum

Figure 4 indicates the capacity release which can be

obtained through balancing loads on the LV network

The figure shows that the load balance can release about 7% of the active and reactive powers

Voltage improvement

Voltage improvement is considered as power quality issue and there are several techniques that can be used

to improve the voltage profile of the feeders The voltage profile is enhanced by three techniques: raising the

Int J Phy Sci 023

Figure 5 Voltage profiles of the 1st Egyptian Feeder

Figure 6 Voltage profiles of the 2nd Egyptian Feeder

sending end voltage, changing the tap settings of the

distribution transformers, and the last method is

accomplished by installation of capacitor banks

Approach 1: Raising the voltage at the feeder

sending-end

The most intuitive way in voltage improvement is to raise

the voltage at the sending end node Even though the

voltage control is done only from the substations, this

method is implemented based upon request from

technical department of Rafah branch Figures 5 to 7

show the improvement of the voltage profiles considering

both the peak and average loading cases

Since the IEC feeder suffers more voltage drop than

Egyptian feeders due to higher loading and longer length,

it is raised to 23 kV as a suitable value for the feeder to

operate within the allowable range of voltage in peak and

off-peak loading The effect of raising the sending-end

voltage is presented in Figure 8

This improvement is achieved at the expense of higher current and power demand, so this approach must be applied carefully

Approach 2: Adjustment of transformers’ tap setting

Another method that can be used to enhance the voltage levels along the feeders in the LV side is to readjust the transformers’ tap changer The transformers’ tap changer

is on high tension side and can be stepped to +1×2.5% or

to -3×2.5% Since the nominal value of the system is 22

kV, then each step can raise or lower the voltage rating

by 0.55 kV The tap changer must be adjusted to suitable

settings to suit both light and heavy loading cases The

1st Egyptian feeder is tested for this method in summer loading case and the voltage profile along the LV nodes

is shown in Figure 9 The tap changers of all transformers connected to LV nodes that experience voltage drop below 90% of the nominal value are adjusted to -3×2.5% position of their tap changers

Figure 7 Voltage profiles of the 3rd Egyptian Feeder

Figure 8 Improvement of voltage profile on IEC feeder

Figure 9 Voltage profile variations by adjusting transformer’s tap-changer settings

Int J Phy Sci 025

Figure 10 Voltage improvement by capacitors’ placement

Table 1 Capacitor banks and power carrying capability

Before adding capacitors After adding capacitors Capacity

release (MVA)

Sending

end PF%

Average Loading 20.5 kV Sending

end PF%

Average Loading 20.5 kV

Peak Load

Avg

Load

MW Mvar MW

Loss

Mvar Loss MW Mvar

MW Loss

Mvar Loss

83.39 6.43 4.25 0.188 0.387 96.17 6.58 1.87 0.153 0.324 1.044 0.86

Approach 3: Installation of capacitor banks

Placement of capacitors has been considered mainly to

enhance the line voltage levels above 90% of the nominal

voltage, power factor correction, and losses reduction

Power factor correction permits additional loads to be

served by the existing system In case if the transformers

or cables get overloaded, improving the power factor will

be the most economical way to reduce the current and

therefore eliminate overload condition This can be

clearly investigated by Equations 2 and 3 (Osama and

Ahmad, 2011):

initial

final

PF

PF

new

new

S

I

3V

(2)

initial

final

PF

PF

new

new

S

I

3V

(3)

Distribution losses in a facility can be reduced by addition

of capacitors as clearly investigated by Equation (4) (Osama and Ahmad, 2011):

2 initial final

PF

PF

(4) (4)

After installing the capacitor banks, the feeder was tested for a combination of different operating scenarios including under voltage of the sending end, overvoltage

of the sending end both at minimum and maximum loading cases The load flow simulation indicates that the capacitors operate with its full rating without leading to under voltage or overvoltage conditions and this is what

we are searching for Figure 10 shows the enhancement

of the voltage profile after installing the capacitor banks regarding different cases of the sending end voltage at the average loading condition

the fifth year, the pure profit starts assuming that the

energy price is fixed at 0.28 NIS for 1 KWH and summer

loading

Conclusion

Power flow analysis is an essential step for operational

purposes to evaluate various operating states of an

existing system Also it is necessary for enhancement

and development projects By using ETAP load flow

program, it is found that the MV network in Rafah

experiences many technical problems including: deficit in

the available power, poor power factor, low voltage levels

and power losses Based on the obtained results, some

technical solutions are suggested to help in the network

improvement The solutions are tested by simulation The

proposed solutions were suggested considering the

financial investment cost and profits such that the

solutions are acceptable from the economic view ETAP

shows powerful functionalities in load flow analysis field

Thus it is strongly recommended to be available for

usage in the technical administration of GEDCO

– A Case Study Online Journal on Power and Energy Engineering (OJPEE), 2: 1

Holm RMU, Chopade PV, Prornod J (2010) Optimal Placement of Capacitor for Power Loss Reduction Using ETAP Software ICSES' 10 International Conference on Science Engineering & Spirituality Studies and Documentation Branch in Technical Administration of Gaza Electricity Distribution Corporation GEDCO (2010)

William HK (2002) Distribution System Modeling and Analysis CRC Press LLC, 39: 269