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Nowadays, renewable energy resources play an important role in replacing conventional energy resources such as fossil fuel by integrating solar, wind and geothermal energy. Photovoltaic energy is one of the very promising renewable energy resources which grew rapidly in the past few years, it can be used to produce electric energy through photovoltaic process. The primary objective of the research proposed in this paper is to facilitate the increasing penetration levels of PV systems in the electric distribution networks. In this work, the PV module electrical model is presented based on the mathematical equations and was implemented on MATLAB to simulate the non-linear characteristics I-V and P-V curves with variable input parameters which are irradiance and temperature based on Riyadh region. In addition, the reliability evaluation of distribution networks, including distributed generators of solar photovoltaic (PV) with varying output power capacity is presented also. The Monte Carlo simulation algorithm is applied to test the distribution network which is RBTS Bus 2 and the same has been conducted on the original case of distribution network substation 7029 which is located at KSA Riyadh. The two distribution networks have been modified to include the PV’s distributed generators. The distributed generators contribute to supply a part of the load during normal mode and supply the entire load during component failure or failure of grid operation supply. The PV stochastic models have been used to simulate the randomness of these resources. Moreover, the study shows that the implementation and integration of renewable resources as distributed generations have improved the reliability of the distribution networks.

The electric power system has been used to generate, transmit and distribute power since the advent of electricity evolution and the foremost important objective of an electrical utility is to deliver economical, reliable, and quality power to its consumer. The electrical power network is extremely complex, a failure may result in loss of power to a large number of customers or sometimes catastrophic events such as blackouts and it is difficult to analyze the entire system at once. For this reason, the power system is divided into three functional zones such as generation facility, transmission facility and distribution facility to evaluate the reliability of the system. These functional zones in series can be considered as the hierarchical levels of the power system reliability studies as shown in

The conventional structure of electrical power systems has been developed mainly to become as following arrangement shown below in

The incorporation and integration of non-conventional or renewable energy sources in the network result in a new term called Distributed Generation (DG). DG can be defined as small-scale power generation units of electricity (a few kilowatts kW) connected directly to the grid, distribution network, on consumer side of the meter to serve a customer on site and at the same time to provide the support to distribution networks or work standalone with different rating levels.

The integration of DGs becomes the most economical solution to meet the increased demand due to load growth in the conventional system. While providing environmentally friendly energy and while helping to meet the increasing load economically. It can play an important role in electrical power network

Item | Class | Size |
---|---|---|

1 | Micro distributed generation | 1 W ≤ 5 kW |

2 | Small distributed generation | 5 kW < 5 MW |

3 | Medium distributed generation | 5 MW < 50 MW |

4 | Large distributed generation | 50 W ≤ 300 MW |

and by employing this technology in power systems has big advantages as following:

· Improving the reliability of the electrical network by providing an alternative source during power disturbance events.

· Reducing the electrical transmission power losses caused by the power traveling through long transmission lines and high voltage transformers.

· Providing better voltage support, used to supporting peak load and supply electricity during peak periods.

· Improving the power quality and enabling consumers to select the source of energy based on the cost and awareness of the environmental issues.

· Relieving the congested of electrical transmission networks and reducing the need to expand the electrical transmission networks.

· DGs considered as a viable alternative of energy storage alternative when interruptions are frequent. It works as Backup supply to ensure the uninterrupted electricity.

Currently, industrial countries generate most of their electricity in large centralized facilities, such as fossil fuel powered by coal or gas, nuclear, large solar power plants or hydropower plants. However, modern embedded systems can provide these traits with automated operation and renewables, such as sunlight, wind, and geothermal. Moreover, the last few years, a number of factors have led to an increased interest in distributed generation schemes such as availability of modular generating plant, ease of finding sites for smaller generators, short construction time and lower capital costs for smaller plant, reduction in gaseous emissions (mainly CO_{2}). Due to the DG sources have a potential solution for some issues, like the deregulation in power system and improving the performance of distribution system as well as reliability evaluation, that is making it more popular. The presence of the DGs, especially when the DG share is significantly high, will obviously impact on way of power system operation. The distribution networks can be designed for (radial) unidirectional power flow, and the electric power systems with DGs spread across the distribution network is shown in

In recent years there has been an increase in the global trend in general and particular in Kingdom of Saudi Arabia (KSA) towards finding and exploiting sources of renewable energy to generate electricity. This may be due to the urgent need for new and renewable sources of energy that are not fading and depleting. One of the reasons for this is that these sources limit the emission of toxic gases as well as saving energy costs and conserving their stocks from fossil sources (oil and natural gas) and not relying on it total. Due to the high prices of fossil fuels with the rapid rises in oil prices, capital costs of conventional power plants, escalation in power demand due to rapid growth in population and industrialization in the last several years especially in Saudi Arabia.

The conventional generation, low quality fuels and the methods of generation typical in KSA is a chief cause of environmental pollution and impacts human health through emissions of harmful gases that are remains a threat to public health around the world. Therefore, it is essential to find green power source that reduce the gasses emission and preserves oil in Saudi Arabia. So, the renewable energy

resources have been globally accepted for power generation, utilizing and integrated them with conventional generation around the globe.

The solar radiation is variable in different parts of the world and the highest value of solar radiation located at Sunbelt. Saudi Arabia is most suitable country over the world for using solar power and its solar irradiances are among the highest in the world because it located in the “Global Sunbelt”, a geographic region situated between 35˚N and 35˚S. The generally characterized by high solar irradiation which can be witnessed in

KSA has widespread desert land, year-round clear skies and abundant solar resource which it to become one of the largest solar photovoltaic (PV) energy producers in future and a world leader in renewable electricity generation. Studies show that the use of solar equipment in KSA is very suitable and can easily provide part of the energy that nation needs.

The Several sources, including Snapshot of Global PV Markets 2016 [^{th} largest consumer of total primary energy in year 2013, of which about 60% is petroleum based and the rest was accounted to natural gas. Energy consumption per capita is twice that of Europe and three times the world average [

The solar resource and weather conditions of a place are often intermittent and prone to vary from one year to another year, so the datasets need to be compiled over a number of years to create historical site data. The statistical model of solar radiation is based on data for the solar irradiance at KSA-Riyadh, during period Jan 2013-Jan 2016 which is available in King Abdullah City for Atomic and Renewable Energy’s (K.A.CARE’s) Renewable Resource Monitoring and Mapping (RRMM).

The solar radiation in Riyadh is characterized by long radiation period with high irradiance during summer and short radiation period with low irradiance during winter due to the differences in the sun’s height at the summer and winter solstice. The solar irradiance is fluctuated from month to month based on the sun orbit. The highest value of monthly solar irradiance was for May, Jun, and July with maximum value 8134 (Wh/m^{2}) for Jun.

It is well known that due to the fact that solar insolation is intermittent and the output power of PV systems is not deterministic. For this reason need for a stochastic model to simulate PV outputs. A stochastic model is a simulation-based technique to describe a non-deterministic behavior and the randomness of the system. The statistical data of solar insolation and temperature have been collected from (K.A.CARE’s) and implemented in our study.

a) Photovoltaic Power System Generation

The Solar photovoltaic (PV) is widely used as renewable energy source. The solar of PV has a high reliability in modules (>20 years), and high public acceptance. PV is the technology that generates direct current (DC) electrical power measured in Watts (W) or kilo Watts (kW) from semiconductors when they are illuminated by photons. As long as light is shining on the solar cell, it produces both a current and a voltage to generate electric power which is known as Photoelectric Effect [

grid, the output DC voltage from PV system should be first regulated by a DC-DC converter. Then DC-AC inverter converts the DC power produced by the PV modules into AC power. A filter is used to eliminate harmonics. The power electronic components have the tasks to guarantee safe and efficient operation, to track the maximum power point (MPP) of the PV system, and to maintain power quality of the PV system output. The transformer is an essential when the generated electricity is to be injected into the utility grid.

b) The PV Module

The ideal model of a solar cell consists of a photocurrent source and diode with an internal resistance connected to a parallel resistor. The current representing the photo generated current (I_{gc}) in parallel with a diode which is directly proportional to the light falling (solar irradiation) on the cell. A PV cell is physically modelled using the equivalent circuit model as represented in

Many researchers have been examined the general mathematical model description of a solar cell current-voltage output characteristic over the past three decades [

I P V = I s c − I o = I s c − I D [ e ( q V D n K T c e l l ) − 1 ] − V + I R S R P (1)

where: I_{PV} is the PV output current. I_{sc} is the cell’s short circuit current at 25˚C and 1 KW/m^{2}. q is the electron charge equal to 1.602 × 10^{−19} C. k is the Boltzmann constant (1.381 × 10^{−23} J/K). V_{D} is the voltage across the diode terminals. V is the output voltage. R_{S} and R_{P} are the series and parallel resistances. n is the ideality factor, also known as the quality factor or sometimes the emission coefficient and usually takes values in the range 1 to 2, which depends on the construction and semiconductor material listed in

The cell’s saturation current (I_{o}) varies with cell’s temperature and depends on the cell temperature. As described by following Equations (2)-(5).

I g c = [ μ s c ( T c e l l − T r ) + I S C ] G (2)

Technology Type | Ideal Factor (n) |
---|---|

Si-Mono | 1.2 |

Si-Poly | 1.3 |

CdTe | 1.5 |

CIS | 1.5 |

I D = I o ( e q V D n K T c e l l − 1 ) (3)

I o = I o α ( T c e l l T r ) 3 e [ e v d K F ( 1 T r − 1 T c ) ] (4)

I o α = I S C e ( e v o α K F T c ) (5)

where: I_{D} is the diode current. T_{cell} is the cell’s absolute temperature. T_{r} is the cell’s reference temperature. I_{oα} is the cell’s reverse saturation current at solar radiation and reference temperature. G is the solar radiation in kW/m^{2}.

A solar cell alone can produce only output power from 1 to 2 Watt [_{S} series and N_{P} parallel connected modules. And in order to calculate the generated photovoltaic current and the output power of a PV module consisted of (N_{s} × N_{p}) cells as following Equation (6).

I P V = N p I g c − N p I o [ e e ( v d R s + s I R s R p ) N s K F T c − 1 ] − N p V d N s R p (6)

c) Non-Linear Characteristics of PV’s

The Photovoltaics have nonlinear characteristics, where the performance and output power are directly affected with the change of the operating conditions solar irradiance, temperature and the angle of the sun. Usually the PV manufacturer supply their products with a data sheet that contains values of current and voltage for three conditions namely, the short circuit, the open circuit, and the maximum power for a given set of reference condition. The major inputs for the proposed PV model were solar irradiation, PV panel temperature and PV manufactures data sheet information. In this study, the Astronergy CHSM6610P Polycrystalline PV Module is taken as example to simulate it. The data sheet of the simulated module is shown in

An insolation-oriented model of PV module is built by using MATLAB to illustrate and verify the nonlinear I-V and P-V output characteristics of PV module. Both I-V and P-V output characteristics of PV module at various insolation and temperatures based on Riyadh region data are carried out.

It is clear from the previous figures that the output power of PV’s is directly proportional with the amount of solar irradiance falling on PV’s panel, and inversely proportional with its temperature. We can see the effect of change in solar irradiation on PV characteristics from

Electrical Specifications | |
---|---|

Maximum output power (P_{m}) | 275 Wp |

Voltage at (P_{m}) (V_{amp}) | 31.12 V |

Current at (P_{m}) (I_{amp}) | 8.85 A |

Short circuit current (I_{sc}) | 9.52 A |

Open circuit voltage (V_{OC}) | 38.45 V |

Temperature coefficient (I_{sc}) | +0.049%/C |

Temperature coefficient for (V_{OC}) | −0.310%/C |

Temperature coefficient (P_{m}) | −0.407%/C |

Number of cells/cell arrangement | 60/6 × 10 |

1.8 A to 8.3 A approximately. The effect of variation of solar irradiation on P-V characteristics result that as solar irradiation increases, power generated increases also. Increase in power is mainly due to increment in current. Decreasing the irradiance will reduce the overall performance of the PV module. On other hand, we observe from

d) The PV Output Power Model

The PV output power system was proposed to assess the reliability of distribution system containing renewable energy resources (solar energy generation plants with different capacity). The PV system output power can be calculated by the following Equation (7) [

P ( o u t ) = { η C K ∗ S ∗ I ( t ) 2 0 < I ( t ) ≤ K η C ∗ S ∗ I ( t ) I ( t ) > K (7)

where ( η C ) is the efficiency of the PV system, K is a threshold and I ( t ) is the hourly solar insolation. The value of ( η C ) is not constant when I ( t ) is less than or equal, it is proportional with the solar insolation. That means when the solar insolation increase the PV system efficiency will increase and vice versa. The following equation can express the hourly solar insolation (8) [

I ( t ) = { I max ( − 1 36 t 2 + 2 3 t − 3 ) 6 ≤ t ≤ 18 0 0 ≤ t < 6 and 18 ≤ t ≤ 24 ≥ 0 (8)

The load modelling is an important part of power system modelling process and has a significant effect on power system simulation results. The load model is created by using daily or monthly or yearly peak loads with respect to time in seconds or minutes or hours. The energy consumption can be displayed with load curve as shows the amount of demand used per period of time but Load Duration Curve (LDC) shows the amount of maximum ranking until the minimum demand is used. LDC is the basic tool used in the analysis of electric power systems such as estimating the operating cost of resource plans, and as tool to integrate the demand side management in the planning of electricity generation and enables to evaluate the operation of the power system more accurately than the traditional approach.

Over the last several years, the peak of electricity demand in Kingdom of Saudi Arabia (KSA) is continuously increasing. The Saudi Electricity Company (SEC) operates an interconnected transmission system for all main areas in KSA. SEC system is divided into four operating areas, named as Central, Eastern, Western and Southern Operating areas. Since Riyadh city is the major load center in Central operating area it faces unique issues in every day operation of the system [

The behavior of power system loads is a frequent pattern during normal conditions, and a time varying load model can be developed by using historical data. The model used in this study is the hourly load curve which can be converted to load duration curve by arranging the data in descending order. The following equation used to find the predicated the load for any load point at any desired time (9).

P i = W h ( h ) × W m ( m ) × P L i (9)

where, W h ( h ) = hourly weight factor, W m ( m ) = monthly weight factor, and P L i = peak load for load point.

The basic function of an electric power system is to supply customers with reasonably economical and reliable electricity. The reliability level of one distribution system is often evaluated from two aspects which are consumer level and system level. These two frequently used reliability indices for users of load points are as follow. The first one is average failure rate (λ_{i}) which is number of outages/year and defined as the probability of failure occurrence during a specific period for load point. The second one is average annual interruption time (U_{i}) which is sum of the outages time/year and defined as the average interruption time of load point in a specific period. These two indices can be expressed as following Equations (10)-(11) [

λ = ∑ i = 1 n λ i (10)

U = ∑ i n λ i r i (11)

where λ_{i} is the failure rate of the series components from the source point to load point, n is the total number of components which affect load point, r_{i} is average restoration time of network component to restore load point i due to the failure of component j.

The commonly reliability induces of the system defined as functions of average failure rate over the total number of customers, and average interruption time [

SAIFI = ∑ i = 1 n λ i N i ∑ i = 1 n N i (12)

SAIDI = ∑ i = 1 n U i N i ∑ i = 1 n N i (13)

CAIDI = ∑ i = 1 n U i N i ∑ i = 1 n λ i N i = SAIDI SAIFI (14)

ϵ ENS = ∑ i = 1 n E i N i (15)

where N_{i} is number of customers at load points i, U_{i} is the annual outage time, N_{i} is the number of customers at load point i, E_{i} is the average of average interruption energy per load point.

Monte Carlo Simulation (MCS) has been used in reliability evaluation of distribution systems to simulate the failures due to that failures in power system networks are random in nature. It is a powerful method for solving a complex system and often used in complex mathematical calculations, stochastic process simulations, engineering system analysis and reliability calculation. MCS is a probabilistic method that can be used to predict the behavior of the system components which in real time will be all different in varying degrees, including the number of failures, times to failure, restoration times. The Simulation techniques used to estimate the reliability indices by simulating the actual process and random behavior of the system. The time sequential simulation is one of the MCS types used when the system behavior depends on past events to examine and predict behavior patterns in order to obtain the probability distributions of the various reliability parameters. The system reliability indices can be obtained from the artificial history that needed in time sequential simulation, and this can be obtained by generating the up (where expressed the element is in the operating state) and down (where expressed the element is inoperable due to failure) times randomly for the system elements. These artificial histories depend on the reliability parameters the elements and the system operating/restoration modes.

Time to failure (TTF) or failure time (FT) is the duration that it would take the component to fail or the time during the element remains in the up state. This time is predicted randomly and calculated by the following Equation (16) [

TTF = − 1 λ × ln ( n ) (16)

where λ is failure rate of system component and n is a random number (range from 0 to1).

Time to repair (TTR) or time to replace (TTR) is the time required to repair a failed component or the time during the element remains in the down state and it is used to indicate the cycle time of failure. Also, this time is predicted randomly by the following (17) [

TTR = − 1 μ × ln ( n ) (17)

where μ is repair rate of system component.

The process of transiting from the up state to the down state is the failure process and can be caused by the failure of an element or by the removal of elements for maintenance time. The parameters TTF and TTR are random variables and it is obvious from Equations (14) and (15) that TTF and TTR follow exponential distributions. So, to predict the artificial history of system components, TTF and TTR can be generated to cover simulation times (e.g. one year) in chronological order. MCS have to be performed for a large number of scenarios, and the simulation time can be expanded to be a very long time (e.g. a thousand years or more) depending on the case study and also the desired accuracy. After that, the average can be calculated.

The element failures might have influence on one or additional load points and the most difficult in the simulation is to search out the load points and their failure period that affected by the failure of an element which are dependent on the network configuration. And in order to get a clear vision of how adding DGs impact on the reliability evaluation results of networks. We have used two reliability

evaluation algorithms which are MV network that does not contain DGs of PV and a MV network that containing DGs of PV.

As the main purpose of this study is to evaluate the reliability of MV network system with renewable energy resource which is the PV distributed generation with vary output power capacity to see their impact on the reliability, the following assumptions were created that shouldn't have a major impact on the results:

1) The permanent faults only are included in the study and all protection devices operate with success to isolate the faults.

2) The primary main feeder failures only are included in the analysis and every section protected by a breaker to isolate the faults.

3) It takes one hour to transfer the loads from the failed feeder to a neighboring feeder through a normal operating point.

4) Each circuit breaker is controlled by a bi-directional protection device.

The simulation procedures of reliability evaluation of distribution networks do not contain and contain renewable energy of PV distributed generators are shown in

Many researchers have been studied the distribution network reliability and used the RBTS Bus 2 or Bus 4. Moreover, substation number 7029 MV distribution network in Riyadh region has selected similarly to conduct the reliability evaluation study and compare the results with RBTS Bus 2. These networks offer the information needed to conduct a reliability study. The RBTS bus 2 is supplied by two 33/11 KV, 16 MVA transformers whereas the substation 7029 is supplied by two 33/13.8 KV, 20 MVA transformers. The 0.415 KV and 0.400 KV low voltage customers are supplied via 11/0.415 KV and 13.8/0.400 KV transformers, whereas the 11 kV and 13.8 KV customers are supplied directly. For the reliability analysis the 33 kV supply has been considered 100% reliable and the feeders are operated as radial feeders but connected as a mesh through normally open sectionalizing points. Following a fault on a feeder, the ring main units permit the sectionalizing

point to be moved and customers to be supplied from alternative supply points.

The data of RBTS Bus 2 distribution network may be found in reference [

power generation to supply the feeders loads during a permanent fault. We have used MCS in this work to evaluate the reliability indices of RBTS bus 2 and substation 7029 without DGs of PV and with DGs of PV. In addition, the impact of implementing these technologies in the reliability assessment of the distribution network is investigated. The annual average rate, average interruption time, and annual interruption energy of load points for RBTS and substation 7029 in two cases without and with DGs of PV are shown in Figures 19-21 respectively.

The RBTS and Substation 7029 failure rate of the feeder section is a function of the length. However, each section has a different length which resulted in the failure rates of the short feeder (F2) are low compared to the long feeders (F1, F2 and F4) of RBTS network. In addition, the failure of loads on the short feeder (F2) is also low compared to loads on the long feeders (F1, F3, F4) in the substation 7029. In the RBTS bus 2 and substation 7029 without distributed generators, it is obvious that the load points located at the end of the main feeder have high failure rates compared to the loads in the beginning of main feeder, because permanent faults result in isolating these load points from the main source. On

the other hand, the load points with distributed generators of PV located at the end of the main feeder have low failure rates. This reduction in failure rate is due to the excess generation capacity provided by the DGs during the outage of the main sources. From

The average interruption time is a function of failure rates and average restoration time of RBTS and substation 7029 network components. Therefore, the all previous observations are applicable to

The average interruption energy of all load points is described as a function of the average interruption time and the load of each load point. Therefore, all of the previous observations and clarifications are applicable to

It is observed that from the previous study and results, the implementation of DGs of PV in distribution networks can improve the reliability of distribution networks by offering a backup source when the main source is not available. In addition, the overall reliability indices have improved after adding DGs of PV which is shown in

The summary of these system reliability indices (SAIFI, SAIDI, CAIDI and ϵENS) are shown in

Index | RBTS W/O DGs | RBTS with DGs | S.S.7029 W/O DGs | S.S.7029 with DGs |
---|---|---|---|---|

SAIFI (Inter./customer yr) | 0.2098 | 0.1918 | 0.3514 | 0.2827 |

SAIDI (hr/customer yr) | 0.8745 | 0.8462 | 0.9017 | 0.8814 |

CAIDI (hr/customer inter) | 4.1682 | 4.4118 | 2.5660 | 3.1177 |

ϵENS (MWh/yr) | 8.3848 | 8.0246 | 8.9307 | 8.5821 |

As the faulted feeder and all load points suffer power outage when a fault occurs either at any location between feeders and main bus or at any feeder circuit breaker or on the main bus. Thus, it has highest impact on ϵENS. We observed that, there is a slight improvement after adding DGs of PV for two distribution networks. RBTS Bus 2 has better system reliability (ϵENS) than S.S 7029. These reliability indices (SAIFI, SAIDI, CAIDI and ϵENS) can be used to determine the system performance of the composite distribution system. The system reliability indices can also be used to make assessment for the severity of system failures on future reliability analysis.

The implementation of renewable resources in distribution networks is promising in many environment and economic aspects, such as reducing green gas emissions, reducing the power losses on distribution networks and improving reliability of power services. The distributed generators of PV could pave the way to integrate solar energy in distribution systems, which can deal with different modes of operation such as interconnected mode and islanded mode. The excess of PV generation does not lead to any negative impact on the system reliability, but they surely have significant economic impacts. Due to high correlation of PV output power during the peak hours of the load, the PV integration results in reduced net loads which lead to higher effective load carrying capability.

In this study, the reliability evaluation was conducted on a test distribution network which is RBTS Bus 2. The data of RBTS Bus 2 have been used in this study because it offers very detailed information for each distribution components (transformers, breakers, lines and busbars) such as the failure rate and average restoration time. The same study has been conducted on the original case of distribution network substation 7029 which is located at KSA Riyadh. Since the original RBTS Bus 2 and substation 7029 did not contain of any distributed generators, the two systems have been modified to include the PV’s with different power output capacity. The probabilistic techniques used to evaluate the reliability of distribution networks containing PV as a distributed generator which is random in its output power production. MCS is a very useful tool, it requires only basic information to generate the artificial history of the distribution system components by generating large and random numbers of scenarios of the system element. The failure rates and average restoration times can be easily found by calculating the frequency and the duration of the down time of each load point in the system. The failures rates and average restoration times were assigned to all distributed generator resources, these indices were considered in reliability assessment studies. The common indices such as SAIFI, SAIDI, CAIDI and ϵENS have been calculated to evaluate the reliability of the distribution network in two cases without and with DGs of PV. The impact of the DGs of PV has been investigated and the simulation studies have shown that the implementation of DGs in the distribution system can improve the reliability of the system. In the RBTS containing DGs of PV SAIFI, SAIDI, CAIDI and EENS were improved by 8.57% 3.23% 5.84% 4.29% respectively. In the Substation 7029 containing DGs of PV SAIFI, SAIDI, CAIDI and EENS were improved by 19.5% 2.25% 21.5% 3.9%respectively.

The authors declare no conflicts of interest regarding the publication of this paper.

Al-Sefri, A.K. and Al-Shaalan, A.M. (2019) Availability, Performance and Reliability Evaluation for PV Distributed Generation. World Journal of Engineering and Technology, 7, 429-454. https://doi.org/10.4236/wjet.2019.73032