TESTING RELIABILITY OF MICROGRID POWER SYSTEM- A SIMPLE APPROACH

e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:03/March-2021

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TESTING RELIABILITY OF MICROGRID POWER SYSTEMA SIMPLE APPROACH Subodh Kumar Gupta, MIE*1, Prof. (Dr) Kalyan Kumar, FIE*2 *1Additional *2Former

General Manager (Retd), NTPC, CC, Delhi, India.

Vice Chancellor, VMS University, Sikkim, India.

ABSTRACT A Microgrid comprises of the low voltage multiple distributed source of generation (DG), energy storage devices (ES), connected multiple loads and control system, connected to national grid / islanded mode. Microgrids are increasingly becoming viable because of rapidly falling costs of generation contributed by wind and solar energy sources. Reliability modeling is the most important step in the design and planning of the distribution system what is expected to operate with economic viability owing to less interruption of customer loads. In the recent years distribution systems have been given utmost attention in terms of continuity and reliability of load power supply, and thus, focusing on the needs to evolve simpler techniques / approaches towards Microgrid reliability modelling and evaluation. The analysis of customer failure statistics shows that distribution systems make the greatest single contribution to the uncertainty / unavailability of customer supply. The paper examines such critical areas to be taken care of in an attempt to ensure maintaining relatively higher reliability of Microgrid power system thereby highlighting economic impact to reduce the cost of interruptions and power outages involving the utility companies and their customers. The results also showcase the importance and necessity of conducting reliability evaluation in the local area Microgrid distribution systems resulting in the improved performance at the customer load points.

Keywords: Microgrids, Reliability Indices, Roy Billinton Reliability Testing System (RBTS), SUBREL.

I.

INTRODUCTION

Reliability of a Microgrid power system can be defined as the ability of the system to perform its intended function under the given operating conditions. When applied to the electric power system, reliability is mainly concerned with the ability of the system to supply the load demands. Reliability evaluation of an electric power system can be divided in two main categories, system adequacy evaluation and system security evaluation.

Reliability assessment of a distribution system is usually concerned with the system performance at the customer end, i.e. at the load points. The basic indices normally used to predict the reliability of a distribution system are: load point failure rate, average outage duration and annual unavailability. System adequacy pertains to evaluation of adequacy of resources to demanded load, whereas system security is concerned with the response of the system to accommodate interruptions. This paper focuses on the local area Microgrids connecting consumer loads for assessment of the power supply system adequacy and security.

II. APPLICATIONS AND ISSUES Reliability evaluation of a system can be performed by two techniques, deterministic and probabilistic. The probabilistic evaluation of the system is preferred for objective analysis, though deterministic technique is computationally faster and requires less data. Due to the advancement of technology and computational techniques, over the years, probabilistic reliability evaluation has become possible and is being followed widely in power system [1-12].

A modified RBTS bus system using microgrid at Bus 5 in Figure 1 below is tested. At Bus 5, the load is 20 MW. Bus 5 is serving several feeders and one of the feeder is IEEE 34 Bus test system [13-15]. The load is 1.769 MW. Both System indices and load indices are calculated using SUBstation RELiability Evaluation (SUBREL) and the results of the same are presented in Tables 1 – 3 below. Three scenarios of RBTS as mentioned below are tested and their findings/results are produced below [19]. 

RBTS NO STATIONS



RBTS BREAKER ONE HALF



RBTS RING

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III.

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RELIABILITY MODEL OF MICROGRID

The IEEE 34-bus feeder system, connected to RBTS as a distribution feeder is simulated using SUBREL software and the relevant Indices analyzed in FORTAN coding environment. SUBREL is a software used for analysis of electric power systems by General Reliability Inc.,USA in countries viz USA, Australia & many others. SUBREL is a computer program to calculate reliability indices for an electric utility substation and generating station switchyard. The main features of the SUBREL are as under: 

Computes reliability indices for a substation with a simulation of all automatic switching, load transfers, and load curtailments, following faults.



Compares alternative substation configurations by providing data for the cost of outages.



Provides a basis for risk/benefit analysis against investments- a value based planning approach.



Helps in developing design standards and in assessing benefits of new technology in substation automation.



User does not need to provide substation equipment outage data and customer outage cost data if it is not available. SUBREL automatically provides default values based on published statistics.



User does not need to provide automatic switching sequence following a fault.

Optimal power flow, transient stability, load flow, short circuit and reliability evaluation. SUBREL can provide reliability indices for individual load points and the overall system.

The initial step is to analyse the network data for load flow analysis and the data for reliability evaluation. In the next step the software generates failure combinations. The failure combinations can be either individual failure or simultaneous multiple failures. The first order failures or individual failures consider a failure of only one component in the system and the second order or multiple failures consider simultaneous failure of two components in the system.

In the next step failure effect analysis is performed on each failure combination. Processing of a failure combination produces a value for the contribution of that combination to the reliability characteristics of the network, expressed as a probability. The contribution of this failure combination is added to the factors already identified, so that after processing all relevant failure combinations, a detailed picture is obtained of the interruptions occurring at each load node.

Table-1: System Indices for Three Cases System Indices

RBTS NO SUBS

RBTS BREAKER ONE HALF

RBTS RING

SAIFI [1/yr]

1.99691

2.71903

3.20337

SAIDI [min/yr]

618.83838

823.16687

1051.01868

CAIDI [h]

309.89731

302.74243

328.09814

ASAI (%)

0.99882263

0.998433828

0.998000324

EUE [KWh/yr]

259547

729138

826127

Outage Cost($)

1213545

26140644

27725088

Customer Interruptions/ Year

245.6205

334.4411

394.0141

Customer Minutes/ Year

76117.125

101249.5234

129275.2891

Table-2: Load Indices for Base case Load

Frequency of Interruptions

Interruption Minutes per occurrences

Interruption Minutes per year

EUE kWh/Year

LOAD 3 1

0.000001

5.997916

0.000007

0.010067

8

0.000012

LOAD 4 1

0.000001

5.998055

0.000007

0.001937

1

0.000001

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Outage Total Customer Costs ($) Interruptions

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LOAD 5 1

0.669794

112.653824

75.454819

6331.054688

707184

1.35535

LOAD 6 1

3.417005

254.257904

868.800476

253216.125

506352

244.265121

Table-3: Primary Outages by Component Type Project

Component Type

Outage Type

Frequency Occ/Year

Duration Hours/year

RBTS NoSubs N-2F N-1S SUBREL

Breaker

Faulted

0.000002476

0.00000025

RBTS NoSubs N-2F N-1S SUBREL

Generator with protection

Faulted

46.91802709

91.98121191

RBTS NoSubs N-2F N-1S SUBREL

Generator with protection

Isolated

37.60000038

1757.80002

RBTS NoSubs N-2F N-1S SUBREL

Line with protection

Faulted

26.30309581

51.84368465

RBTS NoSubs N-2F N-1S SUBREL

Line with protection

Isolated

21

168

RBTS NoSubs N-2F N-1S SUBREL

Transformer with protection

Faulted

0.223934786

0.222094514

RBTS NoSubs N-2F N-1S SUBREL

Transformer with protection

Isolated

0.179999996

138.0599972

IV.

RBTS RELIABILITY TEST SYSTEM

Roy Billinton Test System (RBTS) was developed at University of Saskatchewan, Canada for educational purposes and research. RBTS includes 6 buses and 11 generator units. The transmission voltage is 230 kV and the total installed capacity is 240 MW with a system peak load of 185 MW. Figure 1 shows the single line diagram of RBTS BUS- 6.

Figure-1: Single Line Diagram of RBTS BUS- 6 www.irjmets.com

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RBTS is a smaller test system as compared to RTS. However, data for a distribution system has been defined for the existing RBTS. This makes it very convenient for applications on distribution system reliability analysis and similar studies. The area of research where RBTS has been used the most is introducing and testing new techniques for probabilistic applications. Another advantage of RBTS is that it easily modify the test system due to its size [16-17].

V.

SYSTEM RELIABILITY INDICES

Reliability assessment is the most important factor in the design and planning of any distribution systems that should operate in an economic manner with minimal interruption of customer loads. A distribution system is relatively cheap and outages have a much localized effect. On the other hand, the analysis of customer failure statistics show that distribution systems make the greatest individual contribution to the unavailability of customer supply.

The goal of a power system is to supply electricity to its customers in an economical and reliable manner. It is important to design and maintain reliable power systems because cost of power outages can have severe economic impact on the utility and its customers. In the distribution systems, most of the outages or failures would result in direct impact on the customers. A customer connected to an unreliable distribution system could receive poor energy supply even though the generation and transmission systems are highly reliable. This fact clearly illustrates the importance and necessity of conducting reliability evaluation in the area of distribution systems.

The reliability evaluation is performed in this paper using a hybrid method that combines an analytical method with random sampling techniques. This allows to consider the impact of intermittent energy sources (solar and wind power) in the system reliability indices. The system performance can also be assessed on an overall distribution system including system reliability indices. The system basic reliability indices are defined as follows: SAIDI: System Average Interruption Duration Index; Customer Interruption Duration / Total nos. of Customers served (Hours/yr-customer); SAIFI: System Average Interruption Frequency Index; Total nos of Customer Interruption / Total nos. of Customers served (interruptions/yr-customer); CAIDI: Customer Average Interruption Duration Index; Customer Interruption Duration / Total nos. of Customer Interruption ((h/int or SAIDI/SAIFI); ASAI: Average Service Availability Index; Customer Hours service Availability / Customer Hours service Demand (%); EUE: Expected Unserved Energy (kWh/yr); Outage Costs ($)

VI.

TEST RESULTS

The customer Indices have been cited / represented using bar diagrams shown in Figures 2 to 5 with trend analysis as shown below [18]: Figure 2: This bar diagram depicts customers average interruption frequency per year in three different conditions. The customer interruptions are shown on Y-axis over the one year period which is taken along Xaxis. The lowest value is in case of RBTS No Subs.

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Figure-2: RBTS SUBREL RESULTS - SAIFI Figure 3: This bar diagram is indicative of customer’s average interruption duration in minutes per year. The Yaxis shows the interruption duration in minutes and the X-axis is the one year duration. The minimum interruption duration is seen in case of RBTS No Subs.

Figure-3: RBTS SUBREL RESULTS - SAIDI Figure 4: This bar graph represents expected unused energy (EUE) in kWh per year. The Y-axis shows kWh/year and the X-axis is the one year period. The best condition is of RBTS No Subs.

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Figure-4: RBTS SUBREL RESULTS - EUE Figure 5: This bar graph represents cost involved due to outages in a year. The Y-axis represents cost incurred in US Dollars ($) and the X-axis is the one year period. The significant cost savings could be seen in case of RBTS No Subs.

Figure-5: RBTS SUBREL RESULTS - Outage Costs ( $) Figures 2 to 5 above very well answer the optimum performance condition, i.e., RBTS No Subs to be reached having compared the results based on (a) SAIFI; (b) SAIDI; (c) EUE; and (d) Cost of Outages ($) per year. Higher value of SAIFI (Fig 2 – RBTS Ring) indicates the higher frequency of interruptions on the feeder whereas higher value of SAIDI (Fig 3 – RBTS Ring) shows large duration of load curtailment on that feeder.

VII.

CONCLUSION

The values of computed indices for modified RBTS Bus 5 employing microgrids indicate high reliability. This paper describes the concept of interfacing Microgrids with the main grid with a view to examine the Microgrid performances suggesting consumer end power supply control strategies that can reliably and efficiently operate a balanced 3-phase low-voltage Microgrid in the grid-connected and islanded modes. www.irjmets.com

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Notwithstanding that this paper reviews the standard test system, RBTS benchmarks and its applicability for power system studies. The ultimate goal of power system studies is to supply the end users economically and with an acceptable level of reliability. Beyond the general concepts of the reliability studies, adequacy and security assessment require detailed analysis including stability, power flow, protection, control, and so on.

As a result, the following specific recommendations are proposed for upgrading the test systems for modern power system analysis: The test systems should include wind farms and photovoltaic plants.

The low and medium voltage (both AC and DC) distribution systems should be interconnected to the high voltage bulk system for Microgrid and smart grid related studies.

Power converters and their control mechanism for wind turbines and photovoltaic plants should be provided in the test system.

VIII.

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[19]

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