Benefits of Distributed Generation on Power Delivery System

Distribution Engineering

EE 5250

Rakesh Prasad

Date: April 14th 2006

Contents

Chapter 1 Introduction

1.1 Distributed Generation………………………………..3

1.2 Why Distributed Generation……………………...... 4

Chapter 2 Advantages of Distributed Generation

2.1 Reliability…………………………………………...... 5-8

2.2 Power Quality……………………………………...…..8-12

2.3 Transmission Benefits...... ….. 13

2.4 Environmental Benefits……………………………….14-15

Chapter 3 Conclusion………………………………………...... 16

References……………………………………………………….. 17

Chapter 1 Introduction

1.1 Distributed Generation:

Distributed Generations are parallel and stand-alone electric generation units located within the electric distribution system at or near the end user.

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1.2 WHY Distributed Generation?

For the past 60 years, electricity production and supply has been performed by centralized, regulated electric utilities that owned and operated power generation facilities as well as the transmission and distribution lines.

Investor-owned utilities are regulated by state public utility commissions (PUCs), while cooperative and municipal utilities are governed by local jurisdictions.

Since the 1970s, federal and state public policy has encouraged the opening of the electric power system to entities other than the electric utilities. This has created a competitive landscape for power generation and has opened the transmission system to access. A significant shift in the U.S. regulatory system began with the Energy Policy Act (EPAct) of 1992, which requires interstate transmission line owners to allow all electric generators access to their lines. Many states today are at various stages of electric utility deregulation.

Utility deregulation is one reason for the high level of interest in Distributed Generation. Other benefits associated with distributed generation

Reliability

Power Quality

Transmission Benefits

Environmental Benefits

Some well known types of Distributed Generation

Fuel Cells

Micro turbines

Wind farm

Photovoltaic Cells

Internal Combustion Engine

Chapter 2 Advantages of Distributed Generation

2.1 Reliability:

Power reliability is required for

  • Life-safety systems, such as emergency lighting or ventilation, which must operate properly to prevent the loss of human life.
  • Systems that prevent damage to plant infrastructure (e.g., sump pumps at a wastewater treatment plant), allow monitoring of other systems (e.g., supervisory control and data acquisition, or SCADA, systems), or prevent the loss of vital data during power failures (e.g., at bank data centers) or whose failure to operate could significantly impact public health.
  • Processes that would cause sizeable financial losses if power outages occurred. Power outages cause loss of quality control in batch processes—found at microelectronic component manufacturing, food processing, chemical processing, and oil refining facilities—and force owners to discard entire batches. In addition, power losses to processes that operate 24 hours per day, seven days per week—with no openings to recover lost production time—can lead to cancelled orders.
  • Equipment and processes for which operation is not time-critical. Operating this type of equipment, such as a cooling system with a large, cool storage tank, can be deferred to off-peak times; switched to an alternate source, such as an engine generator; or switched to an alternate fuel, such as an electric heating system with fuel-oil backup.

There are many ways to increase the reliability of power. Redundant power supplies do not always improve reliability. If two redundant feeders supply power to an industrial facility but originate at the same utility substation and are carried on the same set of power poles, reliability will be lower than if they originate at separate substations and travel to the site on different sets of power poles. The problem with redundant feeders carried on the same set of poles is that a single-point failure (e.g., a weather-related event, pole fire, or traffic accident) could cause simultaneous outages on both sources.

To improve power reliability by installing standby generation, uninterruptible power supplies (UPS), flywheels, or fuel cells.

Reliability is the most important feature of electric power distribution system. Quantificationof distribution system indices is the best indices of whether the system with distributed generation has increased reliability or not.

The following indices are generally used by utilities (IEEE standard 1366, 2001) to measure the reliability.

System Average Interruption Duration Index (SAIDI):

Customer Average Interruption Duration Index (CAIDI):

System Average Interruption Frequency Index (SAIFI

Customer Average Interruption Frequency Index (CAIFI)

The following is a short example taken from [7]. The system is a two 22 KV feeders as the main incoming feeders in the station, followed by two 10 MVA, 22KV/11KV transformers. Both the transformers share the total load of about 2 MW with around15000 customers

Customer 327 Customer 220 Customer 327 Customer 220

System without DG System with DG

If there is a fault on feeder 1 all 327 customer of society 1 attached to it get affected, if the fault leads to sustained interruption then there is no alternate feed is for these customers. In such situations a strategically placed DG will be able to take care of all these 327 customers of society 1 in the event of fault in feeder 1, the same will be true in the event of fault in feeder 2 catering to 220 customers of society 2.

The following are data’s obtained from TATA POWER COMPANY.

Total no of customer interruption / Sum of interruption duration in minutes / No of affected customers / Total no of customers served
Without DG / With DG / Without DG / With DG
Feb-03 / 2020 / 74537 / 2018 / 1691 / 12336 / 12663
Aug-03 / 6106 / 241012 / 4334 / 4007 / 15101 / 15428
Dec-03 / 5012 / 66983 / 4916 / 4589 / 15497 / 15824
CAIFI / SAIFI / SAIDI
Without DG / With DG / Without DG / With DG / Without DG / With DG
Feb-03 / 1 / 1.194 / 0.163 / 0.159 / 6.042 / 5.886
Aug-03 / 1.409 / 1.523 / 0.404 / 0.396 / 15.96 / 15.62
Dec-03 / 1.02 / 1.09 / 0.323 / 0.316 / 4.322 / 4.233

The above results show that by optimally placing DG, the reliability indices have improved. The improvement may have been significant in the case of DG supplying a larger part of the network.

2.2) Power Quality Power Quality of any power system can be judged by the voltage profile and line losses. The index VPII and LLRI gives the result of benefits of the system with DG in comparison to system without DG.

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The above fig shows the single line diagram of 12 bus system used to obtain VPII and LLRI. The system consists of three conventional generators at bus 1, bus 5 and bus12 with ratings of 1.0, 0.75 and 0.625 respectively. Total load of 2.013 pu located unevenly on every bus is assumed. The resistance and reactance of all the distribution and transmission lines are assumed to be 0.000625 pu/km and 0.000375 pu/km. The lengths of the distribution lines are as below

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Simulations of following four cases were done

Case (1): D.G located at bus 9.

Case (2): D.G located at bus 10.

Case (3): 50% DG located at Bus 9 and 50% DG located at Bus 4.

Case (4): 50% DG located at Bus 9 and remaining 50% located at Bus 10.

VPII: Voltage Profile Improvement Index

It is defined as the voltage profile index of the system with DG to the voltage profile of the system without DG

Is voltage profile of the system with DG

Is the voltage profile of the system without DG

With

Is the voltage magnitude at bus i in per unit.

Is the load at bus i in per unit.

Is the weighting factor for load bus i.

Is total number of Load bus.

The weighting factors are chosen based on the importance and criticality of different Loads

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To study the impact four sets of bus weighing factor set 1 through 4 as listed above were selected. The results thus obtained are shown below.

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The VPII for all the four cases have value greater than 1 showing the improvement of VPII in comparison to system without DG. The VPII exhibits the highest value under weighting factor set 1(equal weights). With weighting factor set 4(importance given to high load buses), VPII has the lowest value, this is due to the fact that voltages at high load buses before employing DG(base case) are relatively high as compared to low load buses.

D.G rating plays a significant role in determining VPII. As DG rating increases, so does VPII

LLRI: Line Loss reduction Index

It is defined as the ratio of total line losses in the system with and without DG

is the per unit line current in distribution line i, with the employment of DG

is the line resistance(pu/km) for line i

is the ith distribution line length (km)

is the number of lines in the distribution system

is the per unit line current in distribution line I, without the employment of DG.

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The above result of the simulation of the 12 bus system shows that DG significantly reduces the electrical line losses. The rating location and operating power factor of DG are all very important contributing factors in determining the amount of line-loss reduction. However higher DG penetration cannot always guarantee lower line losses

Some results of technical paper:

Study of technical paper “Impact of Distributed Generation on Volt/Var Control in Distribution Networks” 2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy, shows that by proper placement of DG’s, system losses decrease enormously and imbalance of voltage have reduced thereby making for better volt/Var control

Technical paper “On Optimization for Security and Reliability of Power Systems with Distributed Generation” (2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy) gives us some valuable results as how, by systematic and rational placement of distributed generation you can improve feeder voltage profile, reduce losses and increase efficiency, and provide energy to some of the customers, even after the fault in the distribution system.

Paper “Benefit of Distributed Generation: A Line Loss Reduction Analysis”

(2005 IEEE/PES Transmission and Distribution) gives an idea of how line losses reduced as DG is installed closer to the load. However, this fact only applies for the case that DG rating is well matched with the amount of load. If the ratio of DG power output to the amount of load increases beyond the suitable point, the DG that is installed near the load will cause more electrical line loss than the one that installed far away from the load.

2.3 Transmission Benefits/ Substitute of Power Delivery Investments

In some cases, the installation of DG can lower power delivery costs as substitute or deferral of new transmission and distribution investments. That is one reason why some utilities are already encouraging such usewhere economically viable.

Remote areas to which the distribution system cannot reach:

In a number of cases, utilities can save money by installing DG on a customer’s property rather than extending a line to the customer’s remote location.

Areas experiencing load growth:

In some areas experiencing intermittent load growth, it can cost less to install a generator to serve a neighborhood’s load growth than it would to upgrade the power delivery system to import the same power. When a substantial power delivery investment is contemplated and load growth is low or uncertain, DG can be the low cost alternative.

New Large Loads:

A utilitycan save distribution expansion costs if a new large customer is coming up on a weak circuit. Choosing to build new generation to meet its load, instead of upgrading the transmission line can prove economical to the utility. Utilities arealready installing DGfor large stores, factories, Prisons, recreational areas and remotely located resort complexes. These are usually large internal combustion generators. On the other hand, the utilities will not get any benefit if a new large customer builds its own generation to meet its own energy demand but intends to lean on the utility for back up power especially at time of peak. In that instance, the utility may stillhave to reinforce its transmission or distribution facilities to serve the customer’s peakdemand.

2.4 Environmental Benefits:

The quantification of environmental benefits of any system with distribution generation in comparison to the system without distributed generation can be obtained by EIRI

EIRI: Environment Impact Reduction Index

It is the ratio emission of particular pollutant with and without DG

are the amount of electrical energy generated by the jth conventional power plant with and without distribution generation respectively.

is the amount of emission of the ith pollutant for the jth conventional plant per MWh

is the amount of emission of ith pollutant for the kth DG power plant per MWh of energy generated

In reality, power plants emit many pollutants into atmosphere. Thus, it is useful to define a composite index to include all the major pollutants. The index can be formulated as

With And

Where (EI)i is the weighting factor for the ith pollutant and NP is the total number of pollutants of interest.

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The result of simulation includes only three major ones (CO2, SO2, NOx). It is assumed that all the pollutants are equally weighted. The result shows that DG reduces pollutant emissions. The EIRI also depends on rating and locations of DG.

Conclusion:

Proper Planned and operated DG can provide consumers and society a wide variety of benefits, including economic saving, reliability and improved environmental performance. Though it looks wonderful seeing the indices in favor of distributed generation, but there are many other complicated issues involved.

The interconnection of DG with the electric grid involves safety reliability and economic risks. For example if a line goes down, the utility will know whether the line is energized and can respond safely. Consumer ownership and operation of generation can change that. New generation sources can also change the direction and volume of power flows on the system, possibly causing some wires to be underutilized while overloading others. Those changes may require the distribution company to reinforce its system, build new lines, or install new control equipment. Moreover, because DG could replace or reduce the demand for traditional utility service, DG could also pose an economic risk to some utilities too.

Reference:

[1] WHITE PAPER ON DISTRIBUTED GENERATION (National Rural Electric Cooperative Association) Jay Morrison NRECA Regulatory Counsel

[2] “Impact of Distributed Generation on Volt/Var Control in Distribution Networks” 2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy.

[3] “On Optimization for Security and Reliability of Power Systems with Distributed Generation” (2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy)

[4]“Benefit of Distributed Generation: A Line Loss Reduction Analysis” (2005 IEEE/PES Transmission and Distribution)”.

[5] “An Approach to Quantify the Technical Benefits of Distributed Generation”

(IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 19, NO. 4, DECEMBER 2004)

[6]

[7] Distributed Generation Operation under availability based tariff and reliability consideration” IJEEPS/vol 2 iss 1/art 1035.

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