About basics of distributed generation

1. 1
Interconnection Issues
of
Distributed Generation
with
Emphasis on Islanding
Smita Shrivastava
2007-08

2. 2
􀁺
Presentation Outlines
 Introduction of Distributed Generation [ D G ]
 Benefits
 Interconnection Issues
▶ Technical Issues
 Islanding and its Detection
▶ Commercial and Regulatory Issues
▶ Other Issues
 Conclusion

3. 3
􀁺
Distributed Generation [ D G ]
Distributed generation is not a new concept. In fact, the history of power
generation started with the distributed generation.
Early power plant only supplied electricity to customers in its close
neighbourhood.
In the last decade, technological innovations, changing economy &
environment concern have resulted in a renewed interest in distributed
generation.
The ‘economy of scale’ in power industry has shrunk in the recent years and
utility generation pattern is shifting from ‘economy of scale’ to ‘economy of
mass production’
Recent studies suggest that the difference in cost of electricity production
(cost per kWh) between large and small scale generation has reduced to
30% in 2000 from 60% in 1960.
Due to these and various other reasons including environmental concern,
distributed generation are gaining popularity around the globe

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Definition of DG
Distributed generation in simple term can be defined as a small scale
generation. It is active power generating unit that is connected at
distribution level.
IEEE defines the generation of electricity by facilities sufficiently smaller
than central plants, usually 10 MW or less, so as to allow interconnection at
nearly any point in the power system, as Distributed Resources.
Electric Power Research Institute (EPRI) defines distributed
generation as generation from a few kilowatts up to 50 MW.
International Energy Agency (IEA) defines DG as “Power generation
equipment and system used generally at distribution levels and where the
power is mainly used locally on site”.
The International Council on Large Electricity Systems (CIGRE)
defines DG as generation that is not centrally planned, centrally dispatched
at present, usually connected to the distribution network, and smaller than
50-100 MW.

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Present and Future distribution
System
Changing Distribution Structure (Source: Distributed Utility Associates)

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Benefits of DG to Industries
1. Increased reliability: Installation of DG in the industry’s backward
will improve the reliability of the power supply. Industry can have
“Uninterrupted Service” by installing a DG where it can be used as
the backup to the main grid.
2. Improved Power quality: Many types of electrical equipments are
very sensitive to voltage and frequency. With DG, industry can have a
micro-grid operation to prevent its equipment from voltage sag and
swells as well as deviation of frequency.
3. Energy cost savings: Cost of energy is increasing with increasing
fuel price and industries have to pay heavy demand charges. Industry
can save the energy cost by running the DG at peak load. Depending
on the technology and fuel cost, sometime generating energy from DG
is less costly than buying from grid.
Contd….

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4. Increased energy efficiency: Industry can make use of waste heat to
generate power by having a Combined Heat and Power (CHP)
generation. This will improve the overall efficiency of energy use in the
industry.
5. Reduce exposure to price volatility: In case of the deregulated
environment, DG can help the industry to reduce the exposure to the
price volatility.
6. Environmental concern: Environment is major concern today, through
out the world. People are looking for green power. Even in Thailand,
there are rules and regulations to make sure that certain portion of
total power generation comes from renewable sources. DG, especially
those based on renewable energy, helps the environment and it is
eligible to many financial benefits.

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DG Benefits

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Interconnecting DG to Grid
􀁺
 Even though DG is a viable option to meet the power demand of the
industry, there are varieties of technical, commercial, and regulatory
issues that have to be considered before the interconnection of DG
plant with main grid.
 Interconnection is basically meant for back wheeling the excess power
produced by a DG plant to the grid.
 The interconnection issues have potential enough to prevent
distributed generation projects from being developed. Interconnection
issues are broadly classified as:
 􀁺 Technical issues
 􀁺 Commercial and regulatory issues
 􀁺 Other issues

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Technical issues
The Most significant Technical Issues / Barriers for DG
connection are
 Reliability and Quality of supply
 Protection
 Stability and Dynamics
 Islanding Issues

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Reliability and quality of Power of Supply
 Voltage Regulation
 Voltage Flicker
 Harmonic Voltages
 DC Injection

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Protection Issues
The objective of power system protection is to detect a
fault condition (perhaps due to a lightning strike or
equipment failure) and isolate the faulted section of the
system as rapidly as possible while restoring normal
operation to the rest of the system.

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Impact of DG on Protection System Coordination
With the DG connected
IF = IUS + IDG and IR = IF
However, IR ≠IUS
The condition indicated is not seen in the typical radial distribution system.
IUS without the DG does not equal IUS with the DG. With the DG connected
the fault current seen by the recloser (IR) will be greater than without the
DG connected.
IUS is the current from the utility
source. Fault current seen by the
recloser is IR. The fault current at
the fault location is IF.
Without DG
IUS = IR = IF

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Stability and Dynamics
 Few number of DG is considered as harmless to overall stability and
dynamics of the distribution system. But with increasing penetration of
DG, it is believed to effect distribution system's stability.
 In fact, its impact is no longer restricted to the distribution network
but starts to influence the whole system, including transmission
system.
 The small synchronous generators have simple exciter and governor
control schemes compared to large central power generators and
induction generators are not able to control the reactive power at all.
DG units connected via electronic power converters do not have large
capabilities to control.
 These special characteristics of the DG units and their low inertia can
produce many technical and operating challenges regarding the
stability of power systems.

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Islanding Issues
An island is “That part of a power system consisting of one or more
power sources and load that is, for some period of time, separated from
the rest of the system.”
Contd….

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Islanding detection
Why islanding detection?
The importance of detecting islanding operation originates from security
reasons. Inadvertent islanding presents a number of safety, commercial,
power quality, and system integrity problems. In summary, the major issues
are:
 Line worker safety can be threatened by DG sources feeding a system
after primary sources have been opened and tagged out.
 Public safety can be compromised as the utility does not have the
capability of de-energizing downed lines.
 The voltage and frequency provided to other customers connected to
the island are out of the utility’s control, yet the utility remains
responsible to those customers.
 Protection systems on the island are likely to be uncoordinated, due to
the drastic change in short- circuit current availability.
Contd….

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 The islanded system may be inadequately grounded by the DG
interconnection.
 Utility breakers or circuit re-closers are likely to reconnect the
island to the greater utility system when out of phase.

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Techniques in Detection of Islanding
Conditions
Techniques in Detection of Islanding Conditions can be broadly
classified into two types according to their working principles.
This classification is shown in figure below.

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 The first type consists of communication-based schemes-
Such scheme uses telecommunication means to alert and
trip DGs when islands are formed, and
 The second type consists of local detection schemes- It rely
on the voltage and current signals available
Local detection schemes further classified as
 Active Islanding detection method
 Passive Islanding detection method

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Passive Islanding detection method
This method makes decisions based on measured voltage
and current signals only.
• Over/under voltage and frequency detection : Over/under voltage
and frequency detection approach, the inverter is shut down when the
utility voltage/frequency deviates from set values. While the method is
simple, it fails to detect islanding when the inverter generated power
closely matches with the connected loads .
• Phase jump detection : In the phase jump detection method, the
phase of inverter current is instantaneously synchronized at zero
crossing with phase of voltage via phase lock loop (PLL) circuitry.
Considerable phase difference can be identified as an occurrence of
islanding. However, this method fails when the load power factor is
unity. Contd….

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Active Islanding detection method
These methods inject disturbances into the supply system and detect
islanding conditions based on system responses measured locally.
• Output power variation: In the output power variation method, the
inverter real power is periodically perturbed, and the voltage is
continuously monitored. When islanding occurs, voltage fluctuations
become apparent due to real power mismatch. This information is used
to initiate shut down procedure. This method requires multiple DFPGs
connected to the utility to be synchronized. If synchronization is not
done, the method becomes ineffective due to the averaging effect.
• Active frequency drift : Active frequency drift (AFD) method
introduces small increase/decrease in the frequency of inverter
output current while monitoring the frequency of the voltage. A
measurable deviation in frequency of the voltage indicates islanding.
However, this method has been shown to fail when the load phase angle
matches with the phase offset generated by perturbing the frequency.
Contd….

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• Sliding mode frequency shift : Sliding mode frequency shift (SMS)
method applies similar strategy as AFD. In this method, the starting
angle of inverter current is controlled. Under islanding, a
measurable frequency deviation is observed. This method also fails
when the load phase angle equals the starting angle .

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Non-Detection Zone and Associated Risks
All anti-islanding schemes have some limitations which may include:
•􀁺 high implementation cost
•􀁺 need for coordination between the DG operator and the utility;
•􀁺 susceptibility to false detection of islanding (nuisance tripping);
•􀁺 possible non-detection of islanding under some conditions; and
•􀁺 possible reduction of utility power quality and voltage and frequency
stability.

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Conclusion about various methods to detect island
operation.
 Internal passive methods are cost-effective but their non-detection
zone includes balanced islands, which limits dependability.
 Internal active methods are effective and can detect balanced islands,
but can only be applied to DG units interfaced to the network with
power electronic converters.
 External methods are not commonly used, but may be a solution of the
future. When reviewing the basic interests of unit owner, distribution
company and TSO (transmission system operator) in terms of
dependability and security.
 An external method based on power line carrier seems to meet the
demands of all. It also gives the distribution company control of
islanding prevention, of course at the cost of investments.

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Commercial & Regulatory Issues
Net Metering
 Net Metering is the use of a single meter to measure the total
consumption of an electricity consumer against their total onsite
generator production over a fixed period of time.
 Net metering is required as DG relative to consumption cannot justify
the additional administrative and metering cost of typical customer
power sales arrangements.
 The main issue is what should be the policies towards payment to DG
plant owners on account of back wheeling of power.
 Interconnection fee and service charges for activities related to
interconnection is another issue with DG. Application and
interconnection fee were frequently viewed as arbitrary and found
disproportionate for the smaller projects in various countries. Contd….

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Other Economical Issues
 One of the global issues with DG interconnection is that the judicial or
regulatory processes are often highly expensive for relatively small-scale
distributed generation projects.
 High costs compared to alternatives and the solutions to technical
issues may add to costs affecting economics of DG.
 Financing hurdle
 Customer does not capture system (e.g., distribution reliability) and
non-monetary (e.g., environmental) benefits (externalities) of DG to
offset own (internal) costs.

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Siting, Certification and Permitting issues
􀁺
 Regulated variables when siting a power plant of any size include air
quality, fuel supply, noise and safety. These same issues affect each DG
installation.
 Two major siting, certification and permitting concerns that have been
raised are a) how to deal with the requirements of multiple local
jurisdictions and different permitting agencies, and b) what might be
the aggregate effects of locating multiple units in a common
geographical area.

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Other Issues
􀁺
 Lack of utility experience in dealing with the contractual and procedural
interconnection issues is one of the most widespread and significant
issues.
 Absence of a mechanism for giving appropriate credit for the
contributions made by DG proponents in meeting power demand,
reducing transmission losses, or improving environmental quality.

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DG interconnection issue
(Source: The Alpha Group)

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Conclusion
 Distributed generation is a potentially important, new, cost-effective
power supply alternative that can redefine the competitive energy
market.
 Furthermore, substantial technical and commercial issues must be
resolved before DG can be successfully integrated with grid
operations.
 Islanding is a very important technical issue in concern to the safety
of line workers and public. Any method of detection of islanding is not
100% perfect till now.
 The strongest objective of the DG regulatory policy is to encourage
competition and economic efficiency, followed by the need to ensure
safety and grid reliability

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1. Islanding detection and connection requirements. O. Samuelsson, Member,
IEEE, and N. Stråth, 1-4244-1298-6/07/$25.00 ©2007 IEEE.
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July, 2004 CETC-Varennes 2004-074 (TR) 411-INVERT
3. Distributed Generation Islanding – Implications on Power System Dynamic
Performance. R. A. Walling, Senior Member, IEEE, and N. W. Miller,
Fellow, IEEE
4. Stråth, N., Islanding detection in power systems, Licentiate thesis, Dept
of Industrial Electrical Engineering and Automation, Lund University,
ISBN:91-88934-42-X, 2005.
5. California Distributed Energy Resource Guide website at
http://www.energy.ca.gov/distgen/index.html
6. IEEE Standard for Interconnecting Distributed Resources with Electric
Power Systems, IEEE, Standards Coordinating Committee 21, July 2003.
References :-

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Larsen, North Carolina Solar Center, Third Edition 2000.

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12. Department of Energy, National Renewable Energy Laboratory, “Making
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2004
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