Distribution and generation

1. Distributed Generation
EE 700 series
DG Lecture – 7
Impact of DG: Part I

2. Week 1: Brief Get-to-Know session; Introduction to Course, Grading Policy, Introduction to Distributed
Generation (DG), Concept of DG and its significance;
Week 2: Overview of Last week Class; Introduction of DG: History, Traditional Electricity delivery
Methods, DG advantages, DG issues. DG System Components, Microgrid Concept, Smart Grid concept
Week 3: Overview of Last week Class; DG Definitions Part I: Various definitions of DG; Variations in DG
Definitions in terms of Purpose, Location, Rating, Power delivery Technology, Environmental Impact.
Week 4: Overview of Last week Class; DG Definition Part II: Variations in DG Definitions (continued):
Mode of Operation, Ownership; DG Penetration, Distributed Capacity of DG, Connection Issues, Summary
Week 5: Overview of Last week Class. Distributed RE Technologies Part I: Solar Energy, Wind Energy, Fuel
Cell
Week 6: Overview of Last week Class; Distributed RE Technologies Part II: Energy Storage: Various
Schemes and their characteristics.
Week 7: Overview of Last week Class; Impact of DG Part I: Capacity of DG; Impact of DG on Voltage
regulation, losses, Harmonics and short circuit levels of the network, Impact of DG in Grid Business,
Planning and Design
Week 8: Overview of Last week Class; Impact of DG part II: Islanding of Power Network, Impact of DG on
Reliability, System Reliability indices, Indices features.
Week 9: Mid Term
DISTRIBUTED GENERATION COURSE LESSON PLAN

3. DISTRIBUTED GENERATION COURSE LESSON PLAN (contd.)
Week 10: Overview of Last week Class; DG – CG Comparison: Criteria for CG/DG
comparison; CG and DG Values and Recommendations, Benefits and Drawbacks of
DG
Week 11: Overview of last week lecture; DG and Reliability part I: Impact of DG on
Reliability: Location, System Reliability indices, Indices Features; PS Reliability
Assessment tool ETAP.
Week 12: Overview of last week lecture; DG and Reliability part II: Distributed Test
System, Sensitivity studies with DG,
Week 13: Overview of Last week Class; DG and Reliability part III: Case Studies:
Case 1: Reliability Impact to system main feeders; Case 2: Reliability vs Distance;
Case 3: Reliability with Increasing DG units; DG Optimal Placing
Week 14: Economic Considerations: Distributed System Costs; Key Economic
indices for DG: Simple payback, Internal Rate of Return, Life Cycle Costing, ;
Benefits and Drawbacks;
Week 15: Overview of Last week Class; Concept of Microgrid
Week 16: Some Case Studies
Week 17: Final Term Exam

4. Agenda of Week 7
• Overview of Last week Class;
• Impact of DG Part I:
• Impact of DG on Capacity of DG;
• Impact of DG on Voltage regulation,
• Impact of DG on losses,
• Impact of DG on Harmonics and
• Impact of DG on short circuit levels of the network,
• Impact of DG on Impact of DG in Grid Business,
• Impact of DG on Planning and Design

5. Overview of Lecture – 6
•Distributed RE Technologies Part II:
•Energy Storage:
•Various Energy Storage Schemes and their
characteristics.

6. Impact of DG
Part I

7. Combined Heat and Power (CHP)
• Combined heat and power (CHP), also known as cogeneration, is the
simultaneous production of electricity and heat from a single fuel source.
• Approximately two-thirds of the energy used to create electricity in
conventional thermal power plants is lost in the conversion process.
• CHP is a system that reclaims some of this lost energy by using the "waste"
heat to provide heating to the power plant facility or to buildings that are
connected to the power plant by a steam pipe network known as district
energy.
• CHP increases the energy efficiency of power generation to up to 80
percent and is best suited for urban areas, industrial parks, college
campuses, military bases and other communities that are close enough to
their power sources to use the cogenerated heat.
• However, the global penetration of CHP is less than 10 percent.
• The International Energy Agency also has recommended CHP, noting that
CHP “is an integrative technology that can make significant contributions to
reducing emissions of carbon dioxide and air pollution and to increasing
energy security.”

8. Combined Heat and Power (CHP)

9. Capacities of DG
•
The classification of DG depending on its size:
• Micro DG: 1 W < power < 5 kW.
• Small DG: 5 kW <power < 5 MW.
• Medium DG: 5 MW <power < 50 MW.
• Large DG: 50 MW <power < 300 MW

10. Applications of Distributed Generation (DG)
• The power system is prone to failures and disturbances due to weather
related issues, accidents, human errors etc.
• Having the DG as a backup source ensures the reliability of power supply
which is critical to business and industry.
• DG can be used to continuous supply to some of the load feeders using
switch operations.
• As shown in Figure, a fault occurs on feeder 2, but continuous power can
be supplied to load points B and C through DG in the form of an island.
Such an operation is termed as islanded mode.

11. Impacts of Distributed Generation
• The DG can be used as a standby for consumers that cannot tolerate
interruption of service e.g., hospitals.
• The DG can also be used as a stand-alone to supply power to the
customers that are not connected to the grid.
• For example, the DG can be used as standalone in remote areas where
cost of connecting to the main grid is too high.
• A DG installation increases the complexity of the system and impacts the
power flow and voltage conditions of the system.
• The planning of the electric system comprises of several factors
including:
• Types of DG,
• Capacity, Number of the DG units and
• Installation location etc.
• Depending on these factors, the DG can have positive or negative
impacts on the system.

12. Impact of Distributed Generation on Power System Grids
Impact of DG on Voltage Regulation
• The introduction of DG in systems can significantly impact the power
flow and voltage conditions at both, customers and utility equipment.
• Radial distribution systems regulate the voltage by the aid of load tap
changing transformers (LTC) at substations, additionally by line
regulators on distribution feeders and shunt capacitors on feeders or
along the line.
• The connection of DG may result in changes in voltage profile along a
feeder by changing the direction and magnitude of real and reactive
power flows.
• Nevertheless, DG impact on voltage regulation can be positive or
negative depending on distribution system and distributed generator
characteristics as well as DG location.

13. Impact of DG on Voltage Regulation
• The installation of DG units along the power distribution feeders may cause
overvoltage due to too much injection of active and reactive power.
• For instance, a small DG system sharing a common distribution transformer
with several loads may raise the voltage on the secondary side, which is
sufficient to cause high voltage at these customers.
• During normal operation conditions, without DG, voltage received at the
load terminals is lower than the voltage at the primary of the transformer.
• The connection of DG can cause a reverse power flow, may be even raising
the voltage somewhat, and the voltage received at the customer´s site
could be higher than on the primary side of the distribution transformer.
• For any small-scale DG unit (< 10MW) the impact on the feeder primary is
negligible.
• Nonetheless, if the aggregate capacity increases until critical thresholds,
then voltage regulation analysis is necessary to make sure that the feeder
voltage will be fixed within suitable limits.

14. • One of the major impacts of Distributed generation is on the
losses in a feeder.
• According to locating DG units to minimize losses is similar to
locating capacitor banks to reduce losses.
• The main difference between both situations is that DG may
contribute with active power and reactive power (P and Q).
• On the other hand, capacitor banks only contribute with
reactive power flow (Q).
• Mainly, generators in the system operate with a power factor
range between 0.85 lagging and unity, but the presence of
inverters and synchronous generators provides a
contribution to reactive power compensation (leading
current) .
Impact of DG on Losses

15. • The optimum location of DG can be obtained using load flow analysis
software, which is able to investigate the suitable location of DG
within the system in order to reduce the losses.
• For instance: if feeders have high losses, adding a number of small
capacity DGs will show an important positive effect on the losses and
have a great benefit to the system. On the other hand, if larger units
are added, they must be installed considering the feeder capacity
boundaries.
• Most DG units are owned by the customers. The grid operators
cannot decide the locations of the DG units. Normally, it is assumed
that losses decrease when generation takes place closer to the load
site.
• However, as it was mentioned, local increase in power flow in low
voltage cables may have undesired consequences due to thermal
characteristics.
Impact of DG on Losses

16. Impact of DG on Harmonics
• A wave that does not follow a “pure” sinusoidal wave is regarded as
harmonically distorted. This is shown in the Figure.
• Harmonics are always present in power systems to some extent.
• They can be caused by for instance: non-linearity in transformer exciting
impedance or loads such as fluorescent lights, AC to DC conversion
equipment, variable-speed drives, switch mode power equipment, arc
furnaces, and other equipment.
• DG can be a source of harmonics to the network.
• Harmonics produced can be either from the generation unit itself
(synchronous generator) or from the power electronics equipment such as
inverters.

17. Flicker issues due to DG
• A DG installation may increase the flicker level during start/stop or if it has
continuous variations in input power because of a fluctuating energy
source.
• In the case of a solar energy generator, the output fluctuates significantly
as the sun intensity changes.
• Moreover, Squirrel cage induction generators have a high possibility to
make flicker level worse because of an inability to actively control terminal
voltage.
• It is typically caused by the use of large fluctuating loads, i.e. loads that
have rapidly fluctuating active and reactive power demand.
• Flicker effect occurs when one generating source reactive power output
increases or decreases faster than the remaining generators can
compensate.
• Flicker does not harm equipment, but in weak grids with a higher
possibility of voltage fluctuations, the perceived flicker can be very
disturbing to customers.
Impact of DG on Harmonics

18. Impact of DG on Short Circuit Levels of the Network
• The presence of DG in a network affects the short circuit
levels of the network.
• It creates an increase in the fault currents when compared
to normal conditions at which no DG is installed in the
network.
• The fault contribution from a single small DG is not large,
but even so, it will be an increase in the fault current.
• In the case of many small units, or few large units, the
short circuits levels can be altered enough to cause miss
coordination between protective devices, like fuses or
relays.

19. • The influence of DG to faults depends on some factors such
as the generating size of the DG, the distance of the DG from
the fault location and the type of DG. This could affect the
reliability and safety of the distribution system.
• In the case of one small DG embedded in the system, it will
have little effect on the increase of the level of short circuit
currents.
• On the other hand, if many small units or a few large units
are installed in the system, they can alter the short circuit
levels sufficient to cause fuse-breaker miss-coordination.
This could affect the reliability and safety of the distribution
system.
Impact of DG on Short Circuit Levels of the Network

20. Islanding of a Power Network
• The islanding occurs when the
distributed generator (or group of
distributed generators) continues to
energize a portion of the utility
system that has been separated
from the main utility system.
• Furthermore, the network operator
is not able to ensure the power
quality in the island because the DG
is no more controlled by the utility
protection devices and continues
feeding its own power island.
• For instance, if an island is
developed on a feeder during
standard reclosing operations (i.e.,
disconnection), the islanded DG
units will be quickly out of phase
with respect to the utility system
during the “dead period”.

21. Islanding of a Power Network
• Then, the reclose occurs and unless
reclose blocking into an energized
circuit is provided at the breaker
control, the islanded DG will be
connected out of phase with the
utility.
• This can lead to damage of utility
equipment, the DG supporting the
island and customer loads, which
decrease the reliability of the whole
network.
• The last drawback encountered with
islanded operation is the safety
problems to maintenance crews.
• Personnel working on the line
maintenance work or repairing
a fault may mistakenly consider
the load side of the line as
inactive, where distributed
sources are indeed feeding
power to utilities.

22. Power quality
• Two aspects of power quality are usually considered to be important with
distributed generation:
i. Transient voltage variations and
ii. Harmonic distortion of the network voltage.
• Depending on the particular circumstance, distributed generation plant can
either decrease or increase the quality of the voltage received by other
users of the distribution network.
• Transient voltage variations: Distributed generation plant can cause
transient voltage variations on the network if relatively large current
changes during connection and disconnection of the generator are
allowed.
• The magnitude of the current transients can, to a large extent, be limited
by careful design of the distributed generation plant although for single,
directly connected induction generators on weak systems, the transient
voltage variations caused may be the limitation on their use rather than
steady-state voltage rise.

23. • Synchronous generators can be connected to the network with
negligible disturbance if synchronized correctly and anti-parallel soft-
start units can be used to limit the magnetizing inrush of induction
generators to less than rated current.
• However, disconnection of the generators when operating at full
output may lead to significant, if infrequent, voltage drops.
• Harmonic distortion of the network voltage: Incorrectly designed or
specified distributed generation plant, with power electronic
interfaces to the network, may inject harmonic currents which can
lead to unacceptable network voltage distortion.
• The large capacitance of extensive cable networks or shunt power
factor correction capacitors may combine with the reactance of
transformers or generators to create resonances close to the
harmonic frequencies produced by the power electronic interfaces.
Power quality

24. • A number of different aspects of distributed generator
protection can be identified as follows:
i. Protection of the distributed generator from internal faults
ii. Protection of the faulted distribution network from fault
currents supplied by the distributed generator
iii. Anti-islanding or loss-of-mains protection
iv. Impact of distributed generation on existing distribution
system protection
• Proper revised protection scheme management is required
to handle faults in distribution network in the presence of
distributed generations.
Protection

25. Impact of distributed generation on the transmission system
• In a similar manner to the distribution system, distributed
generation will alter the flows in the transmission system.
• Hence transmission losses will be altered, generally reduced,
while in a highly meshed transmission network it may be
demonstrated that reduced flows lead to a lower requirement
for assets (meaning less resources are required to be engaged).
• Generally, the charges for use of the transmission network are
presently evaluated based on a measurement of peak demand at
the transformer linking the transmission and distribution
networks.
• When distributed generation plant can be shown to be operating
during the periods of peak demand, then it is clearly reducing the
charges for use of the transmission network.

26. Impact of Distributed Generation on Central Generation
• The main impact of distributed generation on central
generation has been to reduce the mean level of the power
output of the central generators but, often, to increase its
variance.
• In a large electrical power system, consumer demand can be
predicted accurately by the generator dispatching authority.
• Distributed generation will introduce additional uncertainty
(due to nature dependent sources) in these estimates and so may
require additional reserve plant (at Generation level).
• It is now conventional to predict the output of RE based
sources (e.g., wind farms), which gives significant benefit and
so is useful for energy trading.

27. • As distributed generation is added to the system, its output
power must displace an equal output of central generators in
order to maintain the overall load/ generation balance.
• With limited output of distributed generators, the effect is to
de-load the central generators but maintain them and their
controllable output, on the power system.
• However, as more and more distributed generation is added
it becomes necessary to disconnect central generation with
consequent loss of controllability and frequency regulation.
Impact of Distributed Generation on Central Generation

28. • Distributed generation alters the power flows in the network and so
will alter network losses.
• If a small distributed generator is located close to a large load, then the
network losses will be reduced as both real and reactive power can be
supplied to the load from the adjacent generator.
• Conversely, if a large distributed generator is located far away from
network loads, then it is likely to increase losses on the distribution
system.
• A further complication arises due to the changing value of electrical
energy as the network load increases.
• In general, there is a correlation between high load on the distribution
network and the operation of expensive central generation plant.
Economic impact of distributed generation on the distribution system