Unit1

Unit 1
Introduction
5/19/2016 Grid Integration of DER/BG 1

1.1
What is Distributed Generation?
Distributed generation is an approach that employs small-scale technologies to produce
electricity close to the end users of power. DG technologies often consist of modular (and
sometimes renewable-energy) generators, and they offer a number of potential benefits. In
many cases, distributed generators can provide lower-cost electricity and higher power
reliability and security with fewer environmental consequences than can traditional power
generators.
In contrast to the use of a few large-scale generating stations located far from load centers--the
approach used in the traditional electric power paradigm--DG systems employ numerous, but
small plants and can provide power onsite with little reliance on the distribution and
transmission grid. DG technologies yield power in capacities that range from a fraction of a
kilowatt [kW] to about 10 megawatts [MW]. Utility-scale generation units generate power in
capacities that often reach beyond 1,000 MW.

DER & DG
Generally Distributed energy resources (DER) and Distributed Generation (DG) used synonymously
in electrical power industry as the main oblective is the extraction of electrical energy.
But in wide view;
DER- more into the resource side.
DG- more into the electricity generation side
Some DER like biomass may not be used for extraction of electricity but used in other ways.
Different terms used for distributed generation:
Distributed generation (DG), distributed energy (DE), distributed power (DP), on-site
generation (OSG) or district/decentralized energy.
To be distinctive:
Distributed Generation—Any technology that produces power outside of the utility grid (e.g., fuel
cells, microturbines, and photovoltaics)
Distributed Power —Any technology that produces power or stores power (DGs, batteries and flywheels)
Distributed Energy Resources —Any technology that is included in DG and DP
Renewable and non-renewable DG

DG Applications

History of Distributed Generation

Distributed generation takes place on two-levels:
- the local level
- the end-point level
Local level power generation plants often include renewable energy technologies that are site
specific, such as wind turbines, geothermal energy production, solar systems (photovoltaic and
combustion), and some hydro-thermal plants. These plants tend to be smaller and less centralized
than the traditional model plants. They also are frequently more energy and cost efficient and more
reliable. Since these local level DG producers often take into account the local context, the usually
produce less environmentally damaging or disrupting energy than the larger central model plants.
Fuel cells also provide an alternative route to a DG technology. These fuel cells produce electricity
through a chemical process rather than a combustion process. This process produces tiny
particulate waste.
At the end-point level the individual energy consumer can apply many of these same technologies
in small scale. Examples are small PV systems at homes or the micro turbine. DG technologies can
operate as isolated "islands" of electric energy production or they can serve as small contributors to
the power grid.

History
Traditional technology: the
electric grid:
• Generation, transmission, and
distribution.
• Centralized and passive
architecture.
• Extensive and very complex
system.
• Complicated control.
• Not reliable enough for some
applications.
• Relatively inefficient.
• Stability issues.
• Vulnerable.
• Need to balance generation and
demand
• Lack of flexibility.
Traditional technology: the centralized electric
grid:
• Generation, transmission, and distribution.
• Centralized and passive architecture.
• Extensive and very complex system.
• Not reliable enough for some applications.
• Vulnerable.
• Need to balance generation and demand
Centralized Generation vs Distributed Generation

“Centralized generation” refers to the large-scale generation of electricity at centralized
facilities. These facilities are usually located away from end-users and connected to a
network of high-voltage transmission lines.
- The vast majority of the electricity in world is from centralized generation.
- The earliest electric utilities operated independently from each other. A consumer would
purchase electricity from a utility in their area, which would then provide the electricity through
its own electricity delivery system.
- During the second half of the 20th century, utilities found it more efficient and economical to
connect their delivery systems, resulting in the need to coordinate power plant operations. The
majority of electricity generation in the today is coordinated by regional system operators to
ensure reliability. Therefore, the electricity delivered to consumers by their local electric utility
may be generated at a centralized power plant located in another city or state and owned by a
different company. These power plants are subject to economic, reliability, and environmental
regulations by state, and/or local governments.

Power generation became centralized as most of the
generating facilities are thermal and they use fossil fuels.
- Availability of fossil fuels is not uniform throughout the
world.
- Collecting/storing fossil fuels at many places is
inconvenient and not safe.
- A large centralized system is more economical than
many small decentralized thermal systems.

Conventional centralized grids operation:
• In order to keep frequency within a tight stable operating range generated power needs to be
balanced at all time with consumed power.
• A century working around adding electric energy storage by making the grid stiff by:
• Interconnecting many large power generation units (high inertia = mechanical energy
storage).
• Individual loads power ratings are much smaller than system’s capacity
• Conventional grid “stiffness” make them lack flexibility.
• Lack of flexibility is observed by difficulties in dealing with high penetration of renewable
energy sources (with a variable power output).
• Electric energy storage can be added to conventional grids but in order to make their effect
noticeable at a system level, the necessary energy storage level needs to be too high to make it
economically feasible.
•The grid user is a passive participant whether he/she likes it or not.
• The grid is old: it has the same 1880s structure. Power plants average age is > 30 years.

070412
• The Sendero Luminoso or “Shining Path” guerrillas of Peru attacked six
remote electrical transmission towers with explosives in 1993, felling towers
with the associated long wires and stopping the power flow
• A hacksaw (Arizona case) can be used instead of explosives; hard to defeat a
low tech approach
• In California, Path 15 is the overloaded high line connecting North and South
California that might fail if further overload occurs
• Cyberattack (hacking) of grid systems
http://en.wikipedia.org/wiki/Path_15
Vulnerability of Centralized System to Terrorist Attacks

http://www.eere.energy.gov/der/basics.html
030412
Distributed Generation
• Distributed generation occurs when power is generated (converted) locally and sometimes
might be shared with or sold to neighbors through the electrical grid (or over the fence)
• connected directly to the distribution system or installed at the customer's side of the meter,
non-dispatched by the network operators, not centrally planned, small-scale generators ≤ 10
MW (some experts consider the upper limit of DG to be 50 MW).
• Large central generation is not directly used
• The Public Service Commission may define only one supplier as a utility!
• Distributed generation avoids the losses that occur in transmission over long distances; energy
is used nearby
• Varying wind and sunshine averages across several houses, blocks, cities, or states, stabilizing
the system
• Variability of one source is reduced by dividing by the square root of the number of
sources
• Supply is robust, but automatic precautions are required to protect electricity workers when
main base-load power is out, and a local system might feed back into powerlines

Conventional Distributed
High power
capacity
Very high
>500MW
Limited
~1-10MW
Outage
replacement
No power Some capacity,
but limited
Power sharing One source Utility source plus
locals
Cost Inexpensive,
about $0.12/kWh
Premium, about
$0.15/kWh
Power line
servicing
Controlled
hazards
Possible DG
hazard
How DG Compares with the Centralized Grid

Scattered Users vs. “The Grid”
• Where users are scattered throughout a remote area, extension of the utility grid may be
too expensive
• $18,000 to $50,000 per mile of line depending on terrain; mountain slopes cause
more
• Often, these homes may use small gasoline or diesel generators, or perhaps hydro, wind,
or solar power
• Some utilities in US offer PV power with trailer systems that could be towed to a
residence to provide normal house power without utility lines
• Development in Africa may never require long lines since it’s cheaper to build local
plants than cut jungle
• It’s difficult to establish lines in sandy desert regions since the towers need deep footers.
070412

Distributed generation is what Thomas Edison envisioned with his DC generators: a power
station on every city block, making long-distance transmission and its associated losses
irrelevant. At the time, however, it seemed more feasible to go with Tesla’s AC generators and
long transmission lines. Now that we’ve had the AC grid for a century, we’re starting to see
its shortcomings and a return to the concept of microgrids and distributed energy
generation.

Two-User Grid Example
• Suppose Mr. Windy W. has a wind turbine and Ms. Sunny S. has a solar array
• Each source has a peak capacity of 1 kilowatt
• Sometimes the sky is cloudy a few days, and Sunny hasn’t enough battery capacity to
continue powering her inverter
• Windy gets a good output power on windy days, but sometimes it’s sunny and calm – no
wind
• They agree to combine power resources, using a tie-line between their inverters to share
power at 120 or 240Vac
• Within their battery capacities and wind/solar incoming energy, they extend the period
they can use power before protective shutdown
• The availability outweighs the line losses
070412

Why are Grids Changing?
Technological drivers
- Power Electronics (PE) becomes ubiquitous in loads, generators and grids
- More power produced (and stored) near consumers: Distributed Energy Resources (DER)
- Increased importance of Power Quality (PQ): more disturbances and more sensitive devices
- The digital wave: Digital control systems currently embedded in distributed power technologies enable
operators to remotely optimize operations and minimize costs. Both hardware and software have grown
more sophisticated to the point where distributed power systems can be controlled from a smartphone.
Moreover, the forthcoming marriage of the Internet and industrial machines promises to
transform isolated distributed power technologies into remotely operated and
synchronized fleets of virtual power plants with extended capabilities.
Socio-economic tendencies
- Liberalization of energy markets: free choice of supplier
- New environmental regulations
- More sustainable energy (renewable and ‘high quality’, ‘green’ power)
- Less guaranteed security of supply from grids
- Customer’s intention to have control over the supply
1.2 Distributed Generation: Drivers, Benefits and Issues

History
Traditional technology: the
electric grid:
• Generation, transmission, and
distribution.
• Centralized and passive
architecture.
• Extensive and very complex
system.
• Not reliable enough for some
applications.
• Vulnerable.
• Need to balance generation and
demand
Benefits of DG:
1. Reliability:
Storms, falling tree branches,
brownouts, and acts of terror all
threaten the grid, and when it fails,
it typically leaves tens of thousands
of customers (or millions in
extreme cases) without power for
long periods of time. During
disasters, a few individuals with
solar panels can provide emergency
power to their neighbors. A
distributed generation system with
microgrids can localize the impact
of these failures, reducing the
number of people affected.

2. Flexibility:
Big power plants - whether they’re based on fossil fuels, nuclear energy, or renewable
energy - are expensive to build and have very long payback periods. That means the
utilities are slower to adopt new technologies. If I just spent $40B on a natural gas
electrical generating plant, I’m not likely to abandon that and switch to another fuel or a
renewable source, even if the price of natural gas rises. On the other hand, if one builds
several smaller plants based on renewable sources, he can easily decommission them a
little at a time as he experiments with and adopts new technologies.
3. Economy of Scale:
One reason large power plants are expensive to build is that so few of them are built - it’s a
highly specialized market. If I build a lot of small power plants with whatever technology
is appropriate, the mass production effect will drive down the cost.
4. Diversity
Distributed generation allows me to use a variety of power generating technologies,
decreasing my dependence on any one resource. With stock portfolios, organizations, and
energy, there is strength in diversity.

5. Efficiency
The US Energy Information Administration reports that 7% of the electricity generated is lost
in transmission and distribution. Decrease the distance that it travels and you decrease the
amount that’s lost.
As the grid continues to deteriorate, energy demands keep rising, and corporations focus on
short-term profits, the need for distributed generation will increase. We’ll see smart microgrids
and small power plants - hopefully using renewable energy - appearing on our landscape.
Maybe Edison will get the last laugh after all... 

Issues of DG:
Load following: A load following power plant is a power plant that adjusts its power output as
demand for electricity fluctuates throughout the day. As DG, especially renewable ones are
affected by meteorological conditions, there is very little possibility of using them in load
following.
Stability: Voltage stability issues with DG especially in grid integrated cases are still not fully
understood.
Cost: IEA and many others claim that one of the major remaining issues is the relatively high
capital costs per kW installed power compared to large central plants. Moreover, differences in
capital costs between the different distributed generation technologies are also quite large.
Power Quality:
- Effect on frequency:- Large number of DG might affect the system frequency significantly
causing extra efforts and investments from the side of the utility. So a careful planning and
evaluation of DG is required.

- Voltage level:- According to Ackermann et al. (2001), the impact of distributed
generation connected to the distribution grid on the local voltage level can be significant.
A same reaction was noted through the CIRED (1999) questionnaire, where, next to the
general impact on power quality, a rise in the voltage level in radial distribution systems
was mentioned as one of the main technical connection issues of distributed generation.
The IEA (2002) also mentions voltage control as an issue when distributed generation is
connected to the distribution grid.
Connection Issues:
Change in power flow- Power can flow bidirectional within a certain voltage level, but it
usually flows unidirectional from higher to lower voltage levels, i.e. from the transmission to
the distribution grid. An increased share of distributed generation units may induce power
flows from the low-voltage into the medium-voltage grid. Thus, different protection schemes
at both voltage levels may be required (Dondi et al. (2002))

- Protection: Distributed generation flows can reduce the effectiveness of protection equipment.
Customers wanting to operate in ‘islanding’ mode during an outage must take into account important
technical (for instance the capability to provide their own ancillary services) and safety
considerations, such that no power is supplied to the grid during the time of the outage. Once the
distribution grid is back into operation, the distributed generation unit must be resynchronized with
the grid voltage.
- Reactive power: Small and medium sized distributed generation units mostly use asynchronous
generators that are not capable of providing reactive power. Several options are available to solve this
problem. On the other hand, DG units with a power electronic interface are sometimes capable to
deliver a certain amount of reactive power.
- Power Conditioning Some distributed generation technologies (PV, fuel cells) produce direct current.
Thus, these units must be connected to the grid via a DC-AC interface, which may contribute to
higher harmonics. Special technologies are also required for systems producing a variable frequency
AC voltage. Such power electronic interfaces have the disadvantage that they have virtually no
‘inertia’, which can be regarded as a small energy buffer capable to match fast changes in the power
balance. Similar problems arise with variable wind speed machines.

1.3 Integration of DG in Power Network
Why is integration needed? Why not use in stand alone mode?
In spite of several advantages provided by conventional power systems, the following technical,
economic and environmental benefits have led to gradual development and integration of DG
systems:
(1) Due to rapid load growth, the need for augmentation of conventional generation brings
about a continuous depletion of fossil fuel reserve. Therefore, most of the countries are looking
for non-conventional/renewable energy resources as an alternative.
(2) Reduction of environmental pollution and global warming acts as a key factor in preferring
renewable resources over fossil fuels. As part of the Kyoto Protocol, the EU, the UK and many
other countries are planning to cut down greenhouse gas (carbon and nitrogenous by-products)
emissions in order to counter climate change and global warming. Therefore, they are working
on new energy generation and utilization policies to support proper utilization of these energy
sources. It is expected that exploitation of DERs would help to generate ecofriendly clean
power with much lesser environmental impact.

(3) DG provides better scope for setting up co-generation, trigeneration or CHP plants for
utilizing the waste heat for industrial/domestic/commercial applications. This increases the
overall energy efficiency of the plant and also reduces thermal pollution of the environment.
(4) Due to lower energy density and dependence on geographical conditions of a region,
DERs are generally modular units of small capacity.These are geographically widespread and
usually located close to loads. This is required for technical and economic viability of the
plants. Forexample, CHP plants must be placed very close to their heat loads, as transporting
waste heat over long distances is not economical. This makes it easier to find sites for them
and helps to lower construction time and capital investment. Physical proximity of load and
source also reduces the transmission and distribution (T&D) losses. Since power is generated
at low voltage (LV), it is possible to connect a DER separately to the utility distribution
network or they may be interconnected in the form of Microgrids. The Microgrid can again be
connected to the utility as a separate semi-autonomous entity.

(5) Stand-alone and grid-connected operations of DERs help in generation
augmentation, thereby improving overall power quality and reliability. Moreover, a
deregulated environment and open access to the distribution network also provide
greater opportunities for DG integration. In some countries, the fuel diversity offered
by DG is considered valuable, while in some developing countries, the shortage of
power is so acute that any form of generation is encouraged to meet the load demand.
‘Interconnection’ or ‘Integration’?
Recent utility opposition to DG- insecurity feeling from DG as DG is subsidized

1.4 Impact of DG
Impact on distribution system:
- Voltage profile changes along the network, depending on how much power is produced and
consumed at that system level
- Voltage transients may appear as result of connection and disconnection of different DG
sources
- Short circuit level may increase
- New problems created by bi-directional flow of power
- Coordination problems between utility protection and DG protection schemes
- Effect on phase balance if large part of the DG is single phase and not uniformly distributed
- Power quality issues, harmonics

Impact on Transmission System:
- The impact of DG on the power system transient stability (when connected in small
amounts) will be negligible. When the penetration of DG increases, its impact is no
longer restricted to the distribution network but begins to influence the whole system.
- Large DG penetration is relatively recent phenomena. Researches are being
conducted around the world to evaluate the effect of DG in transmission system and
also to find the largest DG penetration which is safe to the grid.
Impact on Central Generation
Impact on policy side

1.5 Microgrids and Active Distribution Network
Passive and Active Distribution Networks
Passive distribution networks
• Designed to accept bulk power from transmission system and distribute to customers
• Real time control problem resolved at planning stage
• Unidirectional Power flow
• Passive customer, no exchange of information between the consumer and the market
players

Active Distribution Networks:
Any passive distribution system becomes active when DG units are added to the distribution
system leading to bidirectional power flows in the networks.
Active network management allows the planning and operationalization of new power
distribution grids with:
· more dynamic and complex structures,
· pervasive integration of DG,
· new energy source with dual load-generator behaviour,
· DESS, new equipment and services, such as Intelligent Electronic Devices (IEDs) and Smart
Meters,
· demand side management and the information exchange between customers and market
players.

Issues for Active Distribution Networks
• Need to understand and demonstrate the value of flexibility and controllability
- Technical benefits (security, reliability)
- Economic benefits (cost savings, competitiveness)
• Need to quantify the benefits of:
- Enhanced security
- Displaced central generation capacity
- Reduced network investment
- Reduced generation operating costs
- Reduced outage costs
- Increased competitiveness of DER
• Need to explore alternative network control approaches
- Control of network topology (switching technology)
- Coordination of operation of network control facilities
- Coordinated (but decentralized) control of DER

Interconnection of small, modular generation to low voltage distribution systems forms a new
type of power system, the Microgrid. Microgrids can be connected to the main power network
or be operated islanded, in a coordinated, controlled way.
A formal definition from the Conseil international des grands réseaux électriques or (CIGRÉ)
states:
Microgrids are electricity distribution systems containing loads and distributed energy
resources, (such as distributed generators, storage devices, or controllable loads) that can
be operated in a controlled, coordinated way either while connected to the main power network
or while islanded.
- The primary purpose is to ensure local, reliable, and affordable energy security for urban
and rural communities, while also providing solutions for commercial, industrial, and
federal government consumers. Benefits that extend to utilities and the community at large
include lowering greenhouse gas (GHG) emissions and lowering stress on the transmission
and distribution system.
Microgrids

- Any small-scale localized station with its own power resources, generation and
loads and definable boundaries qualifies as a microgrid.
- Microgrids can be intended as back-up power or to bolster the main power grid
during periods of heavy demand.
- Often, microgrids involve multiple energy sources as a way of
incorporating renewable power.
The key differences between a Microgrid and a conventional power plant are as follows:
(1)Microsources are of much smaller capacity with respect to the large generators in
conventional power plants.
(2)Power generated at distribution voltage can be directly fed to the utility distribution
network.
(3)Microsources are normally installed close to the customers’ premises so that the
electrical/heat loads can be efficiently supplied with satisfactory voltage and frequency
profile and negligible line losses.

Two major types of microgrids,
i) those wholly on one site, owned fully by a customer- customer microgrids or ‘true
microgrids
ii) ones that involve a segment of the legacy regulated grid, which are also called milligrids
or minigrids
Microgrid Benefits
- Provides power quality, reliability, and security for end users and operators of the grid
- Enhances the integration of distributed and renewable energy sources
- Cost competitive and efficient
- Enables smart grid technology integration
- Locally controlled power quality
- Minimize carbon footprint and green house gas emissions by maximizing clean local energy
generation
- Increased customer (end-use) participation

Some Microgrid Configurations:
Conceptual ac/dc microgrid architecture with single-point grid interface
(Source: Siemens Corporate Research)

A DC Microgrid
(Source: ‘Electrical Construction and Maintenance’ magazine

The hybrid PV/wind turbine/battery microgrid in Tsinghua University, China

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