Transcript Dr. Sukumar Mishra - Prof IIT Delhi

Micro-grid fundamentals and its
use to CGL
Sukumar Mishra
IIT Delhi
THE POWER BALANCE
Frequency deviation from the nominal value represents mismatch
between the active power (MW) generation and consumption.
How the instantaneous power is balanced
The Power Balance issue in India
• Generation-Load mismatch large
• To fill the gap IPPs are encouraged
• Low investment high return may attract many
people to participate in power generation
• Low power capacity and hence connected at
low voltage
• Encourages renewable energy penetration
• Roof top PV will be the future
A Typical Micro-grid Formation
• A microgrid is a localized grouping of
electricity generation, energy storage, and
loads that normally operates connected to a
traditional centralized grid.
• It can function autonomously when the single
Point of Common Coupling (PCC) with the
micro-grid is disconnected.
• Voltage level of generation and load is usually
low.
Advantages
• Microgrid generation resources can include fuel cells,
wind, solar, or other sustainable energy sources.
• Multiple dispersed generation sources and ability to
isolate the microgrid from a larger network provides
highly reliable electric power.
• Byproduct heat from generation sources such as micro
turbines could be used for local process heating or
space heating, allowing flexible trade off between the
needs for heat and electric power.
• Generate power locally to reduce dependence on long
distance transmission lines and cut transmission losses.
• In peak loads, it prevents utility grid failure by
reducing the load on the grid.
• Significant environmental benefits made
possible by the use of low or zero emission
generators.
• The use of both electricity and heat permitted
by the close proximity of the generator to the
user, thereby increasing the overall efficiency.
• Reduces electricity cost to the user by
generating some or all of its electricity needs.
Disadvantages
• Voltage, frequency and power quality are the three
main parameters that must be considered and
controlled to acceptable standards whilst the power
and energy balance is maintained.
• Electrical energy needs to be stored in battery banks or
as mechanical energy in flywheels thus requiring more
space and maintenance.
• Resynchronisation with the utility grid need to be
made carefully.
• Microgrid protection is one of the most important
challenges facing the implementation of microgrids
• Issues such as standby charges and net
metering may pose obstacles for the
microgrid.
• Interconnection standards needs to be
developed to ensure consistency.
Mera Gao Microgrid in Bihar- 100 houses powered by solar to light two LED
Lights and a mobile charging point.
Five-kW proton exchange membrane
fuel cells at the Microgrid Power Pavilion
in Next Energy Centre, Detroit. The four
fuel cells are used for periodic power
generation.
Village Microgrid- Pamelo, Indonesia- 24kWp PV, 20kWhr battery, 125KVA
genset
Columbia University’s Earth Institute project called Shared Solar in Mali
DG SYSTEMS CAN ADDRESS …
 High peak load shortages
 High transmission and distribution losses
 Remote and inaccessible areas
 Rural electrification (Rajiv Gandhi Rural Electrification
Scheme)
 Faster response to new power demands
 Improved supply reliability and power quality
 Possibility of better energy and load management
 Optimal use of the existing grid assets
DISTRIBUTED GENERATION
DISTRIBUTED GENERATION
 Objective of DG is to promote energy independence and
development of renewable, energy-efficient and low-emissions
technologies.
 DG uses small generators (<10 MW), which are distributed
throughout the power system closer to the loads.
 Generators larger than 10MW are typically interconnected by
transmission lines, forms part of the regular power system.
 More power quality issues because of multiple sources.
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CONVENTIONAL VS RENEWABLE GEN.
Fossil Power Generation
Renewable Power Generation
Concentrate Generation
Distributed Generation
High capacity factor
Low capacity factors
Proven Technology
Still under R&D
Fuel storage relatively inexpensive
All resources cannot be stored
Fuel supplies can be interrupted
Fuel supply weather dependent
Continuous operation
Intermittent operation
Fuel Transportation required
Fuel local available
Severe Pollution
Very less Pollution
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TYPES OF DISTRIBUTED RESOURCES
Distributed resources (DRs) that can be connected to the power grid
can be grouped as:
1. Electronically interfaced generators
2. Rotating machine interfaced generators
 Electronic interfaced DRs are inverter-based units.
 Rotating machine interfaced DRs are induction or synchronous
generator based units.
TYPICAL INTERFACING OF DRs
Distributed Resource
Type of Interface
Flywheel
Inverter
Fuel Cell
Inverter
Microturbines
Inverter or Induction Generator
Reciprocating Engines
Synchronous or Induction Generator
Small Hydro
Synchronous or Induction Generator
Solar Photo Voltaic
Inverter
Super Conducting Magnet
Inverter
Ultracapacitor
Inverter
Wind Turbine
Inverter or Induction Generator
ISSUES OF DISTRIBUTED GENERATION
The interconnection and operation of DG with the grid is very
complex as compared to the traditional system.
Some of the major issues are:
 Operation and control of the DG resources
 Interfacing with the network
 Protection
FUTURE POWER SYSTEM
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DISTRIBUTED GENERATION
Operating Modes
• Grid Connected Mode
– Utility grid is active
– Static switch is closed
– All the feeders are
supplied by the grid
– P-Q Control
• Islanded mode
– Utility grid is not supplying power
– Static switch is open
– Feeder A,B,C are supplied by microsources
– Feeder D is dead.
– V-f control mode
Control in Microgrid
• The controls of a microgrid have to ensure
that
– New microsources can be added to the microgrid
without modification of existing equipment.
– The microgrid can connect to or isolate itself from
the macrogrid in a rapid and seamless fashion.
– Reactive and active power can be independently
controlled.
– The dynamic demands of the load can be met.
• Voltage vs. Reactive Power Droop
– Basic unity power factor controls are required
– Voltage regulation is necessary for local reliability
and stability.
– Without local voltage control, systems with high
penetrations of microsources could experience
voltage and/or reactive power oscillations.
– Should also ensure that there are no large
circulating reactive currents between sources.
• Power vs. Frequency Droop
– Microgrids can provide premium power functions
using control techniques where the microgrid can
island smoothly and automatically reconnect to
the bulk power system.
– In island mode, problems from slight errors in
frequency generation at each inverter and the
need to change power-operating points to match
load changes need be addressed.
– Thus the controller has to maintain the sharing of
power between sources optimally.
Conventional Grid vs. Microgrid
• Efficiency of conventional grid is low
compared to microgrid.
• Large amount of energy in the form of heat is
wasted in conventional grid.
• Power sources in the microgrid are small and
located close to the load.
TECHNICAL ISSUES
 Integration with the existing utility network
 Role of power electronics
 Impact on power quality
 Impact on reliability
 Impact on environment
 DR modeling for improved stability of operation
THE NEED FOR ENERGY BALANCE
Frequency deviation from the nominal value represents mismatch
between the active power (MW) generation and consumption.
Freq. control  Active power control
Voltage deviation from the nominal value represents mismatch
between the reactive power (MVAR) generation and consumption.
Voltage control  Reactive power control
ACTIVE + REACTIVE POWER GENERATION
 Turbine provides the active or real power (P).
 Exciter provides the reactive or imaginary power (Q) .
THE BIG QUESTION
 In the classical energy conversion method using synch. generator,
both real and reactive power can be independently regulated.
 All these resources are controllable.
Can it be possible to get the same kind of controllability from
distributed resources as that of conventional synch. generator..?
POWER SYSTEM CHALLENGES
 Rate of installation of new thermal units is reducing.
 Ageing thermal units are getting decommissioned due to
pollution issues.
 Big units with conventional synchronous generators provide
system frequency support.
 The reduction in generation capacity of these units will
adversely affect the system frequency control capability.
SYSTEM FREQUENCY REGULATION
ISSUES WITH DISTRIBUTED GENERATION
Issues related to type of Generator
 Most of the distributed generation technologies use Asynchronous
(induction) Generators.
 Induction machines derive the excitation from the network and
behave as reactive loads even if they generate active power.
 Hence, voltage control is very difficult with these machines.
 Also because of low power-factor of induction machines, the fault
current level is normally increased.
ISSUES WITH PF CORRECTION CAPACITORS
Self Excitation
 For controlling the power-factor of an induction generator,
capacitors are usually connected at the terminals.
 With these capacitors, all the VAR needs of the IG can be met locally.
 If connection of the ind. generator to the grid is lost, then the ind.
generator will continue to develop a voltage.
 This may develop large distorted voltages as the IG accelerates.
 This phenomenon of 'self excitation' can damage the equipments
connected to the isolated part of a network.
 This may be avoided by limiting the size of pf correction capacitor.
MVAR CAPABILITY OF IND. MACHINES
 The reactive power generation and absorption capability of the
DFIG reduces with the active power flow.
 In order to maintain bus voltage at the point of connection,
reactive power compensation devices such as STATCOM are
needed.
ISSUES WITH DISTRIBUTED GENERATION
Issues related to Power Electronics
Whenever electricity is generated at frequency other than nominal
value, Power Electronic devices are used for utility connection.

Wind Turbines

Micro-Turbines

Solar PV

Fuel Cell
Adv : Converter controllers can be used for functions like VAR control
Disadv: Converts poses many challenges to the system protection
ISSUES WITH SYSTEM PROTECTION
An electrical system can be considered as voltage source (V) behind
impedance, ZTH.
With V = 1 pu, the fault level or fault current iFL 
iFL = 1 pu corresponds to the rated current.
As ZTH reduces with fault, fault current increases.
Typical fault levels in distribution networks: 10 - 15 pu.
1
Z TH
ISSUES WITH SYSTEM PROTECTION
 Fault current will be much higher than the nominal current. This
is the basic precondition for the working of overcurrent relay.
 Fault current has to be distinguishable from normal current.
 This needs a powerful source capable of providing a high fault
current until the relay operates.
 Power electronic converters in the generator output prevent
high currents, even if there is fault.
 Thus the fault is not detected using the over-current protection.
ISSUES WITH SYSTEM PROTECTION
 For a fault at A, fault current If = IGrid + IDG
 Relay R will only measure the current coming from the grid, Igrid.
 Means, the relay detects only a part of the real fault current and
may therefore not trigger properly.
 With a fault at B-2, fault current from DG passes the relay in reverse
direction, which can cause problems if directional relays are used.
REDUCTION IN SYSTEM DROOP
 Increasing penetration of renewable in the electrical system
increases the equivalent droop of the system.
 For a 20% renewable penetration, the conventional generating
capacity will reduce to 1-0.2=0.8 pu.
 The effective droop of the system increases to R/0.8 = 1.25R,
where R is the initial value of permanent droop.
POWER QUALITY ISSUES WITH WTG
Thank you