Sukumar Mishra

Micro-grid fundamentals and its use to CGL

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 LEDLights 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

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

DG SYSTEMS CAN ADDRESS …

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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DISTRIBUTED GENERATION

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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

CONVENTIONAL VS RENEWABLE GEN.

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

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FUTURE POWER SYSTEM

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

Frequency deviation from the nominal value represents mismatch

between the active power (MW) generation and consumption.

THE NEED FOR ENERGY BALANCE

Voltage deviation from the nominal value represents mismatch

between the reactive power (MVAR) generation and consumption.

Freq. control Active power control

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..?

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.

POWER SYSTEM CHALLENGES

SYSTEM FREQUENCY REGULATION

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 related to type of Generator

ISSUES WITH DISTRIBUTED GENERATION

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.

ISSUES WITH PF CORRECTION CAPACITORS

Self Excitation

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.

MVAR CAPABILITY OF IND. MACHINES

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 controlDisadv: Converts poses many challenges to the system protection

Issues related to Power Electronics

ISSUES WITH DISTRIBUTED GENERATION

An electrical system can be considered as voltage source (V) behind

impedance, ZTH.

With V = 1 pu, the fault level or fault current

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.

FLTH

1i

Z

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.

ISSUES WITH SYSTEM PROTECTION

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.

REDUCTION IN SYSTEM DROOP

POWER QUALITY ISSUES WITH WTG

Thank you