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.

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

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 Microturbines Inverter or Induction Generator Reciprocating Engines Synchronous or Induction Generator Small Hydro Solar Photo Voltaic Super Conducting Magnet Ultracapacitor Wind Turbine

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

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 = 1 pu corresponds to the rated current. As ZTH reduces with fault, fault current increases. Typical fault levels in distribution networks: 10 - 15 pu.

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

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