Mg Probable Question Solution.docx

MG PROBABLE QUESTION SOLUTION. 1. What is a microgrid? Explain about typical structure and configuration of a micro grid with neat diagram. 

A microgrid is a small-scale power grid that can operate independently or collaboratively with other small power grids. The practice of using microgrids is known as distributed, dispersed, decentralized, district or embedded energy production.

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Any small-scale, localized power station that has its own generation and storage resources and definable boundaries can be considered a microgrid. If the microgrid can be integrated with the area's main power grid, it is often referred to as a hybrid microgrid.

Fig .Structure and configuration of a micro grid

BASIC COMPONENTS OF MG:

Local generation. 

A microgrid presents various types of generation sources that feed electricity, heating, and cooling to user. These sources are divided into two major groups – thermal energy sources (e.g,. natural gas or biogas generators or micro combined heat and power) and renewable generation sources (e.g. wind turbines, solar).

Consumption 

In a microgrid, consumption simply refers to elements that consume electricity, heat, and cooling which range from single devices to lighting, heating system of buildings, commercial centers, etc. In the case of controllable loads, the electricity consumption can be modified in demand of the network.

Energy Storage. 

In microgrid, energy storage is able to perform multiple functions, such as ensuring power quality, including frequency and voltage regulation, smoothing the output of renewable energy sources, providing backup power for the system and playing crucial role in cost optimization. It includes all of electrical, pressure, gravitational, flywheel, and heat storage technologies.

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When multiple energy storages with various capacities are available in a micro grid, it is preferred to coordinate their charging and discharging such that a smaller energy storage does not discharge faster than those with larger capacities. Likewise, it is preferred a smaller one does not get fully charged before those with larger capacities. This can be achieved under a coordinated control of energy storages based on their state of charge.

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If multiple energy storage systems (possibly working on different technologies) are used and they are controlled by a unique supervising unit (an Energy Management System - EMS), a hierarchical control based on a master/slaves architecture can ensure best operations, particularly in the islanded mode.

Point of common coupling (PCC) 

It is the point in the electric circuit where a microgrid is connected to a main grid.Microgrids that do not have a PCC are called isolated microgrids which are usually presented in the cae of remote sites (e.g., remote communities or remote industrial sites) where an interconnection with the main grid is not feasible due to either technical or economic constraints.

3. Explain about different types of micro grid. ANS: There are five types of microgrid_

1. Campus Environment/Institutional Microgrids 

The focus of campus microgrids is aggregating existing on-site generation with multiple loads located in tight geography in which owner easily manage them.

2. Community Microgrids 

Community Microgrids can serve a few up to thousands of customers and support the penetration of local energy (electricity, heating, and cooling). In a community microgrid, some houses may have some renewable sources that can supply their demand as well as that of their neighbors within the same community.

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The community microgrid may also have a centralized or several distributed energy storages. Such microgrids can be in the form of an ac and dc microgrid coupled together through a bi-directional power electronic converter.[9]

3. Remote Off-grid Microgrids 

These microgrids never connect to the Macrogrid and instead operate in an island mode at all times because of economic issues or geographical position. Typically, an "off-grid" microgrid is built in areas that are far distant from any transmission and distribution infrastructure and, therefore, have no connection to the utility grid.

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Studies have demonstrated that operating a remote area or islands' off-grid microgrids that are dominated by renewable sources will reduce the levelized cost of electricity production over the life of such microgrid projects.

4. Military Base Microgrids: •

These microgrids are being actively deployed with focus on both physical and cyber security for military facilities in order to assure reliable power without relying on the Macrogrid.

5. Commercial and Industrial (C&I) Microgrids: 

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These types of microgrids are maturing quickly in North America and Asia Pacific; however, the lack of well –known standards for these types of microgrids limits them globally. Main reasons for the installation of an industrial microgrid are power supply security and its reliability. There are many manufacturing processes in which an interruption of the power supply may cause high revenue losses and long start-up time.

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Industrial microgrids can be designed to supply circular-economy (near-)zero-emission industrial processes, and can integrate combined heat and power (CHP) generation, being fed by both renewable sources and waste processing; energy storage can be additionally used to optimize the operations of these sub-systems.

4. State the significance of micro grid. Ans: 

Bidirectional power flows: The presence of distributed generation (DG) units in the network at low voltage levels can cause reverse power flows that may lead to complications in protection coordination, undesirable power flow patterns, fault current distribution, and voltage control.

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Stability issues: Interaction of control system of DG units may create local oscillations, requiring a thorough small-disturbance stability analysis. Moreover, transition activities between the gridconnected and islanding (stand-alone) modes of operation in a microgrid can create transient stability. Recent studies have shown that direct-current (DC) microgrid interface can result in significantly simpler control structure, more energy efficient distribution and higher current carrying capacity for the same line ratings.

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Modeling: Many characteristic in traditional scheme such as prevalence of three-phase balanced conditions, primarily inductive transmission lines, and constant-power loads are not necessarily hold valid for microgrids, and consequently models need to be revised.

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Low inertia: The microgrid shows low-inertia characteristic that are different to bulk power systems where high number of synchronous generators ensures a relatively large inertia. Especially if there is a significant share of power electronic-interfaced DG units, this phenomenon is more clear.

5. Write a Short notes on different sources of micro grid. Ans: A microgrid presents various types of generation sources that feed electricity, heating, and cooling to user. These sources are divided into two major groups – thermal energy sources (e.g,. natural gas or biogas generators or micro combined heat and power) and renewable generation sources (e.g. wind turbines, solar)

N.B :( Write some point regarding biogas, solar PV, wind ,DG)

6.With neat diagram explain about hybrid types of micro grid. ANS:

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AC micro grids (ACMG) have been proposed to utilize the existing AC grid technologies, protection and standards. However, power generation from various DGs such as photovoltaic arrays and fuel cells is DC power, which is to be converted into AC power through power electronic interface i.e. DC/AC inverter for connecting them with the AC utility grid. This AC power is again converted back into DC power required by today’s electrical loads such as Uninterrupted Power Supply (UPS), fluorescent lights, variable motor drives and hybrid electric vehicles. Thus, an individual AC micro grid is less efficient due to more power losses are occurring in multiple conversions. Besides this, synchronization, stability, and reactive power requirement are its inherent demerits. DC micro grids (DCMG) , are emerging as a better alternative due to the above mentioned reasons for renewable energy based DGs. However, power generation from the sources such as diesel

generator, small hydro turbine with synchronous generator and photovoltaic panels etc. as well as the electrical loads are a mix of AC and DC power. Thus, an individual DCMG cannot completely eliminate the losses occurring in multiple stage conversions, though the losses occurring in DC/DC conversions are lesser than those occurring in AC/DC or DC/AC conversions.

7. Draw a configuration for power electronics based micro grid. ANS:

8. Write a Short notes on Protection and co-ordination of micro grid. ANS: 

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Micro-grid protection techniques can be classified into local and communication based approaches. Micro-grid protection schemes that do not require or rely on communication have been proposed in. In, a protection scheme for micro-grids which depends primarily on undervoltage relays for islanded operation and overcurrent relays for grid connected operation is proposed. As stated in, one of the salient features of the proposed method is that it does not require communication or adaptive protective devices; however, the proposed method is applicable to low voltage small scale micro-grids.

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A new digital protection relay that utilizes the wavelet packet transform is proposed to classify and detect fault events during islanded and grid connected operation. The proposed approach is realized without the need of adaptive features or communications. However, the protective devices are located at the Distributed Generation side and thus no micro-grid feeder protection and time coordination is considered. Due to the difference in fault magnitudes for grid connected and islanded operation, fault current limiters (FCLs) are positioned and directional overcurrent relays are optimally coordinated considering both micro-grid modes of operation. By using FCLs, the infeasibility of conventional directional overcurrent relays to provide proper protection coordination was overcome; however, some relays experienced high operating times. Micro-grid protection schemes that rely on communication have been proposed in .In a protection scheme that depends on digital relays where its primary protection scheme relies on a communication network is proposed for a micro-grid with grid connected and islanded capability.