Improving The Transient Stability Of An Islanded Microgrid

Running head: IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 1

Improving the Transient Stability of an Islanded Microgrid

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IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 2

Energy systems function in different ways based on the source of energy. Microgrids are

the most commonly used power grids. They are local energy systems that consist of various

sources of energy. In most cases, the microgrids are connected to the main power grids. When

they are isolated from the primary power grids, the microgrids are prone to instabilities. Many

factors cause instabilities in the microgrids. One of the causes of the instability is frequent on and

off switching of the power. Power surges also cause the instability. When several power surges

occur, they disrupt the flow of energy in the microgrid. The parallel operation of machines in a

power system is also likely to cause instability (Natesan et al. 2014).

Power systems have various synchronous generators, which might malfunction due to the

inconsistent flow of power. Massive one-time power surges can also cause instability in the

simultaneous grids. In most cases, this makes it difficult for the microgrid to return to full

functioning (Natesan et al. 2014). It is essential to ensure that the stability of the microgrid is

maintained. Different methods are used to sustain the transient stability of microgrids. These

methods vary depending on the nature of voltage that is used in the system. They also rely on the

kind of the microgrid system. The microgrid systems are mostly used to supply power to the

remote areas (Natesan et al. 2014).

The Q – V droop controller is one of the recent methods that are used to enhance

transient stability. The concept of the Q – V droop controller was developed due to the

advancements that have occurred in modern power systems. Current power generating facilities

have transformed regarding the arrangement of the power generators. Previous power generators

were synchronous. In these scenarios, the power was generated from a central point using a

single source (Usunariz et al. 2015). However, modern power generators have adopted the use of

sources with smaller capacity. The sources result in the distributed generation of power. The

IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 3

microgrids are usually connected to the main power grid. Research indicates that these

microgrids have a higher propensity to lose transient stability when they are isolated from the

main power grid. The instability is mostly caused by the availability of low kinetic energy. These

small decentralized systems have a smaller short – circuit to current ratio compared to the

centralized synchronous generators (Babu et al. 2016).

During the connection of the microgrids to the main power grids, they are mostly

installed with inverters at the end. Novel Q – V dot droop controllers are an effective method of

improving the transient stability of the microgrids. The dot droop controllers are used on the

inverters to enhance the response of the microgrid to frequency (Usunariz et al. 2015). This is

mostly applicable when the microgrid experiences disturbances that involve the deviations of

high frequencies. The incorporation of the Q – V dot droop controllers requires simulation to

ensure that the controllers function efficiently as expected. The simulation is usually conducted

using the MATLAB software to test the efficiency of the control mechanism. During the process,

the functioning of the controller is tested on both the inverters and the synchronous generators

(Babu et al. 2016).

The use of fuzzy local controllers is another method of maintaining the transient stability

of the microgrids. Most microgrids are installed with various types of storage devices. The

primary functions of the storage devices are to secure the power quality of the microgrids. They

are also meant to regulate the flow of power in the microgrid system. Low voltage microgrids are

the most known to experience transient instabilities. This usually takes place during the transition

to the islanded mode if the grid was initially connected. However, the inertia of the existing

sources of power in the microgrid can be used to improve the stability of the microgrid. Wind

IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 4

turbines are the best cases where this approach can be used to improve the transient stability of

the microgrid (Papadimitriou & Vovos, 2010).

The fuzzy logic is used for the controllers in cases where the system does not exhibit

stochastic or nonlinear behavior. The fuzzy local controllers mostly work on doubly fed

induction generators. The main aim of the controllers is to decrease the storage devices to an

appropriate number. This also helps to reduce the setbacks that are involved with having such

many storage devices. Apparently, there are different types of controllers that can be used

depending on the architecture of the microgrid system. In other cases, different types of

controllers can be combined during the transition period. The controllers help to ensure that the

system is working in a steady state. Apparently, the developments in technology have resulted in

the changes like the systems. In some cases, hybrid cases of microgrid systems might require a

different combination of the fuzzy controllers compared to the standard system (Papadimitriou &

Vovos, 2010).

The self – excited induction generator is one of the most commonly used generators in

the microgrid systems. Apparently, compromises in transient stability are also typical in these

generators. Voltage is the primary cause of the instabilities in the self – excited induction

generators. Both unregulated high voltages and unreliably low voltages can cause transient

instability in this generator. In rare cases, offset voltage is also responsible for the lack of

stability (Chakraborty et al. 2012). The static converter (STATCOM) is commonly used to

improve the transient stability of the self – excited induction generators when they are in an

islanded state. STATCOM performs this function by acting as a voltage regulator. This implies

that different devices can be used as a STATCOM. The current controlled voltage source

inverter (CC – VSI) is commonly used as the STATCOM. It is most preferred due to its ability to

IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 5

produce a rapid response that maintains a constant voltage in the self – excited induction

generators (Murthy & Gupta, 2003).

The fact that the performance of the CC – VSI can be calculated using a mathematical

formula makes it more efficient. Voltage build up makes it necessary to analyze the transient

function of the microgrid system that involves the combination of the self – excited induction

generator and the STATCOM. The combined system works by applying and removing loads that

are either resistive, reactive, balanced, or unbalanced (Chakraborty et al. 2012). The main

function of the STATCOM as the controller compensates the loads that are not balanced. On the

other hand, it maintains the balance of the generating system. This helps to maintain a stable

alternating current voltage, which prevents the compromise of transient stability. A mathematical

approach can be used to indicate the model of the STATCOM and how it is used to enhance

stability (Murthy & Gupta, 2003).

The distributed generations is also a common form of the microgrid. In this system,

inverters are used to control the stability of the current. Apparently, the inverters are used in

series and shunt. The converters used in the series arrangement is used to balance the current in

the line while the one arranged in the shunt is used to stabilize voltage. This helps to prevent

short-circuits that constitute a significant cause of transient instability. The battery energy

storage system is the most effective method that can be used in order to avoid transient instability

that is caused by the poor balancing of the loads (Jain et al. 2012).

The battery energy storage system operates in both capacitive and inductive modes to

form the most reliable condenser. Its primary function is the balancing of loads in the microgrid

system. It also levels the loads at different points in the system. It is also involved in the

elimination of harmonics from loads that are either linear, nonlinear, or dynamic. This controller

IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 6

is effective even if a micro source of the microgrid system is removed. As long as the battery is

connected in parallel, the controllers share the load of the microsystem that was removed (Jain et

al. 2012). This helps to maintain the frequency and the voltage of the whole microgrid system

stable. However, the method also has various challenges. It comes with increased costs due to the

high costs of the controller. In most cases, it is also essential to maintain the appropriate energy

storage system of the battery used. This can also increase the size of the batteries used. However,

research indicates that the benefits of this method of improving transient stability outweigh all its

drawbacks (Jain et al. 2012).

IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 7

References

Babu, K.C., Babu, P.S. & Hemakeshavulu, O. (2016). Improvement of Transient Stability in

Micro-grid using Q-V Dot Droop Controller. International Journal of Advanced

Technology and Innovative Research, Vol.08, Issue.06, Pages: 1108-1113

Chakraborty, A., Musunuri, S.K., Srivastava, A.K. and Kondabathini, A.K. (2012). “Integrating

STATCOM and Battery Energy Storage System for Power System Transient Stability: A

Review and Application,” Advances in Power Electronics, vol. 2012, Article ID 676010,

12 pages. doi:10.1155/2012/676010

Jain, M, Gupta, S., Masand, D. & Agnihotri, G. (2012). Analysis of a Microgrid under Transient

ConditionsUsing Voltage and Frequency Controller. Advances in Power Electronics,

Volume 2012, Article ID 208231, 18 pages, doi:10.1155/2012/208231

Natesan, C., Ajithan, S. K., Mani, S., Palani, P., & Kandhasamy, P. (2014). Performance

Analysis of Autonomous Microgrid Subsequent to Symmetrical and Unsymmetrical Fault

Triggered Condition. The Scientific World Journal, 2014, 715963.

http://doi.org/10.1155/2014/715963

Papadimitriou, C.N. & Vovos, N.A. (2010). Transient Response Improvement of Microgrids

Exploiting the Inertia of a Doubly-Fed Induction Generator (DFIG). Energies, 3, 1049-

1066; doi: 10.3390/en30601049

Singh, B., Murthy, S.S. & Gupta, S. (2003). Modelling and Analysis of STATCOM Based

Voltage Regulator for Self – Induction Generator with Unbalanced Loads. New Delhi:

Indian Institute of Technology

IMPROVING THE TRANSIENT STABILITY OF A MICROGRID 8

Usunariz, I., Santamaria, M., Mentesidi, K., and Aguado, M. (2015). “A Modified Control

Scheme of Droop-Based Converters for Power Stability Analysis in Microgrids,” Journal

of Solar Energy, vol. 2015, Article ID 393527, 10 pages. doi:10.1155/2015/393527

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