Improvement of transient stability in microgrids using RSFCL with series active power filter
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Now a day’s non-conventional energy sources like PV power and Wind power sources are well developed because conventional energy sources are reduced day by day and at the same time the population has been increased. These drawbacks can be overcome by developing the microgrids. In current trends, microgrid protection is a very challenging task. Due to the interconnection of several distributed generators, fault currents are produced. Resistive type superconducting fault current limiter is used for the protection of microgrids because its operating time is very less, it stops the abnormal currents with in the first cycle and continuity of supply is possible. But R-SFCL had some disadvantages, it is not compensated for the reactive power. Then voltage levels are not compensated, for that reason series active power filters are used along with R-SFCL. Here series active power filters are compensating the voltage magnitudes and reduce the harmonics and then transient stability is improved in the microgrids. This work is done using MATLAB/Simulink.
KeywordsResistive type superconducting fault current limiter Matlab Series active power filter Circuit breaker Distributed generation Phase-locked loop circuits
Series active power filter
Phase-locked loop circuits
Resistive type superconducting fault current limiter
Nowadays the population has been increased and at the same time a bulk amount of electrical energy is required. But our conventional energy sources are limited, and they reduce day by day. For this reason, people look for non-conventional energy sources and in general solar and wind sources are majorly developed and fuel cell development is in the final stage. This secondary energy source having more advantages is free of cost, free from pollution and no need for input fuel. Here these secondary energy sources having a disadvantage like a continuity of supply becomes impossible, so these plants are called peak load plants. For this reason, integrate the distributed generation with the main grid by using a phase-locked loop circuit (PLL) . Here synchronization process is very difficult and at the same time due to the interconnection of the number of DGs, the short circuit currents will arise.
Up to now, circuit breakers are used for the protection of fault currents but at the time of fault condition, circuit breakers allow up to three cycles of fault currents due to which fault currents system face some damage . At the time of the fault, breakers are opened with the help of overcurrent relay, so that the continuity of supply is not possible. These disadvantages can be overcome by using R-SFCL; it is a fast-operating device, it stops the fault currents for the first cycle and continuity of supply is possible; . Here R-SFCL material is made up of yttrium barium copper oxide (YBCO). During the normal working conditions, YBCO acts as a superconductor, under abnormal conditions YBCO metal is heated and increases the impedance due to which the increasing impedance fault currents are limited . After clearing the fault, YBCO metal is cooled and it comes to the superconducting state .
Here R-SFCL limits only fault currents, and does not compensate the reactive components due to the presence of reactive components, eventually harmonics is presented and the voltage levels are not compensated. These voltage levels are compensated by using SAPF along with R-SFCL’s and then the system comes into the transient stability . Having one hysteresis controller with SAPF, the voltage levels are compensated. This microgrid system having three DG’S which are PV power, Fuel cells, and Wind power. These three DGs are synchronized with the main grid using PLL circuits. In this microgrid, three DG’S are protected from various types of faults using R-SFCL along with SAPF. The following results are good.
The main objective of this work is to improve the transient stability in microgrids. Up to now, literature discussed limiting the fault currents using R-SFCL, they are not discussed about Reactive power and voltage compensation.
The novelty of this work is to compensate the voltage levels for that reason, SAPF is used along with R-SFCL then fault currents are limited, the reactive power is compensated, voltage harmonics are reduced that means voltage levels are compensated. Finally, the system comes into the transient stability.
1.1 Literature review
Zheng et al.  have offered an R-SFCL for the protection of microgrids. But they are concentrated only on fault current limitations, not concentrate on reactive power and harmonics.
Lim et al.  have offered R-SFCL coordination with overcurrent relay, in this work overcurrent relay is used as voltage component and reduced the relay tripping time here they concentrate only relay time limiting, not concentrating on the stability.
Chen et al.  have offered coordination of SFCL with SMES through wireless communication, in this work they did not discuss faults and fault currents. So due to the fault currents system is not in stable condition.
Lan et al.  have offered a Location and size determination method of SFCLs in multi-terminal VSC-HVDC using iterative current reduction ranking, Here they didn’t discuss voltage levels and stability.
Beyond that literature R-SFCL limits only fault currents, not compensate the reactive components, due to the presence of reactive components harmonics is presented finally voltage levels are not compensated. These voltage levels are compensated by using Series Active Power Filter (SAPF) with R-SFCL’s then the system comes into the transient stability
2 SFCL resistive type
As compare to circuit breakers, R-SFCL gives much better performance it is a fast-operating device it stops the fault currents in the first cycle and continuity of supply is possible. Here R-SFCL material is made with yttrium barium copper oxide (YBCO) . At normal working conditions, YBCO acts as a superconductor, during abnormal conditions YBCO material is heated and increases the impedance because of increasing impedance, fault currents are minimized . After clearing the fault again YBCO metal is cooled and it comes to the normal resistance state. This technology is mainly used in South Korea, China and Japan countries.
Where (t0, t1 and t2) be the time intermissions in seconds, scale on X-axis time in seconds and Y-axis resistance in ohms and CB is Circuit breaker.
3 Series active type power filters
R-SFCL limits only fault currents, not compensate the reactive components, due to the presence of reactive components harmonics are presented finally voltage levels are not compensated. These voltage levels are compensated by using Series Active Power Filter (SAPF) along with R-SFCL’s then the system comes into stability . Here SAPF consists of one reference voltage source, inverter, injection transformer, electrolytic capacitors and hysteresis controller.
4 Microgrid system model
5 Simulink model
6 Results and discussion
Harmonic analysis at different loads
THD of R-SFCL (%)
THD of R-SFCL and SAPF (%)
In this article, with this proposed work, fault current limiting ratio at wind DG is increased from 42.1%  to 43.87%.
The verification of fault current limiting ratio had been checked at PV power, here the limiting ratio is increased from 36.2%  to 52.41%.
Fault current from different locations
With fault (A)
With fault & R-SFCL (A)
With fault, R-SFCL & SAPF (A)
Ratio of fault current limiting (%)
Compliance with ethical standard
Conflict of interest
The authors declare that they have no conflict of interests.