In recent days, the generation of electrical energy locally at distribution voltage level is emerging as Distributed Generation (DG). Usage of renewable energy sources such as photovoltaic system, wind power, hydro turbine, tidal and biogas in Distributed Generation (DG) become a vital solution to overrule the problems like energy efficiency and environmental concerns with the traditional electrical grids. The increase of DGs in distribution end leads to the formation of Microgrid.The microgrids can be defined as a subsystem or local distribution system inclusive of microsources for generation, power electronic convertors, storage resources like flywheels, energy capacitors and batteries and associated controllable and uncontrollable loads. Microgrids have the tendency to operate in line with the utility grid (grid mode) or isolated from the utility grid (island mode). Through point of common coupling (PCC) the microgrids have inline connection with utility grid.Microgrids enable an improved energy management systems (EMS) to optimize the power flow within the network when connected with utility grid. As well focus has made to minimize the energy price and improve the power factor at PCC. When microgrids are in island mode the stabilization of voltage and frequency enables the system stability.
Microgrids serve various consumers like residential buildings, commercial entities and industrial parks. They tend to provide various advantages such as improving energy efficiency, minimizing overall energy consumption and improves the quality and reliability of power supply. Apart from these advantages microgrids have three major challenges while connecting with distribution grid. They are as follows:
- Technical challenges associated with control and protection system
- Regulation challenges
- Customer participation challenges
The technical issues with microgrid in grid connected mode are existence of multiple energy resources for power generation, location of PCC and level of penetration of mirogrid with main grid. The increased penetration of DG in micro grid system poses several technical problems in the operation of the grid such as steady state and transient over & under voltages at point of connection, protection malfunctions, increase in short circuit levels and power quality issues. The regulation challenges involves with the regulation policies, microgrid legality and engagement between microgrid firms and customers. The major challenge in microgrid is with the protection system. The response of protection system should be for both microgrid and main grid faults. The quick operation of protection system have to maintain the synchronism after transition to islanded operation of microgrid, to maintain the stability of the main grid. This study have main focus on issues with the protection system in microgrid and possible solutions for the issues in both grid connected mode and island mode.
Protection Issues
The main goal of microgrid is to provide quality and reliable supply to the customers. Hence, it is necessary to isolate the microgrid if fault occurs in main grid and to isolate the minimum faulty part if fault occurs inside microgrid.
Faults during Grid Connected Mode – During the occurrence of fault in Microgrid the protection devices in distributed energy resources (DER) must respond only after the activation of protective devices provided at PCC. With the fault ride through capability (FRT), DER should continue its work. For fault within the microgrid, the designed protection strategy should disconnect the faulty portion from the rest of system. The conventional OC protection scheme is set at fault current of 10-50 times of the full load current. Some non-fault cases result in voltage unbalance at PCC. which are difficult to identify. Hence, there is a need for much effective protection schemes to avoid such unwanted situations.
Faults during Island Mode– In this mode, the nature of issue is different from the previous one. The fault current of an islanded microgrid is of 5 times of the load current.Here, the OC protection scheme is set to get activated at 2-10 times of the full load current. This can be reduced to 2-3 times of the full load current for converter based DERs in microgrid. This difference makes the usage of fuses rather than OC protection devices in microgrid.
Changes in Fault current magnitude– The fault current level is high in grid connected mode rather than island mode. According to types of DG the fault current contribution also varies. Fault current of synchronous type DG is 5 times of the rated current and inverter fed DG is 1.5 times of the rated current. Hence, prediction of fault current is difficult because the magnitude of fault current depends upon many parameters including the mode of operation, type of DGs and number of DGs.
Reduction in reach of impedance relay – The occurrence of fault in downstream of the bus DG connected to utility grid, impedance measured by relay located in upstream is higher than real fault impedance. This affects grading of relays and causes delay in operation or sometimes relay does not operate at all.
Bi-directional Power flows – Unidirectional power flow (Substation to load) takes place in conventional distribution system. The interconnection of DGs in distribution end makes the power flow reverse and leads to power quality issues, voltage variation and protection issues.
False Tripping – It occurs when a DG is located near to the substation in a feeder and if fault current in a healthy feeder is supplemented by the DG connected in neighboring feeder then the protection device in the healthy feeder may isolate the circuit unnecessarily.
Blinding of Protection – When DG is connected to a network and if fault occurs in that feeder, the impedance of the grid is much higher than the DG impedance. This makes the short circuit current less than the pickup current of the feeder relay which leads to the failure in detection of fault.
Re-Synchronisation – After islanding, the process of re-synchronization taking place to reconnect the microgrid with main grid through the re-synchronization equipment at PCC. This can be done either in manual mode or automatic mode. The microgrid synchronization schemes are of three types:
- Active synchronisation
- Passive synchronisation
- Open-transition transfer synch-ronisation
Solutions for Protection Issues
The combination of primary and backup protective schemes should be available in a microgrid protection scheme, so that the unhealthy portions can be isolated from the rest of the system. The introduction of DGs in main grid makes the system more complicated.Hence, usage of fuse and overcurrent relays for protection makes the system very simple. Therefore, an efficient protection scheme must be developed to cope up with the above said issues.
Current Limiter – The placement of Fault Current Limiters near PCC is to limit the fault current supplied by the utility grid to microgrid and vice versa. During normal operating condition the FCL is kept in minimum position to neglect unnecessary losses and in maximum position under faulty condition.
Centralized Protection – In centralized protection, Microgrid Management System is used to supervise the status of microgrid and to set rating of the respective protection equipment. Here the communication of protective devices is based on standards IEC61850. Based on the status of the microgrid (grid connected or island) received through MMS, the protecting devices make comparison between the measured parameter and the operating curves set then provides trip signal.
Adaptive Protection – The introduction of DG units in the utility grid pose changes in fault current magnitude. Hence, the revision of power rating of protecting equipment should be done. Also the mode of operation of microgrid and other network changes must be updated periodically in adaptive protection. This can be effectively done with the help of Transmission Control Protocol/Internet Protocol(TCP/IP)based Ethernet network.
Protection Based on Variables – The protection of microgrid is also possible with the help of various parameters like samples of current, voltage, angles, travelling wave, wavelet packet transform (WPT) – as well as some local variables such as rms current, rms voltage, voltage THD (Total Harmonic Distortion), current THD and symmetrical components of current and voltage.
Distance Protection – The impedance (Zm) from the measured voltage (Vm) and current (Im) at each relay point is used here. At normal condition the measured impedance is higher as it includes load impedance. During fault, the measured impedance is low as it is equal to the line impedance. By comparing the Zm with the preset value the protection is getting activated.
Multi Agent Protection – Distribution of intelligent hardware and software agent in the network promise to achieve an efficient protection of microgrid. This protection scheme is inclusive of three layers such as equipment layer, substation layer and system layer. Through the agents, substation layer communicates with the equipment layer and transmit the information to the system layer and corresponding agents to make microgrid adaptive protection more efficient.
Change in Device Settings – The placement of DG units in main grid impose some impacts on traditional protection scheme hence there is a need to change the device settings to get desirable protection. In this way, the bidirectional power flow due to introduction of DG unit replaces the OC relay to directional OC relay by changing the protective settings.
Smart Transformers – In microgrid protection smart transformers play an important role. While going for wide area protection, synchronization of measured values can be effectively done with smart transformers rather than the protection plan of using local measurement with Solid State Transformer (SST).
From the wide review made on several protection strategies represented for microgrid (Fig.1.), adaptive protection is the best one as it includes all the dynamic changes of DG and relay settings. As well, it works for all working conditions of microgrid. Since the implementation cost of the adaptive protection scheme is high with digital equipments, it should be compromised with the energy price for long planning.
Concluding Remarks
The key factor for the incorporation and enlargement of microgrid is to face the increase in energy demand and the limitations of the unsustainable energy resources. The introduction of microgrid in the utility grid makes the traditional protection schemes ineffective. In this context, the protection issues and the key factors for designing a proper protection schemes to overcome the protection issues with microgrid have been overviewed in this study. Hence, several protection scheme with various parameters are suggested for efficient protection. But, a substantial research is still required for the microgrid protection for the stability of the system and for the reliable power supply.
Acknowledgment
The authors are thankful to the authorities of Thiagarajar College of Engineering, Madurai – 625015 to do this research work. This work was supported by DST-WOS-A fellowship scheme under Ref DST – WOS-A File No: SR/WOS-A/ET-9/2018 (2019-2021).
Dr. Geethanjali M.
Professor at Department of Electrical & Electronics Engineering in Thiagarajar College of Engineering, India.
Research Interests: Power System Protection, Smart Grid, Wide-Area Monitoring and Control and Renewable Energy Resources.
Past recipient of Young Scientist Fellowships- TNSCST, Chennai. Published more than 50 research articles and presented around 10 guest lectures.
Hemavathi R.
Full-time Research Scholar at the Department of Electrical & Electronics Engineering in Thiagarajar College of Engineering, India.
Research Interests: Power System Protection & Control, Smart Grid, Wide-Area Monitoring & Control and Renewable Energy Resources.
A recipient of Department of Science and Technology (DST) – Women Scientists Scheme – A (WOS-A) fellowship from June, 2019.
