- Original Research
- Open Access
Microgrid dynamic security considering high penetration of renewable energy
© The Author(s) 2018
- Received: 15 February 2018
- Accepted: 31 May 2018
- Published: 2 August 2018
This paper presents a coordination strategy of Load Frequency Control (LFC) and digital frequency protection for an islanded microgrid (MG) considering high penetration of Renewable Energy Sources (RESs). In such MGs, the reduction in system inertia due to integration of large amount of RESs causes undesirable influence on MG frequency stability, leading to weakening of the MG. Furthermore, sudden load events, and short circuits caused large frequency fluctuations, which threaten the system security and could lead to complete blackouts as well as damages to the system equipment. Therefore, maintaining the dynamic security in MGs is one of the important challenges, which considered in this paper using a specific design and various data conversion stages of a digital over/under frequency relay (OUFR). The proposed relay will cover both under and over frequency conditions in coordination with LFC operation to protect the MG against high frequency variations. To prove the response of the proposed coordination strategy, a small MG was investigated for the simulation. The proposed coordination method has been tested considering load change, high integration of RESs. Moreover, the sensitivity analysis of the presented technique was examined by varying the penetration level of RESs and reducing the system inertia. The results reveal the effectiveness of the proposed coordination to maintain the power system frequency stability and security. In addition, the superiority of the OUFR has been approved in terms of accuracy and speed response during high disturbances.
- Digital frequency relay
- Load frequency control (LFC)
- Renewable energy sources (RESs)
The frequency control and protection of the electrical systems are the two main sides to investigate the dynamic security of the MG system. There are several studies have dealt this problem from the control side such as conventional controllers with different algorithms and optimization techniques [10, 11], intelligent control, (i.e. Fuzzy Logic Control (FLC)) [12, 13], and robust control [14, 15]. X. Tang et al.  proposed a novel technique of frequency control, which is V/f droop control (VFDC) and P/Q droop control (PQDC) combined for islanded MG based on different energy storage devices. While, Sedghi and Fakharian  used the coordination of robust control and fuzzy technique to address the frequency control issue in . Model Predictive Control (MPC) based LFC for MG based on the coordination of wind turbine and Plug-in Hybrid Electric Vehicles (PHEVs) is proposed in  by Jonglak et al. Furthermore, Wang et al.  studied and analyzed the voltage security issue for MG using Convolutional Neural Network (CNN).
On the other side, the protection systems have changed significantly from the bygone decade and will change continuously as a result of the advancement of technology. Therefore, power systems designers are seeking to apply digital devices to handle the increasing complexity of power system, which improve the cost and usability. Subsequently, the digital technology has appeared in the protection system of microprocessor relays since 1980 and developing to those with communications interfaces in the a990s . Today, digital relays have featured with high speed communication, which helped in replacing wires for safety interlocking, control and also circuit breakers tripping action. Furthermore, there are many applications of digital relays in transmission and generation system protection due to their flexibility, high performance level, and capability of operating under different temperatures compared to the classical electromechanical relays. Therefore, this study focuses on the digital protection device, (e.g. OUFR) to be coordinated with LFC for MG dynamic security. There are several studies have dealt this problem from the short circuit fault side only such as, the optimized time-based coordination of conventional over-current relays; which is the earliest protection technique for utility grids including micro-grids . This method has a limit in its ability of multi-relay protection because of its high sensitivity to components parameters in high fault levels. Sheng et al.  presented a multi-agent method depend on assumptions of high fault current levels. However, this method has been developed to island the MG for any fault in the utility grid and also disconnecting most of distributed generations (DGs) for faults within the MG. Furthermore, some studies handled the frequency protection problems such as; Laghariet et al.  applied an intelligent computational technique for load shedding of the power system under faulted conditions. Moreover, Komsan and Naowarat  discussed the same issue by using the rate of change under frequency relay to improve the load shedding scheme in MGs. Further, Freitas et al.  presented a comparative study of the rate of change of frequency (ROCOF) and vector surge relays for distributed generation applications. However, they faced a very hard task in relays coordination as their design may not detect the islanded conditions within the required time. Teimourzadeh et al.  introduced a new Region of Attraction (ROA) based protection zone for the detection of MG security status. However, the proposed approach is an efficient index for providing a quick detection of MG security status. Jose Vieira et al. in  proposed the coordination of ROCOF and under/over frequency relays. However, this presented coordination has a drawback, which it did not compensate the frequency fluctuations within the allowable frequency limit due to the action of the relay is energized when the system frequency become out of the allowable limit. Such a problem can be overcome by designing the proposed coordination strategy of LFC and digital OUFR for an islanded MG system dynamic security.
According to authors’ knowledge, some gaps still need to be filled in the MG dynamic security issue. Therefore, this paper proposes a design of digital over/under frequency relay coordinated with the LFC for the dynamic security of an islanded MG system, which consist of thermal power plant, PV, wind power generation (WPG), and domestic loads. To prove the effectiveness of the proposed coordination in protecting the MG against frequency variations, it has been tested under different scenarios of disturbances such as, high penetration level of RESs, reducing system inertia, and sudden load variations. The remaining of this paper is arranged as follows, Section 2 discusses the problem description. The structure of the studied MG system with the state equations are presented in Section 3. The coordination of control and protection methodology is described in Section 4. Section 5. shows the simulation results of the proposed coordination which applied to the MG. Finally, the last section concludes the results and advantages of the proposed method.
The dynamic security issue is one of the most critical issues, which face the power system designers. Dynamic security refers to the ability of the electrical power system to maintain the synchronism when subjected to a sever trainset disturbance . Therefore, the dynamic security deals with disturbances that impose momentous changes into the system variables. Among these are short-circuit faults, loss of a dominant generation source, and loss of a large load. The system response to these disturbances includes large deviations in the system variables such as voltage magnitudes and angles, generator speed, and system frequency . Hence, the balance between the input mechanical power and the output electrical power is disturbed. And then, the mismatch makes the synchronous generators (SGs) either accelerate or decelerate.
On the other hand, preserving dynamic security is different between the bulk power systems and MGs. In the case of the bulk power systems, the conventional synchronous generators are considered the source of the dynamics. Likewise, in MGs, the RESs are the host of dynamics. Moreover, most of the available methods for preserving the dynamic security of the bulk power systems are considered inefficient for MGs due to these methods are devised based on the features of the bulk power system, which are significant inertia constant and rather slow dynamics. Therefore, this research studies the dynamic security issue in the microgrids. In the MGs, the RESs exchange power to the MGs through inverters/converters. The power electronic interface-based RESs are static devices without any rotating mass so that the associated inertia constant is roughly zero. On the other hand, synchronous generators-based RESs are small-scale generators with noticeably low inertia constants . Such a low inertia constant renders the MGs more vulnerable to the transients than the bulk power systems. Furthermore, the power generation from RESs are unpredictable and variable, results in more fluctuations in power flow and frequency in the MG, which significantly affects the power system operation. Also, the randomly changes in load power demand caused a bad response to the PCC voltage, active, and reactive powers transfer. To solve the dynamic stability problems, it must be determined the effective factors, which steer the MGs toward the insecurity. These factors include a low inertia constant, frequent fault occurrence, and inadequacy of existing protection schemes. Moreover, the performance of the protection system is one the most significant factors which imperils the dynamic security of the μGs. Therefore, the stability and protection coordination issues have become a center of interest especially for power system researchers. Hence, this research proposes an efficient coordination of secondary frequency control (i.e., LFC) and the digital OUFR for an islanded MG security considering high penetration of RESs.
3.1 Microgrid system
Islanded microgrid parameters
3.2 Mathematical model of the proposed microgrid
where, ∆Pwind, ∆Psolar, and ∆PL are the wind power, solar power, and load power variations, respectively. These variations are considered as the MG disturbance signals. While, the damping (D) and the inertia (H) are the uncertainty parameters. ∆Pm is the thermal power deviation, and ∆Pg is the governor power deviation. The complete state-space model of the presented MG considering high RESs penetration level can be obtained through the state variables and definitions from (1) to (7). The linearized state-space model of the MG from Fig. 3 is as in (8) and (9).
4.1 Frequency control of an islanded microgrid
Frequency and control/protection actions
∆f1 (0.3 Hz)
No contingency or load event
∆f2 (1 Hz)
Generation /Load event
∆f3 > (2 Hz)
Large Separation event
4.2 Protection scheme
4.2.1 Modelling of digital frequency relay
4.2.2 Principal operation of digital frequency relay
Frequency relay settings
fmax = 51 Hz
K = 5 s
fmin = 49 Hz
To investigate the dynamic security of the islanded MG by using the proposed coordination of LFC and digital OUFR, four scenarios are applied on the MG as follows:
5.1 Scenario A
5.2 Scenario B
5.3 Scenario C
The secondary control (i.e., LFC) can restore the frequency to its steady-state value at the first load disturbance at time t = 300 s, while it cannot withstand the change of system frequency caused by high wind penetration (i.e., 35% pu) as noted in Fig. 11(c). Therefore, the digital OUFR sent a trip signal to the generator circuit breaker at that time as shown in Fig. 11(b), whereas the integrator output exceeds the threshold value of 5 s. Hence, the effectiveness of the proposed coordination is approved for the MG dynamic security.
5.4 Scenario D
This paper proposed a coordination strategy of Load Frequency Control (LFC) and digital Over/Under Frequency Relay (OUFR) protection for an islanded microgrid system security considering high penetration of RESs. This coordination strategy is proposed for supporting the frequency stability and protecting the islanded MG against high-frequency deviations, which increased recently due to high penetration of (RESs), random load variations, and system uncertainty. These changes threaten the MG dynamic security and can cause under/over frequency relaying and disconnect some loads and generations, which may lead to cascading failure and system collapse. The simulations results proved that the proposed coordination has been achieved an effective performance for maintaining the MG dynamic security. Whereas, the LFC has been succeeded to readjust the frequency deviations to its allowable limits under different conditions of transients, load disturbances, and RESs penetration levels. However, in some cases of large disturbances and high RESs penetration, the LFC cannot maintain the frequency stability as the frequency fluctuates beyond the normal limits. In that case, the digital frequency relay trips the generation units. Furthermore, the results confirmed that the digital OUFR has superiority in terms of accuracy, sensitivity and wide range controlling.
- LFC :
Load frequency control
- MG :
- ∆f :
Frequency deviation of the microgrid (Hz)
- D :
Microgrid damping coefficient (pu MW/Hz)
- H :
Microgrid system inertia (pu MW sec)
- T g :
Time constant of governor (sec)
- T t :
Time constant of turbine (sec)
- ∆P C :
Regulating the system frequency (Hz)
- GRC :
Generation Rate Constraint, % (pu)
- R :
Droop constant (Hz/pu MW)
- T WT :
Time constant of wind turbines (sec)
- T PV :
Time constant of solar system (sec)
- RES :
Renewable energy sources
- OUFR :
Over/Under frequency relay
- V U :
Maximum limit of valve gate (pu MW)
- V L :
Minimum limit of valve gate (pu MW)
- ∆P M :
Change in Mechanical power
- δ :
- ∆P d :
Change in demand power
- K I :
Integral control variable gain
- K :
Integrator set time
- F max :
Maximum frequency limit
- f min :
Minimum frequency limit
- ω o :
This paper is funded by the higher ministry of education in Egypt.
Availability of data and materials
Data sharing not applicable to this article as no datasets were generated or analysed during the current study.
About the Authors
Emad A. Mohamed received the B.Eng. with a first class honors and M.Sc. degree in electrical power engineering from Aswan University, Aswan, Egypt in 2005 and 2013, respectively. He is working as an Assistant Lecture in the Department of Electrical Engineering, Aswan Faculty of Engineering, Aswan University, Aswan, Egypt. Currently, he is a Ph.D. student in Kitakyushu institute of Technology, Japan. He was in a Master Mobility scholarship at Faculté des Sciences et Technologies - Université de Lorraine, France – 1. The scholarship sponsored by FFEEBB ERASMUS MUNDUS. He was a research student from April to October 2015 in Kyushu University, Japan. His research interests are applications of superconducting fault current limiters (SFCLs) on power systems, power system stability, and protection.
Gaber Magdy received his B.Sc. and M.Sc. degrees in electrical engineering from Faculty of Energy Engineering, Aswan University, Egypt, in 2011 and 2014, respectively. He joined to the Electrical Engineering Department of Faculty of Energy Engineering, Aswan University, Aswan, Egypt, first as an Administrator and then becoming a Research Assistant in January 2012. He is currently a researcher with the Dept. of Electrical and Electronics Engineering, Kyushu Institute of Technology, Japan.
G. Shabib, received his B.Sc. degree in electrical engineering from Al Azhar University. In October 1982 he joins the electrical engineering, King Fahad University of Petroleum and Minerals, Dhahran Saudi Arabia as a research assistant. In December 1985 he received his M.Sc. degree in electrical engineering from King Fahad University of Petroleum and Minerals. In November 1987 he joins the Qassim Royal Institute, Qassim, Saudi Arabia as a lecturer. He received his Ph.D. degree from Menoufia University, Egypt, in 2001. He joined Aswan High Institute of Energy, South Valley University, Aswan, Egypt in 1999. He joined Digital Control Laboratory, Tsukuba University, Japan as a visiting Professor 2006–2007. His research interests are power system stability, control, Self-tuning control, Fuzzy logic techniques, Digital control techniques all as applied to power systems.
Adel A. Elbaset was born in Nag Hamadi, Qena-Egypt, on October 24, 1971. He received the B.Sc., M.Sc., and Ph.D. from Faculty of Engineering, Department of Electrical Engineering, Minia University, Egypt, in 1995, 2000 and 2006, respectively. He is a staff member in Faculty of Engineering, Electrical Engineering Dept, Minia University, Egypt. Dr. A. Elbaset is currently a visiting Professor at Kumamoto University, Japan. His research interests are in the area of power electronics, power system, neural network, fuzzy systems and renewable energy, Optimization.
Yasunori Mitani received B.Sc., M.Sc., and D.Eng. degrees in Electrical Engineering from Osaka University, Japan in 1981, 1983 and 1986, respectively. He was a visiting research associate at the University of California, Berkeley, from 1994 to 1995. He is currently a professor at the department of electrical and electronics engineering, Kyushu Institute of Technology (KIT), Japan. At present, he is the head of environmental management center of KIT and vice dean of a graduate school of engineering, KIT. He is also the President of the Institute of Electrical Engineers of Japan (IEEJ), Power and Energy Society.
Emad Mohamed carried out the all simulation analysis, the design of the digital frequency relay, and wrote the paper. Gaber Magdy helped in the control part of load frequency control for the microgrid system. Gaber Shabib participated in the sequence alignment and renewable energy sources specifications. Adel Abdelbasit participated in some statistical analysis like the state space modeling of the islanded microgrid. Yasunori Mitani conceived of the study, and participated in its design and coordination and revised the draft of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Belwin, J. (2017). Brearley and R. Raja Prabu, “a review on issues and approches for microgrid protection”. Journal of Renewable and Sustainable Energy Reviews, 67, 988–997.View ArticleGoogle Scholar
- Dong, Y., Xie, X., Wang, K., & Jiang, Q. (2017). An emergency-demand-response based under speed load shedding scheme to improve short-term voltage stability. IEEE Transactions on Power Systems, 32(5), 3726–3735.View ArticleGoogle Scholar
- Aristidou, P., Valverde, G., & Cutsem, T. V. (2017). Contribution of distribution network control to voltage stability: A case study. IEEE Transactions on Smart Grid, 8(1), 106–116.View ArticleGoogle Scholar
- Bevrani, H., Watanabe, M., & Mitani, Y. (2014). Power system monitoring and control. New Jersey: Wiley.View ArticleGoogle Scholar
- Rakhshani, E., Remon, D., Cantarella, A., & Rodriguez, P. (2016). Analysis of derivative control based virtual inertia in multi-area high-voltage direct current interconnected power systems. IET Generation, Transmission & Distribution, 10(6), 1458–1469.View ArticleGoogle Scholar
- Bevrani, H., Ise, T., & Miura, Y. (2014). Virtual synchronous generators: A survey and new perspectives. International Journal of Electrical Power & Energy Systems, 54, 244–254.View ArticleGoogle Scholar
- Sortomme, E., Venkata, S. S., & Mitr, J. (2010). Microgrid protection using communication-assisted digital relays. IEEE Transactions on Power Delivery, 25(4), 2789–2796.View ArticleGoogle Scholar
- Zamani, M. A., Sidhu, T. S., & Yazdani, A. (2011). A protection strategy and microprocessor-based relay for low-voltage micro-grids. IEEE Transactions on Power Delivery, 26(3), 1873–1883.View ArticleGoogle Scholar
- Keil, T., & Jager, J. (2008). Advanced coordination method for over-current protection relays using nonstandard tripping characteristics. IEEE Transactions on Power Delivery, 23(1), 52–57.View ArticleGoogle Scholar
- A. Singh, and Sathans, "GA optimized PID controller for frequency regulation in standalone AC microgrid”, IEEE conf, 7th India International Conference on Power Electronics (IICPE), 17–19 Novomber 2016.Google Scholar
- G. Parise, L. Martirano, M. Kermani, and M. Kermani, “Designing a power control strategy in a microgrid using PID / fuzzy controller based on battery energy storage”, IEEE International Conference on Environment and Electrical Engineering and 2017 IEEE Industrial and Commercial Power Systems Europe (EEEIC / I&CPS Europe), 13 2017.Google Scholar
- Bevrani, H., Habibi, F., Babahajyani, P., Watanabe, M., & Mitani, Y. (2012). Intelligent frequency control in an AC microgrid:Online PSO-based fuzzy tuning approach. IEEE Transactions on Smart Grid, 3(4).Google Scholar
- Kang Gong, Jing Shi, Yang Liu, Zuoshuai Wang, Li Ren, and Yi Zhang, “ Application of SMES in the micro-grid based on fuzzy control”, IEEE Transactions on Applied Superconductivity, Vol. 26, No. 3, 2016.Google Scholar
- Kerdphol, T., Rahman, F. S., Mitani, Y., Watanabe, M., & Küfeoğlu, S. (2018). Robust virtual inertia control of an islanded microgrid considering high penetration of renewable energy. IEEE Access, 6(1), 625–636.Google Scholar
- Yi Han, A. Jain, P. Young, and D. Zimmerle, “Robust Control of Microgrid Frequency with Attached Storage System”, 52nd IEEE Conference on Decision and Control, 10–13 December, Florence, Italy, 2013.Google Scholar
- Tang, X., Hu, X., Li, N., Deng, W., & Zhang, G. (2016). A novel frequency and voltage control method for islanded microgrid based on multienergy storages. IEEE Transactions on Smart Grid, 7(1), 410–419.View ArticleGoogle Scholar
- L. Sedghi and A. Fakharian, “Voltage and frequency control of an islanded microgrid through robust control method and fuzzy droop technique”, 5th Iranian Joint Congress on Fuzzy and Intelligent System (CFIS), Qazvin Islamic Azad University, Tehran, Iran, 7–9 March, 2017.Google Scholar
- Pahasa, J., & Ngamroo, I. (2016). Coordinated control of wind turbine blade pitch angle and PHEVs using MPCs for load frequency control of microgrid. IEEE Systems Journal, 10(1), 97–105.View ArticleGoogle Scholar
- Y. Wang, H. P. Painemal, K. Sun, “Online analysis of voltage security in a microgrid using convolutional neural networks,” IEEE Conf, Power & Energy Society General Meeting, Chicago, USA, 2017.Google Scholar
- Meier, S., & Kunsman, S. (2016). Protection and control system impacts from the digital world. Electric Energy T&D Magazine, 12–15. http://www.electricenergyonline.com/show_article.php?mag=117&article=996
- Sortomme, E., Mapes, G. J., Foster, S., & Venkata, S. (2009). Fault analysis and protection of a micro-grid. IEEE Transactions on Power Delivery, 24(3), 1045–1053.View ArticleGoogle Scholar
- Sheng, S., Li, K. K., Chan, W. L., Zeng, X., Shi, D., & Duan, X. (2010). Adaptive agent-based wide-area current differential protection system. IEEE Transactions on Industry Applications, 46(5), 2111–2117.View ArticleGoogle Scholar
- Laghari, J. A., Mokhlis, H., Bakar, A. H. A., & Mohamad, H. (2013). Application of computational intelligence techniques for load shedding in power systems: A review. Energy Conversion and Management, 75, 130–140.View ArticleGoogle Scholar
- Komsan Hongesombut, Naowarat Tephiruk, “Modeling of the rate of change of under-frequency relay for microgrid protection”, International Electrical Engineering Congress (iEECON), 2017, 1–4. Google Scholar
- Freitas, W., Xu, W., Affonso, C. M., & Huang, Z. (2005). Comparative analysis between ROCOF and vector surge relays for distributed generation applications. IEEE Transactions, 20(2), 1315–1324.Google Scholar
- Teimourzadeh, S., Aminifar, F., Davarpanah, M., & Shahidehpour, M. (2017). Adaptive protection for preserving microgrid security. IEEE Transactions on Smart Grid, (99), 1–9.Google Scholar
- Vieira, J. C. M., Freitas, W., Xu, W., & Morelato, A. (2006). Efficient coordination of ROCOF and frequency relays for distributed generation protection by using the application region. IEEE Transactions on Power Delivery, 21(4), 1878–1884.Google Scholar
- Teimourzadeh, S., Aminifar, F., & Davarpanah, M. (2017). Microgrid dynamic security: Challenges, solutions and key considerations. The Electricity Journal, 30(1), 43–51.View ArticleGoogle Scholar
- Bevrani, H. (2014). Robust power system frequency control (2nd ed.). Gewerbestrasse: Springer.MATHGoogle Scholar
- Hassan, A. A. M., & Kandeel, T. A. (2015). Effectiveness of frequency relays on networks with multiple distributed generation. Journal of Electrical Systems and Information Technology, 2, 75–85.View ArticleGoogle Scholar
- Zarei, S. F., & Parniani, M. (2017). A comprehensive digital protection scheme for low-voltage micro grids with inverter-based and conventional distributed generations. IEEE Transactions on Power Delivery, 32(1), 441–452.View ArticleGoogle Scholar