The protection systems in electrical networks play a vital role in sustaining stability and ensuring safe operation. With the development of new technologies, protection systems became more complex. Significant progress has been recorded in power system reliability modeling and computation by applying quantitative analysis based on probability theory [184]. In recent years, different reliability assessment methods have been proposed such as Reliability Block Diagram (RBD) [185], 3RF Technique [186], and Software Reliability Allocation [187]. The failures are classified and new parameters are introduced to design a more comprehensive reliability assessment scheme [184, 188].
These systems consider issues related to relay hardware, relay software, ancillary equipment, communication units and human errors. [186, 188]. All of them consider the probability of a failure occurrence and its impact on system dependability and security as defined by IEEE standard C37.100-1992 [189]. The probability of a failure is defined by [187, 190]:
(5.15)
where all the individual probability components are added to find the overall value. If there is more than one event that is required for a failure to occur then the combined probability can be expressed as in (5.16).
(5.16) where n is the total number of independent events.
As mentioned in earlier sections, in case of a communication failure, the developed protection system implements a “last-setting-valid” approach. Figure 5.8 shows the local
decision making mechanism employed to activate the circuit breakers. As shown, the operation is independent of the communication link and this ensures that relays are always set to operate, though it may not be the most desirable operation point.
Figure 5.8. Local Decision Making scheme in Relays
However, this requires two events: 1) a failure in the communication line, 2) an electrical fault in the network before the communication line is restored. Therefore the probability of a relay operating following “last-setting-valid” method is the multiplication of the probabilities of communication link failure and electrical fault:
P[comm
fault]=P[comm] x P[fault]
(5.17)where P[comm] is the probability of a communication failure occurring in the microgrid while P[fault] is the probability of an electrical fault occurring in the same microgrid. Equations 15 and 16 hold for all conditions except for extreme climate conditions where power system fault follows power law distribution instead of exponential distribution [191]. Extreme weather conditions are not considered in this study.
It is clearly mentioned in the literature that with the improving technology the hardware failures are less frequent [187]. Consequently, the value of probability of communication
failure is very small. The values assumed for these probability values are on the order of 10-2 or 10-3 [188, 190]. For short term analysis these values may drop as low as 10-5[192]. Accordingly, replacing these characteristic values in (5.17) yields:
P[comm
fault]=0.01 x 0.001 = 10
-5
(5.18)
As a result, the developed protection system proves to be reliable in accordance with the requirements of the standards [189]. Considering the value of P[commfault], it is concluded
that the possibility of relays operating when there is a communication failure is very small. Even if they are forced to operate before the communication is restored, the implementation of “last-setting-valid” method ensures that no catastrophic condition occurs.
As long as the communication speed is at permissible levels, the communication speed is not vital. Communication is only required for updating relay settings and this occurs when a DG connects to or disconnects from the grid. In case of a fault, no communication is required as relays operate based on Local Decision Making. If, as an exceptional case, a fault occurs just after a Relay Setting Update process has started, then the regular communication speed is around 80 ms [193] and this ensures that relay settings will be updated and the fault will be cleared before maximum allowed time expires.
5.7. Conclusion
This chapter has presented two procedures to assign two key parameters in a microgrid protection system with a real time protection controller such as an MCPU. The first of these parameters is the DG impact factor „k‟ which is important in accurately anticipating the fault current contributions of DGs‟ over a relay. The calculation of the impact factor involves parameters which are known beforehand. Hence, „k‟ can be calculated before the fault
operation in microgrids has been presented. The concepts of critical relay and 2-pair selectivity have been introduced. The procedure is automated as much as possible by decreasing human input to minimum. The developed system can work autonomously when the possible network structures and corresponding relay hierarchies are supplied in a table. Alternatively, automatic relay hierarchy extraction algorithms, such as the one discussed in Chapter 4, can be utilized and this makes the protection system more robust and independent.
The faults occurring outside the microgrid have also been considered. It is explained how this situation necessitates the protection system to be able to detect and act on directional over- currents. The proposed protection system has been adapted as it is but the calculation and assignment of the k factor and relay selectivity has been modified to suit all cases including the possibility of faults outside the microgrid. As a result, a complex centralized microgrid protection system has been developed and its operation principles and calculation/assignment procedures have been presented in detail.
Finally, the reliability considerations performed in accordance with the reliability assessment methods presented in the literature, has shown that the developed protection system is very reliable in the case of a fault. The selectivity pairs ensure that even if a malfunction occurs in one of the protective devices, total collapse of protection system will be avoided.
In summary, the main intellectual contribution of the work presented in this Chapter has been the development of an adaptive algorithm, which calculates and assigns protection settings on the go. The developed adaptive scheme is able to follow the dynamic changes occurring in the microgrid, easily accommodate new deployments and adjust the necessary settings.
Chapter 6
IEC 61850-Based Modeling of the Microgrid Protection System
Publications pertaining to this chapter:
1) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh, "Modeling of a Centralized Microgrid Protection System and Distributed Energy Resources According to IEC 61850-7-420," IEEE Transactions on Power Systems, , vol. PP, pp. 1-8, 2012.
2) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh, "Distributed Energy Resources (DER) Object
Modeling with IEC 61850-7-420," In the Proceedings of the Australasian Universities Power Engineering Conference, AUPEC '11, Brisbane, Australia, 2011.
3) Taha Selim Ustun, Cagil Ozansoy, and Aladin Zayegh; “Extending IEC 61850-7-420 for Distributed
Generators with Fault Current Limiters”; In the Proceedings of IEEE PES International Conference on Innovative Smart Grid Technologies ASIA (ISGT Asia),, Perth, Australia, 13-17 Nov., 2011.
4) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh,, "Implementing Vehicle-to-Grid (V2G) Technology with IEC 61850-7-420," , IEEE Transactions on Smartgrids, vol.PP, (accepted).
6.1. Introduction
The preceding chapters explain the modular approach assumed in this research, for the design of the protection system. Firstly, a conceptual design has been developed with contemporary protection challenges in mind. It has been shown that the design is flexible and can be adapted for different grid components such as FCLs. Secondly, the automated microgrid structure detection which is a key part of this research has been explained. This feature
enables the developed microgrid protection system to be versatile so that it can be applied on microgrids with changing structures. It also facilitates new deployments. Thirdly, it has been shown how the necessary protection parameters are calculated in this system. It has been elaborated upon how the first two research steps are utilized in the parameter calculation process and thus, a complementary system is achieved.
Similar to previous chapter, this chapter builds upon the previous achievements of this research. It shows how the developed conceptual design with automated approach and parameter assignment features can be implemented with an international communication standard, i.e. IEC 61850 and its recent extension for DER based systems IEC 61850-7-420. This is an important aspect of the developed microgrid protection system, especially from universality, versatility and flexibility perspectives. The work presented in this chapter shows that the developed multi-dimensional protection system can be implemented with international and universal communication standards. Hence, it is suitable for various implementations in different microgrid topologies. It fills a major gap that is related to the need of a standard communication infrastructure which does not depend on the microgrid structure, its components or the manufacturers thereof.