Oportunidad de negocio

To demonstrate the effectiveness of the proposed protection schemes, several fault cases have been simulated in both the islanded and grid-connected modes of operation. How- ever, due to space limitations, the results of only a limited number of cases are presented; the results of other cases are very similar and, thus, not reported in this thesis.

Case 1–Grid-Connected Mode of Operation: As mentioned in Section 6.5.1, the

first step of the protection algorithm is to retrieve the recloser-fuse coordination in the grid-connected mode of operation, if it is lost. Therefore, the coordination algorithm of Fig. 3.1 is employed to coordinate the recloser with the lowest-rating fuse of the feeder in the presence of the DER(s); if the lowest-rating fuse is replaced by a relay, e.g., an MPR, the recloser should be coordinated with the corresponding MPR. Fig. 6.14 shows the characteristic curves of the recloser and the lowest-rating fuse (100-A fuse) of the study microgrid for the case that the synchronous DER is connected to the system. As can be observed in Fig. 6.14, the proposed coordination algorithm is separately exercised for the phase and ground units. It is remembered that recloser-fuse miscoordination is more likely to happen in the presence of the conventional DERs, as EC-DERs have limited fault current ratings. To simulate the worst-case transient condition for the recloser-fuse coordination, the impact of the FCL has not been considered in the calculation of the minimum and maximum fault currents, as discussed in Section 6.5.1–2. This also ensures that the recloser-fuse coordination can be achieved for both scenarios of Section 6.5.1; the calculation of the minimum and maximum fault currents (If min and If max) is explained in Chapter 3. Fig. 6.14 shows that the recloser-fuse coordination is achieved for the grid-connected mode of operation such that the recloser always operates first for each fault incident.

Once the recloser operates in its fast mode, either of the two protection scenarios of Section 6.5.1 (i.e., the use of a transfer trip mechanism through a communication channel or the use of an FCL) can be adopted. The objective of the two options is to eliminate/limit the contribution of the DER(s) for a permanent fault so that the fuse-fuse coordination is preserved (see Chapter 3). However, the first option requires communica- tions which may not be available or it may be costly; thus, the employment of the FCL is studied in this chapter. The FCL is assumed to be of the superconducting type [128], and its impedance value is determined in such a manner that the fuse-fuse coordination

102 103 104 10−2 10−1 100 101 102 current (A) time (s) (a) 102 103 104 (b) Ifmax Ifmin Fuse Recloser Slow Recloser Fast (Revised) Recloser Slow Ifmax Fuse Recloser Fast (Revised) Ifmin

Figure 6.14: Coordination of the recloser and fuse of the study system in the grid- connected mode; (a) phase element and (b) ground element.

of the feeder is not compromised [54]. For the synchronous DER, a resistive FCL whose resistance is 5.8pu ensures fuse-fuse coordination; the resistance value, however, is less than 0.9pu for the wind-power units, due to the limited fault current of the VSCs.

Case 2–Islanded Mode of Operation: The next step is to ensure safe operation of the microgrid in the islanded mode of operation. To do so, the protection scenarios of Section 6.5.2–2 (the use of overcurrent relays with different setting groups) and Section 6.5.2–3 (the use of MPRs) are studied in this section. Therefore, some of the existing protective devices must be replaced by microprocessor-based relays. Although it is hypo- thetically possible to replace any number of protective devices with microprocessor-based relays and coordinate them together (see Section 4.3.5), the number of protective devices that are replaced depends on the system requirements and constraints. In this study, the two fuses at the beginning of Laterals L1 and L2 are replaced by microprocessor-based relays (overcurrent relays or MPRs); in addition, the recloser at the electrical boundary has been replaced by a microprocessor-based relay with reclosing capability (see Section

Figure 6.15: Diagram showing the coordination of digital relays within the study system.

6.6.2–1). It is noted that commercially available digital relays include standard over- current, under-/over-voltage and under-/over-frequency protection functions. They also offer communication and programming capabilities. Therefore, it is possible to model the inverse-time characteristic of the fuses/relays that are being replaced by digital relays.

Considering the location of the DER in the study system, there is no need to equip the downstream relays (Relays 1 and 2 in Fig. 6.15) with directional elements as the issue of “false tripping” does not occur for them [3]; however, to distinguish grid faults from the microgrid faults, a directional element should be employed for the interface relay (i.e., the relay located at the electrical boundary). It is worth mentioning that considering the likelihood of a fault within a distribution system and also the probability of operating a feeder in isolation from the grid, no automatic reclosing has been assumed in this chapter for the islanded mode of operation. The concept of protection coordination for the study system is illustrated by Fig. 6.15 in whichtf and tf+tm respectively denote the forward definite time delays of the microprocessor-based relays, andtm is a grading margin (note that Relay 3 has the reclosing capability).

Tables 6.1 and 6.2 report operating times of the primary and backup protections for different fault cases in the grid-connected mode of operation (see Fig. 6.1 for fault loca- tions), while either of the two protection scenarios (i.e., overcurrent relays and MPRs) have been adopted. The studies are also conducted for the cases where either a syn- chronous DER or an EC-DER is connected to the network. It is borne in mind that, depending on the microgrid operational mode, two different setting groups should be

employed for overcurrent relays, as discussed in Section 6.5.2. Switching from one set- ting group to the other one can be performed by an anti-islanding mechanism embedded in the digital relays and, thus, no communication link is required. Therefore, once the microgrid gets islanded, the anti-islanding signal is activated and the islanded setting group is enabled. It is also noted that the use of MPRS has the advantage of protecting the network against high-impedance faults.

Several cases have also been simulated for the islanded mode of operation but, due to space limitations, the results of only a selected number of them have been reported in Tables 6.3 and 6.4. Similar to the grid-connected mode of operation, it is observed that the coordination between the protective devices is obtained for any fault within the islanded micrgrid, irrespective of the DER type.

6.7

Summary and Conclusion

A realistic distribution network embedding a distributed energy resource was studied in this chapter to identify the conditions and scenarios under which the network can operate as a microgrid. Possible control scenarios that could enable autonomous operation of an existing distribution network were investigated in the chapter, and their advantages and disadvantages were discussed. A suitable control strategy was employed for the EC-DER such that the microgrid can ride through transient incidents, get synchronized with the host utility grid, and slide between two modes of operation. Practical protection strate- gies were also proposed to protect the study microgrid against different fault scenarios in both operational modes. The best control and protection options are selected based on the requirements and constraints of the local distribution companies. Several study cases were conducted based on digital time-domain simulation of the study system, to demonstrate the effective performance of the proposed control/protection strategies. The thesis is summarized and concluded in the next chapter.

Table 6.1: Operating Times of the Main and Backup Protection for Selected Fault Sce- narios in the Grid-Connected Mode of Operation (with OCRs)

Fault Fault Overcurrent Relays (OCRs) Type Loc. Main Protection Backup Protection

Prot. Opr. Prot. Opr.

Device Time(s) Device Time(s) Rotating-Machine-Based DER AG Flt1 R 0.060 cb1 0.171 AG Flt4 R 0.154 F4 0.417 AG Flt5 R 0.096 OCR2 0.486 BCG Flt2 R 0.092 F2 0.689 BC Flt3 R 0.137 OCR1 0.509 ABC Flt6 R 0.105 F6 0.311

Electronically Coupled DER

AG Flt1 R 0.053 cb1 0.174 AG Flt4 R 0.141 F4 0.417 AG Flt5 R 0.087 OCR2 0.486 BCG Flt2 R 0.082 F2 0.690 BC Flt3 R 0.104 OCR1 0.508 ABC Flt6 R 0.090 F6 0.311

Table 6.2: Operating Times of the Main and Backup Protection for Selected Fault Sce- narios in the Grid-Connected Mode of Operation (with MPRs)

Fault Fault Microgrid Protection Relays (MPRs) Type Loc. Main Protection Backup Protection

Prot. Opr. Prot. Opr.

Device Time(s) Device Time(s) Rotating-Machine-Based DER AG Flt1 R 0.060 cb1 0.171 AG Flt4 R 0.154 F4 0.417 AG Flt5 R 0.096 MPR2 0.461 BCG Flt2 R 0.092 F2 0.689 BC Flt3 R 0.137 MPR1 0.460 ABC Flt6 R 0.105 F6 0.311

Electronically Coupled DER

AG Flt1 R 0.053 cb1 0.174 AG Flt4 R 0.141 F4 0.417 AG Flt5 R 0.087 MPR2 0.460 BCG Flt2 R 0.082 F2 0.690 BC Flt3 R 0.114 MPR1 0.460 ABC Flt6 R 0.090 F6 0.311

Table 6.3: Operating Times of the Main and Backup Protection for Selected Fault Sce- narios in the Islanded Mode of Operation (with OCRs)

Fault Fault Overcurrent Relays (OCRs)

Type Loc. Main Protection Backup Protection

Prot. Opr. Prot. Opr.

Device Time(s) Device Time(s)

Rotating-Machine-Based DER

AG Flt2 DER ≃1 N/A N/A

AG Flt3 OCR1 0.488 DER ≃1 AG Flt6 OCR2 0.487 DER ≃1 BCG Flt5 OCR2 0.485 DER ≃1

BC Flt1 DER ≃1 N/A N/A

ABC Flt4 OCR1 0.495 DER ≃1

Electronically Coupled DER

AG Flt2 DER&BESS ≃1 N/A N/A AG Flt3 OCR1 0.499 DER&BESS ≃1 AG Flt6 OCR2 0.511 DER&BESS ≃1 BCG Flt5 OCR2 0.499 DER&BESS ≃1 BC Flt1 DER&BESS ≃1 N/A N/A ABC Flt4 OCR1 0.528 DER&BESS ≃1

Table 6.4: Operating Times of the Main and Backup Protection for Selected Fault Sce- narios in the Islanded Mode of Operation (with MPRs)

Fault Fault Microgrid Protection Relays (MPRs)

Type Loc. Main Protection Backup Protection

Prot. Opr. Prot. Opr.

Device Time(s) Device Time(s)

Rotating-Machine-Based DER

AG Flt2 DER ≃1 N/A N/A

AG Flt3 MPR1 0.461 DER ≃1 AG Flt6 MPR2 0.460 DER ≃1 BCG Flt5 MPR2 0.460 DER ≃1

BC Flt1 DER ≃1 N/A N/A

ABC Flt4 MPR1 0.460 DER ≃1

Electronically Coupled DER

AG Flt2 DER&BESS ≃1 N/A N/A AG Flt3 MPR1 0.460 DER&BESS ≃1 AG Flt6 MPR2 0.460 DER&BESS ≃1 BCG Flt5 MPR2 0.460 DER&BESS ≃1 BC Flt1 DER&BESS ≃1 N/A N/A ABC Flt4 MPR1 0.464 DER&BESS ≃1

Summary, Conclusions, and

FutureWork

7.1

Summary

The economical and environmental merits of microgrids have motivated extensive re- searches and development efforts toward resolving the technical challenges of this new and fast-growing technology. The main objective of this research is to address some concerns related to the control and protection of microgrids. The issues studied in the thesis can generally be classified into two parts: (i) control of EC-DERs under network faults and transient disturbances, and (ii) protection of the microgrid against different fault scenarios in both modes of operation.

In Chapter 1 of this thesis, the research objectives and the contributions of the thesis are presented. Chapter 1 also includes an introduction to the microgrid systems and their advantages and challenges. Chapter 2 mainly focuses on the modeling and control of EC- DERs; in this chapter, a basic control strategy is first adopted and modified for multi-unit microgrids. Then, the chapter outlines the shortcomings of the basic control strategy, modifies it for operation under fault incidents and severe network voltage imbalances, and proposes a phase-angle restoration loop to improve the post-fault recovery of the EC-DER and the host microgrid. A synchronization algorithm is also proposed in Chapter 2 of the thesis.

Coordination of protective devices in radial distribution networks embedding DERs is discussed in Chapter 3. A simple and effective protection strategy is proposed in the chapter that provides coordination amongst protective devices of a typical distribution

system with DERs. Moreover, this chapter presents the steps taken to characterize the impact of future addition of DERs on the protection coordination, for host and neighboring feeders, in a typical distribution network.

Chapter 4 of the thesis investigates the issues associated with protection of microgrids and proposes a protection strategy for low-voltage/residential microgrids. The proposed protection strategy, which does not require communications or adaptive protective de- vices, can be implemented through programmable microprocessor-based relays; it also addresses protection issues of a microgrid in both modes of operation. In Chapter 5, a communication-assisted protection strategy is proposed for large medium-voltage micro- grid, where a fast fault clearance is desired. A backup protection strategy is also offered in this chapter to manage communication network failures. The structure of new relays enabling the proposed protection strategies are also presented in Chapters 4 and 5.

Finally, Chapter 6 of the thesis studies an existing distribution network and presents different control scenarios that enable the network to operate as a microgrid. Practical protection strategies that are effective for the established microgrid (considering the system requirements/constraints) are also discussed in Chapter 6. The main goal of this chapter is to provide the reader with practical scenarios enabling a realistic distribution network to operate as a temporary island.