In order to substantiate the operation of the modeled blocks and the operation of the proposed protection scheme, the system shown in Figure 6.16 is described over a predetermined scenario. The discussion presented is aimed at showing how the communication of various data should take place between various microgrid network elements to enable protection related decision making, e.g. adjustment of the relay protection settings.
Figure 6.16. The system modeled according to IEC 61850-7-420
The system includes two inverter-interfaced DGs and a diesel generator DG which is used to set the frequency and voltage under islanded conditions. The DGs are operated under conventional droop control and share the loads in accordance with their capacities.
A specially designated scenario is applied to show different aspects of the protection scheme. The applied scenario is as follows:
1. The system starts operation with full load, all DGs are on and the MG operates in Grid-Connected Mode.
2. At t=0.2 sec a fault occurs in Load 1 (L1) and Circuit Breaker (CB) 2 opens. Due to less power demand in the system DG1 also turned off.
3. At t=0.5 sec the utility grid experiences power outage for maintenance reasons and MG becomes islanded.
4. At t=0.65 sec, while MG is operating in islanded mode another fault occurs in L4 and CB8 opens.
5. At t=0.75 sec, the connection with the utility grid is restored and MG operates in grid- connected mode.
6. At t=1 sec, the fault in L1 is cleared and it is reconnected to the system. Due to increased power demand in the network DG1 is also put into operation.
7. At t=1.25 sec, a fault occurs in Load 5 and CB3 opens. To protect the integrity of the microgrid, CB4 is closed and alternative path is used to energize right side of the microgrid.
The system shown may assume different structures depending on the status of CB4 and CB5. In the beginning CB4 is open and CB5 is connected. Therefore, the microgrid has two branches in its structure. After t=1.25 sec, CB4 closes and CB3 opens. The microgrid assumes a one-line structure. The changing selective levels of the relays are shown in the tables given below. Comparing different selective level of a particular relay for different microgrid structures shows the need for selectivity coordination. In literature, there are algorithms to ensure dynamic relay hierarchy detection [182] and they will not be discussed here.
As mentioned earlier, it is assumed that microgrid is sufficiently small and `k` is set to 1. This means that DGs supply their maximum fault current for any fault. The parameters of the network components used by the communication system are listed in Table 6.7.
TABLE 6.7.NETWORK COMPONENT PARAMETERS AT T=0 SEC StatusDG DG type Ifault
DG1 DRCS.ModOnConn= True DRCT.DERtyp = 4 (DRCT.MaxWLim/Voltage) * 1.5
DG2 DRCS.ModOnConn= True DRCT.DERtyp = 3 (DRCT.MaxWLim/Voltage) * 1.5
DG3 DRCS.ModOnConn= True DRCT.DERtyp = 2 (DRCT.MaxWLim/Voltage) * 5 CB1 CB 2 CB 3 CB 4 CB 5 CB 6 CB 7 CB 8 XCBR.Pos ON ON ON OFF ON ON ON ON Selective Level 1 2 2 N/A 3 3 4 4
The relay fault current value = Grid contribution + DG1 + DG2 + DG3
= IfaultGrid + (DG1.DRCT.MaxWLim/Voltage) * 1.5 + (DG2.DRCT.MaxWLim/Voltage)*1.5 +(DG3.DRCT.MaxWLim/Voltage)*5
At t=0.2 in line with the changes occurring in the microgrid, DG1 is turned off by the MCPU. This happens by setting DG1.DRCS.ModOnConn false and DG1.DRCS.ModOffUnav true. In order to achieve this, MCPU sends an IEC61850 compliant control message and asks DG1 to switch off. When DG1 switches off, the DG1.DRCS.ModOnConn attribute changes to FALSE and the DG1.DRCS.ModOffUnav attribute becomes TRUE. The change in data parameters of the DG1 model is depicted in Figure 6.17:
MCPU -> DG1.IHMI -> DG1.ITCI -> DG1.DRCS -> DG1.DRCS.ModOnConn and -> DG1.DRCS.ModOffUnav
At t=0.5 sec, the utility grid experiences power outage so the microgrid becomes islanded. CB1 is now open. This information is extracted by the MCPU from the following scheme, as shown in Figure 6.18. Relay1 controlling CB1 would send a GOOSE message to the MCPU and the GOOSE message would contain the data attribute (Relay1.XCBR.Pos) beyond others. The value of the attribute “Relay1.XCBR.Pos” was set in the relay to OFF an
indicator of the OPEN status of CB1. On receiving this GOOSE message from Relay1, MCPU would unpack the contents of the GOOSE message, read the new status of CB1, and adjust its “Network Component Parameter” table as shown in Table 6.8. Essentially, the “Network Component Parameter” table has been updated to reflect the change in the status of
CB1.
MCPU -> Relay1.IHMI -> Relay1.ITCI -> Relay1.XCBR.Pos
Figure 6.18. Reporting Relay status through IEC 61850-7-420 Models
At t=0.5 sec the network component parameter table is as shown in Table 6.8.
TABLE 6.8.NETWORK COMPONENT PARAMETERS AT T=0.5 SEC StatusDG DG type Ifault
DG1 DRCS.ModOffUnav = True DRCT.DERtyp = 4 (DRCT.MaxWLim/Voltage) * 1.5
DG2 DRCS.ModOnConn = True DRCT.DERtyp = 3 (DRCT.MaxWLim/Voltage) * 1.5
DG3 DRCS.ModOnConn = True DRCT.DERtyp = 2 (DRCT.MaxWLim/Voltage) * 5 CB1 CB 2 CB 3 CB 4 CB 5 CB 6 CB 7 CB 8 XCBR.Pos OFF OFF ON OFF ON ON ON ON Selective Level N/A N/A 1 N/A 2 2 3 3
The relay fault current value = DG2 + DG3
At t=0.65 sec, a fault occurs in L4 and CB8 opens. In a similar fashion to Figure 6.18, MCPU receives an IEC 61850 complaint packet and extracts this information:
MCPU -> Relay8.IHMI -> Relay8.ITCI -> Relay8.XCBR.Pos
At t=0.75 sec, the connection is restored with the utility and once again, the microgrid operates in grid-connected mode. This information is extracted by the MCPU from:
MCPU -> Relay1.IHMI -> Relay1.ITCI -> Relay7.XCBR.Pos
At t = 1 sec, L1 and DG1 are connected to microgrid. These will be reported to MCPU through:
MCPU -> Relay2.IHMI -> Relay2.ITCI -> Relay2.XCBR.Pos
and
MCPU -> DG1.IHMI -> DG1.ITCI -> DG1.DRCS -> DG1.DRCS.ModOnConn and -> DG1.DRCS.ModOffUnav
Finally, at t = 1.25 sec, L5 and CB3 are disconnected from and CB4 is connected to microgrid. These will be reported to MCPU through following channels:
MCPU -> Relay3.IHMI -> Relay3.ITCI -> Relay1.XCBR.Pos MCPU -> Relay4.IHMI -> Relay4.ITCI -> Relay4.XCBR.Pos
The final state of the network component table is as shown in Table 6.9.
TABLE 6.9.NETWORK COMPONENT PARAMETERS AT T=1.25 SEC StatusDG DG type Ifault
DG1 DRCS.ModOnConn = True DRCT.DERtyp = 4 (DRCT.MaxWLim/Voltage) * 1.5
DG2 DRCS.ModOnConn = True DRCT.DERtyp = 3 (DRCT.MaxWLim/Voltage) * 1.5
DG3 DRCS.ModOnConn = True DRCT.DERtyp = 2 (DRCT.MaxWLim/Voltage) * 5
CB1 CB 2 CB 3 CB 4 CB 5 CB 6 CB 7 CB 8 XCBR.Pos ON ON OFF ON ON ON OFF ON
Selective Level 1 2 N/A 3 3 4 5 5 The relay fault current value = Grid contribution + DG1 + DG2 + DG3
= IfaultGrid + (DG1.DRCT.MaxWLim/Voltage) * 1.5 + (DG2.DRCT.MaxWLim/Voltage)*1.5 +(DG3.DRCT.MaxWLim/Voltage)*5
For all of the changes occurring in the microgrid, MCPU re-calculates the fault current. These new values are updated in the associated relays through the communication shown in Figure 6.19. MCPU would essentially send an IEC 61850 compliant control message to the Relay. On receiving and unpacking the control message, Relay finds out that it needs to reset its
Relay.PTOC.StrVal attribute to reflect the changes that have occurred.
MCPU -> Relay.IHMI -> Relay.ITCI -> Relay.PTOC -> Relay.PTOC.StrVal
Figure 6.19. Updating Relay operating Currents through IEC 61850-7-420 Models
Over an established communication line between the components and the MCPU, the proposed protection scheme works perfectly with the models that are designed in compliance with IEC61850 and IEC 61850-7-420.
6.7. Conclusion
Microgrids have dynamic structures which change more often than the conventional large networks. In order to ensure protection under these conditions, a centralized management is required to monitor and communicate with the microgrid components and assign suitable operation parameters. The communication lines and systems inevitably utilized in microgrids require standardization so that equipment with different manufacturers or owners can work together; and new deployments can be easily done. Design of a central microgrid management system will be very much simplified if all of the components use the same communication standard. Therefore, communication standards such as IEC 61850 and the
extension IEC 61850-7-420, which is aimed at modeling the information exchange for DER devices, have been published.
In the research project presented in this thesis, two important extensions have been made to IEC 61850 Standard. Due to its promising nature for future control systems, an extension has been made for FCL to facilitate its modeling. Another extension has been made for EVs since the availability of the technologies required and the higher efficiency of electric-driven cars create a genuine interest for EVs in the market. The large acceptance of EVs will definitely have impacts on electrical networks because through V2G technology, EVs not only draw power from the network but also act as distributed storage devices. With these extensions, the mentioned communication standard has been made more comprehensive.
Finally, the information models of the various network elements and their modeling as per IEC 61850 and its most recent extension, i.e. IEC61850-7-420 have been described. The presented relay and DG information models are standard and can be used for modeling the data exchange in any power system network. The universal modeling of information/data models in DGs regardless of their models and manufacturers is very important for having a universal concept for standardizing the data to be communicated within electrical networks for automation, control and data logging purposes. This is the fundamental for the implementation of plug and play concept in microgrids.
The work presented in this chapter has made a very significant contribution to knowledge by demonstrating how a microgrid protection system can be modeled in accordance with the international communication standard the IEC 61850 and its recent extension IEC 61850-7- 420. Sample Data Maps are illustrated on a predetermined scenario which highlights different
aspects of the developed protection scheme. The details of simulation works are given in the next chapter.
Chapter 7
Microgrid Operation and Protection Simulations
Publications pertaining to this chapter:
1) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh, "Simulation of Communication Infrastructure of a
Centralized Microgrid Protection System Based on IEC 61850-7-420", in Proceedings of Third IEEE International Conference on Smart Grid Communications (SmartGridComm), Tainan City, Taiwan, 5-8 Nov, 2012.
2) Taha Selim Ustun, Cagil Ozansoy, Aladin Zayegh, "Investigation of Micro-Grid Behavior While
Operating Under Various Network Conditions", in Proceedings of IEEE International Conference on Smart Grid Engineering (SGE’12), UOIT, Oshawa, Canada, 27-29 August, 2012
7.1. Introduction
Preceding chapters have presented the novel microgrid protection scheme developed during the course of this research. Chapter 2 discussed the microgrid concepts and fundamentals from a number of aspects. Chapter 3 outlined how a conceptual microgrid case study was developed. Chapter 4 investigated the use of OO modeling of microgrid systems. Chapter 5 discussed the development of an adaptive relay protection parameter assignment logic. Chapter 6 explored the modeling of microgrid communications as per the IEC61850 standard. This chapter gives the details of the simulation works undertaken to validate and confirm the intellectual contributions made in the preceding Chapters.
Firstly, general microgrid simulations have been performed to investigate the microgrid behavior under various conditions. Microgrid – utility grid connection has been switched on and off to examine the differences between grid-connected and islanded operation modes. Utility grids with different sizes have been implemented to emulate the impacts of the microgrid connections within transmission and distribution networks. Furthermore, the dynamic nature of the microgrids has been reflected by changing the structure of the connections and analyzing the power flow in the system.
Due to their promising potential in the car market, EVs have also been considered in these microgrid simulations to investigate the effect of Vehicle-to-Grid (V2G) and Grid-to-Vehicle (G2V) technologies on current microgrids. The same microgrid structure has been utilized with the initial topology used for microgrid behavior investigations to be able to contrast the results.
Following these investigative simulations, the protection scheme proposed in the preceding chapters has been implemented for further verification. The individual control and communication blocks of the proposed communication-assisted protection scheme have been modeled and simulated for various microgrid events. Control signals have been named in accordance with IEC 61850 standard and its recent extension IEC 61850-7-420. The results show that the modeling, control signals and the adaptation of the protection system to the changes in the microgrid are satisfactory.
In conclusion, the simulation works have been performed over a wide range of topologies and operating conditions. The results have shown that the preliminary assumptions made during the design of this adaptive microgrid protection system were factual and the overall design
successfully serves to adjust this protection system according to the events occurring in the microgrid.