Since all the switching control and relay models were implemented inside the real-time environment, the wide area protection system was designed to test the new protection schemes for wide area systems.
Power System
G External Grid DG PMU PMU PMU PMU PMUWide Area Monitoring, Protection and Control
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For the test-bed which has been introduced in figure 5.1, an experiment was performed involving the outage of the slack generator G1. The event was the disconnection of the slack generator, G1, which will cause frequency drop of the whole system since generation and demand balance is no longer maintained. The frequency functions of all generators relays are activated. For G2 and G3 frequency settings considered 59 Hz and for G4 is 58 Hz. So, whenever the frequency drops, the monitoring system will calculate the frequency and the relay function will detect the situation and will send the trip signal to the breaker right away. Figure 5.36 shows this procedure with all generators’ relay trip signals and breaker one, respectively in left and right plots. Here, G1 was disconnected at 2:26’:22”.245 and G2, G3 and G4 relays are detected at 2:26’:23”.032, 2:26’:23”.012 and 2:26’:23”.567, respectively. Because of the frequency setting for G4, it detects the event with a larger delay. Figure 5.37 presents the active power change as well as frequency drop for this event for all generators. The generator's G1 frequency was returned to the no load frequency after disconnection from the grid and the other generators’ frequency started to drop until the relays trip them from the grid. The remaining generators take the load share of the disconnected generators. For instance, after disconnection of G1 and then G2 and G3, suddenly, G4 which was producing 250-W in normal condition takes the active power load of whole system (1300 Watt) and leads to large drop of frequency (48-Hz). A wide area protection algorithm was also implemented for this case, which is the disconnection of all generators when G1 is disconnected from the grid right away. In this case we don’t need to worry about the frequency relays settings and their operation delay. Consequently, all the power system data are available by the implemented PMUs for monitoring the system and hence any
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kind of intelligent and smart wide area protection system integrated with system controllers can be developed in real-time to enhance power system operation.
(a) (b)
Figure 5.37: (a) Active power and (b) frequency of all generators during G1 outage (a) (b)
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This system can be used for studying integrated wide area control and protection system to monitor the system status for abnormalities such as over/under voltage, overloads of equipments and any other conditions. In the implemented example, when the loadings of a power system line increases, the relay indicated the system moving condition to the fault situation which may be disconnected by available protection devices. Hence by setting the protection relay under the settings of the physical relay, the control scheme can retrieve the normal status by proper controlling action. This case may be the changing of system topology or power dispatching in alarms in a control center, self healing strategies can maintain the system continuous operation with appropriate control scheme.
The developed system can be used for cascaded failures detection and applying proper remedies on the power system. Following a disturbance, one or more components overload and hence fail. The equilibrium of the load flow will consequently change and the load will then be redistributed to other normal components and this makes additional load transfer to other elements. Thus, a cascading failure is triggered by the overload failures and it cause network's collapse resulting in a blackout. The PMUs data can be used to follow the phase angle of each bus to detect the system failures which may cause cascaded events. In this process, we need to detect upcoming faults by network data and analyze the network reaction to this outage by some algorithms such N-1 Contingency, Fault calculation, OPF, etc. Therefore, the online calculation software is used to analyze system situation and will be presented in next chapters.
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CHAPTER 6
DESIGN AND CONTROL OF HYBRID AC/DC SYSTEMS
6.1 Introduction
DC distribution systems have been proposed to address the challenges of increased energy demand. These systems can also be used to improve or maintain system reliability. DC microgrids and DC distribution systems succeeded to gain popularity and reacquire the interest of several researchers and industry entities. High penetration of stationary renewable energy sources yielding DC output, increased importance of storage elements and increased number of electronic loads, machine drives and other loads operating with DC input are all essential reasons for this reconsideration of DC networks. Moreover, several systems are currently employing DC distribution systems already. For instance, in systems that require high reliability and are involving a big number of electronic loads such as data centers, DC provides a more efficient solution for electric power distribution. Another important example is shipboard power systems with implemented medium voltage DC distribution, zonal DC distribution or other types of DC distributed architectures for power delivery on board. This increased importance of DC distribution systems have encouraged researchers to investigate issues related to them. The hybrid system was discussed in chapter II of this dissertation and its advantages and disadvantages were presented as well. In this chapter, we will focus on the implementation of a DC microgrid setup in test-bed as a Hybrid AC/DC microgrid. This system was implemented and presented in details in [135]-[140] including the results of test and verification for the operation of DC microgrid.
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