As it is discussed in detail, the dynamic structure of microgrids and their versatile operation conditions necessitate the development of alternative protection strategies. Figure 3.6 illustrates the microgrid protection system developed in this research to tackle protection issues encountered in microgrids.
In this developed system, the Microgrid Central Protection Unit (MCPU) communicates with every single relay and DG in the microgrid on interruption basis. The communication with relays is necessary to update the triggering currents of the relays and to detect the direction of fault currents and thus isolate the fault properly. Furthermore, time delays required for proper selective operation of relays is ensured through communication lines. On the other hand, the MCPU communicates with DGs to record their status as ON/OFF, their rated currents IratedDGx
and their fault current contributions IfaultDGx. As it will be discussed, these parameters are
essential for safe and reliable operation of the developed protection system.
Use of MCPU is very advantageous for protection of microgrids. The changing nature of microgrids and the variety of microgrid devices require that the microgrid protection is capable of monitoring and following these changes. The ability to communicate with DGs, loads and relays gives MCPU the possibility to be aware of new deployments, connection changes and changes in operating conditions of microgrid components. It is very difficult to handle all these events with a decentralized protection approach. Because this would require pre-estimation of all cases that may occur in the microgrid. Moreover, a new deployment of equipment or a new installation of transmission/distribution lines would render previous protection approach obsolete. Therefore, the use of MCPU in the proposed protection scheme is necessary to be able to operate microgrids under these changing conditions.
In other words, MCPU can be implemented as a real-time protection controller. Other protection approaches require a list of all cases that may occur inside a microgrid in the form a look-up table or in a database. As explained above, microgrids are not predictable as conventional, passive networks. The connection of DGs increase the amount of players, change the power flow directions, fault current contributions, fault levels for proper
protection etc. Whenever a DG is turned on, all of these parameters are changed and the protection system needs to be updated in real-time. Making it even more difficult, new deployments in a microgrid change all of the predetermined cases. Consequently, a non-real time protection approach would have serious difficulties in following these developments and in adjusting the protection system accordingly. Being a real-time protection controller gives a solid upper hand to the proposed scheme.
As it will be explained in Chapter 5 and Chapter 8, although the proposed protection system requires communication between MCPU and microgrid components, continuous communication is not required and the decisions are made locally. This effectively reduces the dependence on communication and increases the reliability of the proposed system. Another benefit of making decisions locally is negating the effect of communication delays over the communication lines. The proposed system requires communication to update the protection parameters of relays following a change occurring in the microgrid. When a fault occurs, relays operate individually depending on the last operating condition. The amount of data sent and the communication delay experienced over the communication lines are not crucial for the clearance of the fault. This is another benefit of the proposed protection system with MCPU.
The data map of the MCPU is shown in Table 3.1. Information on each component is stored with the relevant control variables. The very first variable of the central unit is the operating condition of the microgrid. Once the microgrid is islanded or re-connected to the grid, the status of the relay R1 is handled as an interrupt by the central controller algorithm. New operating fault currents of relays will be calculated by considering the fault contribution of
the grid, i.e. IfaultGRID and updated. The status of the microgrid is stored in Operating Mode bit
as `1` for Grid-connected operation and as `0` for islanded or stand-alone operation.
TABLE 3.1.DATA MAPS IN THE CENTRAL PROTECTION UNIT
Grid-Connected Islanded
Operating Mode 1 0
Grid Fault
Contribution IfaultGRID Relays Operating Fault
Current
Fault Detection (1 – Y, 0 – N)
Time delay for Selectivity
R1 IR1 0 t1
R2 IR2 0 t2
R3 IR3 0 t3
…
DGs Irated IfaultDGx Status (1-ON, 0-OFF)
DG1 IDG1 IfaultDG1 1
DG2 IDG2 IfaultDG2 1
DG3 IDG3 IfaultDG3 0
DG4 IDG4 IfaultDG4 1
DGs will be monitored and two different current values are stored. One of these is the rated current value of the DG, Irated, whereas the second one is the calculated fault current
contribution of that particular DG, i.e. IfaultDG. The fault current contribution of DGs is
calculated from its rated current value. A third parameter is required to track the connection of DG with the microgrid. Status variable is introduced for this purpose and determines whether a DG will contribute to the fault current in case of a fault in the system.
In Table 3.1, the status of DG3 shows that it is not in operation. This may be due to maintenance, the intermittent nature of RE resources (no sun or wind) or the excess local generation. In case the local consumption increases and DG3 is put back into operation, it immediately sends an interruption signal to the MCPU. MCPU is informed that a new
generator with separate fault current contribution has been connected to the network and thus, needs to be taken into account. The new fault current contribution IfaultDG3 is updated in the
relay operating currents data. Similarly, if DG2 is shut down for a certain reason, it then reports to the central unit that it is no-longer in operation and will not contribute to any fault in the network. Its status bit will then be changed to 0 and new fault current calculations will be performed without IfaultDG2. In this fashion, individual contributions of DGs are handled
and the aggregate affect thereof is reflected on the relay settings to adjust them for the changing conditions of the microgrid.
Three parameters are used for relays in the MCPU. `Irelay` is the operating current of the relay
which is used to program the relays, `fault detection bit` shows the fault detection status of the relay and `time delay for selectivity` indicates the time delays assigned to each particular relay for proper selective operation.
For a particular relay, the operating fault current (i.e. the current level that causes the relay to trip) is calculated as shown in (3.1).
(3.1)
where m is the total number of DGs in the microgrid,
ki is the impact factor of ith distributed generator on the fault current of the relay,
IfaultGRID, IfaultDGi and StatusDGi are as described above.
If the microgrid is operating in islanded mode, then the grid‟s fault contribution will be multiplied with the „Operating Mode=0‟ bit which will be equal to 0. Likewise, the fault
In small microgrids, it may be assumed that the distances between components are small and the fault contribution of a certain distributed generator will be the same for all parts of the microgrid. In this case, the equation may be simplified by taking k = 1, i.e. it is assumed that the fault current contribution of any particular DG is constant for all relays in the network. In larger or more complex networks this may not be the case and a combination of various fault current contributions along with their impact coefficients might be required to be taken into account. This phenomenon is studied in detail in Chapter 5.
Determination of a distributed generator‟s fault contribution can be performed in various
ways. It can be determined by means of simulation studies which require the modeling of the microgrid. However, an easier approach might be adopted on the grounds that most of the DGs need PE interface for grid connection. It is well known that PE interfaces do not supply fault currents as rotating machines do. Their contribution can then be approximated to IfaultDG=1.2 * IratedDG [78, 80, 153]. Should this approach be implemented, new DG
deployments can be made without making fundamental changes in the protection system. They can be treated as plug-and-play devices once their rated currents and fault contributions are reported to the MCPU. MCPU can track their status bit and perform calculations according to the reported fault current contribution. Hence, the communication between a PE interfaced DG and the MCPU could be limited to only status updates.
The MCPU uses an interrupt based algorithm shown in Figure 3.7. Once a new connection or disconnection occurs in the network, an interrupt is received by the MCPU and new fault currents are calculated based on the current situation of the network. Then, new operating conditions are updated in relays for adaptive protection. It is worthy to note that once the MCPU performs these tasks and re-programs the operating conditions of the relays, they
operate independently to open the connections through CBs without the need for further communication. Once the current flowing over the relay exceeds the latest operating current received from MCPU, the relay opens the connection with CB and sends a signal back to MCPU in order to report the fault and set the fault detection bit.
Figure 3.7. Interrupt-based Protection Algorithm
Once there is a fault in the system, the relays on the fault path shall experience the fault current. Each relay shall wait for its own time delay to expire after the detection of the fault. These time delays are set to ensure proper selectivity in the system and stored in MCPU as well as in the relays. Obviously central relays such as R1 and R2 have larger time delays than those of located in branches such as R4 and R8. The system shown in Figure 3.6 does not require rearrangement of time delays for different operating conditions due to the structure of the microgrid. In a more complex system, this might be very well required. In this case, with
each interrupt signal MCPU does not only calculate the operating currents of the relays but also determines the selectivity hierarchy and calculates new time delay settings required. Then, these two parameters are updated in the relays. New Irelay value ensures that relays
detect the fault currents accurately while new time delay setting trelay ensures that proper
selectivity procedures are followed in case of a fault. Selectivity hierarchy detection is discussed in detail in Chapter 4 while time delay calculations are explained in Chapter 4 as well as Chapter 5.