Recomendaciones

The following subsections introduce some important characteristics of a Low-Voltage (LV) distribution network, as they pertain to the operation and protection of a microgrid. The characteristics will be considered in Sections 4.3 and 4.4, for the development of the proposed protection strategy.

4.2.1

Structure

An LV microgrid is based on a designated area of a secondary distribution network which is supplied by a step-down transformer. By assumption, the designated area embeds sufficient amount of generation and is thus able to operate in isolation from the rest of the network. The rating of the step-down transformer, which connects the primary (MV) network to the secondary (LV) network, is typically from a few hundred kVA to several MVA [77], [78]; hence, the peak power demand of an LV microgrid is assumed to be limited to a maximum of several MVA. It is further assumed that microgrid loads are supplied by a number of radial Secondary Mains (SMs), which may be branched by one or more laterals, and that the presence of single-phase loads and/or DERs makes the LV microgrid an inherently unbalanced network.

4.2.2

Conventional Protection

In general, simple overcurrent devices, most commonly in the form of fuses, are employed in secondary distribution networks to protect equipment and ensure safety. Secondary network conductors are typically protected by the so-called limiters. A limiter is a high- capacity fuse that is installed on each phase conductor of the SMs at each junction point. The step-down transformer is protected by a network protector, which is an LV air cir- cuit breaker with a tripping/closing mechanism controlled by a self-contained relay. In addition, the network protector has fuses that provide backup protection for the step- down transformer [86]. Since the fault should rapidly be isolated by the limiters, before the network protector operates, the time-current characteristics of the limiters must be coordinated with those of the network protector [86], [87]. This practice ensures that the smallest possible area of the secondary network is de-energized in response to a fault inci- dent. The secondary side of the step-down transformer may not necessarily be protected

by dedicated equipment, as the SMs are commonly equipped with corresponding dedi- cated network fuses [78]. Hereafter, due to its more common usage, the term “network fuse” or “fuse” is used instead of limiter.

4.2.3

Grounding Practices

An LV microgrid is subject to the same safety requirements and standards as those set for a conventional secondary distribution network. In a microgrid, a fault incident may result in a substantial ground voltage, even if the DERs operate at low voltages [88]. Moreover, the neutral grounding practice in a microgrid can affect protection. Therefore, the grounding strategies of the equipment in an LV microgrid must be adopted judiciously. Typically, a ∆/GY winding configuration is used for the step-down transformer. Thus, either of the two grounding strategies TT and TN [88] can be adopted, since the ground will be available at the LV side of the step-down transformer, even if the microgrid is islanded. However, compared to the TT method, fault currents are higher in the TN approach, due to the low-impedance return path (neutral or protective earth conductor) that exists in the TN method. The reason is that, in the TN approach only a small fraction of the fault current is diverted to the ground, but the rest flows through the neutral path. This characteristic enables the use of ground-fault relays in the TN approach, for the neutral conductor of an SM. For the reason mentioned above, the popularity of TN method [68], and the system safety requirements, the TN-C-S [89] grounding configuration of Fig. 4.1 is assumed in this study. The LV network grounding techniques and their corresponding terminology can be found in [88] and [89].

4.2.4

DER Interface Mechanisms

The DERs of an LV microgrid can be of the single-phase or three-phase type, based on rotating machines or interfaced through power-electronic converters of the VSC types. A single-phase DER is connected between a phase conductor and the neutral conduc- tor, typically through a single-phase isolation transformer. Thus, the TN-C-S grounding strategy ensures that the DER can contribute to a phase-to-ground fault current, through the low-impedance path of the fault current loop. The TN-C-S grounding configuration can also be adopted for a three-phase DER for which a ∆/GY interconnection trans- former is employed.

4.2.5

DER Control Strategies

In this chapter, the droop-based voltage/frequency regulation strategy of Section 2.5 has been employed for the EC-DERs. The conventional rotating-machine-based DERs are also droop-controlled, that is, the excitation and governor systems of the machine are also included in their model. The control scheme of each DER embeds a respective synchro- nization mechanism for safe reconnection of the islanded microgrid to the utility grid. To ensure that traditional basic protection functions, more specifically the directional function, can also be employed here, the built-in controls of DERs are designed in such a way that the DERs behave similarly to the conventional synchronous machines, in the sense that they more or less maintain the balance of their terminal voltages when an asymmetrical fault strikes the network; in an EC-DER, of course, the magnitude of the terminal voltage drops to limit the fault current contribution (see Sections 2.4 and 2.5).