• No results found

with eigenvalue analyzes in [67]. An alternative to thedq transformation comes from [68], that addresses the sinusoidal time-varying behavior of three-phase AC systems via the harmonic linearization approach, introducing some approx- imations.

Balanced and symmetric three-phase AC power systems have been studied and analyzed and well-established techniques for their stability analysis exist. On the other side, single-phase AC system studies are more difficult because the identification of a precise operating point where to perform a linearization is not straightforward: for instance thedq transformation is not easily defined for them. For this reason, different points on the single-phase AC stability analysis are still open and they are beckoning the interests of the research community. These studies are interesting because they allow to address stability issues in LV microgrids, that usually include single-phase connected DERs, and other applications as in the railway system [69].

1.3

Unintentional islanded operation

New European standards state the reference technical rules for the connec- tion of active users to the grid and for their behavior during temporary voltage and frequency variations [70–72]. These standards together with some country- level ones are imposing the participation of DERs to the voltage and frequency regulation, through the P/f and Q/V droop characteristics (Sec. 1.1.3). One of the most relevant modifications is the extension of the frequency range that is al- lowed during normal operation of DERs from the traditional thresholds49.7 Hz and50.3 Hz to the less stringent values 47.5 Hz and 51.5 Hz, and the extension of the voltage levels to the range±15% of the rated voltage [7, 73]. Transient and steady-state voltage and frequency allowed ranges are depicted in Fig. 1.15 for the Italian standard [7].

The power electronic interfaces introduce fast dynamics into the grid and could continue to energize a portion of the electric network also when the main grid disconnects, even if the islanded mode is not explicitly pursued and im- plemented. Thus, undesired islanded portions of the grid can be still oper-

51.5 47.5 50.3 50 49.7 4 t [s] f [Hz] Restrictive thresholds Permissive thresholds 0.2 0.4 0.4 0.85 1 1.15 t [s] V [p.u.] 0.1 1 t [s]

Figure 1.15: Voltage and frequency thresholds imposed by the Italian standard [7, 74]

ating [75, 76], even if currently the distribution grid is not designed for such operation: this operation is called unintentional or uncontrolled islanded opera- tion [73]. Unintentional islands can make damages on the electric equipment of the grid when these parts of the grid have to be reconnected to the mains, since auto-reclosing switches and rotating generators/loads may not be designed to sustain such transient conditions [75, 77].

Different anti-islanding techniques have been proposed so far and imple- mented in the interface inverters for DG applications. Anti-islanding schemes are usually divided in passive, active, hybrid, and communication-based ap- proaches [78–84]. Passive techniques consist of measuring a certain system pa- rameter, such as frequency or voltage, and comparing it with a predetermined threshold. The islanding detection happens if this parameter is out of this prede- termined range. In active methods, the control of the inverter tries to drift some parameters, such as frequency or voltage, or to inject disturbances, as nega- tive sequence components, in order to move the system parameters far from the nominal ones or to destabilize the system itself when the islanding happens. Communication-based methods send and receive signals between different de- vices in order to detect the transition. Active methods are quite attractive be- cause they are less expensive compared to communication-based approaches and they are more effective compared to passive methods. Recently, hybrid passive-active methods can combine advantages of both approaches [85].

Performance evaluation of anti-islanding schemes are usually based on two characteristics [76, 86]:

1.3 Unintentional islanded operation 23

• speed of detection, which is the time interval between the actual islanding instant and the islanding detection instant

• Non-Detection Zone (NDZ), which is a region specified by the system parameters, in which islanding detection fails

Standard [87] suggests the use of a parallel RLC load with a resonance fre- quency equal to the line frequency to test the anti-islanding technique effective- ness, because it can provide higher stabilization of the islanded system. Some interesting studies on the determination of NDZ can be found in [82, 85, 88, 89]. However, when there is a large number of DERs, the behavior of all these provisions, being different from each manufacturer and not specified by stan- dards, is unpredictable and in some cases these techniques can lack to detect the operating mode and so the disconnection of the inverter may not happen [90]. Moreover, in some countries such anti-islanding provisions are not mandatory on MV connections.

In order to address the unintentional islanding issue, the problem can be separated in two sub-problems: the permanent and temporary unintentional islanded operations. The risk of permanent islanding operation is the risk of a steady-state Unintentional Islanded Operation (UIO). With other words, perma- nent islanding means that the steady-state frequency and voltage of the islanded system remain within the allowed thresholds, for instance those in Fig. 1.15. In particular to maintain this operation, the stability of the islanded system is required [8, 73].

The UIO may be dangerous especially in presence of automatic reclosing procedure. This automatic procedure is adopted in some distribution networks of European countries and has the purpose of a fast localization and separation of the faulted line segment of an MV distribution network [7, 91]. It consists of opening and reclosing the breaker of an MV line for established time intervals in order to extinguish faults, for instance single phase to earth faults. This pro- cedure can introduce additional risks for the equipment when UIO happens, be- cause of possible out of phase reconnections. In Fig. 1.16 there is an illustration of the automatic reclosing procedure for the Italian distribution network [91]: observe that the first opening of the breaker lasts for600 ms.

SS 3 SS 2 SS 1 6 s 12 s 18 s 20 s 0.6 s 30 s 70÷ 120 s RR 0.3 s 0.3 s 0.3 s SR TD2 = 5 s SR TD1 = 0 s SS 1◦level opening SS 2◦level opening SS 3◦level opening 1◦PS Circuit breaker opening Last PS Circuit breaker opening Fault t = 0 s PS Opening PS breaker RR MV line

Figure 1.16: Automatic reclosing procedure for fault extinguishing [91]

To address the automatic reclosing procedure issue, the temporary islanding problem has to be considered, as the operation in islanded condition for a short time interval after the disconnection [73]. The study of this problem consists of to understand if the disconnecting transient, i.e. the voltage and the frequency transients, fulfills the thresholds defined by the standards, e.g. those of Fig. 1.15. The considered time interval should be those of the automatic procedure, e.g. 600 ms.

The risk of unintentional islanding should be studied in detail and properly reduced in a distribution grid scenario with more and more DERs. Furthermore, the impact of new standards for DER connection should be evaluated in rela- tion to the unintentional islanding risk, in particular considering the effect of the introduction of P/f and Q/V droop characteristics, which may stabilize the islanded operation leading to an increase of this risk [7, 70–72].

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