Impact of non-simultaneous P/f and Q/V grid code requirements on PV inverters on unintentional islanding operation in distribution network

Abstract

The analysis of unintentional islanding in distribution grids with a large penetration of renewable energy sources is of increasing interest due to the recently introduced requirements of P/f and Q/V supporting capabilities of Distributed Energy Resources (DERs) and the wider frequency and voltage ranges, DERs should be able to operate with, before protection tripping. DER anti-islanding protections, if present, may fail to detect this grid transition and so uncontrolled islanding operation may appear. Since this operating mode may be dangerous for the electric network operation and may cause damages for the grid equipment, in this paper the increase of its risk due to P/f and Q/V droop characteristics of generators is addressed. Analytic results are initially derived for the steady-state unintentional islanding operation and then they are validated on a laboratoryscale grid setup. The proposed investigation will show that the risk of permanent unintentional islanding increases introducing P/f or Q/V droop regulation and that such risk is maximum including both the P/f and Q/V droop regulations, rather than only one of them. I. INTRODUCTIONNowadays the employment of renewable energy resources is driving an increase of the amount of embedded generation that is connected to the Medium Voltage (MV) and Low Voltage (LV) distribution networks via power electronic interfaces (inverters). New European standards state the reference technical rules for the connection of active users to the grid and for their behavior during temporary voltage and frequency variations [1]- [3]. Moreover, these standards together with some country-level ones are imposing the participation of Distributed Energy Resources (DERs) to the voltage and frequency regulation to improve the system stability, through the P/f and Q/V droop characteristics. One of the most relevant modifications is the extension of the frequency range that is allowed during normal operation of DERs from the traditional thresholds 49.7 Hz and 50.3 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 ( Fig. 1) [4], [5].

The Integrated Switching Control Strategy for Grid-Connected and Islanding Operation of Micro-Grid Inverters Based on a Virtual Synchronous Generator

“…According to (18), it is obvious that there is a certain mathematical relationship between the change of frequency and voltage and the variation of active power and reactive power. Therefore, the islanding phenomenon can be detected through frequency or voltage amplitude changes.…”

“…This method, however, has disadvantages of large non-detection zones (NDZs) and slow detection speed. The introduction of the droop characteristic will increase the NDZ of the micro-grid inverter, which is pointed out in [18]. To solve this problem, according to the linear relationship between frequency and active power in droop control, an islanding detection method based on positive feedback of frequency is proposed in [19], which can reduce the time required for detection.…”

Computational geometry‐based methodology for identification of potential islanding initiators in high solar PV penetration distribution feeders

“…Thus and , specifically for the constant current load at 692 as the one at node 675 was removed from the model. Since the islanding behaviour of multiple inverters on a feeder section is peculiar and unpredictable [11], only a single installation was used and the loads were scaled accordingly to represent an instantaneous value on a modelled instant where P match takes place as creating a 100% match due to cumulative penetration could lead to circulating currents among inverters during under‐voltage disturbances. The Q values were changed to match those supplied by the feeder capacitor banks, present already, and the inverter filter circuit.…”