Performance Investigation of a Monitoring Scheme for Low Voltage Grids with a Single Grounded Neutral

Abstract

Low voltage distribution grids (LVDGs) are evolving from their traditional passive behavior into highly active systems. Their secure and reliable operation will require effective control schemes which in return require reliable and accurate monitoring systems. Most of the related existing works rely on the threephase Weighted Least Squares (WLS) state estimation method which is used mainly on Medium Voltage Distribution Grids (MVDGs). However due to different characteristics between MVDGs and LVDGs this method may not be suitable for monitoring LVDGs. This paper investigates the performance of the WLS state estimator on a radial low voltage test feeder and due to the random nature of the measurement errors, the analysis is conducted in a Monte Carlo fashion. The investigation indicates that a significant error is imposed in the monitoring of the LVDG which is related to the loading conditions and the asymmetric operation of the distribution grid.

State Estimation for Distribution Grids with a Single Point Grounded Neutral Conductor

“…It can be observed that the proposed monitoring scheme with the W1 weights is considerably more accurate compared to the . As it was shown in [14], the performance of is highly affected by the zero-sequence current that is flowing in the system and therefore depends on the operating conditions. In 1 , since the neutral voltage is considered in the problem formulation, its performance not only is 6 times higher (in average) than but also significantly more stable as its affected by the operating conditions in a considerably lower degree.…”

“…In fact, in Cyprus and in many other countries the neutral conductor in a LVDG is only grounded at the MV-LV substation. If such a DSSE is used in this kind of LVDGs, then a significant error is imposed on the estimation results [14]. The main reasons for this are: (1) the inaccurate system model due to the Kron's reduction and (2) the phase-toneutral voltage measurements.…”

“…In the case of a four-wire LVDG in which the neutral conductor is grounded only at the transformer substation, this assumption is not valid, and its use can lead to inaccurate results [5]. As a matter of fact, in [14], it was shown that due to the significant zero-sequence current that flows through the neutral conductor (and consequently the voltage drop across it), the yields inaccurate results. Therefore, in such LVDGs the use of Kron's reduction method should be avoided as the resulting system model will not be an accurate representation of the physical system.…”

Operation of a Low Voltage Distribution Grid in Cyprus and the Impact of Photovoltaics and Electric Vehicles

“…According to the transmission/distribution regulations in Cyprus, the phase-to-neutral voltage should be 230 V ± 10%. However, due to the highly asymmetric loading conditions causing a non-negligible neutral to ground voltage at each node, the phase-to-neutral voltage can exceed these limits even if the phase-to-ground voltage is well within the admissible interval [11]. This is illustrated in Fig.…”

Power Flow for a Four-Wire Radial Low Voltage Distribution Grid with a Single Point Grounded Neutral

“…Due to the unbalanced nature of the LVDG and the mainly single-phase loads (asymmetric loading conditions), a significant portion of the total load current flows in the neutral conductor. Especially in a system that the neutral conductor is only grounded at the MV-LV transformer substation, this gives rise to a significant voltage drop across it [11]. By using Kron's reduction to simplify the problem and merge the neutral conductor with the phase conductors not only will yield incorrect results [12] but also important information will be lost as most loads in these kinds of systems are connected between a phase conductor and the neutral conductor.…”