Investigation of the necessary modeling depth in transmission systems with mixed OHL-cable configuration

Traditionally, Extra High Voltage (EHV) transmission systems consist exclusively of Overhead lines (OHL) but in the recent years the utilization of High Voltage AC (HVAC) power cables is increasing due to environmental, political and operational aspects. For transmission networks consisting of purely OHLs there are proposed guidelines concerning the modeling depth needed for acquiring accurate results when simulating transients. However, these guidelines might not be adequate when mixed OHL-Cable-OHL configurations are present in the studied network. This paper presents two methods to evaluate the required modeling depth of a transmission network consisting of both cables and OHLs when studying slow front transients and more specifically line energization. The first method evaluates the maximum overvoltage caused by the energization while the second method evaluates the harmonic content of the energization. The simulations are performed using software package PSCAD/EMTDC. It was found that the modeling depth for both objectives exceeded the advised depth proposed in the guidelines for purely OHL connections.

I. Introduction

In the past, OHLs were the dominant means for transmitting electrical energy at HV and EHV levels. However, some weather-related incidents, technical changes, strong competition in the cable sector, increased urbanization as well as other political and environmental aspects are leading to an increasing utilization of HVAC underground power cables [1]. More specifically, in the west of the Netherlands a 10 km 380 kV power cable is in operation since May 2013 (Randstad380 Southring) and another 10 km 380 kV power cable connection (Randstad380 Northring) is going to be realized in 2017 (Fig. 1). Due to the limited experience in the Netherlands with underground power cables at 380 kV, the Dutch TSO (TenneT) decided to research switching operations in HVAC cables, in order to study possible undesired overvoltages and resonance phenomena.