Application of Converter Transformer Controlled Switching In Nelson River Bipole III HVDC System

摘要

Energization of HVDC converter transformers can generate large transient inrush currents resulting in severely depressed voltages. In addition to power quality concerns in a typical ac system application, considerations have to be given to HVdc performance as voltage depression may cause commutation failures resulting in large temporary power losses. For the Nelson River HVdc system, a voltage depression on Bipole III inverter bus may propagate to nearby tightly coupled Bipole I and II affecting overall HVdc system operation. Switching of transformers with pre-insertion resistor, controlled point-on-wave (POW) switching, or a combination of both can be adopted to mitigate such adverse impact. Bipole III is designed to have pre-insertion resistors at Keewatinohk (rectifier) and POW switching at Riel (inverter) for energizing converter transformers based on technical and economic considerations. There are several POW switching schemes available. The simplest scheme uses fixed closing instants but disregards the impact of remanence. In the second scheme, controlled breaker opening is used to control residual flux to a pre-defined state (such as minimum value), which can be used to select optimal closing instants. However, the remanence cannot be controlled during a protection trip due to the loss of controlled opening. For Bipole III, the POW switching algorithm calculates remanence through integration of the open-circuit voltage measured by dedicated potential transformers and determines optimal closing instants. Such an approach reflects the latest technology applied so far in HVdc systems. This paper discusses the need for POW switching for converter transformer energization at Riel. Various operational scenarios (normal opening, ac faults and dc faults) are evaluated in detail through PSCAD simulation to confirm the accuracy of residual flux assessment and effect of POW switching. Bipole III converter transformers have a wide range of tap positions and the lowest tap position is designed for transformer energization. The paper evaluates the impact on POW switching when different tap positions are used for de-energization of converter transformers and the subsequent energization.