July 6, 2016

Cost-effective and fail-safe integration of distributed grid infeed

Abstract Grid expansion represents a technically obvious but also very expensive solution for the infeed of renewable energy sources. A more intelligent solution is the...

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Abstract

Grid expansion represents a technically obvious but also very expensive solution for the infeed of renewable energy sources. A more intelligent solution is the implementation of controllable distribution transformers whose ratio can be controlled under load, as well as voltage regulators used in the medium voltage grid.

Keywords: voltage regulation, renewables, distribution grid, regulated distribution transformers

Introduction

Photovoltaics, biogas and wind turbines save resources and reduce CO2 emissions. However, they also result in fluctuating energy flow directions, as well as load and voltage fluctuations. Traditional distribution grids with their conventional distribution stations and classic transformers are not designed for these conditions. Capacity could certainly be expanded, but this does not resolve the core issue. The concept was developed for a world with a small number of large power generation facilities and a large number of loads. While expansion of the grid would cover this inherent system deficiency, it would not correct it. Grid operators are currently faced with completely different tasks, being obligated to provide for the infeed of renewable power sources. With no affordable intermediate storage devices currently available, they must also harmonize supply and demand of power, and smoothen peaks using power generation and load management.
With their variable voltage under load, FITformer REG – a regulated distribution transformer – and voltage regulators are two of the available solutions providing a supplement for grids with voltage range issues. Both solutions offer a cost-effective alternative to traditional grid expansion and are easily integrated in existing structures.

Better voltage control with distribution transformers for the MV/LV grid (e.g. 20 kV/0.4 kV)

There are currently different concepts for regulated distribution transformers, where the integration of a tap changer on the high-voltage side of the distribution transformer appears to be the simplest solution for many operators. These units are essentially small versions of controllable power transformers – a concept known from large transformers and therefore easy to understand. The advantage of this solution is precise control enabled by the tap changer on the high-voltage side. However, this requires that the operator accepts somewhat higher losses, caused by the tap changer design. It includes additional reactors, as well as the risk of feedback loops with the medium-voltage transformer occurring when the regulation of the distribution transformer takes place in small steps once a small voltage change from the MV grid happens.

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