Transporting the “harvested” energy from offshore wind farms over great distances to the densely populated areas where electricity demands are high is a difficult undertaking. Subsea high voltage direct current (HVDC) transmission lines are the future for these underwater “electricity superhighways”. HVDC can transport more electricity, but almost more importantly, more efficiently than traditional AC transmission lines. A 2018 study conducted by the U.S. Energy Information Administration states:
DC transmission lines are more cost effective over long-distance applications, have lower electricity losses, are more suitable for underwater applications, can handle longer periods of overload operations, and can prevent cascading failures that propagate across the electric grid.
The demands for “greener” energy generation therefore also require “greener” energy transmission through the use of extra-long HVDC transmission lines.
A typical cross-section of an offshore wind farm power transmission can be seen below, which is from the wind farm “Sofia” off the coast of the UK in the North Sea. Electricity will be generated from the turbines of the wind farm and transported through subsea cables to an offshore converter substation platform where it will be converted from AC to DC. The electricity will then be transported via a 227 km (141 mile) subsea HVDC transmission link to an onshore converter substation, where it will be converted to AC again and fed into the existing AC distribution grid.
The erection of these wind farms and converter substations, as well as the laying of subsea cables is a vastly expensive undertaking. It is imperative that the HVDC cable assets are tested properly at the cable manufacturer to ensure no manufacturer defects are present and also on-site while the cable is laid to ensure no transportation damage has occurred. Furthermore, if a fault does occur while in service, a quick response to locate and repair is imperative so that the transmission of power can continue and potentially huge operating losses are kept to a minimum.
HVDC Cable Testing Solutions from Haefely
The PGR High Voltage DC Test Systems are designed to perform ultra high voltage direct current voltage tests on a variety of electrical assets, including bushings, transformers, and cables. The PGR is built with standard modules of 400 kVDC with the advantage of being able to stack modules to perform tests up to 2,000 kVDC. The modules are built on a frame with a small footprint to optimize test floor space in the laboratory or for designing mobile systems to be used on site. During the DC voltage test the PGR can also be used together with the DDX 9121b Digital PD Detector as a quality control test to measure partial discharge within the insulation of the HVDC cable under test.
Custom Cable Fault Location Systems from BAUR
Traditional TDR-based cable fault locators typically have an accuracy of ± 1%, which can be quite a significant length for extra-long submarine HVDC transmission lines that have lengths in the range of hundreds of miles. Accurate pinpointing (see Protrac) can be achieved fairly quickly and cost effectively for land cables. However, pinpointing of subsea cables becomes far more time-consuming and expensive usually requiring investigations with remote operated underwater vehicles. Every operational downtime amounts to significant losses to the cable operator, as all the energy harvested by the turbines cannot be transmitted. Another aspect to consider is when a fault does occur, the cable fault location equipment must first be transported to the converter substations, further increasing precious downtime of the cable. Especially for the offshore converter substations, this means transportation by ship or helicopter.
The most time-saving measure a wind farm operator can conduct to reduce the downtime created by a fault is to install stationary cable fault location measurement systems. This saves significant time that would otherwise be lost while transporting the required equipment. Moreover, installing cable fault location systems at both ends allows for the ability to measure from both ends significantly improving measuring and fault positional accuracy.
BAUR offers a number of custom cable fault location solutions for extra-long HVDC cables. The powerful IRG 4000 could be used to conduct “fingerprint” analysis of the cable over the course of its lifetime. Complete cable fault location systems could be fitted in cable test vans to quickly mobilize to the onshore converter substation. Systems could also be built on trolleys that are stored in converter substations and used when required. If space is a factor, container solutions are also available so that the equipment could remain outside of the converter substations, regardless of weather conditions.
Below are examples of mobile and container cable fault location systems: