Electrical Tester online
October 2018
Cable faults in older wind farms: Growing source of failures

Cable faults in older wind farms: Growing source of failures

01 October 2018

Megger cable test van systems reliably diagnose underground cables

Oliver Kausch - Head of Technical Management, WPD Windmanager

WPD Windmanager, a leading German wind farm operator, now routinely monitors the condition of buried power cables.

WPD Windmanager is the first German wind farm operator to have its own cable test van system. Since putting the van into service, we’ve been able to reliably locate cable faults in our wind farms and also to detect weak points in the cables before they result in failures. This is helping to protect our investors from the impact of expensive cable damage, and to maximise financial returns. We chose Megger test equipment for this important role because of the advanced, and in some cases unique, technologies it uses.

WPD Windmanager manages both the technical and commercial aspects of wind farms. Our headquarters are in Bremen, and we have around 350 employees in Germany and other countries. We look after 346 wind farms, with 1,940 wind turbines and total capacity of 3,856 megawatts. We are market leaders in Germany, and our clients include national, international and institutional investors. As well as providing operational management services for the wind farms entrusted to us, we give the highest priority to maintaining the value of the substantial investment these wind farms represent.

Cable faults are invisible until a failure occurs

We are increasingly focusing on underground cables in wind farms. These cables are sometimes neglected because they are not readily visible, but this is where problems start: usually there is no indication of a fault developing until it reaches the point of failure, and by then it is too late. The latest cable test technology makes it possible to avoid this situation. This is very important since a large number of wind farms have been in service for many years, and faults associated with ageing cables are becoming more and more common. We act in the best interests of our customers and take a preventative rather than reactive approach to faults. This is why we have invested in a cable test van system.  So far we are the only wind farm operator to do this, and it has already proved an excellent investment.

VLF tests to DIN VDE 0276 are not enough

Repair and maintenance costs are usually incorrectly estimated or not assessed at all. For wind farms, typically only VLF and cable sheath tests are carried out during commissioning, but this is not enough to realistically assess the installation quality of the cables laid. The reason is that installation errors, such as improper shrinkage, air pockets in the joint body or the inclusion of dirt often lead to partial discharge events that begin immediately after installation and have a cumulative negative impact on the insulation properties of joints from day one. VLF tests are not suitable for detecting these partial discharge events. Poor cable installation is the most common cause of cable problems, and it can seriously disrupt investors’ yield planning.

There’s always a lot of money involved

In our experience, faults always occur when they have the potential to create the most damage. In poor weather conditions, plants run at full load so the equipment and cables are under maximum stress. This is when faults occur and at these times they are inevitably very costly because the revenue loss is greatest. If the warranty on the plant has expired, the costs for locating and repairing the fault quickly mount as help is likely to be needed from specialists in several different fields.

Depending on the location and time of the fault, the wind farm operator will almost certainly incur unplanned costs far in excess of €100,000 and, the older an installation gets, the more likely this scenario becomes. Unsurprisingly, wind farm operators start to show more interest when we point this out!

Modern diagnostics detect defective connections

A typical cable route in a wind farm: the sharp stones create a risk of cable damageExperience has shown that cable damage is always due to poor cable installation prior to the commissioning of a wind farm. Around 80% of the problems are the result of incorrect installation of the joints, whilst the remaining 20% are due to the way the cable has been buried in the ground.

Joints are found wherever cables are connected and, in most systems, there are many connections, which increases the probability of errors being made. Furthermore, cables, joints and end closures are being systematically optimised in terms of cost. This does not necessarily make them worse products, but ensuring that they will provide long-term fault-free operation places ever-higher demands on installation skills, and this can also lead to an increase in errors. Regular use of partial discharge diagnostics is the remedy, since a defective joint usually generates partial discharge. With our cable test van systems, we can reliably diagnose and monitor the nature of each partial discharge event, so that remedial action can be taken before expensive failures occur.

Sheath fault testing detects mechanical damage

A typical sheath faultWhile joint defects are by far the largest source of cable problems, damage to the outer sheath during installation is also a significant cause of problems. Underground cables should normally rest in a bed of fine sand, free of large stones or other sharp objects. This naturally requires more installation effort, which increases costs. As a result, even important power cables are often buried quickly with little preparation of the cable bed.

In such cases, the cable works for a while because the cable sheath can withstand the pressure of sharp-edged stones for a considerable time. Sooner or later however, earth movements, typically caused by heavy vehicles such as excavators or agricultural machinery driving over the route of the cable, lead to a gradual or even sudden increase in the pressure applied to the cable by the stones and the sheath suffers mechanical damage. When this happens, the metallic cable screen is exposed and is in direct contact with the ground. Penetrating moisture further degrades the condition of the cable, ultimately resulting in failure. The wind farm will be shut down, and it typically takes around five days of hard work and high expense for the fault to be located and repaired.

Diagnostics within the warranty period

There are always enormous cost pressures during the planning process for projects, to the extent that costs are sometimes given priority over sustainability. Therefore cost pressures can have an impact on the quality of installation work. Nevertheless, the installation company remains responsible for all faults that occur or are detected by diagnostic testing before the warranty has expired. For this reason, we always take into account the guarantee periods for the wind farms entrusted to us so that we can carry out extensive diagnostic testing before the warranties expire. In this way, we are able to offer our customers an effective option for reducing costs and avoiding loss of earnings. In addition, knowing that we will be carrying out these tests incentivises the fitters to work more carefully and sustainably, irrespective of cost and deadline pressures.

Diagnostic and sheath fault tests are also useful during the commissioning and acceptance stages of a project, as they allow the quality of the installation of joints and other fittings such as end closures to be checked before the cable is put into service. Cable sheath damage and faulty joints can often be observed during installation.

Cable-friendly diagnostics with Megger’s TDS NT and PDS 60

Shock wave generator and ground microphoneThere are good reasons why we exclusively use Megger test equipment for analysing the underground cables of our wind farms. We principally use the TDS NT with 50 Hz slope technology, which incorporates a compact voltage source and a PDS 60 partial discharge detector. With the versatile cosine rectangular technology offered by the TDS NT, we can test cables to the DIN VDE 0276 standard, while simultaneously making PD measurements at a frequency close to the normal operating frequency of the cable. This combination of abilities is unique and makes testing very efficient. Automatic evaluation shows weak points immediately, while the measurements are being carried out.

Using the TDS NT, we can also offer our customers non-destructive diagnostics without using 0.1 Hz VLF testing. The frequencies used by the instrument for damped AC (DAC) testing are close to 50 Hz, so the results are directly comparable to the cable performance at supply frequency. This reveals faults that are invisible when using conventional VLF testing at 0.1 Hz. With DAC testing, PD events in defective areas can be identified quickly and evaluated well before damage occurs, since the DAC test voltage (which has a frequency between 50 Hz and several hundred Hz depending on the capacitance of the cable), is applied to the cable for only a few hundred milliseconds.

Expensive repairs aren’t always needed

This diagnostic approach has another important advantage for wind farm operators: they may not need to carry out urgent repairs, which are always expensive. Often it’s enough to detect, evaluate and monitor the defect. If at some future time the PD values start to approach a critical point, action can be taken in a cost-efficient way without time pressures. This means that weak points in the cable installation can often be repaired during planned maintenance shutdowns, which can save a lot of time and money.

Sheath fault testing with Megger’s MFM 5

Oliver KauschWe check the cable sheaths with a Megger MFM 5 sheath fault location system. The menu-driven user guidance and the powerful 5 kV source of the MFM 5, together with its automatic measurement features allow us to work intuitively, and provide dependable pre-location and pinpointing of sheath faults.

We also use shock wave generators and ground microphones for acoustic pinpointing of underground cable faults. The area of the fault is narrowed down using pre-location methods, then acoustic and electromagnetic fault location is carried out with a shock wave generator and ground microphone. The noise damping of the ground microphone optimises its acoustic performance so that it lets through only noise from the fault. This allows us to dig in the exact location of the defective joint. Where previously a very high level of surge energy was required for pinpointing faults, we can now carry out reliable fault location with much lower energy levels. 

The Megger digiPHONE+ acoustically indicates exactly where to dig.

With trained testing personnel who have a wealth of experience, combined with innovative equipment from Megger, WPD Windmanager can now offer its customers the best possible  state-of-the-art operational management for their wind farms.