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Benefits

The use of other condition monitoring systems for wind turbine subassemblies such as the gearbox and rotor has become the regular industry practice. They can be integrated with the SCADA interface as part of the wind

Condition Monitoring Potential Benefits

turbine full control package. Some of the best known providers of condition monitoring solutions are companies like GL Garrad Hassan, Mita Teknik and Bachmann [5], [25]. Condition monitoring does add of bit of extra cost and complexity to the wind turbine control system, but it has proved that its benefits far outweigh the disadvantages [26].

Taking into account factors such as the probability of the occurrence of converter failures and statistic wind distribution in the year a study on the benefits of condition monitoring [27], [28] presents an evaluation for a 2 MW offshore wind turbine unit. It concludes that condition monitoring for the full turbine – including a potential condition monitoring system for the converter subassembly – would result in an annual saving over £70k, which more than the cost of the average cost of an offshore wind turbine controller as given by [25]. Another study based on failure simulation tool for optimising offshore wind farms maintenance strategy also confirms that informed O&M based on visibility and event predictability provides more cost and time effective solution than purely reactive maintenance [29].

Although the area of condition monitoring for power electronics attracts considerable interest at the moment, there are still no commercially available power electronics condition monitoring systems for the offshore wind market and no information has yet been found about any systems under trial. And although power electronic converters are used in a range of other applications, there is as yet little headway in the area of their condition monitoring. (There is only one known

Condition Monitoring Potential Benefits

system (Amantys) for traction applications – currently still in trial – aiming to optimise the operation of the Portuguese railways by tracking IGBT loading and health status using gate driver signals [30].)

It is understandable that before industry makes any serious attempts at the development of such systems, the manufacturers and operators would be interested to the potential value of power electronics condition monitoring as well as its associated extra cost and limitations. As pointed out above, the main justification for power electronics condition monitoring in offshore wind turbines, as opposed to onshore where reactive maintenance is sufficiently cost effective, lies in the fact that while converters are a frequent source of failure the access for their servicing can be problematic. This means that faults may take very long time to rectify and the resulting forced downtimes can cost the operators a large loss of potential revenue. If in such case a condition monitoring were used, its capability to keep track of the system’s (or specific component’s) health and communicate it to the operator, would mean that they would have a much longer warning time before failure could cause shutdown and therefore a much longer window to plan and take action. Thus, shutdown can be altogether avoided.

Here, a straightforward example calculation is attempted focusing on the financial losses associated with the downtime costs for an average 5 MW offshore turbine. It can be used to illustrate the point that the value of power electronics condition monitoring in an offshore wind turbine can greatly outweigh the added cost of having such a system in place. (This was also published in [11].)

Condition Monitoring Potential Benefits

UK offshore wind farms have reported capacity factors as high as 50% for the new higher rating offshore wind turbines – this is mostly due to the fact that the offshore locations have much better wind resource than onshore. We shall use this ballpark figure to calculate the indicative amount of energy the turbine is expected to generate per day is:

5 MW x 24 hours x 0.5 = 60 MWh

The average base level price for energy from offshore is about £30 per MWh, but including the double ROCs (2 times, around £60) [39] it becomes around £150 per MWh in total. We shall use the more conservative figure of £140 (including ROCs) as the current buying price per MWh from offshore wind, as suggested by a private offshore wind development consultant:

£140/MWh x 60 MWh = £8,400

This can represent the average loss of revenue per day that could have potentially been delivered by the 5 MW turbine, if it was operating.

O&M companies typically charge £5k-£6k for a vessel with 2-3 people crew to be sent out on a maintenance trip (from communications with offshore maintenance provider SeaRoc and offshore wind consultants at ORE Catapult). This is expensive in comparison with onshore maintenance, but nevertheless it can be seen that even a single day of unscheduled downtime costs more than that, so it will be in the operator’s interest to restart generation as soon as possible, even if it costs them £5k-£6k go get a boat out. The fact that there is no visibility over what

Condition Monitoring Potential Benefits

may have caused the failure and which components may need replacing is likely to incur more than one maintenance trip.

The reported average downtime associated with power electronic failures onshore is fairly short – about 1 day per fault. Offshore, however, access to site highly depends on the weather condition

For the sake of our calculation, we shall use a conservative figure of 5 days between the time of the converter failure causing full turbine shutdown and the time when the turbine goes live again. That this is a reasonable and even optimistic assumption for an average offshore servicing time delay as it has to include: 1) the remote diagnostics associated with the fault, 2) securing the spare parts and the qualified staff to fix them, and 3) organising the logistics of the servicing trip.

5 days x £8,400 downtime losses per day = £42,000

The longer the forced downtime, the higher the revenue loss will be:

£42,000 + £5,000 (cost of servicing) = £47,000

Just for comparison, the average total cost of a wind turbine controller including certain condition monitoring capabilities as offered by companies such as Bachmann and Mita Teknik costs only about £70,000 [25]. If over its lifetime of 25 years the turbine experiences only two converter failures they will already have cost in downtime more than the capital cost of the entire electronics control equipment. If the turbine experiences only six or seven such downtimes caused by

Condition Monitoring Potential Benefits

the converter, their total cost will outweigh the capital cost for the entire power electronic converter, not to mention the fact that each time a converter module fails, the new replacement itself will cost a portion of the converter price.

Consider the scenario where every wind turbine in a farm of 100 MW wind farm (20 x 5 MW turbines) experiences a power electronics failure once in 2 years (a reasonable assumption as the WMEP database points to 1.5 electrical/electronics failures per turbine per year) with associated downtime conservatively estimated as only 5 days per individual failure. The resultant financial loss is close to a half a million pounds per year:

(20 turbines x £47,000(5-day downtime + O&M cost)/2 years = £470,000 downtime costs per year

And even, if this failure rate is reduced in the modern offshore wind turbines to one third of the quoted-above figure, it can still mean great financial loss over the lifetime of the wind farm and very high associated risk factor for potential investors.

Now, let us consider the cost of a possible condition monitoring system for power electronic devices. The proposed converter level model-based approach will need the sensing of different temperatures as well as current and voltage measurements which are already being used in the controller. Some of the power modules come with incorporated temperature sensors and it is possible that their signatures are already being data-logged and used by the controller or the wind

Condition Monitoring Potential Benefits

turbine’s SCADA (supervisory control and data acquisition) system to schedule maintenance and/or modify the operation of the converter. Existing capabilities need to be thoroughly examined, but on average for hardware the additional costs of the condition monitoring system will only be associated with perhaps 6-12 new temperature sensors, expected to cost less than £1,000 per turbine including retrofitting and calibration. The signal processing, data management and communication and system licencing costs are expected to be a bit higher than that, but probably within the range of £3k-£4k per turbine.

It is true that there are risks associated with condition monitoring related to how accurately it works and the fact that it adds cost and complexity to the wind turbine system (i.e. another bit that could go wrong). But looking at the above average downtime cost and the relatively low cost of such a potential condition monitoring system, it is clear that even if it manages to prevent a single relatively short forced shutdown for the offshore turbine, it would have paid back for its cost. And as the unit capacity of the wind turbine increases, the relative cost of power electronic condition monitoring will become even lower and its cost saving value even higher.

So the answer to the question whether power electronics condition monitoring will be valuable and whether it is worthwhile the efforts to develop it, is definitely yes. The following chapter looks into earlier work on this subject.

EPSRC Project COMPERE

2

Research Background