Control and surveillance

With the commercial climate for new investment in wind farms set to improve, the maintenance of uninterruptible power supply (UPS), systems are playing an increasingly important role in the energy sector. Power Parameters’ Robert Durante discusses the maintenance aspects of UPS systems that, despite being shared with fossil fuel generation, are distinct in technical requirements and demand different emphasis.

A number of wind projects are advancing and many existing plants have ‘come of age’, so to speak. For these, measures required for continued operational reliability are paramount.

Currently wind farms and, indeed, the sum total of ‘green’ distributed generation, represent a very modest impact on total power generation. We still hear the argument, “non-despatchable sources of power can only be permitted at a low percentage impact on grid systems”. The innate variability of power supplied by windfarms would seem to add to this argument. However, this continues to discourage the alternate energy sector, particularly at a government level. More importantly, at the core, there is no impediment for a much higher level of penetration beyond 10 per cent or higher.

Power flow

Windfarms and other distributed generation are almost connected to distribution networks. These networks were historically designed without thought of power sources being inserted. The former passive paradigm is now a very different one: one, where power flow once predictable in a radial distribution system is no longer so.

One only has to think of impedance relaying to grasp the problems in determining sections subject to faults. The field of protective relaying for distributed generation systems in distribution networks is replete with examples that require, if not counter to, the practice for HV transmission in operational principle. This is a very different approach, both in terms of sensitivity to different ‘per unit’ (pu) fault levels, but also in respect of masked faults.

It is a subject we deal with here only in terms of islanding protection, but could well serve for future articles.

No power flow

Historically, the earlier installations of wind generators were the constant speed type. These are characterised by a blade design quickly building up turbulence on the lee side so that a constant speed results.

The electrical generator is usually an induction machine, requiring a soft-start, reliant on the availability of the distribution network for generating power at super-synchronous speed. The other major feature is the network has to be able to supply reactive power, or it needs to be furnished by a local capacitor bank.

A later development is the double-fed induction generator (DFIG), also requiring reactive power, but capable of variable speed by virtue of the adjustable rotor frequency to provide positive slip. The blades are controlled by active stall and usually pitch control to permit feathering during dangerous wind conditions. The DFIG generator is usually capable of two or three percentage points additional energy production.

Being an induction machine, however, it requires the availability of the distribution network. There is very limited ride-through capability by virtue of rotor inertia.

Synchronous generators are also employed (self-excited and permanent magnet). These also feature active stall and pitch control so that they can extract energy under varying wind speed conditions. Unlike the induction generators they are capable of island operation. In some wind farms, internal DC buses can be used, but usually each nacelle is equipped with its own power converter comprising a rectifier-DC link and inverter. Both constant current and voltage-style converters can be equipped with reactive current control.

All three types of wind generators require yaw control if, as is usually the case, they are upwind constructions to reduce column shadow effect.

Islanding

Irrespective of generator type employed, anti-islanding protection generally will necessitate emergency power to be available. The maintenance of this part of a windfarm is critically important. Low capacity UPS, usually battery backed, are employed for tasks like feathering and providing braking power to assure the safety of the hub and blades. Yaw control, which through a confluence of loss of the distribution net and high wind may require rapid action to minimise mechanical damage, can demand more powerful UPS systems because gyroscopic forces are considerable when yawing in excess of 1°/second.

Notwithstanding lightning protection being a standard feature, nacelle fires can still occur and the availability of a sufficiently powerful emergency supply to deal with some form of powder extinguishing system would seem essential.

Depending on the distribution network topology, there may well be the necessity to supply emergency power to defined district levels requiring diesel-backed supply with intermediate battery or flywheel support. In any event, regular preventive maintenance of UPS systems is essential. Areas requiring attention include bypass switches, under-voltage relays and battery impedance monitoring.

Instrumentation

A feature of distributed generation, as mentioned, is protective relaying including anti-islanding. In addition supervisory and control and data acquisition (SCADA) systems will increasingly be required. An assured stable supply for these essential functions as available via an inline double conversion UPS system is highly desirable, as is its regular maintenance.

Some argue the assured collection of network and generator data is not necessary, or at least not essential while penetration of distributed generation remains low. However, this is going to be increasingly challenged. The collection of, for example, microphasor data, GDS synchronised with network data is an important part of improving network stability.

This need for data collection is made more acute because of the large portions of relatively weak distribution networks where stormy conditions can bring on large phase angle differences between local plant and the network, with consequent large power flows and voltage, as well as frequency instability.

More windfarms – more monitoring and maintenance

Windfarms are here to stay, irrespective of changes in the political climate. What’s more, they are likely to proliferate. The accent will increasingly be placed on control and monitoring so as to adequately support significant penetration in established distribution networks.

Stable supplies for instrumentation including SCADA systems will be more and more critical, as will maintenance in order to act promptly when incipient problem are flagged. Safety systems for wind generators will also have to rely in increased self-sufficiency, thus necessitating well-maintained emergency supplies.