DSFO-CONTROLLED DFIGS-ES SYSTEM WITH DIRECT AC GRID INTERFACE

The DSFO is the standard DFIG control in which the generator is field orientated off an external voltage and frequency source. Therefore and are used to control the stator active and reactive power respectively. This section studies ES control strategies in a wind farm based on DSFO-controlled DFIGs which are

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ave

P

rq

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directly connected to AC grid. This case does not seem appropriate for a weak grid. If the wind farm is integrated within a microgrid which is islanded from the main grid, it is necessary to control the local grid’s voltage and frequency using, for example, a STATCOM. In the next two subsections, it is assumed that the voltage and frequency control is provided by the main grid.

3.2.1 Power Smoothing Control for wind turbine-ES connected

directly to the AC grid

The PSC strategy, which is shown in Figure 3.1, has been addressed in many papers [5, 16, 25-27]. In this structure DFIGs are conventionally controlled in the Maximum Power Tracking (MPT) mode. The ES can be either aggregated on to the collector bus of the wind farm or distributed and integrated with each DFIG. The total output power of the wind farm is likely to be smoother than the power generated by each wind turbine due to the possible phase displacement of individual turbine powers. Therefore in the aggregated ES, the required energy capacity of the ES in order to smooth the wind power to a certain level; tends to be smaller than that of the total distributed ES. Hence, the aggregated ES seems to be the appropriate choice in this case. However in large offshore wind farms, accommodating such a large aggregated ES can be quite an engineering challenge. In such cases, distributing ES, for example, in the space available in the turbines’ tower rather than aggregating on a huge central platform; might be practically and economically beneficial.

A number of available ES technologies were explained in Chapter 1. This research concentrates on the short to medium-term ES such as: flywheel, SMES and Supercapacitors. The ES, in this thesis, is simulated by a DC voltage source which is interfaced to the AC system via an AC/DC converter. The converter is called the ESI or ES Interface. The ES power can be controlled by regulating the d- (real) component of the ESI current (see Figure 3.1). The current control is the standard current control which is identical to the DFIG’s current loops explained in Chapter 2.

es d

3 Control of wind turbines-ES system with external voltage and frequency source

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Figure 3.1. PSC in a DSFO-based wind farm with direct AC grid interface

Studies have shown that power system is more sensitive to medium frequency wind power fluctuations (0.01-1Hz) [16] since the power density of wind speed reduces as the wind frequency increases. Therefore the ES is controlled to filter out these frequencies in order to shield the power system. This can be done by subjecting the generated wind power to a High Pass Filter (HPF), the output of which sets the reference ES d-axis current Id-es*. The time constant of the HPF is

used to achieve different cut-off frequencies.

3.2.2 Power Demand Control for wind turbine-ES connected

directly to AC grid

In a PDC strategy, generally, the demand power P* must be imposed on the wind farm. The easiest way to impose the demand power, in a wind farm directly connected to the AC grid, is the ES (see Figure 3.2). The ES is controlled in order to balance the power generated by the wind turbine(s) with the demand. In a DSFO-controlled wind farm, communication is required to make the wind farm generation as close as possible to P* in order to minimize the ES.

Figure 3.2 illustrates a PDC strategy in a DSFO-controlled wind farm which is directly connected to the AC grid. In this case a Supervisory Wind Farm Control (SWFC) unit [57-59] is used to determine the reference demand power and/or

Pg2,Qg2 Pg1,Qg1

~

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pitch angle for each wind turbine based on the total demand power P* and wind speed associated with each wind turbine. The aim here is to minimize the ES. The total demand power is set by the system operator. It is noted that if , no ES is needed which implies that that wind turbines are controlled under Constant Power Mode (CPM). As discussed in section 2.5.3, CPM is not possible when the demand power approaches the average of the extractable wind power. Whenever P1* and P2* are not determined by MPT, stability issues must be taken

into account. The ES power is controlled using the ESI-real current in order to absorb/inject the difference between the wind farm power and P*. If the ES is distributed within the wind turbines, the output of each individual DFIG-ES is smooth.

Figure 3.2. PDC in a DSFO-based wind farm with direct AC grid interface

In [60] a pitch angle control, with reference power derived from the average wind speed, is used to smooth the output power of wind turbine. A similar pitch control can be used here with the reference power given by total demand power considering the associated wind speed in order to reduce the size of ES.

* * 2 * 1 P P P   P=P* Pg2,Qg2 Pg1,Qg1

~

3 Control of wind turbines-ES system with external voltage and frequency source

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3.3 DSFO-controlled DFIGs-ES system with AC/DC/AC