Controls Interaction

Torsional Interaction:

Under the banner of torsional interaction, there are two potential concerns that have been discussed in detail in the literature for conventional thermal turbine-generators. Subsynchronous resonance (SSR) and subsynchronous torsional interactions2 (SSTI).

Subsynchronous resonance is a phenomenon whereby series compensation of a transmission lines leads to electrical resonance frequencies in the subsynchronous frequency range that can thus lead to destabilizing modes of mechanical torsional vibration on the turbine-generator shaft that fall in the frequency range of the electrical resonance (for more detail on the phenomena of SSR the reader should refer to the many texts and papers on this subject such as [1, 2]). Such resonance is less likely to affect wind turbines since the typical torsional mode for a wind turbine is quite low (around 1 to 4 Hz). As such, it would be quite unlikely that the level of series compensation in a system would be high enough to result in an electrical resonance that would interact with such a mechanical frequency (Note: that the electrical resonance needs to be in the range of fo - 1 to fo - 4 Hz, where fo is the system nominal electrical frequency.). The bigger concern is that of self-excitation, see section 4.2.7 below. Thus, some analysis and discussions with the wind turbine manufacturer on a case by case basis is prudent when installing wind farms near series compensated lines.

The phenomenon of SSTI was first observed for the Square Butte HVDC project in 1976 [3]. Subsynchronous torsional interactions (SSTI) is the phenomenon by which controls associated with transmission equipment, such as SVC [4] or HVDC [3], may introduce negative damping torques in the frequency range associated with the torsional mechanical modes of oscillation of nearby thermal turbine-generating units. Again, due to the relatively low frequency range for torsional modes of wind turbine, this may not be a concern in most cases, however, where wind farms are closely couple to a HVDC system some analysis to ensure that controls and/or torsional interaction do not occur is prudent. Such analysis will typically require detailed three-phase models for both the wind farm and the HVDC system. A good recent publication on the subject of torsional issues related to wind turbine generators is [5].

2

Control Instability:

The second possible interaction phenomenon is simply the potential for interactions between the wind turbine controls and controls of other nearby transmission or generation equipment. For example, the DFAG based designs often incorporate a farm wide central control system to regulate voltage at the substation. This is essentially a centralized controller that regulates the voltage at the substation connecting the farm to the grid. The high side (or low side) voltage on the substation transformer together with a current compensation signal are fed into a proportional-integral (PI) regulator, which compares the measured voltage (plus current compensation) to a reference signal. The error is fed in to the PI regulator. The regulators output is fed to the power factor reference input of all wind turbines in the farm. Thus, effectively this centralized controller adjusts the power factor of all the wind turbines in the farm on a continuous basis in order to regulate (within the capability of the farm) the bus voltage at the interconnection point. If there are any other regulating devices nearby (e.g. power plant, SVC, HVDC etc.) that are attempting to regulate the same point, steps need to be taken to ensure the two control systems do not hunt or interact with each other. Often it suffices simply to provide an appropriate level of droop into the regulator through the use of the current compensation setting.

Transient Torque Issues:

The third potential for adverse interactions is system phenomena that may expose the shaft of a wind turbine to cyclical and significant transient torque pulsations. For example, nearby arc furnaces, or high-speed re-closing on a transmission line emanating from the wind farm substation, or repeated commutation failures on a HVDC link connecting the wind farm to the ac system.

If there are nearby equipment that can expose the wind turbine to such repeated transient torques, as a first step, some simple transient stability analysis may be performed to estimate the expected step change in the electrical torque on a wind turbine generator due to the electrical event. Then the wind turbine generator manufacturer must be consulted to identify if the observed level of transient torque is a concern, if the wind turbine were exposed to such a recurring transient torque. Based on consultation with the wind turbine manufacturer, more detailed analysis may be required to assess if a potential problem exists and how it may be remedied.

4.2.4 Harmonics

In general there are two ways in which harmonics can be generated by wind turbine generators:

(1) due to saturation in electrical machines

(2) due to harmonic injection by power electronic equipment

The first item is no different than that of any other electrical generator. The manufacturer of the electrical machine must build their units to comply with the industry standards (IEEE/ANSI in North America and IEC in Europe and other regions of the world).

The second cause may come from one of two sources. Harmonic injection by soft-start thyristor based converters typically used in conventional induction generator designs. The requirement here is that the wind turbine manufacturer should ensure that their design conforms to the applicable standards. The second is by variable speed designs that use frequency converters, such as the doubly-fed asynchronous generators or the full-converter designs. Once again, the requirement would be to ensure that the manufacturers design complies with the applicable standards. Typically, with the variable speed designs the frequency converters are voltage-source converter technologies. This means that the designs are typically based on pulse width modulation (PWM). These converters will mainly generate high order harmonics (several kHz) – see also discussion on harmonics in chapter 5.

In designing the collector system for the wind farm, there may be a concern when interconnecting to a weak node in the system. The concern is that with the charging capacitance on underground cables (typically used in wind farm collector systems) and/or fixed or switched capacitor banks on the collector system, harmonic resonance may occur and thus give rise to significant voltage distortion. In addition, there may be a potential for voltage magnification on the shunt capacitor banks near the wind turbine generators (e.g. at 600 V) when switching higher voltage capacitors on the collector system or at the substation level. The harmonic resonance issue can be resolved by judicious design and/or application of filters. If voltage magnification is deemed possible, solutions might be to minimize the switching surge due to the high voltage capacitor banks by applying synchronously switched breakers. Alternatively, surge arresters may be applied at the lower voltage capacitors banks to protect them. Thus, during the design of the wind farm electrical system these and other equipment application issues should be reviewed to ensure proper design and integrity of the entire wind farm electrical system.