The Advantages Of Using DFIG

In addition to its stator, the DFIG’s rotor is also connected to the grid using a power converter (Picture 3). This type is very common for wind turbines as it offers certain advantages compared to other types:

It can operate like a synchronous generator and at variable speed, although its stator is directly connected to the grid. The converter can adequately control the voltage (phase and magnitude) that is applied to the rotor and as a consequence control the magnetic field’s frequency or speed. It actually forms an AC excitation with a variable frequency, just like the DC excitation used in synchronous generators. This operation gives the DFIG the benefits of a synchronous generator. These benefits include the separate control of reactive and active power, or the control of the wind turbines power factor. The DFIG wind turbines are better than other designs in terms of grid compatibility.

It can operate at variable speed, sub- or super synchronously. The optimum speed can be chosen by adjusting the frequency and phase of the voltage that is applied to the rotor, and is such that maximum power is obtained by the wind, in different wind speeds.

Since it can operate as synchronous generator, a major advantage of large DFIG wind turbines is that they can contribute to the system’s stability after a fault occurs. However, this relies heavily on the control options given by the power converter. Moreover, A DFIG wind turbine can generate reactive power even when the mechanical part is not operating and in not delivering active power.

Relative to other variable speed generators, DFIG’s power converter is rated at lower power (i.e. about 30% of the wind turbine’s rated power), since only part of the total power delivered by the wind turbine is transferred through the converter. This makes the wind turbine cheaper and lighter.

The power mentioned above, can be either delivered by the rotor (when the turbine rotates with a higher than the synchronous speed), or absorbed (when its speed is below the sync speed). This double mode gives the DFIG the ability to operate at speeds below or above even by 50% of the sync speed, although actually lower variations are chosen.

Compared to other generators with variable speed, which use external rotor resistances to allow variable speed, DFIG are more energy efficient, since there is little power dissipated in the converter.

DFIG can be accurately controlled due to the fact that the power converter can adjust both the magnitude and phase of the voltage applied to the rotor. This attribute, combined with pitch-control in wind turbines offers the operator more accurate power control, especially in high winds where high power output can cause severe damage to the equipment.

Finally DFIG shares the benefits of synchronous and inductive generators regarding its contribution to power system’s stability. Appropriate control can improve the damping of power variations in the system, without compromising voltage control.

+voltage control

Picture 1

Active Management in the distribution network [4], [5], [6]

The dispersed nature of renewable resources necessitates their connection at the distribution network, which was designed in order to convey power from high to low voltage. Now, with the introduction of renewable electricity sources, the power can follow any direction, affecting the power and voltage quality as well as the security of the system. Therefore considerable amounts of money should be spent for their upgrade, if the optimum renewable potential is to be achieved. Another, less expensive approach is the transition from the networks’ traditional passive operation to their active management.

Under active management approach, new control and communication technologies are incorporated into the system and allow the operator to control in real time the voltage, the power flows and even the fault levels.

The main options under active management are:

Power flow management

The risks regarding the network power flows must be sufficiently eliminated. The possibility of the distributed generators to deliver power above the system’s ratings-capacity can significantly threat the system’s operation. This is a serious issue, especially in case of a circuit outage. The power flow management protects the system’s parts taking into account their nominal capacity.

Voltage control

The voltage at a bus is affected by the real and reactive power on that bus as well as the R and X values of the line. The voltage change at that bus is approximately equal to. The most significant issue accruing from the introduction of distributed generators at the distribution networks is the voltage rise at the connected bus. The operator can choose appropriate R and X values but this would require an expensive network upgrade. The control of P and Q is a less expensive option, and this is why it is preferred. The main actions usually taken are:

On-load transformer tapping: The operator might reduce/increase the voltage at the primary substation reducing/increasing all the subsequent voltages. Nevertheless, in case of reducing the voltage, a possible generator disconnection might drive the voltage below its allowable limit.

Power factor or/and voltage control: see question 4

Reactive power compensation: Appropriate devices (capacitor banks, STACOM etc) can be connected on critical buses, so that they deliver/absorb reactive power and fix the voltage profile.

Generation curtailment: another way to mitigate a voltage rise, would be to curtail real power generation from the distributed generators. However, this sets a significant constraint in the renewable energy that can be exploited.

Fault level management : see below

Load management

Reducing or shifting adjustable loads can alleviate the pressure on the network’s operation, especially in case of very low renewable generation.

Power quality management

The operator exploits various capabilities at the generators, loads or other special devices to maintain the power’s high quality in terms of harmonics, voltage disturbances etc.

Power Factor vs Voltage Control [7], [8], [9]

The dominant negative impact of connecting generators in the distribution network is the distortion of the voltage profile. The simplest, but at the same time the most expensive, solution for this problem is the upgrade of the local grid. However, there are more efficient ways, (borrowed by the operation of the transmission networks) that could allow for larger DG penetration without big distortions at the voltage profile. Two of them, concerning the generator’s operation, are:

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Power factor control: the generator operates at a constant power factor. The reactive follows the real power output so that their ratio remains constant. This kind of operation was required by the distribution network operator (DNO) in the context of the ‘fit and forget’ rationale, with which all the low voltage networks were initially designed. The major advantage of this method is that it does not disrupt the operation of other regulation devices, such as OLTC. However, it may have a negative impact on the voltage at the connected bus, since it tends to contribute to the voltage variation which is influenced by the changes in power flows. As a consequence, this approach limits the total generation capacity that can be connected on the network.

Voltage control: the generator adjusts its reactive power output so that the voltage at the connection bus remains within the allowable limits. This adjustment is done according to the real power output and the voltage drop or rise occurring at the bus. Obviously, the generator’s power factor is not constant anymore, although an upper and a lower limit would normally apply. The DNO are not very keen on allowing generators to operate in that way, since this operation could considerably affect the operation of other voltage regulation devices, such as OLTC transformers. Moreover, if a small generator tries to correct a high voltage change, it might need to set its reactive power output at a critical high or low level. This sort of dangerous operation close to the thermal or overcurrent limits entails a significant increase in the maintenance cost, or even worse, it can activate the protections and cause a sudden trip.

A combination of the two approaches described above is considered the best solution regarding the optimum operation of a distributed generator. The main idea is that the generator operates with a constant power factor until the point where the voltage at the connected bus reaches the upper or lower allowable limit. At this point, the power factor control is deactivated, letting the generator to adjust its reactive power output and fix the voltage. Again, the power factor must be kept within it’s the allowable limits. When the voltage returns within its limits, the power factor control is reactivated.

The consequences of new DG capacity on network fault levels and protection [4]

The connection of distributed generation, using either synchronous or inductive generators, causes an increase in the systems fault levels, owing to the additional generation that could feed a potential fault current. This is especially the case at the very edges and weak parts of the distribution network, where the renewable generators are usually connected. The new fault levels might exceed the rating of the existing protections. Such an implication could cause significant damage to the protection equipment and set the system’s security at high risk or ultimately incur supply interruptions. But most importantly could set personnel’s life in to severe danger. Therefore, every time a new distributed generator is connected, the fault levels must be re-examined and wherever is deemed necessary, protections must be upgraded. In some cases it might be necessary to upgrade the respective part of the network (reduce the R and X values – new lines or/and transformers), which would usually require significant amount of money.

Nonetheless, in the context of the active management operation of the distribution network, the operator can take some alternative measures:

Advanced converter technology: the use of advanced power inverters, makes a generator’s contribution to the fault current much lower. The more advanced a converter, the higher its cost.

Network reconfiguration: changing the topology of the network could change the fault level at some buses. This operation is already available in most of the networks for maintenance purposes.

Is Limiter: a device that can instantly increase the system’s impedance (lower fault level) in case of an incident, but it needs replacement after each use.

Sequential switching: the contribution at the fault current by one or a team of distributed generators can be isolated in case of a fault in a different section.

The need for active power balancing in networks with high penetration of renewable energy resources

In order for the frequency of a system to be maintained, the total real power generation must always be equal to the total real power load and losses. A distortion of this balance could cause a frequency deviation beyond the allowable limits and hence, damage the equipment and the loads. Therefore, it is of major importance that the system operator must always retain the active power balance.

Most of the prevailing renewable energy resources, such as wind or solar, have an intermittent behaviour and their output relies on the weather/climate conditions and hence, cannot be controlled. Although there were significant advances in the weather forecasting, errors are still present. Moreover, any sudden incidents that could trip a big part of the total generation (not only renewable) could also contribute to a real power unbalance. Adding to this the fact that the demand can also vary, the large penetration of renewable resources increases considerably the risk for the system’s stability.

The main techniques that a system operator can use in order to maintain the power balance are:

Storage: Large (compressed air, hydro-pump storage, flywheels etc) or smaller (EV etc) storage facilities can contribute to the real power balancing. They can store energy when the renewable generation exceeds the total load and deliver it back to the network when the total generation is not sufficient to meet the demand. Some storage technologies are better than others in terms of their performance, however all of them are quite capable of quickly adjusting their output to support the system’s frequency. Nonetheless, most of the storage technologies are still very expensive. Pump storage, a cheaper option, is naturally constrained, while the massive introduction of EV is not feasible in the foreseeable future.

Demand Side Management: The active power equation has two sides. Available for adjusting or shifting demand could adequately contribute to the system’s stability when the renewable sources are lower than expected, if appropriate incentives are given to the consumers.

Dispatchable units: The unbalances risk that might emerge by the large penetration of renewable energy resources could be offset by large and fast, in terms of start-up and output, generating units, such as hydro or gas plants. However, hydro plants are naturally constrained and gas power plants are still causing carbon emissions, albeit fewer than the coal generators.

Interconnections: Interconnections between large electricity grids could significantly reduce the risk associated with the large penetration of renewable energy sources. Each system could either absorb or deliver real power from its counterpart depending on its total generation and demand. DC interconnections are more common, since they can transfer more real power and isolate the two systems in terms of frequency control, although they are more expensive than conventional AC interconnections.