Mixing Renewable Energy Technologies for mitigating grid perturbance or grid loss operation in Wind Power

Once the Wind Farm (WF) is in operation. The target is the get the Return on Investment (ROI) forecasted in the business plan.

If it is considered the onshore Wind Turbine Generator (WTG) class I, II and III. The machine has been designed for a minimum of 20 years, following the standard IEC 61400-1. In case of offshore WTG, the standard defined the S class. In that situation, the standard is not defining any specific time, but at least is considering the protection for offshore tropical storms such as hurricanes, cyclones and typhoons.

For getting the mentioned ROI, it is necessary to mention the relevance of the Operation and Maintenance (O&M). This O&M must be optimal for decreasing the operational cost.

Technically speaking, Wind Power has one important consideration. It is called utilization or use factor. It is regarding the high level of intermittency in the electricity production. It means, the difficult to control the energy yield, just by the nature of the wind.

For that reason, Wind Power has relevant implications for the management of the electric system by Transmission System Operator (TSO). Even more, when the generation of Wind Power, are increasing in the total of the mix generation year by year.

In consequence, it has been implemented what is called “Grid Code Compliance (GCC).” The GCC is a framework to establish the conditions to connect WF to the grid. See what is established in the Standard IEC 61400¹ and the proposal for ENTSO-E (for EU countries).

Figure 1_ Wind Power Generation in real-time in Spain in two different days. In the top, for September 30th 2015, and in the bottom for 1st October 2015. In the pictures is possible to see the difference in the power generation between two days _source: Spanish TSO - Red Electrica de España (Ree)

Normally, in the past, the technical requirements for a WF to be coupled into the grid was established an accordance by two parts; by one side, by the WF operator, and by the other side, by the purchaser, an Utility, in the most of cases.

With this model, the issues came, when the TSO claims in case of black-outs, and the high associated cost.

For that reason, TSO establish GCC. Just to mitigate issues in relation to the power system operation management and its expensive consequence in case of issues.

GCC is basically related with the quality of wave delivered by the WTG, or in general by WF to the grid in the Point of Common Coupling (PCC). Similarly, GCC are considering also the conditions for WTG to inject electricity into the grid in case of fault of the grid.

It is mandatory mentioning, when there is a dip voltage, one WTG can be disconnected to the grid. This issue also can affect the complete WF indeed. This phenomenon is called “the cascade effect.” When there is a dip voltage in the grid, the WTG can recover the connection depending the time of the dip voltage. If this time is longer than the capacity of WTG to stay in connection, the WTG gets unplugged from the grid until the grid is recovered. 

This effect also can affect others WTG of the WF, because this WTG out of connection increases the dip voltage in the grid. In first term, for non supplying power, and also for the consumption of power in the state of recovering the normal performance. In this scenario, there is the possibility to affect the complete WF. 

In the worst possible case, the cascade effect of the all complete WF can make great consequences for the management of the electric balance by the TSO. Just for being necessary to inject or increase the generation available to replace the missing electric yield of the mentioned WF.

Figure 2_ View inside the Nacelle of Enercon WTG during Hannover Messe in 2004. In the picture is possible to see the high number of pols in the generator, the cabinets in the top right side, and on the bottom right side, three motors of the yaw system. As a consideration, all those systems must be supplied with the main electricity connection for the properly performance of the complete WTG_source; J. Sánchez Ríos

Getting in more detail in the technical frameworks, one of the most important consideration is Fault Ride Through (FRT). FRT is relevant in case of stability of the grid voltage during faults or unbalanced in the grid. Those unbalanced mostly are caused by starting and shutting down heavy loads such as large motors, but also by Wind or Photovoltaic farms.
FRT can be separated in low and high voltage faults. Having Low Voltage Ride Through (LVRT) or High Voltage Ride Through (HVRT).

In case of voltage dip, the conditions which must comply every WTG must be the following;

• in first term, the WTG must stay connected to the grid,
• secondly, the WTG mus avoid any consumption of active or reactive power and,
• finally, and depending of the level of wind. The WTG and WF in general, must produce active and reactive power to help recovering the grid at levels of normal functionality, following the International Grid Codes*.

After the voltage has been recovered, and due to the unbalance generated in the grid by the dip voltage. The WTG must deliver Active Power, proportionally to the voltage values in the present situation of the grid.

It is necessary to remark that, a part of the well known meteorological effects in the mechanical structure of the WTG, the electric transients by the dip voltage creates as well, serious problems for the torque stress to the gear and the drive shaft, just for the inertia in the magnetic winding in the generator during the fault or dip voltage.

Figure 3_ Two WTG with a Meteorological Mast between them in southern Tarragona (Catalonia - Spain) _source; J. Sánchez Ríos

To solve LVRT issues, one possibility is to mix other Renewable Energies in the same WTG, to supply the ancillary systems and the generator in the WTG or in the WF.

In second term, it also can supply power to the grid, helping the TSO to control the possible unbalance in the electric system.

The WTG has incorporated an Uninterruptible Power Supply (UPS). With this UPS, some essential systems of the WTG are being supplied in case of power loss, mainly the pitch control, yaw system, communication systems and other ancillary systems in relation, basically to safety or essential operational systems.

The proposal is to add other Renewable Energy Technologies with its respective storage system, in its different technologies, to help in case of grid loss to the WTG or WF.  Just to recover the grid loss as soon as possible, injecting reactive or active power, depending of the electric system needs and to help in the recovering process when there is a fault in the grid.

Figure 4_ Example of  Off-Grid Street Lighting system composed by a Solar Photovoltaic tracker systems, Small Wind Energy system in the top of the lighting systems and the respective storage system with an autonomy of 58 hours (by the company Eolgreen) in a Beach of Barcelona City _source; J. Sánchez Ríos

It is well known the foundations systems in Wind Offshore; monopile, substructure and gravity-base. 

Furthermore, and regarding the floating foundations, it is feasible to talk about barge floater, tension leg platforms and spar floater. 

As information, these new implementations in the foundations represent a great cost in the total investment of the Wind Farm Project.

For mitigating the FRT in Wind Offshore, the proposal is to use storage systems, in concrete by ultracapacitors (ultracaps) for short voltage dips and batteries systems for longer dips.

For that, it is possible to insert tidal, marine or wave energy in the foundations systems, instead of being monopile or triple structure in fixed or floating foundations.

Installing those power systems in the foundations, it is feasible to get electricity and storage it. With this power, it can be supplied electricity in situations of FRT or even grid loss state.

With that implementation of tidal, marine or wave energy, the new system is reducing the impact of waves in normal functionality, grid loss or low wind (idling). In consequence, reducing the fatigue in the substructure, the tower and in the complete WTG.

On the other hand, it is important to remark some issues caused by the meteorological conditions. In concrete, the consequences of aerodynamic damping in WTG. The WTG are design to resist the impact of wind in the complete WTG in the normal functionality.

Figure 5_ Example of consequences in Aerodynamic Damping. In the left side of the figure is shown the scenario when there is no enough wind or the WTG is under grid loss state. In the right side, the normal functionality of the WTG versus idling or grid loss state_source: J. Sánchez Ríos

However, in the case of having low levels of wind or the WTG is in a grid loss state. It is feasible to have the impact of other forces. For that, it is mandatory thinking about the misaligned between three forces that can impact to the whole WTG structure; the wind, the waves and the current. Those three forces can get a vector in different direction and module, depending of high (wind) or depth (currents).

To introduce the behavior of the wind, waves and current. The wind and waves can be aligned (co-directional) and acting from a single force. In the worst case, there is only one direction (uni-directional).

Regarding waves, waves are irregular in shape, varying in height, length and speed of propagation. Furthermore, it is necessary to consider the water depth and the seabed topology. Just to mention the model of the waves; which are defined such as normal sea state (NSS), normal wave height (NWH), extreme sea state (ESS) and extreme wave height (EWH).

Regarding currents, must be considered as a horizontally uniform flow field of constant velocity, varying only as a foundation of depth. At the same time, it is necessary to contemplate the sub surfaces currents generated by tide storm surge, atmospheric pressure variations, wind generated surfaces currents, and braking wave induced surf currents. In the case of currents classification, it is feasible to mention; normal current models (NCM) and extreme current models (ECM).

Consequently, the considerations for the correct site assessment for the offshore WTG, such as indicated in IEC 61400-1, are; 

• the Metocean data, 
• the assessment of waves, 
• currents, 
• water levels, including tides and storm surges, 
• sea ice, 
• scour and seabed movement, 
• weather windows and weather downtime, and 
• eabed soil conditions.

In cases of normal functionality in the WTG, the misaligned is mostly, the result from the vector subtraction of the wind direction and the wave direction. In some cases, it is needed to add the water current direction.

Nevertheless, in some cases, the wind is low for power production, and then; the aerodynamic damping is almost null. However, it is feasible in this case, to pay attention in the wave level, which is not negligible in terms of the exposition of the complete WTG to the fatigue.

Similarly, it is mandatory considering, the reduction of the possible fatigue in the WTG structure, and to take advantage of the energy produced by the waves or stream.

All this implementation can be a study to insert in the offshore substations, to maintain the complete WF in operation, even in dip voltage or grid loss and for the ancillary systems in case of grid loss.

Figure 6_ Example of Substation in a Wind Onshore Farm to connect to the electric yield from the Wind Farm to the Point of  Common Coupling (PCC)_source: J. Sánchez Ríos

As a conclusion, mixing Renewable Energies with its respective storage system can be a great solution to mitigate the yield with high intermittency.

The example exposes can be extended to other technologies, depending the emplacement or the technology applied (see other examples by Geothermal and Biogass).

It is well known, the efficiency of the Renewable Energy technologies and the storage system is increasing its performance and cost day by day. However, similarly, other kinds of Renewable Energies are being introduced into the field.

In Japan, the offshore solar farms are a reality. This technology with its storage system can help to supply the ancillary systems in the WTG or in the Substations in the Offshore Wind Farms.

If the example is regarding Wind Onshore. Some projects in Canary Islands are mixing pumped-storage systems for the Wind Onshore in an Off-grid System. Otherwise, and following with Onshore Wind, the geothermal energy can help to mitigate the issues in extreme conditions, being necessary to supply heat to get the properly temperature for some electronic equipments. In the study of the foundation systems, it would be possible to introduce the geothermal energy to adecuate the temperature inside the tower and the nacelle.

Figure 7_ Example of Wind Onshore Farm in La Muela (Zaragoza - Spain), an emplacement with high temperture in summer and very cold in winter. Emplacement where will be possible to insert geothermal energy for getting the properly work temperature in WTG for the electronica, mechanical systems (even lubrication) and the ancillary systems in case of extreme temperature_source: J. Sánchez Ríos

The mixing of Renewable Energies for new applications are open for discussion, depends on the evolution and the willingness to invest in these technologies. 

Nowadays are being developed by an isolated way. The technological development, the maturity of some Renewable Energy technologies, the reduction of the cost (LCOE and the necessity of the TSO to find solutions to mitigate the use factor, can help to develop the mixing of Renewable Energy technologies with storage systems, even with the possibility to mix also different storage technologies.

REFERENCES

2. Sistemas Eólicos de Producción de Energía Eléctrica – Editorial Rueda S.L. - J. L Rodriguez Amenedo, J.C. Burgos Díaz, S. Arnalte Gómez.
3. Smart Grids – Technology and Applications – Wiley – Janaka Ekanayake, Kithsiri Liyanage, Jianzhoung Wu, Akihiko Yokohama, Nick Jenkins.
4. Aerodynamic Damping in the design of support Structures for Offshore Wind Turbines – Delft University of Technology – David Cerda Salzmann, Jan Van der Tempel.
5. Description of the relation of Wind, Wave and Current Characteristics ot the Offshore Wind Farms Egmond aam Zee (OWEZ) location in 2006 – S.Bath, P. J. Eecen
6. Grid Code Compliance beyond simple LVRT – Tobias Gelhaarg
7. Comparison of Fault Ride-Through Strategies for Wind Turbines with DFIM Generators – Andreas Dittrich, Alexander Stoev
8. Standard IEC 61400 – 3 (Wind Offshore)
9. ENTSO-E Network Code for Requirements for Grid connection applicable to all Generators

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¹ Normal Electrical Power Network conditions apply when the following parametrs fall within the ranges stated bellow:

1. Voltage-nomial value (according to IEC 60038) ± 10 %
2. Frequency-nominal value ± 2 %
3. Voltage imbalance, the ratio of the negative-sequence component of voltage which must not exceed the value of 2 %
4. Auto-reclosing cycles and auto-reclosing cycle periods of 0,1 to 5 seconds for the first reclose shall be considered
5. Outages – electrical network outages shall be assumed to occur 20 times per year. An outage of up to 6 hours shall be considered a normal condition. An extreme condition is one week.

* Example of International Grid Codes in voltage and frequency for Portugal:

1. Maximum Voltage 1 and 2:

• > 110% of nominal voltage for more than 1 second,
• > 115% of nominal voltage for more than 0,1 seconds
1. Frequency before disconnecting:
• above 53 Hz for more than 0,3 seconds
• bellow 47 Hz for more than 0,3 seconds

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Index Terms:

Generation, Wind Power, Transmission System Operator (TSO), Wind Turbine Generator (WTG), Wind Farms (WF), used factor, Point of Common Coupling (PCC), voltage, active power, reactive power, frequency, Fault Ride-Through (FRT), High Voltage Ride-Through (HVRT), Low Voltage Ride-Through (LVRT), balance of power, tidal energy, marine energy, wave energy, floating systems, substructure, intermittence, Metheorological Mast (Metmast).

J. Sánchez Ríos

javier.sanchezrios.1978@ieee.org