Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey
Summary (4 min read)
I. INTRODUCTION
- HE increasing number of renewable energy sources and distributed generators requires new strategies for the operation and management of the electricity grid in order to maintain or even to improve the power supply reliability and quality.
- The first one is the development of fast semiconductor switches, which are capable of switching quickly and handling high powers.
- New trends in power electronics technology for the integration of renewable energy sources and energy storage systems are presented.
- Appropriate integration of renewable energy sources with storage systems allows for greater market penetration and result in primary energy and emissions savings.
A. Variable Speed Wind Turbines
- Wind turbine technology has undergone a dramatic transformation during the last 15 years, developing from a fringe science in the 1970's to the wind turbine of the 2000's using the latest in power electronics, aerodynamics and mechanical drive train designs [1] [2] .
- The legislation in both countries favors continuing growth of installed capacity.
- Wind power is quite different from conventional electricity generation with synchronous generators.
- There is also less mechanical stress and rapid power fluctuations are scarce, because the rotor acts as a flywheel (storing energy in a kinetic form).
- Variable speed turbines also allow the grid voltage to be controlled, as the reactive power generation can be varied.
B. Current Wind Power Technology
- Variable speed wind turbines have progressed dramatically in recent years.
- Variable speed operation can only be achieved by decoupling electrical grid frequency and mechanical rotor frequency.
- This scheme allows, on one hand, a vector control of the active and reactive power of the machine, and on the other hand, a decrease by a high percentage of the harmonic content injected into the grid by the power converter.
- The silicon is isolated to the cooling plate and can be connected to ground for low electromagnetic emission even with higher switching frequency.
- In conclusion, with the present semiconductor technology, IGBTs present better characteristics for frequency converters in general and especially for wind turbine applications.
C. Grid Connection Standards for Wind Farms
- 1) Voltage Fault Ride-Through Capability of Wind Turbines:.
- To enable large-scale application of wind energy without compromising power system stability, the turbines should stay connected and contribute to the grid in case of a disturbance such as a voltage dip.
- Though the definition of fault ride through varies, the E.ON (German Transmission and Distribution Utility) regulation is likely to set the standard [8] .
- Modern forced-commutated inverters used in variable-speed wind turbines produce not only harmonics but also inter-harmonics.
- The last one is the harmonic analysis, which is carried out by the FFT algorithm.
D. Trends on Wind Power Technology 1)
- Offshore wind turbines may have slightly more favorable energy balance than onshore turbines, depending on local wind conditions.
- This implies that the maximum feasible length and power transmission capacity of HVAC cables is limited.
- The HVDC transmission offers many advantages over HVAC [13] : a).
- As the ratings of the components increases and the switching and conducting properties improve, the advantages of applying multilevel converters become more and more evident.
III. PHOTOVOLTAIC TECHNOLOGY
- This section focuses on a review of recent developments of power electronics converters and the state of art of implemented photovoltaic (PV) systems.
- PV systems as an alternative energy resource or energy resource complementary in hybrid systems have been becoming feasible due to the increase of research and development work in this area.
- Several standards given by the utility companies must be obeyed in the PV modules connection.
A. Market Considerations
- Energy demand has grown consistently by 20-25% per annum over the past 20 years, mainly due to the decreasing costs and prices.
- This decline has been driven by a) increasing efficiency of solar cells b) manufacturing technology improvements, and c) economies of scale.
- In 2001, 350 Megawatts of solar equipment were sold to add to the solar equipment already generating clean energy.
- If the growth rates of the installation of photovoltaic systems between 2001 and 2003 could be maintained in the next years, the target of the European Commission's White Paper for a Community Strategy and Action Plan on Renewable Sources of Energy would already be achieved in 2008.
B. Design of PV Converters Families
- An overview of some existing power inverter topologies for interfacing PV modules to the grid is presented.
- Before discussing PV converter topologies, three designs of inverter families are defined: central inverters, module-oriented or moduleintegrated inverters, and string inverters [34][35] .
- The use of a transformer leads to the necessary isolation (requirement in US) and modern inverters tend to use a high-frequency transformer.
- The DC/DC converter performs the MPPT (and perhaps voltage amplification) and the DC/AC inverter is dedicated to control the grid current by means of Pulse-Width Modulation (PWM), Space Vector Modulation (SVM) or bang-bang operation.
- A variant of this topology is the standard full-bridge three-level VSI, which can create a sinusoidal grid current by applying the positive/negative DClink or zero voltage, to the grid plus grid inductor [42] .
D. Future Trends
- The increasing interest and steadily growing number of investors in solar energy, stimulated research that resulted in development of very efficient PV cells, leading to universal implementations in isolated locations [44] .
- A cost reduction per inverter watt is, therefore, important to make PV-generated power more attractive.
- Problems associated with the centralized control appear and it can be difficult to use this type of systems.
- These topics have been deeply studied during last years but some improvements still can be done using new topologies such as multilevel converters.
- In addition, the incorporation of new technologies, packaging techniques, control schemes and an extensive testing regimen must be developed.
B. Hydrogen
- The purpose of this section is to analyze new trends in hydrogen storage systems for high quality back-up power.
- The use of fuel cells in such applications is justified since they are a very important alternative power source due to their well-known specific characteristics such as very low toxic emissions, low noise and vibrations, modular design, high efficiency (especially with partial load), easy installation, compatibility with a lot of types of fuels, and low maintenance cost.
- An example of the hydrogen storage application to improve the grid power quality through smoothing large and quick fluctuations of wind energy is reported in [60] .
- Applications to identify and investigate advanced concepts for material storage that have the potential to achieve 2010 targets of 2 kWh/kg and 1.5 kWh/L.
C. Compressed Air Energy Storage (CAES)
- Energy storage in compressed air is made using a compressor, which stores it in an air reservoir (i.e. an aquifer like ones used for natural gas storage, natural caverns or mechanically formed caverns, etc.).
- When a grid is operating off-peak, the compressor stores air in the air reservoir.
- Such systems are available for 100-300 MW and burn about one-third of the premium fuel of a conventional simple cycle combustion turbine.
- An alternative to CAES is the use of compressed air in vessels (called CAS), which operates exactly in the same way as CAES except that the air is stored in pressure vessels, rather than underground reservoirs.
- Recent research is devoted to maximum efficiency point tracking control [64] or integrated technologies for power supplies applications [65] .
D. Supercapacitors
- Supercapacitors, also known as ultracapacitors or electric double layer capacitors (EDLC), are built up with modules of single cells connected in series and packed with adjacent modules connected in parallel.
- Single cells are available with capacitance values from 350F to 2700F and operate in the range of the 2V.
- The module voltage is usually in the range from 200V to 400V.
- They have a long life cycle and are suitable for short discharge applications and less than 100kW.
- New trends focused on using ultracapacitors to cover temporary high peak power demands [66] , integration with other energy storage technologies and development of high−voltage applications.
E. Superconducting Magnetic Energy Storage (SMES)
- Also, there is no need for conversion between chemical or mechanical forms of energy.
- Recent systems are based on both general configurations of the coil: solenoidal or toroidal.
- Such devices require cryogenic refrigerators (to operate in liquid helium at -269°C) besides the solid-state power electronics.
- When a load must be fed, the current is generated using the energy stored in the magnetic field.
- Typical applications of SMES are corrections of voltage sags and dips at industrial facilities (1MW units) and stabilization of ring networks (2MW units).
F. Battery Storage
- There are several types of batteries used in renewable energy systems: lead acid, lithium and nickel.
- Batteries provide rapid response for either charge or discharge, although the discharge rate is limited by chemical reactions and type of battery.
- New trends in the use of batteries for renewable energy systems focused on the integration with several energy sources (wind energy, photovoltaic systems, etc.) and also on the integration with other energy storage systems complementing them.
- Also, there are attempts to optimize battery cells in order to reduce maintenance and to increment its lifetime [69] .
G. Pumped -Hydroelectric Storage (PHS)
- When no extra generation is needed, the water is pumped back up to recharge the upper reservoir.
- One limitation of PHS is that they require significant land areas with suitable topography.
- There are units with sizes from 30 MW to 350MW, with efficiencies around 75%.
- New trends in PHS are focused on the integration with variable speed drives (cycloconverters driven doubly-fed induction machine) [70] and the use of underground pumped hydroelectric storage (UPHS), where the lower reservoir is excavated from subterranean rock.
- Such a system is more flexible, more efficient, but requires a higher capital cost.
V. CONCLUSION
- The new power electronics technology plays a very important role in the integration of renewable energy sources into the grid.
- It should be possible to develop the power electronics interface for the highest projected turbine rating; optimize energy conversion and transmission and control reactive power; minimize harmonic distortion; achieve at a low cost high efficiency over a wide power range; have a high reliability and tolerance to the failure of a subsystem component.
- In addition, the applicable regulations favour the increasing number of wind farms due to the attractive economical reliability.
- These systems are nowadays being studied and only research projects have been developed focused on the achievement of mature technologies.
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Citations
2,254 citations
Cites background from "Power-Electronic Systems for the Gr..."
...industry [8], grid integration of renewable-energy sources [9]– [11], reactive-power compensation [12]–[14], and other applications [15], [16]....
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2,115 citations
Cites background from "Power-Electronic Systems for the Gr..."
...The current paradigm in the control of wind- or solar-power generators is to extract the maximum power from the power source and inject them all into the power grid (see, for example, [1]–[3])....
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1,158 citations
Cites background from "Power-Electronic Systems for the Gr..."
...Among these, the MIC system offers “plug and play” concept and greatly optimizes the energy yield [4], [12]....
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...2) The dual grounding becomes a difficult issue in the transformerless inverters [4]....
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...However, compared with large converters, MICs have smaller power ratings and tend to have lower efficiencies [4], [31]....
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...Electronic power inverter is one of the enabling technologies required for utilizing PV energy and its cost is becoming more visible in the total price of the PV system [3], [4]....
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...However, a low-power line-frequency transformer is bulky and may not be very efficient [4], [51]....
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Cites background from "Power-Electronic Systems for the Gr..."
...introduces some new issues on the operation of the power system, such as potential unbalancing between generation and demand [98]....
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"Power-Electronic Systems for the Gr..." refers background in this paper
...Sometimes they are a multiple of 50 Hz, and sometimes they are not....
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"Power-Electronic Systems for the Gr..." refers methods in this paper
...A variant of this topology is the standard full-bridge three-level VSI, which can create a sinusoidal grid current by applying the positive/negative dc-link or zero voltage, to the grid plus grid inductor [42]....
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3,530 citations
Additional excerpts
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Related Papers (5)
Frequently Asked Questions (19)
Q2. What are the contributions in this paper?
In this paper, new trends in power electronics for the integration of wind and photovoltaic power generators are presented. A review of appropriate storage systems technology used for the integration of intermittent renewable energy sources is also introduced.
Q3. What are the future works in this paper?
Finally, for the energy storage systems ( flywheels, hydrogen, compressed air, supercapacitors, superconducting magnetic and pumped-hydroelectric ) the future presents several fronts and actually they are in the same development level.
Q4. Why is the current control scheme used more frequently?
The current control scheme is employed more frequently, because a high power factor can be obtained with simple control circuits, and transient current suppression is possible when disturbances such as voltage changes occur in the utility power system.
Q5. What is the main reason for the increase in the voltage in the converter?
In particular, multilevel cascade converters seem to be a good solution to increase the voltage in the converter in order to eliminate the high frequency transformer.
Q6. What are the advantages of variable speed turbines?
The advantages of variable speed turbines are that their annual energy capture is about a 5% greater than fixed speed technology, and that the active and reactive power generated can be easily controlled.
Q7. Why are residential neighbourhoods becoming a target of solar panels?
Due to improvement of roofing PV systems, residential neighbourhoods are becoming a target of solar panels and some current projects involve installation and set-up of PV modules in high building structures [45].
Q8. What type of converters can be used to develop a PV system?
Classical solutions can be applied to develop these converters: Flyback converters (single or two transistors), Flyback with a Buck-Boost converter, resonant converters, etc.
Q9. What is the main advantage of a permanent magnet synchronous machine?
For small wind turbine, Permanent Magnet Synchronous Machines are more popular because of their higher efficiency, high power density and robust rotor structure as compared to induction and synchronous machines.
Q10. What is the common disadvantage of the multilevel converters with split DC-link?
The most commonly reported disadvantage of the multilevel converters with split DC-link is the voltage unbalance between the capacitors that integrate it.
Q11. What is the main advantage of a direct drive generator?
As compared to a conventional gearbox-coupled wind turbine generator, a direct drive generator has reduced overall size, lower installation and maintenance cost, a flexible control method and quick response to wind fluctuations and load variation.
Q12. How long has it been used to limit power interruptions?
For approximately 20 years it has been a primary technology used to limit power interruptions in motor/generator sets where steel wheels increase the rotating inertia providing short power interruptions protection and smoothing of delivered power.
Q13. What are the advantages of applying multilevel converters?
As the ratings of the components increases and the switching and conducting properties improve, the advantages of applying multilevel converters become more and more evident.
Q14. What is the main drawback of the multilevel cascaded topologies?
The continuous reduction of the cost per kW of PEBBs, is making the multilevel cascaded topologies to be the most commonly used by the industrial solutions.
Q15. What are the benefits of removing the gearbox?
There are several benefits of removing the gearbox: reduced losses, lower costs due to the elimination of this expensive component, and increased reliability due to the elimination of rotating mechanical components.
Q16. Why is hydrogen gaining popularity in industrial applications?
The hydrogen-fuel economy has been rapidly increasing in industrial application due to the advantages of the hydrogen of being storable, transportable, highly versatile, efficient and clean energy carrier to supplement or replace many of the current fuel options.
Q17. What are the main trends in the use of batteries for renewable energy systems?
New trends in the use of batteries for renewable energy systems focused on the integration with several energy sources (wind energy, photovoltaic systems, etc.) and also on the integration with other energy storage systems complementing them.
Q18. Why is the wind turbine required to be integrated with grid or energy systems?
Because these electrolysers require a constant minimum load, wind turbines must be integrated with grid or energy systems to provide power in the absence of wind [28].
Q19. What are the disadvantages of variable speed wind turbines?
As disadvantages, variable speed wind turbines need a power converter that increases the component count and make the control more complex.