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Power Electronics for Renewable Energy Systems: Wind Turbine and Photovoltaic Systems U. M. Choi, and K. B. Lee

F. Blaabjerg

Division of Electrical and Computer Engineering Ajou University Suwon, Korea [email protected], [email protected]

Department of Energy Technology Aalborg University Aalborg, Denmark [email protected]

Abstract— The use of renewable energy sources are increased because of the depletion of natural resources and the increasing pollution level from energy production. The wind energy and the solar energy are most widely used among the renewable energy sources. Power electronics is needed in almost all kinds of renewable energy system. It controls the renewable source and interfaces with the load effectively, which can be grid-connection or working in stand-alone mode. In this paper, overview of wind and photovoltaic energy systems are introduced. Next, the power electronic circuits behind the most common wind and photovoltaic configurations are discussed. Finally, their controls and important requirements for grid connection are explained. Keywords- power electronics, renewable energy system, wind turbine, photovoltaic

I.

INTRODUCTION

In the last decade, renewable energy systems have experienced the largest growths in percentage with more than 30% per year, similar to the growth of coal and lignite energy systems, because of the exhaustion of natural resources and the increasing pollution levels caused by the use of fossil fuels and nuclear energy [1]. The ever-increasing demand for energy can lead to problems for power distributors, such as grid instability and outages [2]. The importance of producing more energy and the interest in clean technologies has provoked increased development of renewable energy systems. One driving force in Europe was initiated in March 2007, when EU Heads of State adopted a binding target of 20 % of energy generated by renewable sources by 2020. Similarly, a plan for 20 % renewable energy sources by 2025 has been adopted in the US [1]. Among renewable energy sources, wind energy and solar energy have recently become the most interesting. There is a widespread use of wind turbine systems in power distribution networks as well as increasing numbers of wind power stations connected to the transmission networks. Denmark, for example, has a high wind energy power capacity penetration over 30 % in major areas of the country, and today 28 % of all electrical energy consumption in the country is covered by wind energy. The aim is to achieve a 100 % non-fossil-based power generation system by 2050 [3]. According to the GWEC (Global World Energy Council) [4], the cumulative worldwide installed wind power by the end

of 2011 was 237.7GW. In 2011, the wind power grew by about 6% compared to 2010, and the 40.6 GW wind power brought on line. The China (26.2 %), USA (19.7%), and Germany (12.2%) comprise the large proportion of world cumulative wind power capacity. The world wide cumulative and annual installed photovoltaic power in 2011, according to the EPIA, was 69.68 GW and 29.7 GW, respectively [4]. It can produce 85 TWh of electricity every year and this energy volume is sufficient to cover the annual power supply needs of over 20 million households. In terms of cumulative installed capacity, Europe still leads the way with more than 51 GW installed as of 2011. Next are Japan (5 GW) and the USA (4.4 GW), and then China (3.1 GW) is followed. Europe has increased its cumulative capacity base by 50% compared to 2010 with almost 22 GW of grid-connected PV installations until 2011. The Italy (9.3 GW) and Germany (7.5 GW) comprise the large proportion about 60% of global market. The overall PV penetration is quite low but the EPIA estimate it could be as high as 12 % in 2020. Another important aspect is that the cost of PV panels dropped during 2008 by around 40 % to levels under €1/W [1], [5]. The increasing use of renewable energy systems requires new strategies for the operation and management of the electricity grid in order to maintain or to improve power-supply reliability and quality. Additionally, regulation of the grid leads to new management structures in which trading of energy and power is becoming increasingly important. Power electronics, the technology to convert electric power efficiently from one stage to another, is essential for distributed renewable energy systems to achieve high efficiency and performance. The power electronics field has grown during the last decades for two main reasons. Firstly, the development of fast semiconductor switches that are capable of switching quickly and handling high power. Secondly, the production of microcontrollers that can implement advanced and complex control algorithms. These factors have led to the development of costeffective and grid-friendly power converters [6]. This paper discusses wind turbine and photovoltaic (PV) systems representative of these renewable energy systems. In section II, the overview of wind turbine system and grid converter structure adopted in wind turbine system are discussed. The structures are classified into reduced power, full power, and multiple-cell full power.

Fig. 1. Basic power conversion principle in a wind power system.

Further, control strategy of wind turbine systems and the faultride-through requirement are explained. In section III, the overview of wind turbine system and design of PV converters are discussed. Then control methods of PV systems and several standards for anti-islanding are expressed. II.

WIND TURBINE SYSTEMS

Wind turbine systems consist of two parts: a mechanical part and an electrical part, as shown in Fig. 1. The mechanical power system extracts the kinetic energy from the wind and makes it available to a rotating shaft, while the electrical power system transforms the electrical energy so that it is suitable for the electrical grid. The mechanical energy is converted to electrical energy by an electric generator connected between the mechanical system and the electrical system [1]. There are three stages used to optimize extraction of the wind energy. The first stage is the mechanical stage, where the pitch of the blade, yaw of the turbine shaft and speed of the motor shaft are regulated. The second stage, electromechanical, can have variable components such as pole pairs and rotor resistors, an external excitation and/or a power converter that adapts the speed or the torque of the motor shaft and waveforms of the generator voltages/currents. The electrical stage is the third one, where the waveforms of the grid currents are adapted. Power electronic converters may be present in the second and/or third stage [2].

Fig. 2. Reduced power converter structures for wind turbines (a) rotorresistance converter (b) doubly-fed induction generator.

A. Structures of wind power system 1) Reduced power converter structure [2], [7], [8] Fig. 2 (a) shows a wind turbine system where the generator is an induction generator with a wounded rotor. An extra resistance controlled by power electronics is added in the rotor, which gives a speed range of 2% to 4%. The power converter for the rotor resistance control is for low voltage but high currents. At the same time, an extra control freedom is obtained at higher wind speeds in order to maintain a fixed output power. This solution needs a soft-starter and a reactive power compensator. The other reduced power structure is that of a doubly-fed induction generator with a wounded rotor (DFIG), as shown in Fig. 2 (b). In this case, slip rings are needed to connect the converter with the rotor and control its currents. A gear-box is still needed and speed regulation via the rotor is used to optimize power extraction from the wind.

Fig. 3. Full power converter structures: (a) induction generator with gear-box (b) multipole synchronous generator.

If the generator is running super-synchronously, electrical power is delivered through both the rotor and stator. If the generator is running sub-synchronously, electrical power is only delivered into the rotor from the grid. A speed variation of ±30 % around the synchronous speed may be obtained by the use of a power converter rated 30% of nominal power.

Fig. 4. Multi-cell full power structure (a) connected in parallel and interleaved on the grid side. (b) n-leg diode bridge producing a high DC voltage shared among converters connected in parallel and interleaved on the grid side.

Fig. 5. General scheme of wind turbine control.

2) Full power converter structure [2], [7], [8] These wind turbines have a full power converter between the generator and grid, which gives extra losses in the power conversion but also added technical performance. Fig. 3 shows the turbine structure with full power converters. The converter may perform reactive power compensation and maintains a smooth grid connection over the entire speed range. Fig. 3(a) shows a full-scale power structure with gear-box. The generator can be asynchronous, electrically excited synchronous (WRSG), or a permanent magnet excited (PMSG). If a multi-pole generator is used, such as a multiple wound rotor synchronous generator or permanent-magnet synchronous generator, the gear-box can be eliminated, as shown in Fig. 3(b) [13]. The trend for large scale wind turbines are full scale power conversion. 3) Multiple-cell full power structure [9]-[11] To increase the overall power of the system, several power converters are used in parallel or in cascade. Using multiplecell full power structure, the capacity of each switching device can be reduced and these provide redundancy because if one of power converter is failure, the system can still provide a part of the power. Furthermore, the commutation of different Identify applicable sponsor/s here. If no sponsors, delete this text box. (sponsors)

converters could be synchronized to reduce output harmonics using interleaved modulation. Fig. 4(a) shows the multiple-cell structure with full power converter connected in parallel and interleaved on the grid side. Fig. 4(b) shows the n-leg diode bridge producing a high DC voltage shared among converters connected in parallel and interleaved on the grid side. B. Wind turbine system control [1], [2], [11] The goal of Wind Turbine System (WTS) control is to maximize the power production. The electrical and mechanical systems are characterized by different control goals but interact in view of the main aim that the control of power injected into the grid. The mechanical system is controlled to limit the power, capture the maximum energy, limit the speed and reduce the acoustical noise. The electrical system control is responsible for the interconnection with grid and active/reactive power control, and the overload protection. Fig. 5 shows the general scheme of wind turbine control. The pitch control is used to limit the aerodynamic power generated by the rotor above rated wind speeds. The generator side converter is controlled to extract the maximum power

Fig. 7. Typical structure of PV systems.

Fig. 8. Simplified PV cell model.

Fig. 6. Fault Ride Through requirement given by E. ON-Netz. (a) FRT requirement (b) Requirements of reactive power provision.

from the wind. The control of grid-side converter is keeping the DC-link voltage fixed. Both converters use the internal current and voltage loops. The state variables of the LCL filter are controlled for system stability. The grid fault ride through and the support is needed for grid voltage restoration. The inertial emulation is a control function aiming at emulates the relation between active power and frequency. Power smoothing is accomplished by supplying a compensation power from an energy storage system. The energy storage can be connected to the alternating current grid or to the DC-link of the variablespeed WTS. C. Fault ride-through requirement for wind turbine systems. Wind energy plays an important role in the world’s energy market. As the wind energy capacity connected to the grid increase, their dynamic behaviors and performances are critical to the stability and quality of grid. To enable a large-scaled application of wind energy without compromising the power system stability, the WTs should be connected continually and contribute to the grid in case of grid fault. The WTs should supply active/reactive power for grid frequency and voltage recovery, immediately after the grid fault occurs. Therefore, several grid-connection codes are introduced for WTs especially in places such as Denmark, Spain, UK, US and Germany. Among these requirement, fault ride-through (FRT) is regarded as the main challenge to the WTs. E. ON-Netz in Germany has the hardest FRT requirements. Fig. 6(a) shows the FRT requirement of E. ON-Netz. According to Fig. 6, the generator must be operated continuously for 150 ms at zero grid voltage. In dotted area, if the facility is facing stability issues, short time interruptions with resynchronization are allowed for a maximum 2sec. In the light grey area, later than 2 sec, short disconnection with resynchronization can be allowed after agreement with the transmission system operators. For faults longer than 1.5 sec, stepwise interruptions are allowed. After the fault clearance,

active power in-feed must increase with a rate of 10% of the rated power per second. For all generators that do not disconnect from the grid during the fault, the active power must be continued immediately after fault clearance and increased to the original value with a gradient of at least 20% of rated power per second. The generator should not produce any active power and all of the output should be reactive power when the grid voltage is less than the 50% of rated value as shown in Fig 6(b). From less than 90% to less than the 50% of rated grid voltage, increase the reactive current at a rate of 2% per a percentage of grid voltage reduction as shown in Fig. 6(b). More than 90% of grid voltage is considered to the normal condition [12], [13]. III.

PHOTOVOLTAIC SYSTEMS

While the world’s energy demand is steadily increasing, PV power supply to the utility grid is gaining more and more visibility. With reductions in system costs, such as for PV modules, inverters, cables, fittings and man power, the PV technology has the potential to become one of the main renewable energy sources for future electricity supply [7]. Fig. 7 shows the typical structure of a PV system. The generated dc voltage is boosted by the dc-dc converter and the current is converted to a suitable ac current by the dc-ac inverter. A. PV cell The PV cell is an all-electrical device that produces electrical power when connected to a suitable load and exposed to sunlight. Fig. 8 shows a simplified PV cell model. Without any moving parts inside the module, wear-and-tear is very low and module lifetimes of more than 25 years can easily be reached. However, aging may reduce their power generation capability to 75–80 % of the nominal value. A typical PV module is made up of around 36 or 72 cells connected in series, encapsulated in a structure made of, for example, aluminum and Tedlar [7]. The curves of PV-cell current-voltage and power-voltage characteristics are changed by cell temperature and incident solar radiation.

Fig. 10. Structure of different PV systems (a) centralized inverter (b) string inverter (c) module inverter (d) multi-string inverter.

voltage DC cables between PV panels and inverter, power losses due to common MPPT, power losses due to module mismatch, losses in the string diodes, and the fact that the reliability of the whole system depends on one inverter [17].

Fig. 9. Characteristics of PV cells (a) V-I curve (b) V-P curve.

Since the power produced by the cell influences other parameters, Maximum Power Point Tracking (MPPT) is required to maximize energy capture to the highest possible efficiency over a wide range, due to morning-noon-evening and winter-summer variations [14]. Fig. 9 shows the characteristics of a PV cell. B. Design of PV converter [15]-[17] During the last decade, there have been significant advances in PV inverter technologies. Inverter prices have decreased around 50% and efficiency and reliability have increased considerably. To decrease the cost and increase the efficiency of PV system, many inverter designs have been developed. 1) Centralized inverter Fig. 10(a) shows a centralized inverter layout interfacing a large number of PV modules with the grid Here, the PV modules are divided into series connections called strings. The strings are arranged in parallel and connected to one common central inverter, in order to reach high power. This centralized inverter has significant disadvantages, such as a need for high-

2) String inverter The string inverter, shown in Fig. 10(b), is a reduced version of the centralized inverter where single strings of PV modules are connected to separate inverters that are paralleled and connected to the grid. If the string voltage is high enough, voltage boosting is not necessary and the efficiency can be improved. Fewer PV panels can also be used, but a DC-DC converter or a line frequency transformer is then needed to boost the voltage. The advantages of the string inverter compared with the centralized inverter are as follows: no losses in the string diodes, separate MPPTs for each string, better yield due to separate MPPTs, and lower price due to the ease of mass production. 3) Module inverter As shown in Fig. 10(c), the AC module is made up of a single solar panel connected to the grid by its own inverter. The advantage of this configuration is that mismatch losses are significantly reduced and it is possible to maximize the power production of each separate MPPT. This results in better optimization of power extraction than in the case of the string inverter. Furthermore, the modular structure simplifies the modification of the whole system because of its plug-and-play nature. However, it has a low efficiency due to the high-voltage amplification, low power conversion and the price per watt is still higher compared with the previous inverters [16]. 4) Multi-string inverter The multi-string inverter, illustrated in Fig. 10 (d), is a further development on the string inverter. Here, several strings are interfaced with their own dc-dc converter to dc-ac inverter [8], [9]. It combines the advantages of both string and module inverters by having many dc-dc converters with separate MPPTs that feed energy to a common dc-ac inverter [18], [19]. The multi-string concept is a flexible solution, having a high overall efficiency of power extraction because it is possible to control each PV string individually.

Fig. 10. Topologies of converters for Photovoltaic system (a) H-bridge (b) H5-bridge (c) HERIC (d) NPC (e) Conergy NPC.

C. Topologies of converters for Photovoltaic system The transformerless PV (photovoltaic) converters are widely used in grid-connected renewable energy systems because of its advantages. They can reduce the costs, size, and weight. First of all, they can increase the total efficiency of PV system. Many topologies have been proposed during the last few years. Among them, in this chapter, the H-bridge, H5bridge, HERIC, Neutral-Point clamped (NPC), and Conergy NPC are introduced [1], [15], [18]. 1) H-bridge converter Fig. 11 (a) shows the H-bridge converter topology. Most single-phase H-bridge converters use unipolar modulation. The output voltage is positive when the switches S1 and S4 are turned on. Adversely, output voltage is negative when the S2 and S3 are turned on. The zero output voltage states are possible when the switches S1 and S3 are turned on or S2 and S3 are turned. The unipolar modulation has various advantages. The ripple current is significantly reduced. Further, it yields lower filtering requirements and lower core losses because the output voltage has the three-levels. However, if this topology and modulation are applied, common mode voltage VPE has switching frequency components and it yields high leakage current and EMI. 2) H5-bridge converter The H5-bridge topology is derived from H-bridge by SMA. It is made up of standard H-bridge topology with an additional fifth switch S5 in the positive bus of DC-link as shown in Fig. 11 (b). The switch S5 is turned off when the zero voltage state. Using this topology, the efficiency can be increased up to 98% because the reactive power exchange between the filter L and DC-link capacitor is prevented during the zero switching state. Additionally, this topology isolates the PV module form the grid during the zero switching state. Because of this, the

common mode voltage VPE has only grid frequency components and no switching frequency components. It leads a very low leakage current and EMI. However, the H5 requires additional switch. 3) HERIC converter Fig. 11 (c) shows the HERIC (highly efficiency and reliable inverter concept) converter topology. It also derived from Hbridge topology by adding a bypass leg in the AC side using bidirectional switch or two IGBTs. The AC bypass provides the same function as the fifth switch in case of the H5-bridge topology. During the positive period of grid voltage, switch S6 is turned on and it is used during the freewheeling period of S1 and S4. Adversely, during the negative period of grid voltage, switch S5 is turned on and it is used during the freewheeling period of S2 and S3. Using this topology, the efficiency can be increased up to 97% and leads a very low leakage current and EMI. However, two switches are required. 4) NPC converter The NPC converter topology consists of four IGBTs, two clamped diodes and two capacitors as shown in Fig. 11 (d). This topology has the three voltage states. The output voltage is VPV/2 when the switch S1 and S2 are turned on, on the other hands, when the switches S3 and S4 are turned on, the output voltage state is –VPV/2. If the output current is positive, the output voltage is zero when the switch S2 is turned on. If the output current is negative, the output voltage is zero when the switch S3 is turned on. The NPC has very similar performance in comparison with H5 and HERIC. Using this topology, the efficiency can be increased up to 98%. The VPE is constant and it is equal to –VPV/2 without switching frequency components. It leads a very low leakage current and EMI. Additionally, it is possible to reduce the voltage capacity to VPV/2. However, this topology requires additional two diodes and two IGBT. Further, the neutral –point voltage unbalancing can be occurred.

Fig.12. Generic control structure for PV inverter with boost stage.

5) Conergy NPC converter Fig. 11(e) shows the Conergy NPC converter topology where the bidirectional switch is connected between the neutral-point and its output and a full-bridge is used instead of a half-bridge. The main concept of the Conergy NPC inverter is that zero voltage can be produced by the clamping the output to neutral-point of the DC-link using switch S2 or S3 depending on the current direction. The Conergy NPC converter topology has slightly higher efficiency in comparison with NPC. The conduction loss of Conergy NPC is lower than the NPC because the current is conducted through a single switch. The other performances are similar with NPC converter. However, the voltage capacity of switch S1 and S4 are double as compared with switches used in NPC converter. D. Photovoltaic system control [1], [19], [20] The PV inverter converts the DC power produced by the solar modules to grid synchronized AC power. Fig. 12 shows the generic control structure for PV inverter with a boost stage. On the PV array side, a Maximum Power Point Tracker (MPPT) control is implemented in order to collect the maximum available power at every operating point because the characteristic of the PV modules are influenced by the solar radiation and the temperature as mentioned earlier. The MPPT can be performed through DC voltage, AC current or AC voltage control [7]. Through the current control, photovoltaic systems harmonic rejection is achieved to satisfy the THD requirement imposed by standards. Furthermore, the systems can be stable in case of large grid impedance variations by the current control. In order to synchronize the injected grid current with the grid voltage to achieve unity power factor, the Phase Locked Loop (PLL) control is performed. The AntiIslanding (AI) protection control is fulfilled to stop the power production of the photovoltaic systems in the case of abnormal conditions. Grid monitoring is also needed for synchronization and fast voltage/frequency detection for passive AI. E. Anti-Islanding requirement for PV systems Islanding for grid-connected PV systems takes place when the PV inverter is not disconnected for a very short time after the grid is tripped. The grid disconnection can occur as a result of a local equipment failure detected by ground fault protection or an intentional disconnection of the line for servicing.

Fig. 13. The test setup for anti-islanding requirements in IEEE 1547.1.

In the above situations, if the PV inverter is not disconnected to the grid following problems can occur. For the first, it can cause a safety-accident of line workers who misapprehended the situation as blackout. Second, when the grid connection is restored, it provokes the damage to the connected equipments due to out-of-phase between grid and PV systems. In order to avoid these serious problems, Anti-Islanding (AI) requirements have been issued in standards. 1) Anti-Islanding defined by IEEE Std. 1547.1 [21], [22] In IEEE 1547.1, the test setup for anti-islanding requirements is described as shown in Fig. 13, where the EUT represents the Equipment Under Test. The RLC load is connected in parallel between the PV inverter and the grid. The LC load should be adjusted to resonate at the rated grid frequency fgrid, and to have a quality factor Qf =1. The values of RLC load can be calculated as R=

PQ f V2 V2 , L= ,C= 2π f grid PQ f P 2π f gridV 2

(1)

The RLC load shown in Fig. 13 should be set so that the grid current which flows through S3 should be lower than 2% of the rated value on the steady-state condition. In this condition, the disconnection should be detected within 2 sec after S3 is opened. 2) Anti-Islanding defined by VDE 0126-1-1 [23] There are two anti-islanding methods in the VDE 0126-1-1. a) Impedance measurement Fig. 14 shows the test circuit for anti-islanding requirements in VDE 0126-1-1. The procedure is based on local balancing of active and reactive power using variable RLC load. The switch S is opened in order to increase the grid

[4]

[5]

[6] Fig. 14. The test circuit for anti-islanding requirements in VDE 0126-1-1 in PV systems [1].

impedance by 1 Ω. The inverter should be disconnected within 5 sec. The test should be repeated for different values of R2 and L2 in the range of 1 Ω. b) Disconnection detection with RLC resonant load The test circuit is same with the Fig. 13. the RLC load should be set that the quality factor Qf > 2 using (1). In balanced power condition, the inverter should be disconnected within 5 sec after S3 is opened for the 25%, 50% and 100% of rated power. 3) Anti-Islanding defined by IEC 62116 [24] The test circuit is same as in the IEEE 1547.1 test shown in Fig, 13 and the power should be balanced before the island detection test. The test is tested at three levels of output power but conditions to confirm the island detection is similar to IEEE 1547.1 test. The test is performed divided into three cases according to the power level: case A (100~105%), case B (50~66%), and case C (25~33%). For case A, the inverter is tested in step of 5% both real and reactive power in an interval of ±10 % from the inverter’s operating output power. The case B and C are evaluated by reactive power deviation from -5 to 5% at a step of 1% of inverter output power. The maximum trip time is 2 sec. it is same as in IEEE 1547.1 standards. IV.

CONCLUSION

This paper has discussed wind turbine and photovoltaic (PV) systems which are representative of these renewable energy systems. First, the overview of wind turbine systems and grid converter structure adopted in wind turbine system are discussed. The structures are classified into reduced power, full power, and multiple-cell full power. Further, control strategy of wind turbine systems and the fault-ride-through requirement are explained. Next, the overview of wind turbine system and design of PV converters are discussed. Then control methods of PV systems and several standards for anti-islanding are expressed which are important for safety. REFERENCES [1]

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