CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 8, NO.

1, JANUARY 2022 39

Power Electronics: The Enabling Technology for

Renewable Energy Integration
Zhongting Tang, Member, IEEE, Yongheng Yang , Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE

Abstract—The markedly increased integration of renewable by strong policy support, Germany has rapidly increased the
energy in the power grid is of significance in the transition to share of renewable energy in its electricity generation in the
a sustainable energy future. The grid integration of renewables recent past [3]. Seen from the global landscape of renewable
will be continuously enhanced in the future. According to the
International Renewable Energy Agency (IRENA), renewable energy, the RES capacity has grown remarkably in the past two
technology is the main pathway to reach zero carbon dioxide decades, as shown in Fig. 1 [4]. In addition, the development
(CO2) emissions by 2060. Power electronics have played and of global RESs in the immediate past, i.e., from 2000 to 2020,
will continue to play a significant role in this energy transition is depicted in Fig. 2, where wind and solar PV sources have
by providing efficient electrical energy conversion, distribution, the highest growth rates [4].
transmission, and utilization. Consequently, the development of
power electronics technologies, i.e., new semiconductor devices, There are mainly two challenges that accompany the high
flexible converters, and advanced control schemes, is promoted penetration of RESs. One is how to friendly integrate large-
extensively across the globe. Among various renewables, wind scale RESs into the electrical grid, ensuring network stability
energy and photovoltaic (PV) are the most widely used, and when injecting varying renewable power, as well as in case
accordingly these are explored in this paper to demonstrate of grid disturbances. The other is how to achieve efficient,
the role of power electronics. The development of renewable
energies and the demands of power electronics are reviewed intelligent, and reliable power conversion, transmission, dis-
first. Then, the power conversion and control technologies as tribution, and utilization of electrical energy by using power
well as grid codes for wind and PV systems are discussed. Future electronics. Accordingly, power electronics technologies have
trends in terms of power semiconductors, reliability, advanced been developed at a fast pace, and the grid-integration stan-
control, grid-forming operation, and security issues for large- dards are continuously being updated for RESs, especially in
scale grid integration of renewables, and intelligent and full user
engagement are presented at the end. the case of wind and PV systems [5]–[13].
Regarding power electronics technology, many advance-
Index Terms—Advanced control, grid codes, grid integration, ments have been made along with the development of power
photovoltaic system, power electronics, reliability, wind turbine semiconductor devices, as shown in Fig. 3 [14]. From the
system. first-generation power semiconductor devices (i.e., thyristors)
in 1957 to the third generation of fully controlled power
switches, e.g., insulated gate bipolar transistor (IGBT) and
metal-oxide-semiconductor field-effect transistor (MOSFET)
I. I NTRODUCTION in the 2000s [15], [16], research efforts have mainly focused on
gate drivers, circuit topologies, modeling, and control strate-
R AW material shortages and environmental pollution due
to conventional energy sources (e.g., coal and oil) are
the main obstacles to the global strategic sustainability plans.
gies to achieve high switching frequencies (for high power
density), low losses, and high power handling capability in
Following the Paris Agreement of 2015, there is a need to power electronics-based power converters. As demonstrated in
achieve energy transition by the development and utilization Fig. 3, the development of wide-bandgap (WBG) devices, e.g.,
of renewable energy sources (RESs). Accordingly, many coun- silicon carbide (SiC) and gallium-nitride (GaN) power devices,
tries have made substantial efforts to change their energy brought the second revolution due to their superior perfor-
paradigms by intensively integrating RESs, e.g., wind, solar mances in terms of high voltage/current stress, low power
photovoltaic (PV), bioenergy, and ocean wave energy, into losses, high switching frequencies, and high-temperature op-
their energy systems [1]–[3]. For instance, Denmark plans to eration capability [17], [18]. On the other hand, in practical
be 100% independent of fossil fuels and 100% carbon-neutral applications, WBG devices are usually accompanied by new
based on RESs by 2050 [2]. IEA (2020) reported that driven challenges, e.g., packaging, thermal management, and elec-
tromagnetic interference (EMI). Nevertheless, advancements
Manuscript received April 12, 2021; revised June 25, 2021; accepted August in semiconductor technologies have enhanced the large-scale
17, 2021. Date of online publication September 10, 2021; date of current
version October 23, 2021. grid integration of RESs, however, lowering the system costs
Z. Tang and F. Blaabjerg are with the AAU energy, Aalborg University, is still of concern.
Aalborg DK-9220, Denmark. In the past, power converter topologies for low power RESs
Y. Yang (corresponding author, e-mail: yang yh@zju.edu.cn; ORCID:
0000-0002-1488-4762) is with the College of Electrical Engineering, Zhejiang mainly focused on high power density and high efficiency,
University, Hangzhou 310027, China. and they had to satisfy the requirement of electrical isolation
DOI: 10.17775/CSEEJPES.2021.02850 between the low voltage side and the grid [19]. Generally, such
2096-0042 © 2021 CSEE
40 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 8, NO. 1, JANUARY 2022

100 280
82%

Share of new electricity generating

Renewables

New generating capacity (GW)

50 210

capacity (%)
0 140

18%
50 70

Non-Renewables
100 0
2002 2004 2006 2008 2010 2012 2014 2016 2018 2020

Non-renewable share (%) Renewables share (%) Increase in renewables (GW)

Fig. 1. Comparison of RESs and non-RESs as a share of the total global annual additions based on the data available from IRENA [4], where the net
increase in global renewable generation reached 261 GW in 2020.

GW 1st generation 1957

3000 Research
● Thyristor
Hydropower Wind Solar
2500 Bioenergy Geothermal Marine ● Circuit topologies
2nd generation 1970s ● Modeling and control
● GTO,GTR,BJT ● Gate driver
2000 ● Digital control
● Power MOSFET
Thyristor
1500 Features

Power Semiconductor Devices

Power Electronics Technology

3rd generation 1980s
● Switching frequency
1000 ● Power losses
● IGBT,MOSFET
● Power density
500
4th generation 2000s
Research
0 ● High-performance
2000 2010 2020 IGBT,MOSFET ● Topologies and control

Reliability-oriented
● Wide-bandgap devices ● Gate driver
Fig. 2. Global accumulated capacity of RESs from 2000–2020 based on (SiC,GaN devices) ● Packaging
the data available from IRENA [4]. Here, hydropower covers pumped storage Characteristics ● Thermal management
and mixed plants, wind contains both onshore and offshore wind energy, solar ● EMI and EMC
● Switching times ● Modularization
includes photovoltaics and solar thermal, and marine includes tide, wave, and ● Losses
ocean energy. ● Power stress Features
● Cost
● Temperature ranges ● Reliability and lifetime
● Controllability ● Smart and intelligent
grid-connected converters can be classified into transformer- ● Renewables application
● Reliability and Lifetime
based and transformer-less topologies. By comparison, the
transformer-less topologies are more efficient, more compact,
smaller in size, and less costly than transformer-based con- Fig. 3. Development of power electronics technology along with the evolution
verters. In addition, micro-inverters employed for low-power of RESs and power semiconductor devices (Adapted based on the discussions
in [14]).
PV systems have the advantages of high voltage gain, plug-
and-play, and the ability to maintain maximum power point
tracking (MPPT) for each PV panel [20]. On the contrary, Moreover, protection is mainly designed for ensuring the stable
power converters for large-scale RESs, e.g., wind power plants, and safe operation of RESs, but not the grid. However, with the
pay more attention to high power level, high voltage, and continuous increase in grid-connected renewable energy, the
high reliability [21]. It has been seen in practical industrial stability of the grid is challenged in some areas. Consequently,
applications that the use of power electronics in wind turbine control strategies for large-scale grid-connected RESs are
systems has shifted from partial-scale to full-scale levels, now focusing on grid-forming capabilities, to enhance grid
bringing more flexibility and controllability into the system. resiliency [5]. Furthermore, the RESs coopted with power
Furthermore, to enhance the grid integration of renewable en- converters need to also intelligently respond to global man-
ergy, multiport converters are being studied for integrating into agement commands from system operators (e.g., limited power
energy storage devices [22]. Due to system cost limitations, injection) as well as specific demands from the end-users (e.g.,
these mainly focus on low-power grid-connected RESs. uninterrupted power supply). In addition, reliability-oriented
With the relatively small installation capacity of renewable control will be more significant in the consolidation of grid
energy in the past, the motivation of the control strategies for integration.
RESs was to satisfy the grid-following demands, e.g., high This paper provides an overview of the development of
power quality and grid synchronization [6], [9]. In present-day power electronics for efficient and reliable energy conversion
scenarios, grid-supportive control is increasingly in demand. from RESs, with a focus on wind and PV technologies. In Sec-
TANG et al.: POWER ELECTRONICS: THE ENABLING TECHNOLOGY FOR RENEWABLE ENERGY INTEGRATION 41

tion II, the typical architecture of RESs is briefly introduced,

Wind Power
followed by a comprehensive review of the demands on wind Conversion System
and PV power systems. Then, technologies for wind and PV Pin Po
power systems are reviewed in terms of converter topologies Generator Qin Qo Utility grid
and control strategies, in Section III. In Section IV, challenges
and research trends on future power electronics technologies 1.Controllable igen 1.Energy balance/storage 1.Fast/long Po response
for large-scale grid integration of RESs are presented. Finally, 2.Variable freq.& Cgen 2.High power density 2.Controllable Qo
3.Strong cooling 3.Freq.& Vg stabilization
Section V gives the conclusions. 4.High reliability 4.Fault ride-through

Fig. 5. Common demands on wind power systems, where Pin and Qin are
II. R EQUIREMENTS FOR RES S the active and reactive power transferred from the generator to the power
A. Typical RES Architecture converter, respectively, igen and Vgen are the generator current and voltage,
respectively, Po and Qo are the active power and reactive power exchange
A typical RES architecture is shown in Fig. 4, in which between the power converter and the utility grid, respectively, and Vg is the
the power electronics converter is a critical interface to con- grid voltage [11].
nect the renewable energy, the utility grid, the end-users,
and even the energy storage devices. As shown in Fig. 4, maximum energy but also to ensure energy balance when
the power electronics converter undertakes the mission of there is an inertia mismatch between the mechanical and the
transferring varying amounts of renewable energy into the electrical power [23].
utility grid with a constant voltage amplitude and a fixed 2) Utility grid side: The requirements for wind energy grid
frequency, and/or provide energy to local users. Therefore, integration, including grid synchronization, response under
demands on power electronics are diverse and complex. It abnormal grid conditions, and for grid supporting, aim to
can be generally summarized as—1) harvesting the maximum ensure safe operation with high integration of wind energy [5],
energy possible according to the characteristics of renewable [8], [26]. The most widely concerned demands are power qual-
energy; 2) producing the most energy with the least cost ity, reactive power injection, frequency regulation, and fault
through power converters (related to power semiconductor ride-through operation. Furthermore, communication, power
devices and topologies), i.e., being high efficiency, high power forecasting, ramp rate limitation, and other requirements are
density, and low cost, along with high reliability; 3) grid seen in practice as far as offshore wind power plants are
supporting capability (e.g., flexible power control and power concerned [27].
management). The specific demands for wind and PV power 3) Wind power conversion system side: As the power con-
systems as well as certain grid integration requirements are version system is the core of the wind power system, failures
discussed in the following. at the power conversion stage will affect the entire system
operation and lead to high maintenance costs, especially with
Flexible Power Flow a relatively large wind power capacity. Therefore, reliability
becomes increasingly significant in wind power systems [28],
2/3 2/3 [29]. Furthermore, to ensure normal power transmission when
connected to the grid, a transformer is usually adopted to
Renewable Energies Power Electricity Grid
Electronics boost the voltage level. In this case, power density and heat
dissipation issues should be well addressed due to the limited
PWM
physical space of the nacelle and tower in wind power sys-
Storage Intelligent tems. Moreover, energy storage/balancing capability should be
Control Appliances/loads
Batteries References Industry integrated into the power conversion stage to avoid additional
Measurements Communication
(Feed back) costs caused by the power mismatch between the wind turbine
and the utility grid in the very short term [23], [30].
Fig. 4. Configuration of a typical grid-connected RES with power electronics
converters and intelligent control. C. Demands on PV Power Systems
Solar PV energy is directly obtained through PV cells/panels
B. Demands on Wind Power Systems using the photovoltaic effect in PV power systems without any
For wind power systems, a wind turbine harvests the wind mechanical energy conversion stage as in the case of wind
energy as mechanical energy, and a generator converts it power systems. As a result, the demands on PV power systems
to electrical energy. Then, a power converter regulates the are less rigorous than those on wind power systems, although
electrical energy to meet the requirements of the utility grid far stricter requirements need to be complied with due to the
and/or local loads [23]–[25]. The specific demands on wind remarkable expansion of solar PV installation [31]–[34]. The
power systems are shown in Fig. 5, which can be summarized demands for PV power systems can be categorized as shown
into three aspects: in Fig. 6.
1) Wind generator side: The generator rotor or stator current (1) PV panels side: Maximum energy harvesting and good
is controlled by the power electronics converter to regulate maintenance of the PV panels (i.e., PV panel monitoring)
the electromagnetic torque of the generator. The demands of should be ensured to enhance high energy utilization as well
the generator side current control are not only to harvest the as a long lifetime of the system. Generally, a DC-DC converter
42 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 8, NO. 1, JANUARY 2022

For instance, an unexpected disconnection may be triggered

Photovoltaic Power by a sudden grid voltage decrease, threatening the equipment
Conversion System and grid security, or even leading to a large-scale outage under
Pin Po Utility grid
PV panels a high-level penetration of renewable sources. To tackle this,
Qo
the RESs must remain connected during this short period,
1.MPPT 1.High efficiency 1.Power quality
which is exemplified as the mandatory ride-through operation
2.DC current/voltage 2.Temp.management 2.Anti-islanding protection area in Fig. 7. Figure 7 demonstrates the response time
3.PV panel monitoring 3.High power density 3.Low voltage ride-through and voltage ride-through requirements for distributed energy
4.High reliability 4.Frequency control
5.Communication 5.Flexible power control resources (DERs) (including wind and solar PV energies) in
IEEE Std. 1547–2018 [5]. It can be illustrated that the response
Fig. 6. Common demands on PV systems, where Pin is the active power for the abnormal voltage conditions becomes more flexible and
from the PV panels to the power converter, and Po and Qo are the active and
reactive power injected into the grid. controllable.

Over-voltage zone (May ride-through or trip)

employed as the first stage of PV inverters enhances the 1.3
Shall Trip
flexibility of power tracking [31]. Also, it extends the operation 1.2
Momentary cessation capability
1.1
hours to some extent. Continuous operation

Voltage (p.u.)
(2) Utility grid side: The requirements of PV systems 0.9
0.88
have also been enhanced in terms of power quality, volt-
0.7 Mandatory ride-through operation
age/frequency regulation, and abnormal grid voltage protection
and recovery, as illustrated in Fig. 6. For instance, the total 0.5
harmonic distortion (THD) of the grid current must be lower Under-voltage zone
0.3 Momentary cessation (May ride-through or trip)
than 5% [7], [12], [32]. Moreover, many of the existing capability
grid-supporting demands in wind power systems have now Shall Trip
0.1
become mandatory for PV systems, as the power capacity 0
0.01 0.1 0.16 1 2 10 13 21 100
is increasing. For example, low-voltage ride-through (LVRT), Time (s)
frequency regulation and reactive power injection demands are
now seen in IEEE Std. 1547–2018 (i.e., revision of IEEE Std. Fig. 7. Response for distributed energy resources (DERs) to abnormal
grid voltages in the IEEE Std. 1547–2018 [5], including voltage ride-through
1547–2003) [5], [35]–[38]. requirements.
(3) PV power conversion system side: Although the price
of PV panels is continuously decreasing, the cost-efficiency Furthermore, the RESs should have voltage/frequency sup-
of the power capacity per generating unit in PV systems is port during the fault ride-through operation, including voltage-
relatively low. Lowering the overall costs while increasing the reactive power regulation and frequency-active power regula-
efficiency in power converters should be specially considered. tion [5], [32]. More specifically, in the case of LVRT operation,
The transformer-less PV inverters at low power levels are RESs can operate as per one of the following regulation
promising alternatives with high efficiency as well as high modes of reactive power: 1) constant power factor mode; 2)
power density [31], [39]. In this case, the leakage current voltage-reactive power mode; 3) active power-reactive power
issue becomes critical due to the parasitic capacitance between mode; and 4) constant reactive power mode [5]. Moreover,
the PV panels [5][26]and the ground. Accordingly, grid codes the transmission system operator can send commands for
in [6] and [40] require that the leakage current should be the reactive power injection to regulate and support the grid
suppressed below the limit (e.g., the root mean square (RMS) voltage. Notably, this reactive power control should be realized
value should be lower than 300 mA) to ensure the safety of slowly (e.g., under the time constant of minutes) in steady
equipment and personnel. It is worth noting that reliability, state. Referring to the requirements for the active power,
which directly affects the stable operation and indirectly the the RESs should regulate the active power according to the
system cost, becomes more important in power electronics for grid frequency at the Point-of-Common-Coupling (PCC). As
PV systems [41], [42]. Since the PV inverter is always being exemplified in Fig. 8, the production of the wind turbine can be
exposed to harsh environments or smaller housing, thermal limited to any power setpoint remotely. When the production
management should be considered to enhance reliability. is 100% of the rated power, the frequency control can only
reduce the output power for over-frequency events (i.e., the red
D. Grid Integration Requirements line in Fig. 8). In contrast, when the wind turbine operates with
It is well known that the most inherent characteristic of a certain level of power reduction, the output power can both
wind and solar energy is weather dependency, which means be increased and decreased to regulate the frequency flexibly
uncertainty and unpredictability are expected. To alleviate the (i.e., the blue curve in Fig. 8) [26]. It means that the wind
impact of intermittency, the RESs, e.g., wind and PV power power systems with reduced output power operation can pro-
systems, should support the grid [11]. The main pathway vide more flexible grid frequency regulation. In all, grid codes
includes predicting power production, flexible power control in many countries have been modified to ensure stronger grid-
capability, and fast dynamics to the varying weather and supported capability of renewable energy systems, promoting
operation conditions. reliable and stable large-scale grid integration.
TANG et al.: POWER ELECTRONICS: THE ENABLING TECHNOLOGY FOR RENEWABLE ENERGY INTEGRATION 43

Production in % of the rated power Available power structure with a limited power rating [23]. As depicted in
100
Deadband Fig. 10(a), the structure of a back-to-back (BTB) 2L-VSC is
introduced, where the advantage is the full power controlla-
75 Without reduced
bility with a relatively simple structure and few components.
production This converter is a well-proven, robust, and reliable solution
50 to low-voltage wind power systems.
Setpoint with reduced production
25 (20%~100% Prate) DFIG Transformer Grid
Gear
0
47 48 48.7 49 49.85 50 50.15 51 51.3 52 AC/DC DC/AC Filter
Grid frequency fg (Hz)
Wind
Fig. 8. Frequency regulation requirements for wind turbines connected to
the grid [26], where fg represents the grid frequency, and the wind turbine Partial-scale power electronucs
production can be limited to any power setpoint in the range of 20% ∼ 100% (a)
of the rated power Prate .
SG/IG Filter AC/DC DC/AC Filter
Transformer
Gear

III. P OWER E LECTRONICS FOR RES I NTEGRATION Grid

As mentioned previously, the power electronics for RES grid Full-scale power electronics
Wind
integration should not only consider the inherent characteris- (b)
tics of renewable sources, but also the grid requirements and
Fig. 9. Configurations of wind power systems based on variable speed
energy transmission/distribution demands. Furthermore, more wind turbines: (a) partial-scale power converter with a doubly fed induction
end-users’ preferences should be integrated to provide intelli- generator (DFIG) and (b) full-scale power converter with a synchronous
gent energy management, e.g., uninterrupted power supply. As generator (SG)/induction generator (IG) [11].
one of the typical RESs, wind power systems have experienced
a change from non-power-electronics-based concepts to full-
scale power converter-based systems with the power rating +
per turbine being significantly increased [24], [43]. At the
Wind Generator

same time, the power electronics for PV systems have had a Vdc

Grid
remarkable evolution in terms of topologies and control strate-
gies [31], [44]–[47]. Especially, reliability-oriented control and Filter Filter Transformer
inertia enhancement strategies have been popularly researched (Optional)
−
for both large-scale wind and solar PV power converters. 2L-VSC 2L-VSC
(a)
A. Wind Power Systems
+
Wind Generator

1) System Configurations Vdc/2

There exist several wind power system concepts depending

Grid
on the types of generators, power electronics, speed control-
lability, and the way in which the aerodynamic power flows. Filter Vdc/2 Filter Transformer
Correspondingly, the power converters are of various types (Optional)
and configurations in those wind power systems with different −
3L-NPC 3L-NPC
generators and power rating levels. As of now, the Doubly- (b)
Fed Induction Generator (DFIG) with partial-scale power
converters is still the mainstream configuration of wind power Fig. 10. Common power converters for wind power systems: (a) 2L-VSC
back-to-back topology and (b) 3L-NPC back-to-back topology (Adapted
systems, as presented in Fig. 9(a) [23]. To develop efficient, according to the discussions in [23]).
reliable, and compact wind turbine systems, full-scale power
converter-based synchronous generators (SG)/permanent mag- Comparatively, multi-level power converters show promise
net synchronous generators (PMSG) or induction generators for wind power systems with higher voltages and higher
(IG) are playing an increasing role in the wind power system power ratings [24], [25]. A three-level Neutral Point Clamped
market, which is shown in Fig. 9(b). It is anticipated that the (3L-NPC) BTB power converter is exemplified in Fig. 10
full-scale configuration will further cover the market of wind (b). Compared to the 2L-BTB converter, the 3L-NPC BTB
power systems in the future along with the fast development topology can achieve less dv/dt stresses on the semiconductor
of power electronics [23], [28]. devices and smaller filter inductors due to the multi-level
2) Power Converter Topologies output voltages. In this case, multi-level power converters
Depending on the wind turbine system concepts, the power are suitable to achieve the medium-voltage (MV) level power
converter topologies vary. For instance, the most common conversion with lower currents. It is worth noting that the
power converter in the DFIG wind power systems is the two- mid-point voltage fluctuation of the DC-link should be well
Level Voltage Source Converter (2L-VSC), which has a simple addressed for reliable operation, as investigated in [25], [48].
44 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 8, NO. 1, JANUARY 2022

With the continuous expansion of the installed capacity in voltage source inverters with LCL filters [56].
wind power systems, multi-cell converter configurations (i.e., (3) Advanced control: Notably, many advanced control
converter unit modules connected in the form of an array) are functions for wind power converters have been introduced
also becoming promising and will be used in wind turbine to enable an intelligent, reliable, and grid-supportive system.
systems in the future as power levels increase [21], [49]–[51]. For instance, reliability-oriented control may be considered to
3) Control ensure high availability and low maintenance costs, lowering
For wind power systems where the time scales are different the cost of energy in a long run, such as thermal control and the
for various reasons, the mechanical turbine and the power reliability evaluation approach for the wind power converter
converters need to be controlled [11]. According to the special with energy storage [57], [58]. Response to grid faults (e.g.,
demands in Fig. 5, the control of wind power systems typically voltage ride-through operation) and grid support capability
includes three levels, as shown in Fig. 11. The control func- (injecting or absorbing reactive power) should be provided
tions for the power converter system, i.e., the power interface to ensure grid-friendly wind power systems. Subsystems in
between the wind turbine and the grid, are detailed as follows: the wind turbine, e.g., generator/grid side converters, braking
(1) Basic control: Like all grid-connected converters, the chopper/crowbar, and pitch angle controller, also need to be
basic control for wind power converters mainly considers coordinated to ride through abnormal grid conditions, such as
current regulation, stabilization of the DC-link voltage, and power oscillation damping, inertia emulation, and grid voltage
grid synchronization [9]. The most common control strategy balancing [59]–[62].
is still the proportional-integral (PI) control, while proportional
resonant (PR) control, repetitive control [52], and model B. PV Power Systems
predictive control are also used [53]. The objective of the basic 1) System Configurations
control is to obtain efficient and reliable power conversion. In Grid-connection PV configurations can be summarized into
addition, the basic control should provide good steady-state three types according to the power level, as presented in
and dynamic performances to ensure stable and safe operation. Fig. 12 [47], [63]. As shown in Fig. 12(a) and (b), considering
(2) Specific control: Since the wind speed varies, the gener- an AC grid with the RMS voltage of 230 V per phase and
ated power also fluctuates. Therefore, the mechanical system the fundamental frequency of 50 Hz, the DC bus voltage
and power converter should be properly controlled to maxi- for the two-stage PV inverters is typically 400 V for single-
mize energy harvesting by adjusting the rotational speed of the phase systems, and 700 V for three-phase systems, where a
turbine. When the wind speed is lower than the rated value, the tradeoff between the power quality and switching stress is also
wind turbine can find an optimal pitch angle to achieve power taken into account. Referring to the central PV inverters in
optimization. If the wind speed exceeds the rated value, the Fig. 12(c), a wide range of the DC bus voltages up to 1500 V is
pitch angle should be regulated to limit the generated power, required for PV systems, to minimize the costs [64]. For low-
such as through frequency control utilizing the angle regulation power PV systems (e.g., below 1 kW), module-level converters
presented in [54]. In the normal grid-connected operation, the are usually adopted to achieve the high MPPT efficiency of
wind power systems should adopt proper current controllers to each PV panel, as demonstrated in Fig. 12(a). However, system
meet the power quality requirements, where various advanced costs are relatively high, and more power losses are generated
current control methods, e.g., sliding mode control [55] and when a large number of module converters with a high voltage
observer-based state space control, have been studied for the gain are used.

Gear/Gearless DFIG Filter DC/DC DC/AC Filter Grid

DC-link Transformer
vdc
SG/PMSG
Braking Chopper

Wind IG Pulse Width Modulation (PWM) Signals

Basic Control
igen ig
Voltage/Current Control Grid Synchronization
vdc Wind System Specific Control vg
Ωgen MPPT Power Limiting Pich Angle Control Power Quality Po,Qo
θpitch Advanced Feature Xfilter
Power Oscillation
Inertia Emulation Damping Fault Ride Through
Communication Supervisory
Grid Support Energy Storage Black Start Protection Command
Control and Monitoring

Fig. 11. General control structure for wind power systems.

TANG et al.: POWER ELECTRONICS: THE ENABLING TECHNOLOGY FOR RENEWABLE ENERGY INTEGRATION 45

DC/AC AC bus cording to DIN VDE 0126 [40], [45]. Generally, PV inverters
can adopt transformers to provide isolation as they have low
DC/DC leakage currents, however, the overall system efficiency is low.
DC bus +
PV module DC/DC − + Thus, transformer-less inverters have been introduced in the
−
DC load PV industry, where the leakage current issue must be well
1- addressed [45].
400 VDC DC/AC phase Micro-inverters are increasingly used to directly interface
DC/DC Transformer
(a) + PV modules to the utility grid [44], [66]. These can en-
DC/DC − + hance the energy harvesting per PV module. Micro-inverters
− AC Grid
DC load have to boost the PV module voltage for grid connection.
PV string High-frequency transformers are used in practice, such as
400/700 DC/AC 1/3- the commercial inverters in [67]. On the other hand, many
VDC phase AC load
(b) DC/DC step-up transformer-less micro-inverters have also been in-
+
− + troduced [44]. The buck-boost integrated full-bridge micro-
PV array − AC/AC inverter is illustrated in Fig. 13(a) [68].
DC load
3-
700/1500 DC/AC phase DC-AC
VDC P
Blocking diodes (c) S1 D1 S3 D3
D5 D6
Fig. 12. Grid-connected PV configurations: (a) module-level converters for Lb1 Lb2
L1 L2
low-power applications (micro-inverter), (b) string inverters for medium or
high-power PV systems, and (c) central inverters for utility-scale PV power Cdc A
stations, where optional DC-DC converters can provide a wide range of PV Cf
PV Module B AC Grid
input voltages.
S2 D2 S4 D4
N
Consequently, string inverters are often preferred in resi- Cp
dential and commercial grid-connected PV systems. The con- Ground
(a)
figuration of the string inverter is demonstrated in Fig. 12(b).
DC-AC
Each string of PV panels employs a DC-DC power optimizer, P
and it is then connected to a string inverter. Although the S1 D1 S3 D3 L1
MPPT efficiency for PV panels has to be compromised to
S5 D5
a certain extent, the PV systems can obtain high cost-effective A
performances in terms of conversion efficiency, power density, D6
B S6 AC Grid
reliability, and flexible control. Notably, multiple strings can L2
be adopted to increase the overall system power. PV Strings S2 D2 S4 D4
For large-scale PV power systems (e.g., commercial and
N
utility-scale PV stations), the central inverter is widely em- Cp
ployed due to its simpler structure and control with lower Ground
overall system costs [47], as depicted in Fig. 12(c). For such (b)
DC-DC
a configuration, there are several challenges: (1) high voltage Converter Central LV/MV
and current stresses on PV panels due to the high DC-link PV Arrays (Optional) Inverter Transformer
voltage and power level; (2) risk of low efficiency due to DC DC
MV/HV
a global MPPT and mismatch of PV panels; and (3) lower DC
Cdc
AC Transformer
reliability caused by the high-power diodes and one central
inverter. To address the above issues, many central inverters
DC DC AC Grid
adopt multi-level power converters for large-scale PV power
systems with high voltage and high power, as well as parallel Cdc
DC AC
central inverters [65]. This may complicate the entire system
to some extent. (c)
2) Power Converter Topologies
Fig. 13. Power converters for PV systems: (a) buck-boost integrated full-
Correspondingly, there are various inverter topologies in bridge micro-inverter, (b) transformer-less PV string inverter (HERIC), and
different grid-connected PV configurations, as shown in (c) central PV inverters (3L-NPC).
Fig. 14 [44]–[46], [65], [66]. As PV systems harvest solar
PV energy through PV panels, the parasitic capacitor between To process high power while maintaining high efficiency,
the PV panel and the ground should be carefully considered in many advanced transformer-less inverters have been devel-
practice. All grid codes/requirements for PV systems have a oped [45], [46] for PV strings. Transformer-less PV string
strict limitation on the leakage current, e.g., the RMS leakage inverters can be divided into two groups, i.e., DC-decoupling
current limitation and sudden leakage current limitation ac- and AC-decoupling converters. For instance, the H5 inverter is
46 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 8, NO. 1, JANUARY 2022

PV array

Filter DC/DC DC-link DC/AC Filter Grid

Transformer
vdc

DC Chopper

Pulse Width Modulation (PWM) Signals

Panel Monitoring

Basic Control
Weather condition

ipv Voltage/Current Control Grid Synchronization ig

vpv PV System Specific Control vg
vdc Seamless
MPPT Anti-islanding Protection Power Quality Transition Po,Qo

Advanced Feature Xfilter

Si,Ta
Panel Monitoring Inertia Emulation Weather Forecasting
Energy Storage Black Start Low Voltage Ride-through
Communication Supervisory
Flexible Power Control Power Oscillation Damping command
Control and Monitoring

Fig. 14. General control structure for grid-connected PV systems.

a typical DC-decoupling inverter, however, its leakage current weather/climate conditions. Thus, specific controls of the
suppression is affected by the parasitic capacitors of the inverters adopted in PV systems contain the MPPT control,
power switches [45]. The highly efficient and reliable inverter islanding protection, and seamless transition. In addition, the
concept (HERIC), as shown in Fig. 13(b), is an excellent AC- specific controls for wind systems are now mandatory in PV
decoupling topology with good performance in leakage current systems, as presented in IEEE 1547–2018 [5].
suppression, conversion efficiency, and reliability [69]. (3) Advanced features: Nowadays, reliability, fault ride-
Several PV powerplants have come into service over several through, and grid supporting capability are emphasized in
years that use central inverters (e.g., SMA Sunny Central CP PV systems [71]–[77]. Correspondingly, more flexible power
XT inverter), and more are under construction. As mentioned control is required [71], e.g., reactive power to regulate the grid
previously, multilevel inverters are employed to tackle the is- voltage and active power to regulate the frequency. For exam-
sue of high voltage stresses, e.g., the 3L-NPC inverter adopted ple, the delta power production control and power reserved
as the central PV inverter in Fig. 13(c) [65]. In addition, several control are the recently studied active power control strategies
central inverters are connected in parallel to share the current for frequency regulation [72]. The inertia emulation can be
stresses and provide flexible power management. It should achieved by either the virtual inertial control of the PV system
be noted that the line-frequency LV/MV transformers may be or implemented by the integrated energy storage [73], [77].
considered when being connected to the grid. To achieve long lifetime as well as high reliability, PV panels
employ system-level condition monitoring schemes. Besides,
3) PV System Control
reliability-oriented controls (e.g., power limiting control with
Most of the demands on PV systems depicted in Fig. 6 can weather forecasting and junction temperature control in the
be achieved by controlling the PV inverter. Especially with the power modules [74]) are considered, to enhance the reliability
remarkable growth in the installation capacity, more and more and lifetime.
advanced features have had to be provided in addition to basic
control of grid-connected inverters and PV system-specific C. Grid Support
control. The multi-layer control functions, as presented in To realize the “carbon neutrality” strategy [1] and the
Fig. 14, are detailed in the following: sustainable energy transition, grid support capability is highly
(1) Basic control: As with wind power systems, the com- critical for achieving stable and reliable grid-integrated RESs.
mon basic control includes voltage/current control and grid This means that the energy must be balanced between the gen-
synchronization, for which PI control, PR control, repeti- erators and the loads under any conditions, such as uncertain
tive control, and model predictive control can be used [9], and non-dispatchable renewables, abnormal grid conditions,
[52], [53]. Moreover, the basic control generally achieves and special demands from the operator or end-users. Con-
good steady-state and dynamic performances, guaranteeing the sequently, many active and reactive power control strategies
power quality of the grid-connected PV system [70]. The basic have been studied to improve grid integration [77]–[81]. For
controls are fundamental to PV system-specific controls and instance, the power references can be obtained intuitively
advanced features. through the power control based on the PQ theory [71]. Droop
(2) Specific control: Power generation from solar PV control with the reactive power injection function is usually
systems is also uncertain and highly dependent on the adopted when the line impedance is inductive [81].
TANG et al.: POWER ELECTRONICS: THE ENABLING TECHNOLOGY FOR RENEWABLE ENERGY INTEGRATION 47

In addition to controlling the power generation of renewable efficiency, high power density, low device count, and simple
energies [75], [80], the energy storage system is a feasible control. Moreover, the centralized energy storage device and
option to enhance the active power control capability [77]– the reactive power compensator are directly connected to the
[79]. In California, energy storage must be provided when AC grid. The above auxiliary devices, i.e., energy storage
installing grid-connected RESs [82]. To provide reactive batteries and reactive power compensators in Fig. 15, can
power, the reactive power management capability has been achieve an optimal operation point between cost and revenue
integrated into RESs [81], [83]. Moreover, the centralized growth [79]. In all, grid-friendly RESs have to minimize the
reactive power compensator is also a common device. As impact on the grid operation, while also providing additional
for reactive power compensators, the traditional methods are services.
based on synchronous condensers and switch capacitors or
inductors [84]. In contrast, more flexible and fast dynamics IV. T RENDS IN P OWER E LECTRONICS FOR RES
power electronics-based compensators are being developed, I NTEGRATION
such as the static var compensator and thyristor-switched
capacitors or reactors. Reactive/harmonic compensators with A. Power Device Technologies
IGBTs are also being explored for high compactness, e.g., the Power semiconductor devices are the key components to
transformer-less series compensator for HVDC transmission achieve energy conversion in terms of system cost, efficiency,
systems [85]. It is believed that more reliable and cost- power density, reliability, and modularity. The further devel-
effective power electronics will shortly be developed in power opment of power semiconductor devices can be summarized
transmission applications. Figure 15 shows that wind and PV as follows:
power systems adopt batteries and capacitors to achieve active (1) Materials: High-power silicon-based semiconductors
and reactive power regulation, respectively. As demonstrated have been the main components in power converters of
in Fig. 15, the distributed energy storage systems can be RESs for several decades, e.g., IGBT and Integrated Gate
connected to both the DC-link and AC bus of wind and PV Commutated Thyristor (IGCT). The development of WBG
power systems by DC/DC converters or DC/AC converters, devices, e.g., SiC and GaN power devices, has led to more
respectively. Recently, many stand-alone multiport converters advantages as well as fresh challenges. On one hand, high
have been developed to integrate energy storage batteries switching frequencies and low power losses of WBG devices
into RESs, achieving flexible configurations, high system can improve the power density of power converters. On the
other hand, challenges in the design of gate drivers and EMI
issues should be considered, especially when the switching
AC DC frequency of the WBG devices is becoming much higher, e.g.,
several MHz.
DC AC Transformer
(2) Packaging: The conventional packaging technology for
Wind DC DC
+ IGBT, which has soldering and bond-wire connection in
− DC AC their internal chips, has the disadvantages of large thermal
Energy storage Energy storage resistance, low power density, and high failure rates [86],
Distributed energy storage system

on DC bus on AC bus [87]. To increase the lifetime of the IGBT modules, improved
DC DC technologies include press-pack-based plate soldering, sinter
technology to avoid the chip soldering, as well as replacing
DC AC Grid the bond wire with new materials to reduce the coefficient
PV of thermal expansion [88], [89]. The press-pack technology
DC DC
+
improves the connection of chips by directly press-packing
− DC AC the contact, leading to low short-circuit failure, high power
Batteries
density, and better cooling capability. Consequently, the press-
DC pack devices, including the silicon-based and WBG devices,
are expected to be utilized more widely in the future [90].
AC
PV Packaging technologies are significantly relevant to the life-
Energy storage
+ interation through time of power semiconductor devices, further affecting their
− multi-port converter
applications in RESs. Power semiconductors for new power
Batteries
DC converters, which would need to meet the high power level
+
−
demands of future large-scale grid-integrated RESs, would
AC
encounter much higher levels of voltage/current stress. In
Centralized energy storage system
addition to thermal management, compactness, and failure
DC
rates, packaging technologies would require much higher
AC performance in terms of parasitic parameters (e.g., especially
Capacitors for WBG devices with high switching frequencies), explosion
Reactive power compensator
resistance, and costs. Moreover, the better means of connection
Fig. 15. Energy storage integration and reactive power compensator for grid of power semiconductor devices in series or parallel is also an
support of wind and PV power systems. important aspect to handle high power and high currents.
48 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 8, NO. 1, JANUARY 2022

B. Reliability level of RESs. In such a system, all forms of energy storage

must be integrated, including electric vehicles or stationary
One of the main goals of power converters for RESs is
batteries. The infrastructure of the Energy Internet contains
to achieve the highest energy conversion efficiency with the
the physical energy (e.g., RESs and the utility grid operator)
lowest system cost. Therefore, high reliability has drawn more
and information networks (e.g., communication, data, and
and more attention [28], [29], and this will continue in the
control center). With comprehensive information and flexible
future as RESs have to operate for 25 ∼ 30 years [14].
power control capability, global, efficient, and optimal energy
In addition to improving the power converter structure and
management can be achieved for better power generation,
developing more advanced semiconductor devices (e.g., WBG
transmission, and distribution. With further integration of
devices), reliability-oriented control strategies show promise
storage, the development of energy storage materials is now
in enhancing reliability and performance.
more and more important [98].
Reliability-oriented design and control, including effective
thermal management, robustness design, and validation with D. Grid Integration
the knowledge of mission profiles, are gaining much interest.
To continue the energy transition with large-scale RESs,
For instance, many attempts have been made to develop the
more stringent demands in terms of grid integration, cooper-
thermal models of power devices and power converters to
ation, protection, and end-user engagement are being made.
estimate the lifetime of the system, thereby enabling the
This makes RESs with 100% power electronics operate in
reliability-oriented design [58], [91]–[93]. Moreover, many
the grid-forming mode or become more flexible [99]. As
control strategies have been developed that aim to improve
illustrated in Fig. 16, these RESs (e.g., consisting of wind/PV
thermal performance, such as junction temperature control dur-
energies, converters, storage devices, and loads) should con-
ing LVRT operation and hybrid control to reduce the thermal
sider the following aspects in the future:
loading by flexible power regulation [91], [93]. Reliability
(1) Storage: Without synchronous generators, RESs with
will be considered more and more in future large-capacity
100% power electronics have low inertia, resulting in poorer
renewable energy systems, and such reliability will be more
voltage and frequency regulation capability. Thus, storage
and more dependent on power electronics.
energy devices should be integrated with RESs to replace the
C. Advanced Control role of synchronous generators in grid regulation. This energy
storage can be provided either by using high-specific energy
Deep integration of RESs and energy storage is an efficient batteries, electric vehicles, and/or loads with certain energy
path to realize energy balance, fault-tolerance, and grid sup- storage [100], [101]. In this context, how to size the integrated
port capability, and for tackling issues caused by renewable energy storage devices in the design phase, and then how to
fluctuation and abnormal grid conditions [94]. In addition, better control the entire system to enhance the active/reactive
RESs with energy storage can be considered as an energy power regulation capability to achieve enough inertia, are
hub in the concept of the Energy Internet, which features of interest in future power grids. Coordinative operation of
intelligent cooperation management with the end-users and multiple energy sources should be optimized to maximize the
operators [95], [96]. In this case, power electronics will be economic benefit.
equipped with many advanced control strategies, enabling (2) Converters: As shown in Fig. 16, power converters
thermal analysis and control, and energy-cooperated control for RESs with grid-forming operational roles should integrate
strategies (e.g., artificial intelligence-based and data-driven energy storage devices into RESs. Multiport converters are
controls [97]). Figure 16 exemplifies an Energy Internet ar- promising solutions that enable flexible power control, high
chitecture, where the future power grid has a high penetration system efficiency, high power density, and high reliability [22].
At the same time, many challenges, e.g., high power ratings
and strong intermittency, should be considered when devel-
oping multiport converters for large-scale grid integration of
RESs.
(3) Control: To realize the effective functional operation of
Utility Grid Data and control center RESs with 100% power electronics, the frequency and voltage
Energy Interaction controls may significantly differ from the grid-following ones.
The power converters should be controlled to operate in the
Information Interaction grid-forming mode, where the frequency and voltage control
can be achieved as conveniently as that in the synchronous
Energy burden generators. In this case, the frequency and voltage control in
the grid-forming mode should be properly addressed consid-
ering the stability of the utility grid and interaction with other
loads and sources. Correspondingly, the operation range of
RES 1 RES n
the frequency and voltage, as well as future power systems’
stability indices, may be redefined. Moreover, the cooperation
Fig. 16. Intelligent energy architecture for large-scale renewable energy grid and communication with the distribution/transmission opera-
integration. tors should be re-prioritized according to the time scale. Com-
TANG et al.: POWER ELECTRONICS: THE ENABLING TECHNOLOGY FOR RENEWABLE ENERGY INTEGRATION 49

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52 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 8, NO. 1, JANUARY 2022

Yongheng Yang (SM’17) received the B.Eng. de- Frede Blaabjerg (F’03) was with ABB-Scandia,
gree in Electrical Engineering and Automation from Randers, Denmark, from 1987 to 1988. From 1988
Northwestern Polytechnical University, China, in to 1992, he got the Ph.D. degree in Electrical Engi-
2009 and the Ph.D. degree in Energy Technology neering at Aalborg University in 1995. He became an
(power electronics and drives) from Aalborg Uni- Assistant Professor in 1992, an Associate Professor
versity, Denmark, in 2014. in 1996, and a Full Professor of power electronics
He was a postgraduate student with Southeast and drives in 1998. From 2017 he became a Vil-
University, China, from 2009 to 2011. In 2013, he lum Investigator. He is honoris causa at University
spent three months as a Visiting Scholar at Texas Politehnica Timisoara (UPT), Romania and Tallinn
A&M University, USA. Since 2014, he has been Technical University (TTU) in Estonia.
with the Department of Energy Technology, Aalborg His current research interests include power elec-
University, where he became a tenured Associate Professor in 2018. In January tronics and its applications such as in wind turbines, PV systems, reliability,
2021, he joined Zhejiang University, China, where he is currently a ZJU100 harmonics and adjustable speed drives. He has published more than 600
Professor with the Institute of Power Electronics. His current research interests journal papers in the fields of power electronics and its applications. He is the
include the grid-integration of photovoltaic systems and control of power co-author of four monographs and editor of ten books in power electronics
converters, in particular, the grid-forming technologies. and its applications.
Dr. Yang was the Chair of the IEEE Denmark Section (2019–2020). He is an He has received 32 IEEE Prize Paper Awards, the IEEE PELS Distinguished
Associate Editor for several IEEE Transactions/Journals. He is a Deputy Editor Service Award in 2009, the EPE-PEMC Council Award in 2010, the IEEE
of the IET Renewable Power Generation for Solar Photovoltaic Systems. He William E. Newell Power Electronics Award 2014, the Villum Kann Ras-
was the recipient of the 2018 IET Renewable Power Generation Premium mussen Research Award 2014, the Global Energy Prize in 2019 and the 2020
Award and was an Outstanding Reviewer for the IEEE Transactions on Power IEEE Edison Medal. He was the Editor-in-Chief of the IEEE Transactions on
Electronics in 2018. He was the recipient of the 2021 Richard M. Bass Power Electronics from 2006 to 2012. He has been Distinguished Lecturer
Outstanding Young Power Electronics Engineer Award from the IEEE Power for the IEEE Power Electronics Society from 2005 to 2007 and for the IEEE
Electronics Society. In addition, he has received two IEEE Best Paper Awards. Industry Applications Society from 2010 to 2011 as well as 2017 to 2018. In
He is currently the Secretary of the IEEE Power Electronics Society Technical 2019–2020, he was the President of the IEEE Power Electronics Society. He
Committee on Sustainable Energy Systems and a Council Member of the is Vice-President of the Danish Academy of Technical Sciences.
China Power Supply Society. He is nominated in 2014–2020 by Thomson Reuters to be between the
most 250 cited researchers in Engineering in the world.