Bidirectional DC–AC Converter-Based Communication Solution for Microgrid

Abstract

The communication system of a microgrid can transfer the information of electricity price, power consumption and so on between users and the control centre. This capability is of great significance to improve the efficiency and sustainability of power facilities. In this paper, a bidirectional DC–AC converter topology is proposed to achieve the composite transmission of power and signals in microgrids. Since the transmitted signals are modulated by power switches of converters and integrated into the currents, the cost of signal couplers can be saved and the circuit structure can be simplified. In order to verify the feasibility of the proposed method, a simulation model of the proposed converter is implemented in MATLAB/Simulink. With the power supply frequency of 50 Hz, when the converter operates in the inverter mode and rectifier mode, the data transmission rate can reach 120 bit/s and 48 bit/s, respectively.

Full text

Power Electronics and Drives
Power Electronics and Drives
Volume 5(41), 2020 DOI: 10.2478/pead-2020-0013
* Email: yihua.hu@york.ac.uk
Research Article
1Department of Electronics, University of York, York, United Kingdom
2School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan, China
3Dynex Semiconductor Ltd., Doddington Road, Lincoln, United Kingdom
Yixuan Zhang1, Kai Ni2, Yangang Wang3, Yihua Hu1,*
Bidirectional DC–AC Converter-Based
Communication Solution for Microgrid
1. Introduction
Given the requirements of high reliability and low cost in developing power grid systems, the concept of microgrid is
proposed to realise large-scale energy conversion and power transmission through resource optimisation (Tripathi
et al., 2020). The microgrid system takes advantage of the two-way ow of power and information to achieve real-
time demand–supply regulation in a consistent, secure and efcient manner (Fang et al., 2011; Hau et al., 2013;
Mocanu et al., 2016; Sun et al., 2011). Since a large amount of data are generated and transmitted in microgrids,
an efcient information transmitting approach is essential for not only dealing with such massive data but also for
addressing network security issues such as customers’ information leakage and large-scale blackout (Ghorbanian
et al., 2019; Lu et al., 2010).
Although the existing communication technologies which are applied in microgrids can satisfy the requirements
of data exchange, each method has its limitations. For instance, an optical bre can provide 100 Mbps to ~40 Gbps
data rates at the access layer, core level and aggregation level, which makes it popular in the eld of public
utilities (Fang et al., 2011; Lo and Ansari, 2011). However, if there is no existing infrastructure, especially in
rural areas, utilising such a technique will cause slow deployment and high installation costs. Besides, wireless
technologies such as cellular networks and Wi-Fi networks are well-developed communication approaches. With
the merits of large coverage, fast response and low operational and maintenance costs, wireless communication
is applicable for real-time applications in urban areas (Gungor et al., 2011; Wang et al., 2011). However, the
associated high interference-sensitivity is a major weakness characterising wireless communication because of its
unxed communication channel (Gungor et al., 2011; Wang et al., 2011). Furthermore, power line communication
(PLC) is the technology that employs power lines as the transmission medium for both power and communication
signals. Since bulky communication cables are omitted in this approach, it has higher mobility and exibility for end
177
Received: October 15, 2020; Accepted: November 27, 2020
Keywords: bidirectional DC–AC converter • frequency shift keying • MATLAB simulation • microgrid • two-way communication
Abstract: The communication system of a microgrid can transfer the information of electricity price, power consumption and so on between
users and the control centre. This capability is of great signicance to improve the efciency and sustainability of power facilities.
In this paper, a bidirectional DC–AC converter topology is proposed to achieve the composite transmission of power and signals in
microgrids. Since the transmitted signals are modulated by power switches of conver ters and integrated into the currents, the cost
of signal couplers can be saved and the circuit structure can be simplied. In order to verify the feasibility of the proposed method, a
simulation model of the proposed converter is implemented in MATLAB/Simulink. With the power supply frequency of 50Hz, when the
converter operates in the inver ter mode and rectier mode, the data transmission rate can reach 120bit/s and 48bit/s, respectively.
Open Access. © 2020 Zhang et al., published by Sciendo. This work is licensed under the Creative
Commons Attribution NonCommercial-NoDerivatives 4.0 License.

DC–AC converter-based communication solution for microgrid
devices. Nevertheless, the attenuation, distortion and noise are critical issues for the PLC-based radio frequency
communication, which make it difcult for the data rates of narrowband PLC and broadband PLC to exceed
500 Kbps and 200 Mbps, respectively (Gungor et al., 2011; Kabalci, 2016; Lo and Ansari, 2011; Wang et al., 2011).
In the conventional PLC-based microgrid, capacitive coupler, inductive coupler and resistive coupler are
commonly employed to couple signals to the power cable (Costa et al., 2017). In capacitive coupling, one or
more capacitors are series connected with the power cable to provide a low-impedance path for high-frequency
components of the power line signal (Costa et al., 2017). The capacitive coupler is widely employed in low-voltage-
based PLC systems because of its low insertion loss (Costa et al., 2015). In an inductive PLC coupler, the current
ows through the winding coil and generates an electromagnetic eld that induces the signal into the conductor
(Lee et al., 2010). The inductive couplings are suitable for medium-voltage-based PLC systems, since they have
the capability for electromagnetic isolation between the PLC transceiver and the power cable (Costa et al., 2017).
With their simple circuitry and low-cost features, resistive couplers are suitable for low-voltage-based PLC systems
(Swana et al., 2015). However, resistive couplers are only practicable for signal extraction from the power cable as
a result of the intrinsic nature of conductive properties (Swana et al., 2015). Although these coupling approaches
can efciently transmit data through cables, the increased cost and the complexity of the system structure needs to
be taken into account prior to the usage of additional couplers in microgrids.
In order to simplify the method of applying couplers to couple signals to the power line, some signal and energy
integration approaches have been explored in related researches. Stefanutti et al. (2006, 2008) proposed the
idea of using the internal power switch of a converter to achieve data and energy composite modulation. Without
additional coupling circuits, data modulation is realised only by controlling the frequency or phase parameters of the
power switches. Wang et al. (2016) combines direct sequence spread spectrum and phase shift keying modulation
strategies, and uses the boost and buck converter voltage ripple as a carrier to achieve synchronous transmission
of data and energy while reducing electromagnetic interference. According to Choi and Jung (2017), the power ow
information sharing between the converters is realised by modulating the switching frequency of the converter and
loading the power ow information of the microgrid on the voltage ripple of the DC bus. This approach is benecial
since it helps to increase the light load efciency and zero voltage switching capacity of the dual active bridge
converter. Since most of these researches investigate the data and energy composite modulation strategies of DC
systems, and the application of this technology to the AC microgrid can not only simplify the system structure, but
also save costs, the relevant modulation methods in AC microgrids are still worthy of further study.
This paper proposes a bidirectional DC–AC converter topology for two-way communication in AC microgrids. In
this design, power conversion is achieved by a full-bridge converter, and the frequency shift keying (FSK) approach
is used to modulate the transmission signals. Since the transmitted data are modulated by the switching frequency
of the converter instead of applying additional coupling circuits in the proposed approach, the infrastructure cost
and structural complexity of the microgrid can be greatly reduced. Moreover, the coupling noise can be eliminated
by using this transmission method.
The rest of this paper contains the following contents. Section 2 presents the topology of the proposed converter.
Section 3 describes the mechanisms of power transmission and signal transmission. The simulation results are
exhibited in Section 4. Finally, a brief conclusion is reached in Section 5.
2. Integrated Power and Signal Transmission Model
Power electronic converters are essential components in a microgrid. As shown in Figure 1, a microgrid can be
compatible with wind, photovoltaic and other power generation technologies to provide power exibly for the end
users. These energy generation methods are mostly based on power electronic converters, to ensure that the
same specications of power are transmitted through the power line (Chakraborty et al., 2007). With the proposed
approach, power can be transmitted along with the communication signals through the converters. For instance, the
residual power information of the power grid can be transmitted to the controller of energy storage system (ESS)
in time through the bidirectional DC–AC converter; thus the ESS can store energy in the low-power consumption
periods and return the energy to the grid during the summit-power consumption periods.
The proposed bidirectional DC–AC converter topology is presented in Figure 2. There are two series connected
full-bridge converters at the DC side, where the upper converter is applied for signal modulation and the lower
one is used for power modulation. In addition, two AC voltage sources V4 and V5 are connected in series with the
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Zhang et al.
AC power grid V3 to generate different frequency carriers. R3 and L in series, respectively, represent the inherent
impedance and inductance of the transmission line. When the topology operates in the rectier mode, the switch
S3 will turn on to isolate the DC source and the load R1 will be connected to the AC power source. Similarly, when
the circuit is operating in the inverter mode, the load R2 will be connected to two full-bridge converters to isolate the
AC power source.
3. Operation Mechanisms
3.1. Inverter Mode
The topology of the proposed converter in the inverter mode is shown in Figure 3. In this circuit, the upper full-
bridge circuit containing the power supply V1 is used to modulate the transmission signal, and the lower full-bridge
circuit involving the power supply V2 is employed to modulate AC current. The AC output power is acquired by the
Fig. 1. Conceptual structure of AC microgrid using the proposed power and signal integration strategy.
Fig. 2. Schematic of the proposed bidirectional DC–AC converter.
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DC–AC converter-based communication solution for microgrid
Fig. 3. Schematic of the proposed converter in inverter mode.
Fig. 4. The relationship between the transmitted signal and the current waveform.
conventional sinusoidal pulse width modulation approach, which uses a reference sine wave to compare it with the
triangle carrier and employs the comparison results to control the four IGBT switches. To utilise the FSK method
for signal modulation, the high-frequency carriers are generated by fast switching of the four switches of the upper
full-bridge converter. For example, if a 4-bit signal ‘1100’ requires to be transmitted, the switches Q1 and Q2 will
turn on and turn off, respectively, at the same time with a specic frequency to modulate ‘1’ whereas the switches
Q3 and Q4 will operate simultaneously using another frequency to modulate ‘0’. Furthermore, the working states
of the switches Q1 and Q2 are contrary to those of the switches Q4 and Q3, respectively, to prevent short-circuiting.
Since the two full-bridge converters for signal transmission and energy transmission are connected in series, the
modulated signal is superimposed on the modulated current waveform, as exhibited in Figure 4. Since the amplitude
of the power source V1 is much smaller than that of the power source V2, the modulated signal is superimposed on
the current in the form of small amplitude uctuation, which will not cause large distortions to the current waveform.
3.2. Rectier Mode
The proposed topology will operate in rectier mode when the AC power source is connected to the load at the DC
side. As shown in Figure 5, all IGBT switches are off, and the current will ow through the anti-parallel diodes of
these switches to realise the rectication process. Since the current is transmitted through two series-connected
diodes Q1 and Q5 in Figure 5, the output current waveform obtained in this process is the same as that achieved
by the conventional single-diode half-wave rectication method. In this circuit, two AC power supplies V4 and V5
are utilised to generate different frequency carriers. Specically, if a 4-bit signal ‘1100’ is transmitted through this
topology, the switches S5 and S6 will turn on to modulate digital ‘1’ and ‘0’, respectively. As the power supplies V4
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Zhang et al.
and V5 are connected in series with the grid power source V3, the modulated signal is integrated with the AC current
and transmitted through the grid. The integration process of transmission signal and power is shown in Figure 6.
3.3. Demodulation
After the integrated power and signal are transmitted through the AC grid, an efcient demodulation approach is
essential to restore the data at the receiver. The proposed demodulation process is presented in Figure 7. First, a
bandpass lter is used to extract the carrier from the current waveform of the receiver. Because the digital ‘1’ and
‘0’ of the transmitted signal are modulated by different frequency carriers, any carrier can be separated from the
current by using a suitable bandpass lter. The Fourier series expansion of a random periodic signal F(x) is given by
Fx aanx
bn
x
n
n
n
() (cos si
n)
=+ +
=
∞
∑
1
20
1
ωω
(1)
where
w
is the angular frequency, T is the signal period and the coefcients
a0
,
a
n and
bn
are represented by the
following expressions, respectively.
aTfxdx
aTfx nxdx
bTfx n
T
T
nT
T
n
02
2
2
2
2
2
2
=
=
=
∫
∫
()
()cos
()sin
-/
/
-/
/
ω
ω
xxdx
T
T
-/
/
.
2
2
∫









(2)
Fig. 5. Schematic of the proposed converter in rectier mode.
Fig. 6. The integration process of transmitting signal and power in the rectier mode.
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DC–AC converter-based communication solution for microgrid
Fig. 7. The signal demodulation process of the proposed model.
Fig. 8. The schematic diagram of the demodulation process of the signal ‘1010’, where SI is the initial transmitted signal, SM is the integrated current
and signal waveform, SC represents the demodulated carrier for digital ‘1’, SE exhibits the upper envelope of the demodulated carrier and SR is the
restored signal.
Therefore, if a square wave
ft()
shown in Eq. (3) is employed as the carrier to modulate the transmission
signal in the inverter mode, the Fourier series expansion of
ft()
can be expressed as
Ft()
in Eq. (4).
ft
Tt
tT
()=
≤<
≤≤







020
1
2
-
0
(3)
Ft tttt
n
n() sinsin sinsin sin=+ ++ +⋅⋅⋅+
1
2
22
3
32
5
52
7
72
π
ω
π
ω
π
ω
π
ω
π
ω
+tt (4)
Since the rst-order harmonic of Eq. (4) has the largest amplitude and has the same frequency as that of the
square wave, the extracted rst-order harmonic can represent the position of the corresponding carrier in the time
domain. Thus, the square carrier used in the inverter mode can be extracted by a bandpass lter. Besides, the
sinusoidal carrier employed in the rectication mode can also be extracted by this approach.
An envelope detector is then applied to the demodulated carrier for upper envelope extraction. In this process,
the input carrier is rst squared and then passed through a low-pass lter. Squaring the carrier can raise half of the
signal energy to higher frequencies and transfer the energy of the other half of the signal to the lower frequencies
which are close to DC. Next, a low-pass lter is utilised to eliminate the high-frequency energy generated by the
squaring process. The remaining curve after ltering is the upper envelope of the carrier. After comparing the upper
envelope with a threshold value, the transmitted data can nally be restored by down-sampling the curve after the
comparison process at the initial data rate. Figure 8 shows a schematic diagram of the demodulation process of
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Zhang et al.
the signal ‘1010’, where SI, SM, SC, SE and SR represent the original signal, integrated current and signal waveform,
demodulated carrier, upper envelope and restored signal, respectively.
4. Simulation Results and Analysis
4.1. Parameters and Output Waveforms
A simulation model of the proposed converter topology is implemented in MATLAB/Simulink using the power and
signal transmission method demonstrated in Section 3. The parameters utilised in this model are given in Table 1.
The DC and the AC sides use a 200 V DC voltage source and a 200 V AC voltage source, respectively, as the
power supply for the power conversion part. The signal carriers of the DC and the AC sides are generated by an
additional 10 V DC voltage source and two additional 10 V AC voltage sources, respectively. Since the amplitude of
the carrier generated by the 10 V power supply will not greatly distort the current waveform of the power conversion
section and since the carrier energy is large enough to be detected in the demodulation process, it is appropriate to
employ a 10 V voltage source for carrier generation. In this model, the inherent impedance and inductance of the
transmission line are set to 2 W and 200 mH, respectively. When the topology works in inverter mode and only the
‘data’ converter is considered, its rated power can be calculated by the following equation:
PU ID W
id id id
=××=×× =10 55
02
5% (5)
where D is the duty ratio (50%) of the converter switches. Besides, the instantaneous power of the power converter
can be obtained with the following formula:
PU IU tI t
ip ip ip
mm
=×
=×
=−
si
ns
in
(co
ωω
20000 1s
s)
200
π
t (6)
Since the inductance value of the transmission line in the simulation model is much smaller than the impedance
value, the inuence of the inductance on the power is neglected in Eq. (6) to simplify the analysis. Therefore,
the rated power of the power converter is approximately equal to 10 kW. Similarly, when the topology operates
in rectier mode, the rated power of the ‘data’ converter is denoted by Eq. (7). Additionally, the expression of the
instantaneous power of the power converter is the same as that presented in Eq. (6).
PU IU
IW
rd rd rd mm
=×
==
1
2
25 (7)
It can be observed that the rated power of the ‘data’ converter is smaller than that of the power converter.
However, since the ‘data’ converter is connected in series with the power converter, it actually conducts full current
and its actual power rating is greater than the derived value. Specically, if the current owing in the power converter
is taken into account, for both inverter mode and rectier mode, using Eqs (5) and (7) we can approximate the rated
power of the ‘data’ converter as 525 W.
Table 1. Parameters value utilised in the simulation model.
Parameter name Value
AC power source frequency 50Hz
DC voltage source for power conversion 200V
AC voltage source for power conversion 200V
DC side carrier voltage 10V
AC side carrier voltage 10V
DC side carrier frequency for signal modulation 4kHz for ‘1’ and 5kHz for ‘0’
DC side carrier frequency for AC current modulation 20kHz
Modulated AC current frequency 50Hz
AC side carrier frequency 1.5kHz for ‘1’ and 3kHz for ‘0’
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DC–AC converter-based communication solution for microgrid
After establishing the simulation model and setting the parameters, the output current waveform of the AC grid
is obtained as shown in Figure 9. The simulation lasts for 2 s, in which the converter works in the inverter mode for
the former 1 s and in the rectier mode for the latter 1 s. The modulated signals are superimposed on the current
waveform as expected, which proves that the proposed energy and signal integrated transmission approach is
feasible.
4.2. Signal Transmission
An 8-bit signal ‘11010001’ with the data rate of 16 bps is transmitted while the converter operates in the inverter
mode. Before modulating the signal, a 4-bit cyclic redundancy code (CRC) is added at the end of each 8-bit data
string for error checking. Specically, the 4-bit CRC code created with the polynomial
xxxx
432
1
++
++
is ‘0100’.
Then the 12-bit data frame ‘110100010100’ is transmitted and divided by the divisor ‘11111’ at the receiver by
modulo-2 division. If there is no remainder after the division operation, it indicates that the transmitted frame is
correct. Since the added CRC code does not take extra time to transmit, the actual data rate in the rst 1 s of the
simulation is 24 bps. The original 8-bit signal and the 12-bit combined data frame are exhibited in the curves SO and
SP in Figure 10, respectively.
The 4 kHz and 5 kHz carriers are employed to modulate digital ‘1’ and digital ‘0’, respectively. After the modulated
signal is transmitted along with the energy, the 4 kHz carrier is extracted from the current waveform at the receiving
terminal using a bandpass lter. The extracted 4 kHz carrier is presented in the curve ST of Figure 10. Next, the
upper envelope of the demodulated 4 kHz carrier is obtained by the envelope detection process, and the result is
shown in the curve SY in Figure 10. Since the attenuation at the cut-off frequencies of the bandpass lter is set at
6 dB, the attenuation ratio x is obtained as 0.5 based on Eq. 8.
20logx=−6
(8)
Therefore, the amplitude of the extracted carrier curve ST in the passband frequencies region is twice of
its amplitude in the cut-off frequencies region. Moreover, since the amplitude of the carrier envelope obtained
by envelope detection has not changed signicantly, the average value ‘1.5’ of the amplitudes of two different
frequency regions is taken as the threshold with which to convert the carrier envelope into a digital signal. If the
carrier envelope is higher than the threshold value, the digital signal ‘1’ will be generated. Otherwise, the digital
signal ‘0’ will be generated. The digital signal after comparison is displayed as SQ in Figure 10. Finally, the curve SQ
is sampled using the original data rate for signal recovery, and the recovered signals with and without CRC code are
represented by curves SF and SG in Figure 10, respectively. After comparing curve SF with the original data string SP
,
a delay that is mainly caused by the ltering process can be found. Thus, the recovered data actually begins at the
second sampling point (simulation time t = 1/12 s) of curve SF. Moreover, since the signal is transmitted in frames,
the restored signal is manually delayed for one frame to ensure the consistency of the data size of each frame.
When the CRC code is removed, the restored signal has the same data rate as the original signal SO, and it appears
after t = 1 s, as shown in curve SG in Figure 10.
Similarly, the 16-bps signal ‘00101110’ is transmitted through the converter in the rectier mode using the same
modulation and demodulation methods as those applied in the inverter mode. The original 16-bps signal and the
signal combined with 4-bit CRC code are shown in curves SJ and SK in Figure 11, respectively. The generated CRC
Fig. 9. The output current waveform of the AC grid.
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Zhang et al.
code for the 8-bit signal using the polynomial
xxxx
432
1
++
++
is ‘1000’. Since the converter in the simulation works
in the rectier mode between 1 and 2 s, considering the signal delay characteristics, Figure 11 shows the simulation
results between 1 and 3 s. After the modulated signal is transmitted along with the current, the demodulated carrier
for digital ‘1’ and its upper envelope are obtained as displayed in curves SL and SX in Figure 11, respectively. Finally,
the recovered signal SW is acquired by resampling curve SU, and the received signal with 4-bit CRC code removed
is exhibited in curve SN in Figure 11.
4.3. Data Transmission Capability
To determine the maximum signal rate that the proposed method can transmit, the data transmission rate in the two
modes is gradually increased to investigate the relationship between the bit error rate (BER) and the signal rate.
After calculating the BER by comparing the initially transmitted and recovered data strings bit by bit, the BER curve
is sketched as shown in Figure 12. It can be observed from Figure 12 that the BER starts rising when the data rate is
higher than 120 bit/s in the inverter mode. Besides, the error bits in the rectier mode begin to appear when the data
rate is higher than 48 bit/s. Since the modulated signal is superimposed on the current, and the designed converter
uses diodes to achieve half-wave rectication, the signal cannot propagate when the diodes are reverse biased,
and this results in a high BER in the rectier mode. Specically, a 50 Hz AC power supply is used for rectication
in this simulation model; thus, the current period is 0.02 s and the theoretical maximum signal transmission rate
is 100 bit/s. Considering the delay of the lter and the error of the envelope detector, the appropriate actual data
rate is much smaller than the theoretical value. Therefore, the accuracy of the restored signal can be improved
by employing lters with higher orders. Although the proposed signal transmission method is slightly insufcient
in terms of the data rate when compared with the conventional PLC approach, the simplied circuit structure can
satisfy the requirements of power and signal bidirectional transmission while reducing the cost. Thus, it is benecial
to apply this method to AC microgrids.
Fig. 10. The signal transmitted in inverter mode, where SO is the original signal, SP is the signal combined with CRC code, ST is the demodulated carrier
for digital ‘1’, SY is the upper envelope of the extracted carrier, SQ is the curve after comparison, SF is the resampled signal and SG is the received signal
without CRC code. CRC: cyclic redundancy code.
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DC–AC converter-based communication solution for microgrid
Fig. 12. The relationship between the signal bit rate and error rate when the converter operates in two different modes.
Fig. 11. The signal transmitted in rectier mode, where SJ is the original signal, SK is the signal combined with CRC code, SL is the demodulated
carrier for the digital signal ‘1’, SX is the upper envelope of the extracted carrier, SU is the curve after comparison, SW is the resampled signal and SN is the
received signal after CRC code removed. CRC: cyclic redundancy code.
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5. Conclusion
A bidirectional DC–AC converter topology is proposed in this paper to realise two-way communication in microgrids.
In this topology, the communication signal is modulated by the power switches using the FSK method, by which the
communication signal can be superimposed on current and transmitted through the AC grid. Two series-connected
H-bridge converters are employed to convert the current type from DC to AC. Conversely, the AC current is rectied
through the anti-parallel diodes of the IGBT switches of the H-bridge converter. Since the proposed method uses
power switches for signal modulation, the signal couplers used in the conventional PLC-based microgrid are omitted.
Therefore, such a method has the characteristics of simple circuit structure and low cost. After implementing the
proposed converter model in MATLAB/Simulink, the feasibility of modulating the communication signal through
power switches is veried. Moreover, when the power frequency is 50 Hz, 120 bit/s and 48 bit/s, signals can be
transmitted in the inverter mode and rectier mode, respectively.
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