The AC charging pile is the main energy supply facility for household electric vehicles, which uses a vehicle mounted charger to charge the power battery. The current standard of the State Grid Corporation of China clearly stipulates the function of the AC charging pile and does not take into account the impact of the harmonics of the vehicle mounted charger on the power grid. Therefore, in view of the deficiency that AC charging piles cannot suppress the current harmonics of the vehicle mounted charger, application of the active power filtering technology to the design of AC charging piles is proposed to form a new type of AC charging pile with better functions. In the experimental prototype that was built, for vehicle mounted chargers with two load characteristics, the composite control method of traditional PI control and repetitive control is adopted, where the new AC charging pile effectively suppresses the harmonics of the vehicle-mounted charger. Experiments show that the AC charging pile using active power filtering technology cannot only improve the power quality of the grid side but also reduce the impact of harmonics on the power metering and billing system, ensuring the stability of the charging communication system.

Topics
Rectifier, Linear filters, Electronic filters, Electric vehicles, Energy production, transmission and distribution, Electrical grid, Batteries, Pulse width modulation, Mathematical modeling, Matrix calculus

At present, the problems of environmental pollution and energy shortage are becoming more serious. Due to the high energy consumption and serious environmental pollution of traditional fuel vehicles, more people are concerned and worried.1,2 Therefore, countries around the world are actively implementing energy conservation and emission reduction strategies.3,4 As an important part of it, electric vehicles have been developed rapidly in recent years. The power source of the drive system in electric vehicles is all- or partly electric. Compared with traditional fuel vehicles, electric vehicles have obvious advantages such as environmental protection, cleanliness, energy saving, and so on. Electric vehicles will become the main direction of contemporary automobile development and the most potential means of transportation in the 21st century.5,6 The energy supply device of electric vehicles is an indispensable and important piece of equipment for the electric vehicle industry, mainly including DC chargers and AC charging piles. The DC charger has a large power (about 100 kW) and a short charging time. Due to its large size, it is generally installed in a special electric vehicle charging station. The AC charging pile directly provides AC mains power and uses a vehicle mounted charger to charge the power battery.7,8 Generally, the AC charging pile has a small power (about 10 kW) and a long charging time. Due to its small size and small carbon footprint, it can be installed in every corner of the city.

Electric vehicle DC chargers (including vehicle mounted chargers) are actually switching power supplies using power electronics technology, which inevitably generate harmonics and reactive currents during the charging process, affecting the power quality of the power grid. From the current construction of the charging station to the harmonic and reactive power interference generated by the high-power DC charger, the charging station is equipped with a special harmonic control and reactive power compensation device.9,10 Ordinary low power vehicle mounted chargers, in consideration of quality, volume, and cost, generally do not deal with their own harmonic problems. As for the power grid, with the widespread use of AC charging piles, the vehicle mounted charger will be a source of harmonics, which will affect the power quality and stability of the power grid. Moreover, the existence of harmonics will inevitably affect the accuracy of the charging pile metering and billing system and the stability of the communication system.

Judging from the current standards and actual technical level of AC charging piles, the existing charging piles have perfect metering and billing measures, perfect communication capabilities, strong monitoring capabilities, and simple power supply functions, and they have not considered the effective harmonic control of the vehicle mounted charger. Therefore, this paper proposes to apply active power filter (APF) technology to AC charging piles and develops a new single-phase AC charging pile based on single-phase parallel active filter technology.

The circuit structure of a single-phase AC charging pile with an APF function is similar to the single-phase APF circuit of the internal output power line of the existing AC charging pile. The power supply of the metering system, the communication system, and the control system are taken from the front stage of the APF circuit branch, which can eliminate harmonic interference of the vehicle mounted charger. The fact that whether the single-phase APF circuit works normally or not does not affect the normal power supply function of the charging pile. Its primary circuit topology and secondary control function are shown in Fig. 1. In Fig. 1, SD is the signal detection, MBCC is the metering and billing and communication circuit, CP is the control power, DSP CC is the DSP control circuit, IC PC is the isolation circuit and protection circuit, and APF D is the APF drive. The main circuit of the APF is composed of a full-bridge circuit composed of switch tubes PT1 to PT4, an AC-side inductor L, and a DC-side capacitor C, which is consistent with a high-frequency pulse width modulation (PWM) converter. Among them, Rs, Ds, and Cs constitute the DC bus buffer circuit, which is used to suppress the overvoltage of the switching tube. Rs is the snubber resistance; Ds is the snubber diode; Cs is the snubber capacitor; K1 is the power grid input control switch; K2 is the charging pile output control switch; K3 is the APF switching control switch.

In Fig. 1, us represents the grid voltage; is is the grid current; iL is the output current of the charging pile, that is, the input current of the vehicle mounted charger; ish is the output current of the APF used to compensate the harmonic and reactive current generated in the charging process of the vehicle mounted charger; and uL is the output voltage of the charging pile. The voltage Hall element VHL and the current Hall element CHL are used to detect the voltage and current of the system, respectively, in real time. The DSP control circuit collects the voltage and current signals in real time for the metering and billing and communication systems and outputs PWM driving pulses according to the control algorithm. The protection and drive isolation circuit can photoelectrically isolate the primary circuit and the secondary circuit.

According to the voltage and current directions defined in Fig. 1, according to the Kirchhoff voltage theorem, for the AC side circuit of the APF, we can get
(1)
where uAB is the AC side voltage of the APF, r represents the equivalent impedance of the inductor internal resistance and the bridge arm dead zone voltage drop, and uAB = nUdc is the controlled AC side voltage, where Udc is the APF DC side voltage and n is the switching function.
In order to facilitate the design of the digital control system, in the switching period T, the period average processing is performed using Eq. (1). Since the switching frequency of the APF system is high enough compared with the power frequency, the average value of us can be replaced by its instantaneous value, the period average value of uAB is consistent with its instantaneous value, and the influence of r is ignored; the period average model of the APF is obtained as
(2)
(3)

The APF output current ishtk+1 in Eq. (2) is replaced by the harmonic current command, and the output voltage command value uABk of the converter at time k can be calculated to realize the digital control of the APF.

In the single-phase system in this paper, the unipolar frequency doubling SPWM modulation mode is adopted. Under the same switching frequency condition, the output voltage pulse is twice the switching frequency. Figure 2 shows the output waveform under this modulation mode, where uc is the triangular carrier wave signal and ur is the standard modulated sinusoidal signal. The expression of the switch function n is
(4)
This paper adopts the control strategy of lagging one beat and the control mode of the voltage outer loop and the current inner loop. The error of the current inner loop generates the control signal uABk through the controller, and the output signal uABk+1 is caused by lagging one beat. It acts on both ends of the AC inductor L together with the grid voltage, making the inductor current controllable. Therefore, the transfer function expression of the discrete domain of the controlled object is
(5)
Z in Eq. (5) represents the discrete z-transform of the continuous system s function, and s is the complex frequency in the system frequency domain analysis. Due to the factors of sample holding and lagging one beat, the use of traditional PI control algorithms will affect the stability of the system. Figure 3 shows the Bode diagram analysis of the closed-loop transfer function of the converter controlled object after passing through the PI controller with or without lagging one beat. In Fig. 3, G1z is the amplitude phase frequency characteristic of the lagging one beat system, G2z is the amplitude phase frequency characteristic of the system without lagging one beat, the ordinate A represents the amplitude frequency characteristic, φ represents the phase frequency characteristic, and the abscissa f represents the current inner loop error signal frequency. If the proportional parameter of the PI controller is increased to widen the bandwidth of the system, the G1z system will tend to be critically stable or even destroy the stability of the system. From the analysis of phase frequency characteristics, it can be seen that in the medium frequency band, the phase shift of the system is serious, which will affect the performance of the system in tracking high-order harmonics.

The load characteristic of the AC charging pile is mainly the uncontrolled rectifier circuit inside the vehicle mounted charger, which is a nonlinear load, including a small amount of reactive current and a large amount of harmonic current. Therefore, the current command of the APF is often a signal superimposed by the power frequency and its multiple frequency signals. As can be seen in Fig. 3, under the premise that the system is stable, as the frequency increases, the amplitude–frequency characteristics of the system are gradually attenuated, which will affect the magnitude of harmonic compensation. However, the phase–frequency characteristic of the system has a large attenuation, which will affect the phase accuracy of the harmonic compensation. Therefore, the traditional PI controller is difficult to meet the application requirements of the APF, which must be compensated by more effective methods.

The basic idea of repetitive control is that under the premise that the system is stable, in order to achieve good tracking ability (that is, the steady-state error tends to zero), it must satisfy the mathematical model containing external signals in its open-loop transfer function and form a closed-loop control system.

Because the current command of the APF is superimposed by many AC sinusoidal signals of the fundamental frequency and its multiplier frequency, according to the principle of the internal model, a mathematical model describing these harmonic signals is constructed in the control model, the power frequency period is used as the repetition period of these signals, and the expression of the repetition signal generator that can be applied to the digital control system is obtained as
(6)
where N is the number of samples per fundamental frequency period, which is equal to the modulation wave period divided by the carrier wave period.

Due to the influence of lagging one beat and the accuracy of the APF compensation harmonic current, the traditional PI controller will generate a certain harmonic compensation tracking error, the integral I control coefficient is not very effective in reducing the tracking error, and it has an adverse impact on the stability of the system. Therefore, on the basis of the original PI controller, only the proportional P controller is used, and the compound controller is composed of parallel repetitive controllers to improve the ability of harmonic current tracking.

Figure 4 shows the closed-loop control diagram of the compound control current loop based on the proportional and repetitive controller. kp is the expression of the proportional controller. In the dashed box is the repetitive controller, which includes the repetitive signal generator, the repetitive control gain link kr, and the compensation link Sz⁠. Qz is used to weaken the effect of the integral and increase the stability margin of the system, and it generally takes a constant close to 1. ish* represents the harmonic current command, ish1 is the harmonic current output by the proportional control, and ish2 is the harmonic current output by the repetitive control. Simultaneously, the transfer function of the current closed loop is obtained as
(7)
According to Eq. (7), the characteristic equation of the system is sorted out as
(8)
It can be seen from Eq. (8) that the characteristic equation of the system is composed of two-part polynomials. Among them, Δ1z represents the characteristic equation of the original proportional control system, which represents the stability criterion of the original system. Δ2z is the stability criterion representing the addition of the repetitive controller part. To make the whole system stable, the characteristic roots of Δ1z and Δ2z must be in a unit circle. Between the addition of the repetitive controller, the system using the proportional controller is stable, so the key to the stability of the compound control system is whether the characteristic root of Δ2 is in the unit circle or not; the criterion expression is
(9)
where ω∈0,π/T⁠. Equation (9) is expressed in vector form, and the geometric vector diagram of the compound control stability criterion can be obtained.

As shown in Fig. 5, the stability of the system requires the magnitude of the vector a to be less than 1. When QejwT=1⁠, the center of the unit circle is fixed at point (1,0), the left arc of the unit circle will be tangent to the imaginary axis at the origin, and the area contained in the unit circle is limited to part of the first and fourth quadrants. In the middle and low frequency bands, due to the effect of the compensator, it can be considered that the vector b has approximately zero gain and zero phase shift. However, in the high frequency bands, due to the modeling errors, the compensation effect cannot be guaranteed. If at a certain frequency the phase angle of vector b is close to ±90°, even if its amplitude is small, the end of the vector a will stay on the unit circle, and the system becomes critically stable. Therefore, if QejwT is set to a constant less than 1, the whole unit circle is shifted to the left so that even if the phase angle of vector b exceeds ±90° in the mid-to-high frequency band, the end of vector a can be adjusted in the unit circle to ensure the stability of the system. Therefore, the addition of Qz makes the stability region of the system cover the four quadrants of the complex plane. Even if a serious phase error is caused by modeling errors in the middle and high frequency bands, the stability of the system can be ensured, and the robustness of the system is enhanced.

According to Eq. (7), after sorting out the current loop transfer function under compound control, we can get
(10)
From the sorted current loop transfer function, it can be seen that there are two transfer functions, namely,
(11)
(12)
where F1z is the current closed loop transfer function of the proportional controller acting on the system and F2z is the current closed loop transfer function when the repetitive controller acts.
Therefore, combining Eqs. (9)–(11), under the action of the compound controller, the relationship between the output current of the system and the current command is
(13)
The proportional controller can quickly respond to the error of the system and then produce the regulating effect. The repetitive controller, on the other hand, delays a fundamental wave period, and when the system becomes stable, it begins to gradually produce the regulating effect and finally eliminates the steady-state error of the system. In the middle frequency band, the system F2z can realize the output characteristics of zero gain and zero phase shift, track the steady-state error ish2 without error, and superimpose it on the original output ish1 so that the system can finally realize the tracking without static error. Therefore, it can be seen from Eq. (11) that the expression of the equivalent control object of the repetitive controller is
(14)
In order to realize the zero gain and zero phase shift characteristics of F2z in the middle frequency band, the lead and lag compensation link S1z in the correction link Sz is designed. The typical transfer function form of this link is
(15)
The zero point a1 is designed as the pole of the equivalent controlled object to reduce the order of the system. The compensated system should meet the characteristics of zero gain in the low and middle frequency bands, so when ω → 0, z=ejωT→1⁠, the compensated system satisfies the following relation:
(16)

The value of c1 affects the bandwidth of the corrected system, and the value of b1 is used to correct the gain characteristics of the system. The system designed in this paper can have zero gain characteristics between 50 and 1200 Hz.

Due to the high quality requirements of the APF output waveform, Qz is taken as a constant of 0.98. Moreover, the phase frequency characteristic of the system is compensated, and two beats ahead are used to compensate the lagging one beat of digital control and the phase lag of the control object, that is, S1z is selected as z2. Therefore, according to Eq. (8), it can be deduced that the value range of the repetitive controller gain kr is
(17)

Figure 6 shows the Bode plot of the transfer function of the current inner loop with the constructed composite control. Compared with the simple PI controller, the phase shift and gain attenuation of the system at the fundamental frequency and its multiplier within 1200 Hz are smaller, which can meet the application requirements of APF tracking the harmonics of vehicle mounted chargers.

According to Fig. 1, the AC charging pile circuit with a single phase APF function is built for experimental verification. Table I shows the circuit structure parameters of the single phase parallel APF. The uncontrolled rectifier is connected to RC and the uncontrolled rectifier is connected to RL to simulate the front stage rectification link of the vehicle mounted charger, as shown in Fig. 7, and the harmonic compensation test is carried out.

As shown in Fig. 7, the DC side inductance L of the harmonic source is 100 mH, the resistance R is 20 Ω, and the DC side capacitor C is 4700 µF.

Figure 8 shows the compensation current waveform of the uncontrolled rectifier connected to RL load with the APF. Figure 9 shows the Fast Fourier Transform (FFT) frequency spectrum of the load current, and Fig. 10 shows the power grid current frequency spectrum. It can be seen that the Total Harmonic Distortion (THD) of the grid current decreases from 41.7% to 4.6%. The abscissa m in Figs. 9 and 10 represents the harmonic order, and the ordinate represents the percentage of the harmonic current amplitude im in the fundamental current i1.

Figure 11 shows the compensation current waveform of the uncontrolled rectifier connected to the RC load, Fig. 12 shows the load current spectrum, and Fig. 13 shows the power grid current spectrum. It can be seen from the figure that the THD of the power grid current decreases from 51.7% to 4%.

In this paper, the construction and stability criteria of the single phase active filter compound controller are theoretically derived, and the feasibility of harmonic suppression is verified from the prototype experiment, but the effect of harmonic suppression depends on the accuracy and rapidity of the harmonic current command. The harmonic current command comes from the extraction of the harmonic components of the input current of the vehicle mounted charger. The main methods applied to the single phase system are the fundamental wave component method, the FFT analysis method, the adaptive detection method, and the wavelet theory detection method. Due to the small amount of calculation of the adaptive detection method, it is easy to realize the digital system, and its dynamic detection performance can meet the actual requirements of charging, so this paper adopts the adaptive harmonic detection method to uniformly compensate all harmonic components of the charger except the fundamental wave. If the FFT analysis method can accurately extract each harmonic component, it can accurately compensate the specified harmonic.

In the experiment carried out in this paper, the harmonic current component and the output of the voltage outer loop controller are given as the current inner loop. The phenomenon in the experiment is that the DC voltage on the DC bus side of the APF main circuit is stable, and the AC side output compensates the harmonic current of the vehicle mounted charger. If the reactive component of the vehicle mounted charger is included in the given current inner loop, the APF can also compensate the reactive current of the charger. For the power grid, the whole charging device is a resistive device.

  1. In this paper, the topological structure of the new AC charging pile with an APF function is analyzed, and the state period average model of a single phase APF is established to realize the digital control of the APF.

  2. The control performance of the current inner loop in the control system is analyzed, a compound control system combining proportional control and repetitive control is designed, and the expression of the system stability criterion and the method of geometric vector judgment are derived.

  3. The prototype experiment shows that the method of using the AC charging pile with an APF function to compensate the harmonics of the vehicle charger is feasible. Through the comparative analysis of system Bode plots in the frequency domain, it can be seen that the compound control system increases the system response bandwidth and improves the robustness of the system. Therefore, the compound control method can compensate the harmonics of the vehicle mounted charger better so that the harmonic content of the current on the power grid side can be reduced to less than 5%, which meets the requirements of national standards.

  4. The harmonics of the vehicle mounted charger can be effectively suppressed, which will help improve the efficiency of the whole charging system, reduce system losses, ensure the accuracy of electric energy metering and billing, and ensure the stability of the charging pile communication system. The charging device meets the requirements of a strong smart grid and promotes the development of the electric vehicle industry better.

This work was supported by the National Key R&D Program of China under Grant (Grant No. 2016YFF0201201).

The authors have no conflicts to disclose.

Jin Bao: Data curation (equal). Meimei Duan: Project administration (equal). Jun Li: Formal analysis (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
S.-J.
Huang
,
C.-T.
Hsieh
, and
C.-L.
Huang
, “
Application of Morlet wavelets to supervise power system disturbances
,”
IEEE Trans. Power Delivery
14
(
1
),
235
–
243
(
1999
).
https://doi.org/10.1109/61.736728
Google Scholar
Crossref
Search ADS
 
2.
Y.
Nishida
 and
M.
Nakaoka
, “
Simplified predictive instantaneous current control for single-phase and three-phase voltage-fed PFC converters
,”
IEE Proc.-Electr. Power Appl.
144
(
1
),
46
–
52
(
1997
).
https://doi.org/10.1049/ip-epa:19970798
Google Scholar
Crossref
Search ADS
 
3.
K.
Zhang
,
Y.
Kang
,
J.
Xiong
 et al, “
Direct repetitive control of SPWM inverter for UPS purpose
,”
IEEE Trans. Power Electron.
18
(
3
),
784
–
792
(
2003
).
https://doi.org/10.1109/tpel.2003.810846
Google Scholar
Crossref
Search ADS
 
4.
K.
Keliang
 and
D.
Wang
, “
Digital repetitive controlled three-phase PWM rectifier
,”
IEEE Trans. Power Electron.
18
(
1
),
309
–
316
(
2003
).
https://doi.org/10.1109/tpel.2002.807150
Google Scholar
Crossref
Search ADS
 
5.
D.
Li
,
Q.
Chen
,
Z.
Jia
 et al, “
A novel active power filter with fundamental magnetic flux compensation
,”
IEEE Trans. Power Delivery
19
(
2
),
799
–
805
(
2004
).
https://doi.org/10.1109/tpwrd.2003.823194
Google Scholar
Crossref
Search ADS
 
6.
S.
Fukuda
 and
T.
Yoda
, “
A novel current-tracking method for active filters based on a sinusoidal internal model
,”
IEEE Trans. Ind. Appl.
37
(
3
),
888
–
895
(
2001
).
https://doi.org/10.1109/28.924772
Google Scholar
Crossref
Search ADS
 
7.
J. C.
Montano Asquerino
,
M.
Castilla Ibanez
,
A.
Lopez Ojeda
, and
J.
Gutierrez Benitez
, “
Measurement of apparent power components in the frequency domain
,”
IEEE Trans. Instrum. Meas.
39
(
4
),
583
–
587
(
1990
).
https://doi.org/10.1109/19.57237
Google Scholar
Crossref
Search ADS
 
8.
M.
El-Habrouk
 and
M. K.
Darwish
, “
Design and implementation of a modified Fourier analysis harmonic current computation technique for power active filter using DSP
,”
IEE Proc.-Electr. Power Appl.
148
(
1
),
21
–
27
(
2001
).
https://doi.org/10.1049/ip-epa:20010014
Google Scholar
Crossref
Search ADS
 
9.
J. F.
Chicharo
 and
H.
Wang
, “
Power system harmonic signal estimation and retrieval for active power filter applications
,”
IEEE Trans. Power Electron.
9
(
6
),
580
–
586
(
1994
).
https://doi.org/10.1109/63.334772
Google Scholar
Crossref
Search ADS
 
10.
S.
Luo
 and
Z.
Hou
, “
An adaptive detecting method for harmonic and reactive currents
,”
IEEE Trans. Ind. Electron.
42
(
1
),
85
–
89
(
1995
).
https://doi.org/10.1109/41.345850
Google Scholar
Crossref
Search ADS
 
© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).