Design of a Power Flow Control Method for Hybrid Active Front-End Converters

Design of a Power Flow Control Method for Hybrid Active Front-End Converters

Tzung-Lin Lee Zong-Jie Chen Shang-Hung Hu Department of Electrical Engineering

National Sun Yat-sen University 70, Lienhai Rd., Kaohsiung 80424, TAIWAN

Email: tllee@mail.ee.nsysu.edu.tw

Abstract—Active front-end converters with bidirectional power

flow capability have been extensively used for utility applications of power electronics. Large passive filters are normally required at the grid side of the converter to mitigate switching EMI noise.

This may become a critical issue, in terms of installation space and potential resonance of the passive filter. This paper proposes a hybrid active front-end converter and its power flow control method. The hybrid converter is composed of a capacitor and a voltage source converter in series connection. Bidirectional real power of the converter can be controlled by the output voltage vector perpendicular to the grid voltage, and reactive power delivery of the converter for grid voltage regulation can be determined by the output voltage vector parallel to the grid voltage. Due to series connection capacitor, the converter can be operated between a low-voltage dc side and high-voltage grid side without any low-frequency transformer, which is the significant advantage of the proposed method. A harmonic resistance is also emulated in the proposed method to assure stable operation of the converter for unintentional voltage spike coming from the power system. Operation principles are explained in detail, and computer simulations and experimental results are provided to validate the effectiveness of the proposed approach.

K

Hybrid active front-end converter, power flow control

I. I

Active front-end converters have been extensively used in various power conversion applications requiring bidirectional power flow capability, such as adjustable speed drivers, un- interruptible power supply systems, inverter based distributed generators, and active power filters [1], [2]. Passive power filters, such as LC or LCL filters, are usually deployed between the converter and the grid to mitigate switching EMI noise produced by the converter [3], [4]. However, the required pas- sive filters is a critical issue in terms of installation space and weight, and potential harmonic resonance between the passive filters and the grid may cause instability of the converter.

Various active damping approaches have been proposed to suppress the harmonic resonances of passive filters, but power rating of the converter may be limited due to high switching frequency [5], [6], [3]. Multilevel inverters with reduced switching voltage would provide acceptable EMI noise at the cost of high component count and complex control [7], [8].

A shunt hybrid circuit has been proposed in active filtering applications of medium-voltage range, where a series capac- itor replaces the bulky low-frequency transformer to reduce

kVA rating of the active filter and the resulting switching ripples [9], [10]. Authors have presented simulation results of transformerless interface converters applied in distributed generation systems with unity power factor operation [11].

This paper proposes a power flow control method for a hybrid active front-end converter. The proposed converter is composed of a capacitor and a voltage source converter in series connection. Bidirectional real power of the converter can be controlled by the output voltage vector perpendicular to the grid voltage, and reactive power delivery of the converter for grid voltage regulation can be determined by the output voltage vector parallel to the grid voltage. Since the series capacitor sustains a part of the fundamental voltage, the converter can be operated with a reduced dc voltage compared with the conventional grid-connected inverter. This algorithm allows the fundamental power control between the low-voltage dc side and the high-voltage grid side without any low-frequency transformer, which is a significant advantage in terms of both installation space and switching EMI noise of the converter.

The proposed interface converter also emulates harmonic resistance to reduce transient voltage of the converter resulting from the capacitive switching of the power system.

II. O

P

A simplified one-line diagram of the proposed hybrid active front-end converter (HAFEC) is shown in Fig. 1. The proposed HAFEC is composed of a capacitor C and a voltage source converter in series connection to the grid U. The operation of the proposed HAFEC includes both real power control (S is at A) and reactive power control(S is at B) modes. The reference voltage generator determines the voltage command E

according to the operational mode. The voltage controller then produces the current command i

of the converter. Sub- sequently, the current controller generates the output voltage command v

for the PWM of the converter. Operational principles are detailed as follows.

A. Operation modes

In the real power control mode, the angle command θ

is set as −90

for maximum real power conversion and the

voltage command E

is determined by using a PI controller

to regulate the real power output according to the real power

command P

. The real power is calculated by using the

instantaneous power theory [12]. If the HAFEC is operated

R S

B

A

to d^eq^e

d^eq^e to abc

Uabc

Eabc Eabc

E∗abc

iabc E∗A iabc

E∗B θ∗A

θ∗B

Eeqd

Ee∗qd

U eqd U e∗qd

ω

ω ω

P P ∗

ieqd ˜ieqd iabc,h

i∗abc

i∗qd v∗abc,f v∗abc

v∗abc,h

−90

0

HAFEC

HPF PLL

Current

regulator PWM

Sinusoidal Generator

Sinusoidal Generator

PI

PI

PI

Reference voltage generator Voltage controller Current controller

Vdc

Vs

C I

Loads

Li U E

Fig. 1. The proposed HAFEC and its associated control.

in the reactive power control mode, the angle command θ

should be set as 0

for maximum reactive power delivery. The voltage command E

is based on the required reactive power for restoring the grid voltage to the nominal value, which can be implemented by a synchronous reference frame (SRF) PI controller [13].

Note that, if a dc capacitor is used in the dc side of the converter, the dc voltage can be controlled by drawing reactive current from the grid [2]. Therefore, the voltage command E

can be obtained for different operating modes.

B. Voltage and current controls

A PI-based voltage controller in the SRF is implemented to generate the current command i

of the converter according to the measured voltage E

. Based on the current command i

, the measured current i

, and the measured voltage E

, the current regulator in the stationary frame calculates the voltage command v

of the fundamental frequency as follows [1],

v

= E

− L

ΔT (i

− i

). (1) L

is the output inductor of the inverter, and ΔT is the sam- pling period. Harmonic damping for suppressing the transient voltage coming from the upstream of the power system is also emulated, whose operation is defined as follows,

v

= R · i

. (2) R represents harmonic resistance, i

is harmonic current component, and v

is harmonic voltage command of the HAFEC, respectively. Harmonic current component i

can

be extracted by a high pass filter (HPF) in the SRF. Finally, the space vector PWM is employed to synthesize the gating signals of the inverter. Based on this algorithm, the HAFEC current can be controlled with the desired power flow require- ment.

U = |U |^ 0^o Vc E = |E|^ θ

Xc

P Qr P Q

I

Fig. 2. A single phase equivalent circuit of the HAFEC.

C. Power flow analysis

Fig. 2 shows the single-phase equivalent circuit of the proposed HAFEC at the fundamental frequency. The inverter and its output inductor are considered as an ideal voltage source E, due to the proposed voltage control. U is the grid voltage, θ is the angle of E leading U, and X

is the reactance of series capacitor, respectively. Inverter output real power P , inverter output reactive power Q, the reactive power injected into the grid Q

can be expressed as follows,

P = −3X

(|E||U|sinθ) Q = 3X

(|E|

− |E||U|cosθ) Q

= 3X

(|U|

− |E||U|cosθ)

(3)

and maximum reactive power delivery is at θ=

0 or

180 , respectively. Therefore, the real power or the reactive power can be separately and adequately controlled by regulating voltage command E

or E

with the corresponding angle condition. In this paper, θ

=−90 and θ

=0 are used for power controlling.

−180 −120 −60 0 60 120 180

−0.4

−0.3

−0.2

−0.1 0 0.1 0.2 0.3 0.4

P@E=0.2pu Q@E=0.2pu Q_r@E=0.2pu P@E=0.3pu Q@E=0.3pu Qr@E=0.3pu Power

Degree

(a) P , Q, and Qr for |E|=0.2 pu and |E|=0.3 pu, respectively, with fixed Xc=0.2 pu.

−180 −120 −60 0 60 120 180

−0.4

−0.3

−0.2

−0.1 0 0.1 0.2 0.3 0.4

P @ X c=0.2 Q @ X

c=0.2 Qr @ X

c=0.2 P @ X

c=0.3 Q @ X

c=0.3 Qr @ X

c=0.3 Power

Degree

(b) P , Q, and Qr for Xc=0.2 pu and Xc=0.3 pu, respectively, with fixed

|E|=0.3 pu.

Fig. 3. Examples of power flow analysis.

Fig. 3(a) and Fig. 3(b) show power-flowing examples be- tween E and U for various conditions. Fig. 3(a) illustrates maximum real power and maximum reactive power are in- creased with |E| when X

is fixed. The controllable range of both real power and reactive power is also extended with increasing X

as shown in Fig. 3(b) if |E| is fixed. Fig. 4 shows phasor diagrams of power flow in the proposed HAFEC.

and at the charging mode P < 0 in Fig. 4(b), respectively. In addition, the converter is operated as an inductor Q > 0 in Fig. 4(c) or a capacitor Q < 0 in Fig. 4(d), respectively.

U

E I

Vc

(a) P>0.

U E

I

Vc

(b) P<0.

E U

I Vc

(c) Q>0.

U E

I Vc

(d) Q<0.

Fig. 4. Phasor diagrams of the proposed HAFEC.

HAFEC

Vdc Vs

C

I

Ls

Ll

Li E U

Fig. 5. Simulation circuit.

III. S

Fig. 5 shows the simulation circuit and circuit parameters are given as follows:

•

Power system: 220 V(line-to-line), 20 kVA, 60 Hz.

•

C=200 μF, L

= 1.0 mH, L

= 1.0 mH, L

= 30 mH.

•

V

=100 V, U

=127 V.

•

The converter is a conventional three-phase voltage source inverter. The PWM frequency and the sampling frequency are 10 kHz and 20 kHz, respectively.

Fig. 6 shows 1 kW power conversion for both discharging

and charging modes. At t=2s, the HAFEC starts in operation

with P

=1 kW. The converter voltage E is increased and

maintained 90

lagging the grid voltage U for supplying

real power to the grid as shown in Fig. 6(a). At the steady

1.9 2 2.1 2.2

−200

−100 0 100 200

1.9 2 2.1 2.2

−200

−100 0 100 200

1.9 2 2.1 2.2

−30

−15 0 15 30

1.9 2 2.1 2.2

−2

−1 0 1 2

1.9 2 2.1 2.2

−2

−1 0 1 2

Time U

E

I

P

Q

(a) 1kW discharging operation from dc side to grid side.

2.9 3 3.1 3.2

−2

−1 0 1 2

Time

2.9 3 3.1 3.2

−200

−100 0 100 200

2.9 3 3.1 3.2

−200

−100 0 100 200

2.9 3 3.1 3.2

−30

−15 0 15 30

2.9 3 3.1 3.2

−2

−1 0 1 2 U

E

I

P

Q

(b) 1kW charging operation from grid side to dc side.

Fig. 6. Simulations of real power conversion.

state, E

=35 V, I

=10 A, P =1 kW, and Q=-280 var. Sub- sequently, P

is changed to -1 kW at t=3s. Fig. 6(b) shows the HAFEC enters the charging mode and draw 1 kW from the grid when reaching the steady state. At this time, E is kept 90

leading the grid voltage U.

Fig. 7 shows the reactive power control for grid voltage regulation. Due to inductive load L

, the grid voltage U drops to 126.4 V, which is slightly lower than nominal value. After the HAFEC is started at t=2s, the converter voltage E is regulated to 180

out of phase with the grid voltage U. Finally, the grid voltage can restore to the nominal value, and the HAFEC absorbs 0.7 kvar at E

=21 V and I

=11 A.

1.9 2 2.1 2.2

−200

−1001002000

1.9 2 2.1 2.2

−200−1001002000

1.9 2 2.1 2.2

−30−1515300

1.9 2 2.1 2.2

125 126 127 128

1.9 2 2.1 2.2

−2−1012

1.9 2 2.1 2.2

−2−1012

Time U

E

I

Urms

P

Q

Fig. 7. Simulations of reactive power delivery for grid voltage regulation.

IV. E

The experimental setup is similar to the simulation circuit in Fig. 5 except the inductive loading L

=30 mH is absent. The converter control is implemented by using TI TMS320F28335 chip to perform signal processing, such as power calculation, frame transformation, PI controllers, filters, and PWM algo- rithm.

Fig. 8 shows both steady-state and transient results of real power conversion operation from the dc side to the grid side for E

=30V. The converter output current I lags the grid voltage U by 90

and maintains in phase with the converter output voltage E as shown in Fig. 8(a). Fig. 8(b) shows the transient of the real power P and reactive power Q after the HAFEC is in operation. TABLE I summarizes test results of P , Q, I when E

=25V, 30V, 35V, 40V, respectively. With increasing E

, more real power is delivered to the grid.

TABLE I REAL POWER CONVERSION.

E^∗_A 25V 30V 35V 40V

I 11.7A 12A 12.4A 12.8A

P 620W 775W 934W 1095W

Q -88var -121var -160var -205var

Fig. 9 shows the reactive power delivery for grid voltage regulation. The reference grid voltage is set as 182V(peak).

Before the HAFEC is started, the grid voltage is 185 V(peak)

due to no loading. After the HAFEC is engaged, the grid

voltage is restored to its reference value, 182 V(peak), with

Q=822 var and I=8.1 A as illustrated in Fig. 9(b). In contrast,

the converter output current I lags the grid voltage U and the

converter output voltage by 90

simultaneously as shown in

Fig. 9(a). TABLE II gives Q and I when E

=25V, 30V, 35V,

40V, respectively. More reactive power delivery to the grid

with increasing E

is verified.

U

E I

(a) Inverter output voltageE, inverter output current I, and grid voltage U of phase a.

P

Q

(b) Inverter output real powerP and inverter output reactive power Q.

Fig. 8. Voltage, current, real power output and reactive power output when the HAFEC is in real power operation mode. (P :^{500 W}/^div,Q:^{500 var}/^div,E, U:^{100 V}/^div,I:^{10 A}/^div)

TABLE II REACTIVE POWER DELIVERY.

E_B^∗ 25V 30V 35V 40V

I 8.8A 8.7A 8.5A 8.3A

Q 471var 559var 637var 718var

V. S

A power flow control method for a hybrid active front- end converter is presented in this paper. The maximum real power flow can be converted between the dc side and ac side with bidirectional capability, and the maximum reactive power can be controlled for grid voltage regulation by adequately adjusting the converter voltage vector. Thanks to a series capacitor between the converter and the grid, the converter can be operated with a reduced dc voltage without any low- frequency transformer, compared with the conventional grid- connected inverter. This is a significant advantage, in terms of

U

E I

(a) Inverter output voltageE, inverter output current I, and grid voltage U of phase a.

Q

Upeak

(b) Inverter output reactive powerQ and grid voltage peak value Upeak(with 180V offset).

Fig. 9. Voltage, current and reactive power output when the HAFEC is in reactive power operation mode. (P :^{500 W}/^div,Q:^{500 var}/^div,E, U:^{100 V}/^div, I:^{10 A}/^div,Upeak:10 V/^div)

both installation space and switching EMI noise of the con- verter. Fig. 10 shows switching ripples between the proposed hybrid active front-end converter and the conventional grid- connected converter, where both converters deliver the same real power (1kW) to the grid. Obviously, the conventional grid- connected converter produces about 5 times current switching ripple compared with the proposed method.

As shown in (3), the power delivery of the proposed

HAFEC is dependent on both E and X

, where E is related to

the required dc voltage of the converter and X

is the reactance

of the series capacitor. Fig. 11 shows the relationship of power

conversion for E to X

. E is roughly inverse to X

for fixed

power delivery, and required E or X

is increased with power

output. Based on Fig. 11(a) and Fig. 11(b), the maximum dc

voltage and the series capacitor can be determined for the

required power output.

(a) Current spectrum of the proposed HAFEC.

(b) Current spectrum of the conventional grid-connected converter.

Fig. 10. Comparison of switching ripples between the proposed HAFEC and the conventional grid-connected converter when they produce 1kW real power output concurrently.

A

This research is funded by the National Science Council of TAIWAN under grant NSC 98-2221-E-110-078.

R

[1] T. G. Habetler, “A space vector-based rectifier regulator for AC/DC/AC converters,” IEEE Trans. Power Electron., vol. 8, no. 1, pp. 30–36, Jan.

1993.

[2] H. Akagi, “Active harmonic filters,” Proc. IEEE, vol. 93, no. 12, pp.

2128–2141, Dec. 2005.

[3] P. C. Loh and D. G. Holmes, “Analysis of multiloop control strategies for LC/CL/LCL-filtered voltage-source and current-source inverters,” IEEE Trans. Ind. Appl., vol. 41, no. 2, pp. 644–654, Mar./Apr. 2005.

[4] I. J. Gabe, V. F. Montagner, and H. Pinheiro, “Design and implementa- tion of a robust current controller for vsi connected to the grid through an lcl filter,” IEEE Trans. Power Electron., vol. 24, no. 6, pp. 1444–1452, June 2009.

[5] V. Blasko and V. Kaura, “A novel control to actively damp resonance in input LC filter of a three-phase voltage source converter,” IEEE Trans.

Ind. Appl., vol. 33, no. 2, pp. 542–550, Mar./Apl. 1997.

[6] E. Twining and D. G. Holmes, “Grid current regulation of a three- phase voltage source inverter with an lcl input filter,” IEEE Trans. Power Electron., vol. 18, no. 3, pp. 888–895, May 2003.

[7] J. Rodriguez, J. S. Lai, and F. Z. Peng, “Multilevel inverters: a survey of topologies, controls and applications,” IEEE Trans. Ind. Electron., vol. 49, no. 4, pp. 724–738, 2002.

0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8

1 P=0.05pu

P=0.1pu P=0.5pu P=1pu

E

Xc

(a) The relationship ofE to Xcfor real power conversion.

0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8

1 Q=0.05pu

Q=0.1pu Q=0.5pu Q=1pu

E

Xc

(b) The relationship ofE to Xcfor reactive power conversion.

Fig. 11. The relationship ofE to Xcfor various power output.

[8] B. Wu, High-Power Converters and AC Drives. Piscataway, NJ: IEEE Press, 2006.

[9] H. Akagi, S. Srianthumrong, and Y. Tamai, “Comparison in circuit configuration and filtering performance between hybrid and pure shunt active filters,” in IEEE Industry Applications Conference 38th IAS Annual Meeting, 2003, pp. 1195–1202.

[10] S. Srianthumrong and H. Akagi, “A medium-voltage transformerless ac/dc power conversion system consisting of a diode rectifier and a shunt hybrid filter,” IEEE Trans. Ind. Appl., vol. 39, no. 3, pp. 874–882, May/Jun. 2003.

[11] T.-L. Lee and Z.-J. Chen, “A transformerless interface converter for a distributed generation system,” in 13th Power Electronics and Motion Control Conference (EPE-PEMC), 2008.

[12] H. Akagi, Y. Kanagawa, and A. Nabase, “Instantaneous reactive power compensator comprising switching devices without energy storage com- ponents,” IEEE Trans. Ind. Appl., vol. IA-20, pp. 625–630, May/Jun.

1984.

[13] S. Bhattacharya, D. Divan, and B. Banerjee, “Synchronous frame har- monic isolator using active series filter,” in the 4th European Conference on Power Electronics and Applications, 1991, pp. 30–35.