Download 2.3resonance damping by the AHR

Active Resonance Damping for Power Capacitor in HV Distribution Grid
NIE CHENG, LEI WANJUN, WANG HUAJIA, CHEN MINGFEN, WANG YUE
State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University
Abstract:
In distribution grid, power capacitor is wildly used to compesate reactive power. The resonance between power
capaciotor and line inductance will cause over current/voltage and endanger the safety of power capacitor. This paper
focus on the active resonance damping for power capacitor in distribution grid. With a specific current-controlled
method, the AHR (active harmonic resistor) perform as positive resistor only in the continuous harmonic frequency in
order to damp the potential resonance, and the resistor value can be changed dynamically. While in fundamental
frequency, the AHR not only have no consumption , but can also output active power to grid. Moreover, with the
inject structure, the AHR can apply in high voltage level distribution grid conveniently.
1.
Introduction
In the distribution power system, harmonic components generated by non-linear loads may excite resonance
between the power capacitors and the transmission lines. Fig.1 shows a simple distribution system. Here, Vs is
the voltage source of the power system and Ls is the line inductance. The compensation capacitor C is series
connecting with an inductor L, which is used to limit the inrush current.
Fig.1 Diagram of a simple distribution system.
Passive resistor bring power consumption problem. Damping in active ways obtain more and more
attentions. There have already some solutions, like R-APF, hybrid-APF and AHR. AHR is a technique that
control a power electronic converter act as resistor in harmonic frequency. Among those active methods, AHR
way only need small current ration and voltage ratio.
2.
Modeling and impedance character of the AHR
2.1 structure of the AHR
The system configuration and control strategy for AHR are shown in Fig.2.
PCC
Vs
Ls
Ih
IM
Linear
Loads
u
IC
C
L1
+ i
L u
Harmonic Capacitors
Loads
C1
AHR
Vdc
1/Kh
I ref1
+
+
PLL
ref
dc
V
+-
PI
If
iref
+-
Kp
KPMW
*
VPWM
1
sL1
i
Vdc
(a)
(b)
Fig.2 Diagram of current-controlled AHR. (a) System configuration of AHR; (b) Proposed current control strategy
In Fig.2 (a), the converter is parallel connected with the series inductor L though an output inductor L1. This
structure can reduce the fundamental voltage and current that the converter needs to suffer. Fig.2 (b) is the
current control strategy, the harmonic extraction is needless.
2.2 modeling of the AHR
The AHR is used to damp the harmonic resonance, only the harmonic impedance characteristic need be
concerned. Equation (1) is the transfer function of harmonic current to harmonic voltage.
uh
L * Kh
s 1
 Kh
ih
Kp KPWM
(1)
From equation (1), the close loop harmonic impedance characteristic of the AHR is similar to a RL load.
Consider that the L1 is small, then the equivalent harmonic resistor of AHR is expressed in equation (2).
(2)
Reql =K h
2.3resonance damping by the AHR
With the AHR damping, Fig.3 is the bode plot of transfer function from capacitor current to harmonic
current. With different equivalent harmonic resistor, the damping effect is not the same in Fig.3.
Bode Diagram
Magnitude (dB)
40
Without damping
20 Reql=1
Reql=10
0 Reql=20
-20
-40
-60
-80
10
1000
10000
Frequency (Hz)
Fig.3 bode plot of transfer function from capacitor current to harmonic current with different damping resistor
From Fig.3, AHR has well performance in resonance damping. With appropriate resistor value, it can
decrease the capacitor harmonic current both in the low and high frequency region.
3.
Parameter design criterions of the AHR
The ac side voltage of the AHR satisfy equation (3), and maximum value is only 10% of the grid voltage.
U 
Z pa
Z pa  Z C
(3)
U PCC
Here Z pa is the parallel impedance of the series inductor L and AHR, Z C is the impedance of capacitor,
U PCC is the grid voltage.
4.
Simulation results
Fig.4 is the simulation result. When only switch in the reactive compensation branch, the line current and
capacitor current become resonance as shown in Fig.4 (a). Fig.4 (b) is the result after damping by AHR.
(a)
(b)
Fig.4 The simulation result. (a) without damping; (b) with damping by AHR
From simulation result, the harmonic currents of the line and capacitor are amplified evidently without
damping. With the proposed AHR, resonance problem is disposed effectively.
5.
Conclusion
The AHR is used to suppressing the resonance of reactive power compensation branch. With the inject
structure, it can be used in high-voltage level applications. The control strategy cancels the harmonic extraction
process, so the equivalent resistance valid at the continuous harmonic frequency spectrum. It makes the AHR
senseless of grid parameters change.
Keywords:
active harmonic resistor; resonance damping; HV distribution grid; modeling; parameters design
NIE CHENG：received the B.Eng. degree in 2008 from Hunan University, Changsha, China, and M.Eng.
degree in 2013 from Xi’an Jiaotong University, Xi’an, China, where he is currently working toward the
Ph.D. degree. His current research interests include microgrid, power quality, analysis and control of
converters. Telephone number: 15809243405; E-mail address: [email protected]; mail address: School
of Electrical Engineering, Xi'an Jiaotong University, No.28, Xianning West Road, Xi'an, Shaanxi, 710049,
P.R. China.