Download 6.3.4 The Design of Harmonic Filter and Reactive Power

6.3.4 The Design of Harmonic Filter and Reactive Power Compensation
in Power Supply System
1, Introduction
The Electro-Magnetic-Compatibility (EMC), i.e. Power Quality (PQ), for a power supply
system in Tokamaks is one of the essential issues in fusion technology because of its impact
of high demands of pulsed power and reactive power, and harmonics as well.
The power supplies in HT-7U super-conductive Tokamak mainly consist of thyristor
converters, pulse-width-modulation inverters, switching power supplies, and so on. All of
those power converters with power electronics in HT-7U power supply system are inherently
impulsive and non-linear.
The study and design of power quality control in power supply system of HT-7U superconductive Tokamak have been carried out in order to ensure the safety and reliability in
operation and to realize the compatibility between the power supply system and high voltage
grid. On the one hand, in the design of power supplies of magnets, one of the principles is to
reduce the impact of pulsed power and reactive power, harmonics, and unbalanced
components in AC power system as much as possible. On the other hand, Static Var.
Compensation (SVC) and Harmonic Filter (HF) are very important and indispensable.
2, Design principle and features of SVC & HF
The analysis and estimation of pulse power level and harmonic contents have been carried
out. The reactive power profile may see Fig. 1.
35
30
Q
P
MVAR
MW
20
10
0
-10
-20
-30
0
2
4
6
8
10 12 14 16 18
time(s)
Fig. 1 Pulse power demands of HT-7U
And the maximum level reaches about 30MVAR in Double Null operation (48MVAR in
Single Null operation*). The harmonic currents in 110KV Point of Common Connection
(PCC) and 10KV intermediate line can be found in Tab. 1 and Tab. 2. They are beyond the
1
Limit Value of Standards (LVS) of China national standard and IEC standard and will cause
large voltage fluctuation and distortion obviously.
Tab. 1 Harmonic Current (A) and Voltage Distortion in 110KV PCC
LVS
No PPF
With PPF
5th
25.3
27.3
4.3
7th
17.9
27.3
2.7
11th
11.4
16.4
1.2
13th
9.8
8.2
0.7
17th
7.4
4.7
1.4
19th
6.6
2.3
0.7
23rd
5.5
1.4
0.5
25th
5.0
2.5
0.9
THDU％
1.5
3.2
0.5
Tab. 2 Harmonic Current (A) and Voltage Distortion in 10KV Line
LVS
No PPF
With PPF
5th
65.4
300
47.2
7th
49.0
350
29.4
11th
30.4
180
13.5
13th
25.8
90
7.3
17th
19.6
55
15.2
19th
17.7
45
8.0
23rd
14.7
30
5.3
25th
13.4
30
10
THDU％
3.2
19.5
3.0
Based on the progress of Custom Power Technology (CusPow) in power system
engineering. The design principal is to integrate static compensation with dynamic
compensation, and Passive Power Filter (PPF) with Active Power Filter (APF) together in
10KV intermediate line. The design features are as the followings:
(1) A proper amount of PPF will be installed first and kept without switching during the
whole time of Tokamak pulse operation.
(2) Multi-group switching capacitor banks and thyristor-controlled reactor (TCR) are
designed to realize the dynamic compensation in a wide range but with a reduced capacity
of TCR comparatively.
(3) A novel high voltage AC compound switch that is based on the ordinary vacuum switch
and thyristor valves for frequent operation in capacitive circuits is developed for group
switching capacitor. It may be named as Compound Switching Capacitor (CSC) here.
(4) A novel topology of Hybrid Active Power Filter (HAPF) is proposed to minimize the
capacity of APF.
(5) A new definition of instantaneous power theory in the integration vector plane is derived
and a novel approach to detect reactive power and harmonic current is investigated.
(6) The dynamic compensation control with high precision and fast response is applied.
The system schematic of SVC & HS may see Fig. 2.
3, Passive Power Filter (PPF)
The PPF is designed to minimize fundamental var. capacity within a limited value of
Voltage Total Harmonic Distortion (THDu), and then to distribute it to different harmonic
branches and check up the current, voltage and capacity balance respectively.
2
Fig. 2 System schematic of SVC & HS
The design parameters of PPF are given in Tab. 3. The assessment of PPF can be found in
Tab. 1 and Tab. 2 as well. The voltage fluctuation in the 10KV intermediate line is reduced
from –10.9% to (+2.4 ~ 6.7)% within LVS ( ±7%).
Tab. 3 Design parameters of PPF
R
SF(Kvar）
(Ω)
(Fundamental
Capacity)
th
5
1097
0.21
7th
865
0.19
th
11
451
0.23
13th
254
0.35
L
(mH)
C
(μF)
UN
(KV)
I
(A)
4.11
2.61
2.00
2.54
100.0
80.88
42.74
24.11
8.4
9.7
10.9
8.8
346
321
190
87
SI (Kvar)
(Installed
Capacity)
2246
2406
1585
592
4, Compound Switching Capacitor (CSC)
The switching capacitor banks are divided into three groups with 4Mvar, 6Mvar and
8Mvar capacity respectively. The step difference is just 2Mvar as the same capacity as TCR.
The design parameters of CSC are given in Tab. 4. The harmonic resonance is checked up
and the harmonic current amplifications are within the limited value of standard. The voltage
fluctuation in the 10KV intermediate line will be reduced further to ( 0.61 ~ 1.2)% because
of the operation of CSC.
Tab. 4 Design parameters of CSC
SN
C
LS
UCN
(Kvar) (μF） (mH）
(V)
4000
112
11.0
6600
6000
168
7.24
6600
8000
224
5.43
6600
SI (Kvar)
(Per phase)
1515
2273
3030
3
5, High voltage AC compound switch
The key equipment for the CSC is the high voltage switch in frequent operation. In high
voltage AC system, as well known, conventional Mechanical Switches (MS) such as Vacuum
Circuit Breaker (VCB) and Vacuum Contactor (VC) are not so good for frequent and precise
operation; and the Solid State Switches (SSS) are advanced but too expensive to use. Can we
combine MS and SSS together to take their advantages and to avoid their shortcomings? That
is the compound switch combining MS with SSS together. It can be operated just as “soft
switching” with low electric tension and long life. A prototype made by vacuum contactor
and thyristor valve with ratings of 10KV/630A and a 1.2Mvar capacitive load has been
developed and will be tested soon. The principle scheme of CSC and its simulation result are
shown in Fig. 3 and Fig. 4 respectively.
A
B
C
12
Current in thyristor
Current in mechanism breaker
8
DS1
4
DS
T1
T3
T4
T5
T6
T2
I(kA) 0
-4
-8
-12
0
Fig.3 Principle scheme of CSC
50
100
T(ms)
200
150
Fig. 4 Simulation result of CSC
6, Hybrid Active Power Filter
Active power filter (APF) is a new power electronics technology for harmonic
suppression and reactive power compensation with good controllability and fast response. A
novel topology for HAPF is proposed to reduce the capacity of APF, i.e. to reduce the
fundamental voltage and current of APF. The topology of proposed HAPF and its Equivalent
circuit are shown in Fig. 5 and Fig. 6.
～ Ea
～
～
Ec
APF
Vdc
Ish
C
Eb
Zs
Ls
n:1
IIh Z1
Vsh
£«
Y
Y
IFh
Zl
PF
£«
£-
A
VAP F
ILh
Z2
£-
D
4
B
Fig. 5 The new topology of HAPF
Fig. 6 Equivalent circuit of HAPF
7, Reactive power detection and SVC control
Studying on instantaneous power theory by integrated vector, a novel approach for
reactive power detection and control has been found. Assuming the grid voltage distortion is
negligible in a non-linear system, the grid voltage line current can be expressed as:
u a  U cos(t )

o
u b  U cos(t  120 )

o
u c  U cos(t  120 )
(1)


i

 a  I n cos( nt   n )
n 1



o
ib   I n cos[ n(t  120 )   n ]
n

1



o
ic   I n cos[ n(t  120 )   n ]
n 1

(2)
Using a sine wave with 90 degree lagging behind the phase voltage to modulate the phase
current, the result will be:
A
m i
k  a ,b, c
k k
3
 I1 sin 1  f ( )
2
(3)
where the f(ω) is the alternative component and the (3I1sin1)/2 is the direct component
which denotes the peak value of the fundamental reactive current. With a low-pass filter as
shown in Fig. 7, the fundamental reactive power current can be detected and used for SVC
and TCR control in real time easily.
A microcomputer-based system for SVC control is designed to determine the control logic
to ensure the control precision and to avoid the switching oscillation. The stabilization of line
voltage can be further improved with closed-loop control of TCR as shown in Fig. 8.
ia
reactive current
Sin(ωt)
+
ib
o
Sin(ωt-120 )
ic
+ +
+
f(α,I)
α
TCR
Grid
-
LPF
Peak value of
reactive current
voltage adjuster
Sin(ωt+120 o )
U
ΔU -
+
Uref
Fig. 7 Block diagram of reactive current detecting
5
Fig. 8 Block diagram of TCR control
8, Summary
With the power quality control in HT-7U power supply system, the grid voltage
fluctuation and total harmonic distortion are within the limitation of standards. The
compatibility between the power supply system and high voltage grid is realized. The
feasibility and availability of the design are demonstrated.
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