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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( nt 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 (3I1sin1)/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. 6