1 Active Power Tracking Control Strategy to Suppress 2 DC-Link Voltage Rising with Enhanced Fault Ride-3 Through Capability using Superconducting Fault 4 Current Limiter 5

Building a new power plant to address the growing demand for power due to population 11 concentration in the metropolitan area is one of the world's major concerns. However, since a large 12 power plant can not be located around the city due to burden of economic cost, building power 13 plant outside metropolitan and cities is necessary. Therefore, new power generation facilities are 14 promoting policies to provide distributed generator(DG) with a small capacity relatively near the 15 metropolitan. When the DG (photovoltaic, wind farm, etc.) is connected with the grid using medium 16 voltage direct current (MVDC) system, voltage sourced converter(VSC) should supply reactive 17 power to the grid, because of fault ride through(FRT) operation in grid fault. If the voltage drop is 18 severe, the converter should be disconnected from the grid immediately without supplying the 19 reactive power, resulting in a considerable economic loss. In general, superconducting fault current 20 limiter(SFCL) is introduced as a measure to enhance FRT capability. In this paper, we use trigger 21 type SFCL which protects superconducting element and reduces low voltage. On the other hand, 22 the active power unbalance of the DC-link and the DC voltage rise due to the reactive power supply 23 of the grid-side converter. The rise of the DC voltage causes the P (active power), Q (reactive power) 24 control of the converter to deviate, causing malfunction and damage of the DC equipment. 25 Therefore, the rise of the DC voltage must be prevented. In this paper, we consider the suppression 26 the DC voltage rising caused by the FRT operation through the active power tracking control 27 (APTC). 28


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On the AC grid-side, using the trigger type superconducting fault current limiter (SFCL), we 70 identify that the fault current in the feeder is reduced and the voltage drop of Bus1 and Bus2 is 71 suppressed [8][9][10]. The SFCL used in the grid is usually resistive type SFCL. This is because the 72 resistive type is more economical than other SFCLs, and has a high limiting impedance and a large 73 capacity. However, resistive SFCLs are vulnerable to large fault current because the burden on 74 superconducting elements is directly applied. The trigger type SFCL limits the fault current to the 75 CLR by commutating the current to the parallel circuit of the superconducting element when the 76 superconducting element is quenched and overloaded. Consequently, the trigger type SFCL 77 suppresses the AC voltage drop through the CLR resistor, making it easy to change the impedance 78 and thereby effectively enhance the FRT capacity [8][9].

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In order to construct a system in which DG is connected to a grid by DC-link, DG and VSC are 82 primarily modeled. The VSC has different functions depending on the purpose of use. As shown in 83 Figure 2, VSC1 controls the DC voltage and reactive power (Q), and VSC2 controls the active power 84 (P) and Q. Since DG is not a model for control purposes, it is conceived to have a synchronous 85 generator with a constant P output. The over-current relay (OCR), circuit breaker, and SFCL are 86 modeled based on the system constructed as shown in Figure 2. Finally, a control block diagram for 87 controlling P of VSC2 in condition during fault is modeled [10].

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Unlike CSC, which has been used in the past, each VSC stage in a DC serves as a constant voltage 91 source. In other words, the DC voltage is kept constant even if the power direction changes or the 92 size changes. The VSC model used in this paper can be configured as a CSC because of the structure 93 in which two converters are connected in series. However, since the global trend is aimed at a multi 94 terminal where multiple converters are connected to one another, there is a problem in operating it.

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In addition, since CSC uses SCR instead of IGBT or GTO, it has an inherent problem that independent 96 control of reactive power and active power can not be done. These VSCs can selectively control DC 97 voltage, active power, reactive power, and AC-side voltage, as mentioned briefly above. "Selectively" 98 means that the DC voltage and the active power are physically related It is. That is, since the active 99 power is used to control the DC voltage, the two things can not be simultaneously controlled to a 100 desired reference value. The reactive power and the AC voltage also have the principle of supplying 101 reactive power to control the AC voltage, so that independent control is impossible.
Since the DC voltage and the active power control can not be simultaneously performed in one When making the d-q axis current signal, it passes through the reference signal generator as 119 shown in Figure 3 and its main input signal is active power and reactive power. The relationship 120 between P, Q and each d-q axis current is expressed in the following equations (3) and (4). In 121 particular, equation (4) shows the relationship between q-axis current and reactive power.

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Equations (3), (4) can be expressed in terms of the d-q axis for P and Q, as in equation (5) and (6).

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Here, some of the terms can be eliminated because the q-axis voltage is synchronized to zero.

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As mentioned in Section 2.0, P and Q values are dependent variables for controlling DC voltage 129 and AC voltage, respectively. Therefore, transfer function for DC voltage (s) and active power 130 _ ( ) should be obtained.

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Where subscripts 0 represents steady-state value of the

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Trigger type SFCL can take the voltage signal from the superconducting resistor through the PT 144 and operate the SW, so that the SCE can be protected and the limiting impedance can be set high. The

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topology of the trigger type SFCL is shown in Figure 5. 153 154 Mathematical modeling of this superconducting resistance for the last century was one of the 156 greatest interests for researchers and several prototypes have been developed and researched [11}.

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One of the studies is to model the superconducting phenomenon mathematically. Implementing this 158 superconducting phenomena is essential for modeling the superconducting resistance. However,

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accurate modeling is very difficult because the quenching phenomena is affected by whole the 160 magnitude field, current through it and the temperature [12]. Supposing that it is only excited by the current, the superconducting resistance can be expressed as the following equation (9) where is 162 the normal state resistance [11].  (11). The is the presetting 167 value that the OCR user desires the trip performance to start [13].

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The positive current at the feeder's OCR ( ) increases to 3.5 [kA] over , which causes the 242 OCR to proceed. Figure 9 shows the OCR operation with and without a trigger type SFCL under the

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VSC has a limited amount of reactive power and active power that can be converted. This is why 296 the amount of active power decreases with reactive power supply during FRT operation. When 297 reactive power is supplied, if the amount of active power is not reduced, the VSC may be overloaded 298 and the VSC may be damaged. Figure 12 shows the P-Q curve when SFCL and APTC are applied or 299 not. If APTC is not applied, it can be seen that the active power decreases and then increases again 300 before the reactive power reaches 15 [MVar]. However, when APTC is applied, it can be seen that the

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The application of the APTC the operation of the converter, making it economically more 337 efficient than other methods and allowing for faster control than controlling the multiplying factor.

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In addition to the operation of the Trigger type SFCL, the capacity of the FRT greatly increased. On 339 the other hand, the operation of the trigger type SFCL delayed the trip time of the overcurrent relay