## 4 Proposed Cooperative Operation Method
### Capacity Selection  
Implementation of the proposed method requires adequate selections of the inductor and capacitor capacitance for two reasons, as follows. First, if the capacitor is additionally grounded, while the operation of the IMD can be secured, the time expended to measure the insulation resistance increases. The additional grounding of the capacitor on the primary side of the GPT increases the parallel capacitance component of the entire system. Hence, being integrated with the capacitance of the existing system, it acts as a larger time constant for the voltage of the IMD measurement circuit. Therefore, instead of enabling the use of the IMD, it is necessary to consider how much the increase in insulation resistance measurement time will be acceptable. Second, the reductions in the sensitivity of ground fault detection using GPT should be avoided. Ground fault detection of GPT is accomplished by detecting the tertiary side zero-phase voltage induced by the current flowing in the GPT primary side neutral line in case of a fault. If the inductor and capacitor are additionally grounded, the GPT primary-side grounding impedance changes, which affects the zero-phase current flowing through the GPT and reduces the ground-fault detection sensitivity due a change in the GPT zero-phase voltage. Therefore, it is necessary to select an inductor capacitor capacity that can minimize the decreases in ground fault detection sensitivity. Considering the two discussion points, an ideal inductor-capacitor (LC) capacitance selection method proposed in this paper is to set the LC series resonant frequency at 60 [Hz] and maintain the capacitor capacitance as small as possible. The LC series resonant frequency is suggested to be set at 60 [Hz] in consideration of the zero-phase current in the case of a 1-line ground fault in the system possessing the characteristics of a commercial frequency (60 [Hz]). If the LC series resonant frequency is set at 60 [Hz], the magnitude of the reactance for the current component of 60 [Hz] becomes 0, and only the existing CLR resistance component is involved as the primary ground impedance of the GPT. As a result, even though the LC is additionally grounded in series on the primary side of the GPT, the sensitivity of ground fault detection may be equivalent to when the primary side is directly grounded. In addition, since the reactance component increases in other frequency bands, there is an additional advantage of a reduced harmonic current flowing to the primary side of the GPT. Moreover, a decreased capacitor capacitance was suggested to minimize the increases in duration for insulation resistance measurement using IMD. However, designing the LC series resonant frequency at 60 [Hz] has limitations in device size and cost. This is because a larger passive element is required for a lower resonance frequency, and a larger inductor is required as the capacitor capacitance is reduced. Hence, realistic inductor and capacitor capacities should be selected considering increases in IMD insulation resistance measurement time and decreases in GPT detection sensitivity. In this paper, when an inductor or capacitor element of arbitrary capacity is used, the increase in IMD insulation measurement time and the change in the GPT zero-phase voltage are described, as follows. When a capacitor of arbitrary capacitance is utilized, the time constant involved in the IMD insulation resistance measurement time can be expressed as Equation (1). The additional grounding of the capacitor on the primary side of the GPT increases the parallel capacitance component of the system, increasing the time constant of the RC series/parallel circuit for IMD to measure insulation resistance. Hence, the duration for IMD to measure the insulation resistance of the system is extended. \[TimeConstant=R_{e}\times(C_{e}+C_{GPT}) \tag{1}\]  
\(R_{e}\) = Combined System Insulation Resistance  
\(C_{e}\) = Combined System Capacitance  
\(C_{GPT}\) = Additional Grounded Capacitance.