## 4 Cyber Physical System Threat Modeling
### Mitigation Control Method  
One of the many crucial factors affecting balancing arrangement control in terms of cyber security operation is the variation of HVDC interconnection capacity, as was stated in the introductory section. Additionally, we explored various HVDC interconnection types, which also constitute another crucial aspect of MIDC power system operation. Moreover, the various HVDC connectivity types could be used to perform cyber security detection of balancing cooperation concepts according to the droop value of E-FCR as an active power control management strategy. Nevertheless, a couple of references in the introductory section neglect to consider active power management regulation for cyber security detection and mitigation, notably in MIDC power systems. As illustrated in Figure 11[9; 31], the activation of active power control management through E-FCR control could be monitored through a DC sensor and due to the different specifications of voltage scheduling and rating capacity for each HVDC. The authors proposed a measurement based on per-unit conversion which could hopefully increase cyber attack detection in the MIDC system. Per unit, conversion could easily be achieved by using the calculation of Idc rate(Icc/vsc) as described in Equation (4), as follows:  
\[\text{Idc rate}_{\text{(Icc/vsc)}}\left(\text{p.u.}\right)=\frac{\text{Prating }_{\text{(Icc/vsc)}}}{\text{Vsched}_{\text{(Icc/vsc)}}} \tag{4}\]  
where \(\text{Vsched}_{\text{(Icc/vsc)}}\) is the fixed voltage value of the HVDC system, i.e., 254 kV and 200 kV for the second LCC and VSC, respectively. The modulation power through E-FCR control could be calculated as follows:  
\[\Delta\text{f}_{\text{b}}=\text{ f}-\text{f}_{0} \tag{5}\]  
\[\Delta\text{P}_{\text{E-FCR}}=\text{Kp}_{\text{(Icc/vsc)}}\times\Delta\text{ f}_{\text{b}} \tag{6}\]  
where \(\Delta\text{f}_{\text{b}}\) is the deviation between normal frequency along with the frequency when the contingency occurs. In addition, the author proposed a DC measurement that only reflected E-FCR activation, which could eliminate the initial power error calculation for each HVDC line. The calculation for DC current measurement based on per-unit conversion is described as follows:  
\[\text{Idc FCR Measurement}_{\text{(Icc/vsc)}}=\frac{\Delta\text{P}_{\text{E-FCR}}}{ \text{Vsched}_{\text{(Icc/vsc)}}} \tag{7}\]  
Figure 10: False data injection attack during E-FCR operation control. where \(\Delta\)P\({}_{\text{E-FCR}}\) is the addition of power modulation support through the multiplication gain value and frequency deviation. \[\text{Idc}\;\text{FCR}\;\text{Measurement}_{(\text{(lcc/vsc)}}\;(\text{p.u.})= \frac{\text{Idc}\;\text{FCR}\;\text{Measurement}_{(\text{lcc/vsc})}}{\text{Idc}\; \text{rate}_{(\text{lcc/vsc})}\;(\text{p.u.})} \tag{8}\]  
where Idc FCR Measurement(lcc/vsc) is the real current value of additional power modulation and Idc rate(lcc/vsc) (p.u.) is the per-unit conversion value of the rating current. The ramp-rate deviation between LCC and VSC was used to create an adaptive droop control when mitigation operation was applied. The formulation is defined as follows:  
\[\Delta\text{Pr}_{\text{rr}}=\left|\text{Idc}\;\text{FCR}_{\text{lcc}}-\text{ Idc}\;\text{FCR}_{\text{vsc}}\right|\;(\text{p.u.}) \tag{9}\]  
where Idc FCR\({}_{\text{lcc}}\) (p.u.) is the per-unit conversion current value of additional power modulation through the LCC line and Idc FCR\({}_{\text{vsc}}\) (p.u.) is the per-unit conversion current value of additional power modulation through the VSC line. As illustrated in Figure 12, the mitigation of low-cost and blind data injection attacks could be an approach using DC current prediction through a normalized correlation concept since E-FCR gain has different values between LCC and VSC types. The normalized correlation concept represents the comparable data between two actual DC current measurements, which has the goal of determining how similar the signals (DC currents) are.