# Dynamic Stability Analysis of Isolated Power System
## 2 Structure and Operating Conditions of the Researched Isolated EPS
The frequency of each region is measured and compared with the reference frequency of the respective region to obtain the frequency error. \(f_{n1}\) and \(f_{n2}\) above refer to the frequency of networks 1 (rectifier system) and 2 (inverter system), respectively. For a reverse power operation, the polarities should change. The resulting signals are then passed through a dead-band of 0.01 Hz and then an amplifier with gain set to 2500 MW per Hz corresponding to 0.4% droop characteristics. It is only possible to activate frequency control from one AC network at a time. If a frequency controller is activated in the isolated EPS region for a power flow direction from the isolated EPS, an increase in frequency in the isolated EPS region should be counteracted by an increase in power flow and vice versa. The incremental (or decremental) change in power order is separately generated as per the operating frequency of each region and is applied to the pole controller, where they are summed together, along with the output of the power oscillation damping controller and the base power order. The following settings for the frequency controller have been defined in the study: \(db=\pm 0.01\) Hz; \(t_{1}=2\) s; gain \(K=2500\) MW/Hz. The simulation time interval to be used is 5 ms, which is a suitable time interval when using the HVDC model. The need for the frequency controller is found to be negligible,  
Figure 3: Structure of a frequency controller. even when the normal load flows are subjected to the worst-case disturbances, such as large generation outages or sudden load losses in the system. However, in certain worst cases (such as where EPSs are islanded from the rest of the network), the criterion for designing the frequency controller is to utilize the full capacity of the HVDC BtB link to address the 0.2 Hz of frequency deviation that occurs in the isolated EPS AC network due to the load and generation unbalance created in the network. G1, SC, P model: VGDS-1025/245/40 UHL represents generator GI, which operates in Hydro-generator mode, Synchronous condenser mode, Motor pump mode, and does support the FFR function, but only when the hydro-generator has temporary droop settings and time constants. These settings are manually set. A mathematical model of generator G1's structure is shown in Figure 4.  
In this model, the evaluation of rotor magnetic saturation occurs in the direct-axis \(d\) and in the quadrature-axis \(q\). Saturation is described by a quadratic function. The model assumes that the magnetization acts on all of the generator's existing mutual and leakage inductances. In modeling, the saturation value is considered to depend on both the rotor (excitation) and the stator currents [14]. Therefore, the effective values of the generator parameters in the model at each moment are converted from:  
1. Unsaturated values of all of the generator's reactive resistances;
2. Instantaneous values of the coupled magnetic flux. The G1 parameters are shown in Table 1.  
Figure 4: Structure of the electromagnetic model of a bright pole generator. The G2 model uses a combined cycle gas turbine and turbogenerator and does support the FFR function, but it works on limited value. The G3 model is a TVV-220-2 turbogenerator and does support the FFR function. The G4 model is a TVF-150-2 UZ turbogenerator and does not support the FFR function. The G5 model is a TVF-120-2 turbogenerator that does support the FFR function. A mathematical model of the structure of generators G2-G5 is shown in Figure 5.  
This model assumes that saturation affects both the direct-axis and quadrature-axis inductive resistances, and that the mutual inductances vary with the bounded magnetic flux generated beyond the transient inductive resistance [14]. The parameters for G2-G5 are shown in Figure 2.0