# Dynamic Stability Analysis of Isolated Power System
## 2 Structure and Operating Conditions of the Researched Isolated EPS  
The analyzed isolated electric power system consists of 330 kV open switchgear busbars, lines, and a switching apparatus, and autotransformers, power transformers, and 110 kV open switchgear busbars. The HVDC BtB converter is connected to the 330 kV grid and the 330 kV line. The operating devices of the isolated EPS are:  
* Generator G1 = 200 MW, operates through 330 kV switchgear;
* Synchronous condenser SC1 mode (\(-\)10 MW), operates through 330 kV switchgear;
* Synchronous condenser SC2 mode (\(-\)10 MW), operates through 330 kV switchgear;
* Motor P--pump mode (\(-\)220 MW), operates through 330 kV switchgear;
* Generator G2 = 200 MW, operates through 330 kV switchgear;
* Generator G3 = 180 MW, operates through 330 kV switchgear;
* Generator G4 = 120 MW, operates through 110 kV switchgear;
* Generator G5 = 100 MW, operates through 110 kV switchgear;
* HVDC BtB converter (\(-\)225 MW) exports from the islanded EPS to the neighboring EPS and operates through 330 kV switchgear. The scheme of the isolated power system is presented in Figure 1.  
During simulation, it is assumed that the operating conditions of the EPS are:  
1. Active power generation capability of the isolated system is 800 MW;
2. One unit (case A) and two units (case B) are operating in SC (synchronous condenser) mode;
3. HVDC BtB converter operates in (\(-\)225 MW) export mode from the isolated EPS to the neighboring EPS. The structure of the model design of the frequency and damping controller is shown in Figure 2.  
A two-terminal DC model with a frequency controller model was added in the _dyr_ file to model the dynamic aspects of the HVDC. A FRQDMP model summarizes the power modulation from the frequency controller and then sends the modulation signal to a standard classic model CDCAB3 to implement the power. An HVDC transmission system can be used to support the frequency of a disturbed power system (due to loss of generation or large loads or sections of the system) by adjusting the power taken from the other (healthy or supporting) system connected to the HVDC transmission system. Thus, one system that supports the other will have its frequency  
Figure 1: Isolated power system scheme. Figure 2: Structure of the model design of the frequency and damping controller. changed. A large system can easily support a small system, while in the case of similar sizes of system capacity, this influence needs to be considered. The frequency controller is designed in such a way that its preference is to retain frequency limits in the healthy (primarily undisturbed) system. The frequency controller principle is thus to assist with balancing the power loss/change between the systems and still reduce the impact on the healthy system in order to avoid deviations beyond its frequency limits. When the frequency of the faulty (primarily disturbed) system exceeds any of the dead-band limits, it will activate a power order change (\(\Delta P_{DC}\)) to the converters to reduce the frequency deviation. The HVDC link will adjust the power to control the frequency in the faulty AC grid. As a result, the frequency of the healthy system will also be changed. If its frequency also exceeds any of the dead-band limits, it will activate a power order to reduce (counteract) the initial change (\(\Delta P_{DC}\)) to the converters in order to reduce its frequency deviation. The frequency controller uses information about the frequency in both grids. Consequently, the operation of the frequency controller requires communication between the rectifier and the inverter of the HVDC link. This frequency controller is designed for certain worst-case conditions, where EPSs are islanded from the rest of the network. It gives a delta power contribution to the Active Power Controller (APC) in proportion to the frequency deviation, as shown in Figure 3. The controller has a transfer function with a low pass characteristic with a dead-band on the input signal and a limiting function on the output signal.