Dynamic Stability Analysis of Isolated Power System
Round 1
Reviewer 1 Report
Brief summary:
In the article isolated operation mode of an electrical power system is analyzed. The isolated power system consists out of conventional thermal power plants, a pumped storage power station, two synchronous condensers and one HVDC back-to-back converter that connects analyzed isolated electrical power system with a neighboring power system. The influences of a sudden and spasmodic load changes are analyzed in isolated electrical power system, according to the HVDC back-to-back converter’s settings and frequency control functionality.
Contribution of the article:
The article discuses important topic of isolated operation of an electrical power system with different power generating devices, also including a pumped storage power station, two synchronous condensers and especially HVDC back-to-back converter with possibility of frequency and voltage control. All mentioned devices with their control settings contribute to the stable operation of the isolated electrical power system. The article provides some recommendations for frequency control settings of the HVDC BtB converter and other generators, that can improve the stability of an isolated electrical power system in case of sudden disturbances. The article includes many frequency and power generation time series that describe the dynamic behavior of an isolated electrical power system under different operating conditions and frequency control settings.
Major criticisms of the article:
-The article discusses interesting and important subject of electrical power system, when it operates in isolated mode with different kinds of devices, that requires different mathematical models to properly describe their dynamic behaviors when sudden load changes are applied, and frequency deviations appear. In section 2 of the article a structure and operating condition of the researched isolated electric power system are presented and in the figure 2 the isolated power system scheme is shown, but this section is very short and many important information are missing. The article does not provide any information about the mathematical model of the generators G1 to G5, motor pump, synchronous condensers and HVDC back-to-back converter, that are used in numerical simulations. The article does not provide any parameter values (resistance, inductivity, time constants, inertia, …) of used mathematical models of different devices, only nominal active power and voltage values are provided in lines 76-85. The article discuses frequency control, but does not include any function block diagram or a scheme of primary control and voltage control of generators and HVDC back-to-back converter used in numerical simulations. The article does not provide clear information, what type of controllers (P, PI, PID, …) is used for generators G1-G5 and HVDC BtB converter. From the subsection 4.2 can be assumed that the only proportional controller is used, because only gain K is changed to find optimal control parameters, but in subsection 5.2 the turbine regulator temporary time constant is mentioned, therefore is not clear from the beginning what type of controller is used. All that mentioned missing important information has significant impact on dynamic behavior of the frequency in isolated electrical power system, but they are not provided.
-The article discuses stability of isolated electrical power systems, but not provide clear definition or criteria for stability. From lines 131-133 can be assumed that the only criterion for the stability is, that the frequency deviation is smaller than +-200 mHz. In practice this is not enough, the oscillatory behavior of the frequency needs to be damped enough, so that the amplitude of oscillations is small enough in appropriate time. For that reason, the frequency responses on figure 2 to figure 6 are too short and not provide information if the amplitude of the oscillation is decreasing or increasing and if the damping is high enough. The values of frequency deviation in the new steady state are not known (in section 3 all frequency controllers are off or only of HVDC BtB?). These missing values can be included in table 1 and 2.
-In lines 126-129 is described, that the total inertia of an isolated EPS in equal to the sum of the inertias of all the generators in the system, but the inertia of the loads is not mentioned. It is not clear if the inertia of loads is considered and what is with the self-regulation of the load, that is mentioned in the equation (5), but the values are not provided.
-In figure 7 twelve different time series of frequency are shown, but only 5 can be seen due to overlapping of some of them. It is not clear which time series are overlapping, therefore is hard to compare them.
-In lines 233-238 is discussed, that any value of gain factor K can be used, because there is no big difference in time responses in figure 8, but the reason for that is not explained. As already mentioned, the block diagram of frequency control is not provided, therefore some assumptions can be made.
-In figure 9 can be seen, that with frequency control ON, there is almost no difference in frequency responses, despite of very different settings of control parameters. No explanation is given for such behavior.
-In figure 23 the frequency response is shown for the example of a disconnected HVDC converter, but it is not clear if any other generator in the isolated electrical power system is controlling the frequency with its own frequency control? What is the reason for frequency behavior between 10-15 s, where frequency suddenly changes and start to increase faster for a short time?
Minor criticisms of the article:
-Between the figure 1 and the description (caption) of figure 1 can be seen two marks (a) and (b) that have no sense according to the figure, this is probably a mistake.
-In the line 145 the variable “PN” and in equation (3) “fN” are mentioned but is not clear what exactly represents these two variables. Nominal values?
-In lines 186-187 the reference to figure 5 is mentioned, but this reference is probably a mistake, because in these lines the (case A) is discussed, but figure 5 is for the (case B)?
-In lines 275-279 some temporary values are provided (droop and time constant) but is not clear what are standard values for these parameters.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors deal with electric power system (EPS) operation in islanded mode. The dynamic system response is studied under load changes and during transient switching due to on and off synchronous condensers as well. This reviewer has the following comments:
· The authors claimed that the developed models can be employed for stability studies of isolated systems. Please add details because it seems that the models used comes from PSS Sincal Electricity Basic software.
· Include the timescale of the analysis presented due to the main studies focused on primary and secondary control functions.
· Add more details about their contribution because the grid parameters' analysis should be used to determine critical scenarios or to indicate that additional control actions are required.
· Typically, the grid voltage are around 0.95 and 1.05 p.u. The Figure 2b shows voltages close to 1.08 p.u. Please add additional details.
· The frequency changes reported in Table 1 should be displayed in a Figure to show the demand system response (at least one change).
· Include a reference for equation (1).
· Based on Figure 12, the voltage fluctuations are smaller with a Ramp=1000 MW/s than the others values. Include physical restraints to get it.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Major criticisms of the article:
-Authors of the article provided additional information according to my comments and answered most of the questions, therefore I have no more major criticisms of the revised article.
Minor criticisms of the article:
-Figure 4 appears to be in two different pieces on two different pages, is this OK? Perhaps should be two different captions of each figure or at least (a) and (b) parts.
-Table 1 and table 2 should be reviewed, Xd’’ is named as Quadrature-axis, that is probably a mistake? Many variables in these two tables are named up-transient instead of sub-transient or subtransient?
-In line 212 the reference to Table 1 is made, that is probably a mistake, the correct reference is table 3?
-In line 293 the reference to Figure 5 is made, that is probably a mistake, the correct reference is figure 9?
Author Response
Thank you for your valuable and important notes and suggestions. The responses and comments are:
Minor criticisms of the article:
-Figure 4 appears to be in two different pieces on two different pages, is this OK? Perhaps should be two different captions of each figure or at least (a) and (b) parts.
Yes, Thank you. Fixed.
-Table 1 and table 2 should be reviewed, Xd’’ is named as Quadrature-axis, that is probably a mistake? Many variables in these two tables are named up-transient instead of sub-transient or subtransient?
Absolutely right, you are right. Thank you. Xd’’ - Direct-axis sub-transient reactance. Fixed.
-In line 219 the reference to Table 1 is made, that is probably a mistake, the correct reference is table 3?
Absolutely, you are right. Thank you. Fixed. Table 3.
-In line 293 the reference to Figure 5 is made, that is probably a mistake, the correct reference is figure 9?
Absolutely right, you are right. Thank you. Fixed. Figure 9.
