1. Introduction
Power-balance and power-quality problems are caused by the difference between power supply and demand. An energy storage system (ESS) can be used to reduce the difference between the supply and demand of electricity. Moreover, utilizing ESS, it is feasible to enhance power quality and respond to a power supply and demand crisis. The number of ESS applications in the power industry is increasing. ESS can play a role in managing peak load, stabilizing the output of renewable energy sources [
1] and regulating system frequency. Various demonstrations of ESS in connection with renewable energy sources are in progress through international efforts to reduce greenhouse-gas emissions. ESS is considered a countermeasure for supply shortage due to aging power facilities. The detailed ESS applications in the power system are described in
Section 2.1.
Currently, the ESS industry mainly focuses on batteries of a scale used for electric vehicles. However, the utilization of large-capacity ESS in the power system is also increasing [
2]. Batteries [
3,
4], flywheel energy storage [
5,
6], supercapacitors [
7,
8,
9], compressed air energy storage (CAES) [
10,
11] and superconducting magnetic energy storage (SMES) [
12] are the most popular items. In addition, various studies on hybrid types combining two or more ESSs are being conducted [
13,
14,
15,
16,
17,
18].
The installation of ESS affects the power quality of nearby systems [
19,
20,
21,
22]. Therefore, an analysis procedure that can investigate the impact before installation is required for a system’s stability. Depending on the capacity of the ESS, the effect on the power system is different, and the input of a large-capacity ESS may require a transmission-level system analysis [
4]. Therefore, an ESS analysis model has to include the proper characteristics for the simulation [
23]. The “CBEST” model is one of the ESS analysis models in commercial analysis programs. In transmission-level analysis, the “CBEST” model is widely used because its configuration is simple and easy-to-use. However, this model has limitations because it reflects only the fundamental characteristics of ESS. The characteristics of the already developed and popularly used ESS analysis model, including “CBEST”, are summarized in
Section 2.2.
In this study, an ESS analysis model was developed for transmission-level power-system analysis. The developed model is not simply a test model for research but is usable in PSS®E, a commercial power-system analysis program. In addition, it is not complicated because it has only essential parts for the simulation.
PSS®E uses the programming language FORTRAN for modeling. The modeling process and detailed configuration of the developed model was described in this study. Two test systems were used to validate the model. First, some simulations were performed to verify the developed model in the simple test system. Second, in order to verify that the developed model operates properly in a general system, a simulation was performed in the IEEE 14-bus power system, and the result was confirmed.
The rest of the paper is organized as follows: In
Section 2, this work briefly explains the background of this study, such as ESS application in a power system or ESS models in transmission-level analysis. In addition, this study describes the user modeling overview in PSS
®E. In
Section 3, the developed ESS analysis model is presented. The description of the overall model and each part are described in detail. In
Section 4, the developed ESS model is verified by simulation. Finally, the conclusion is drawn in
Section 5.
3. ESS Model Development
3.1. Model Overview
As mentioned earlier, most analysis models for power-system simulations are divided into control and generator parts. However, this study aims to construct a simple model which contains only essential functions for transmission-level analysis. For this reason, the ESS analysis model was developed as a single generator model, and the schematic form is shown in
Figure 3.
Figure 3 shows a schematic diagram of the developed ESS analysis model. In order to inject electricity into the power system in the PSS
®E program, a form of a current injection model is required. Therefore, the developed ESS model was also implemented as a current injection model. Considering the current ESS characteristics, the active/reactive power can be controlled independently, so each part was developed separately.
Figure 3 shows that the control part generates two control signals by receiving active/reactive power reference, power-factor reference, terminal voltage, and system frequency as inputs. In the control signal generation process, various processes for control are performed as needed. The generated control signal is transmitted to the current control part, and a signal for current infection is generated through appropriate conversion. The current output control that meets the device’s specifications is also applied in this part.
Figure 3 shows the schematic configuration, and detailed information, including block diagrams for each part, is described in the following sections.
3.2. Active-Power-Control Part
In
Section 3.1, it was described that there is an active-power-control part which creates a control signal by receiving a system-frequency and active-power reference. The specific form of this part is shown in
Figure 4 and its components are described in
Table 2.
In PSS®E simulation, STATE is a kind of dynamic simulation array and contains state variables. Instantaneous values of state variables are determined by differential equations. In this model, STATE(K) means a simple filter that receives a frequency, and FREQ means deviations between the base and current frequencies.
Most facilities require analysis to understand the impact before installing them in the power system. Similarly, for ESS, such power-system analysis is required before actual installation. The power-system operator may request an analysis model suitable for the vendor who wants to install the ESS into the power system based on the grid code of the system. For example, the Australian Energy Market Operator (AEMO) defines an analytical model’s requirements for power-system facilities such as ESS. In [
35], the requirements for parts such as deadbands, saturation characteristics, limits, and mathematical functions are described. This study constructed a model by appropriately inserting a deadband and a limiter part to suit these contents.
ICON means integer quantities which may be either constants or algebraic variables. Some important matters that the user has to decide when performing simulation are marked with ICON. In this part, whether the active-power-control model reads the frequency and reflects it in the control signal can be selected using ICON(M). If the ICON(M) value is 1 (frequency part is considered), a signal is generated which is combined with the reference value input by the user. If the value is 0, the signal reflecting the active-power reference value is generated regardless of the frequency. As the control range of active power is determined due to the characteristics of the facility, the limiter is necessary so that the generated signal can pass through it. Through this process, Pcmd considering essential control elements can be generated, and this signal is sent to the current control part.
3.3. Reactive-Power-Control Part
This part receives reactive power reference, RMS value of voltage, Pord (the output of active power control part), power factor reference, etc. A control signal related to reactive power is generated in this part. Details are shown in
Figure 5, and each element is shown in
Table 3.
This control part aims to generate a reactive-power-control signal, Qcmd. Firstly, Qcmd can be generated with a value reflecting the received Qref. In this case, the externally designated reactive-power value is maintained as an output. There is a method of purely outputting Qref, and a method of generating Qcmd by mixing Qref with feedback considering the voltage value of the system. For considering voltage, read the RMS voltage of the bus to which the device is connected and pass it through the filter. After calculating the difference from the reference value of 1.0 (p.u.) to derive the voltage deviation, a signal is generated through the deadband composed of an appropriate value. When the value of ICON (M + 1) is equal to 1, the generated signal is added to Qref to generate Qcmd. For the deadband section, the user specifies all four necessary values. Secondly, reactive power can be controlled by a specified power factor. The required power-factor value is in the form of receiving an input from the user, and it is composed of a form in which reactive power is calculated by applying power factor to Pcmd, an output signal of the active-power control introduced above. When the ICON (M + 2) value is equal to 0, Qcmd is generated using the signal generated from the Qref side, and when it is equal to 1, Qcmd is generated using the value calculated by the PF. The generated Qcmd passes through the limiter considering the device’s actual performance and is then sent to the current control part.
3.4. Active-Current-Control Part
The form and components of the active-current-control part are described in
Figure 6 and
Table 4. The primary function of this part is to convert the Pcmd received from the active-power-control part into Idcmd, the current injection signal. The current limiter and RAMP rate control in consideration of device performance belong to this part. A current output signal is generated using the received Pcmd and system voltage. After that, the limiter and ramp-rate control reflecting the device’s characteristics is performed, and the final Idcmd is generated after this process.
3.5. Reactive-Current-Control Part
The part that generates the reactive-current-control signal is described in
Figure 7 and
Table 5. The overall configuration is similar to the part that generates the active-current-control signal in
Section 3.4. The main function of this part is to receive the signal from the reactive-power-control part and generate the reactive-power-current injection signal, Iqcmd. The previous part dealt with frequency and active power, but this part is different in that it is composed mainly of voltage and reactive power. Additional functions, such as the current limiter, are included in this part, so simulations can be performed by adding the necessary parameters to the dyr file.
5. Conclusions
Although various ESS models have been proposed in previous studies, there are some non-critical elements for transmission-level power-system analysis. This research developed an ESS analysis model that can be used in commercial programs. The developed model is suitable for transmission-level analysis, and it is easy to use because it shows a relatively uncomplicated configuration compared to the previous models. The programming language Fortran is used to compile the analysis model code. The detailed configuration of the developed model was described using block diagrams and tables. Simulations were performed for four modes to verify the developed model in the test system. In order to verify that the developed model operates properly in a general system, a simulation was performed in the IEEE 14-bus power system, and the result was confirmed. The developed ESS model can be used for power-system analysis, and it can be used as reference material for model development. In future work, the detailed aspects of the ESS analysis model will be upgraded, and it will be used for various case studies, including stability analysis.