Abstract
In recent years, the expansion of renewable energy in electric power systems has been increasing at such a rapid pace that it has started affecting frequency stability. Renewable generators connected to the grid produce variable amounts of power, and in most cases have no inherent inertia response (IR) to the system frequency. Therefore, the high penetration of renewable generators in the system results in low inertia and frequency distortion. If renewable generators account for a high proportion of the supply in a power system, the use of energy storage systems (ESSs) with frequency-support algorithms (in the place of synchronous generators) can stabilize the network. The participation of ESSs in frequency support must be organized precisely, so that they are fully devoted to their own purpose. In this paper, the frequency-support parameters of ESSs are calculated for achieving stable frequency response from a network. An estimation and calibration process is conducted during the active power-order change of the ESSs in the substation, and is verified through electromagnetic-transients-including-DC (EMTDC)-based simulations.
IN recent years, the installed capacities of wind and photovoltaic (PV) generation have been increasing globally because of environmental and economic concerns. Europe, and a portion of North America, have already reached a significant level of renewable energy penetration. Other developing countries have also shown progress in terms of deploying renewable generators in their power system sectors [
Owing to the variability of renewable generation output, the planning and balancing of a power system by transmission system operators (TSOs) have become more complex. For system planning, operators must examine short- and long-term adequacy studies, considering various weather conditions. This means that some variability of wind and PV power production should be considered in the studies, with different time scales. Long-term variations may last from 4 to 12 h, and these variations can be balanced using the forecast information [
The increasing use of renewable generators in power systems results in the displacement of conventional synchronous generators. The displacement of synchronous generators reduces frequency stability of the network, as most of the renewable generators do not have inherent inertia or a primary frequency response [
Curtailing renewable generation output is still the most reliable and commonly used reliability countermeasure applied in power systems. The advanced features of this option have been utilized in various countries with high renewable participation levels. In the Nordic grid code, wind turbines provide the primary reserve by adopting power production limits and ramp rate limits for their production [
The synthetic inertia response (IR) from renewable generators is another frequency-support strategy used in networks with high wind power participation [
For the maximum utilization of renewable sources, additional reserves must be prepared using energy storage systems (ESSs) such as pumped hydro system and battery energy storage system (BESS) [
When installing renewable generators into a grid, cost is an important factor that determines the sizes of the facilities. The enlargement of wind and PV farm sizes is a recent trend observed in renewable industries. When interconnecting large offshore wind turbines to the main grid, a significant base cost is incurred for facilities such as connection cables and offshore and onshore substations. Unlike small-scale distributed renewable generators, large wind or PV farms are obligated to apply strict grid codes, considering their influence on the power system. In the cases of interconnection, a point of common coupling (PCC) of a wind or PV farm is a point where the assessments of influence by generation of those sources on the system are conducted. For this reason, ESSs are often installed at PCCs to smooth out variations in output power and time shifting, and to either store surplus wind energy during low-demand periods or discharge energy during high-demand periods. In the Korean power system, ESS-mounted modular substations have been developed for the integration of renewable generators. These ESS substations will be implemented at places where the installation of renewable generators is expected to increase significantly.
For now, most ESS installation is oriented toward the time shifting of production rather than toward supporting the integration of renewable generators and grids [
The frequency of a power system is directly related to the balance between supply and demand of active power in the network. The lack of supply or demand leads to a frequency change as a result of the imbalance between the mechanical and electrical power of the generators. Load demand varies with time, and it has served as a major driving factor of frequency deviation in previous power systems. By contrast, in a modern power system, renewable generators are primarily considered in the dispatch process of other generators to balance the network.
The relationship between active power change and frequency deviation can be explained by the inertia and governor droop constant (R) of the synchronous generators in the system. In a contingency situation such as a large generator trip, load shedding, or a transmission line trip, the frequency changes rapidly until the imbalance between the supply and demand is cancelled. Synchronous generators in the grid release kinetic energy from their rotating masses until they achieve a new synchronous speed. The relationship between rates of frequency change and generator inertia is defined [
(1) |
where Ssys is the capacity of the online generators; ∆P is the power imbalance or active power output change from the generators; is the initial frequency following the input-output imbalance in active power; and Hsys is the equivalent inertia constant of the power system. It should be noted that nonsynchronous generators such as wind turbine, PV, and ESS are not included in the calculation of Ssys . To make the wind turbines and PV contribute to the IR, an additional control system is required to override the control order of active power that follows the operation point with maximum efficiency. Hsys is defined as:
(2) |
where N is the number of generators; Si is the rated capacity of the individual generators; and Hi is the inertia constant of each generator. By substituting (2) into (1), the rate of change of frequency (ROCOF) can be defined as:
(3) |
After the contingency situation is resolved and the system reaches a new steady state, several generator outputs change, and the individual generator output is determined by the governor droop constant. Unlike the IR, governor response is proportional to the deviation of frequency value. The relationship between the frequency and generator output is defined as:
(4) |
where fnom represents the nominal frequency of the system and SGEN represents the total capacity of generators in the system. In a power system with multiple generators, the equivalent sum of each governor droop constant should be considered. The frequency and governor response in a multi-generator network is generally defined as:
(5) |
where ΔfSS is the deviation of the steady-state frequency when a mismatch of ΔP is induced in the network; and is the droop constant of generator i.
The characteristics of frequency response can be explained using (3) and (5), and the general behavior of the grid frequency response is depicted in

Fig. 1 Characteristics of frequency response.
The frequency drop after a power loss P
loss is arrested when the sum of IR and governor response matches P
loss. If the ramp up of the generator output is not sufficiently fast, or if the generator reserves are depleted, the system state would drive the frequency down to the load-shedding point. In this respect, the ROCOF is an important parameter for assessing the characteristics of frequency response of the grid. In normal operation, a small change in the load demand or renewable generation output can be mitigated if enough grid inertia and system capacity exist. As the increasing participation of renewable generation displaces the synchronous generators, the inertia and system capacity will decrease to dangerous levels [
As shown in
The integration of renewable generators into a grid requires a PCC station, which usually includes a step-up transformer and measuring devices. The main purpose of ESS installation at this point is to conduct smoothing and time shifting of the renewable generation output.

Fig. 2 Operation strategy of ESS for wind farm interconnection.
The measured frequency needs to be filtered in order to calculate ROCOF, as the direct differentiation of measured frequency can be noisy. In this paper, the discrete least-squares approximation of frequency is used, rather than direct filtering. Direct filtering may contain oscillatory components produced by synchronous generators [
(6) |
where n is the number of sampling data; and xi and yi are the time and frequency values, respectively. Using (6), a given set of discrete frequency points can be approximated using an algebraic polynomial equation, as follows:
(7) |
where a 0 is the measured frequency at the moment when frequency is sampled, and a 1 becomes ROCOF at that moment. By rearranging (3), we obtain
(8) |
In (8), all terms on the right-hand side are known values, as the active power order for the ESS substation is planned in advance based on the renewable forecasting information. Thus, the current value can be determined as the sum of all products of Hi and Si by measuring the ROCOF. Based on (8), the sums of all products of each Hi and Si are directly related to the ROCOF of the system when the balance between the input and output of active power is lost.
Thus, assuming that the ROCOF of a stable power system does not exceed 0.5 Hz/s, the minimum requirement of the total products of Hi and Si can be calculated using (8). In this case, ΔP represents the capacity of the largest generator in the given system, and can be used to consider the largest imbalance of active power that could be lost in the network.
The ESS can emulate the IR by implementing active power control, the value of which is proportional to the ROCOF, as depicted in

Fig. 3 Frequency-support operation of ESS.
In addition, the ESS can also have an active power-control loop, which can be used to generate an output proportional to the change in frequency. This is identical to the governor response of synchronous generators. Using (5) and (8), the optimal droop constant of ESS (RESS
) and gain of IR values (KIR
) for the ESS can be calculated, as shown in
(9) |
In [
(10) |
where m is the number of ESS substations in the network; is the maximum allowable value of ROCOF; and ΔP max is the maximum value of generation loss that can be tripped from the network. It should be noted that SESS,i is the margin of ESS capacity, not the rated capacity of the ESS itself.
The primary frequency response support of ESS needs to be assessed along with the IR, to satisfy the steady-state operation constraint of the given network. The most important security constraint related to frequency is the under-frequency load shedding (UFLS). Generally, ESSs have a variable droop constant for frequency support, and it is considered in the target network, as shown in
(11) |
where is the minimum frequency of system requirement.
The primary frequency response support of the ESS is a temporary measure for contingency operation. Therefore, it is expected that the secondary or tertiary frequency response must be supported within some specified time. In general, the rebalancing of the network by automatic generation control or by the TSO takes at least 5 to 15 min [
(12) |
where SESS , MWh is the MWh capacity of the ESS; is the duration of the droop operation of ESS; and SSOC is the SOC of the ESS.
The operation of the ESS can be optimized using (10), (11) and (12). As mentioned above (in
To minimize the effect of the proposed algorithm on the normal operation of the ESS, the operation point values of the PCS of ESS is considered for the calculation. The operation margin of the PCS capacity determines the variable Si in (8) in the calculation of HESS . Because the capacity margin of the PCS in the ESS is altered continually, depending on the operation conditions, HESS is continuously assessed and modified.
Similarly, the primary frequency-support parameters are set in order to satisfy the grid operation conditions. In every iteration, the current ESS operation value is assessed to determine whether the margin of ESS capacity is sufficient to supply the required primary frequency support. If the required ESS capacity is not sufficient, the TSO can curtail or limit the operation bandwidth of the ESS.
The simulation of frequency stability assessment was conducted based on an actual power system.

Fig. 4 Configuration of studied case.
Load demand is 708 MW.
As the first step of simulation, it was assumed that the synchronous generators matched the supply and demand in the system, while four large wind farms supplied active power without curtailing their outputs. Each ESS substation smoothed out the volatility of production from the interconnected wind generators. During operation, the active power reference of the ESS substation was changed because of a decrease in the wind farm production in the network, as shown in

Fig. 5 Frequency response due to change of active power reference of ESS. (a) Output power of wind farm smoothed by ESS. (b) Grid frequency. (c) ROCOF.
Table II shows the detailed simulation case parameters prepared in order to verify the proposed algorithm. For each scenario, the number of generators was planned to vary from six to four. The scenarios were designed to displace synchronous generators continuously as the renewable participation in the network increased. The load demand remained constant while WGT #1 and WGT #2 disengaged gradually.
When a small step change was induced in the active power reference in the power system by the ESS, the change in frequency was measured to calculate the ROCOF and system stability parameters depicted in Table III. As the number of generators decreased, the sum of products of Hi and Si declined eventually.
Simultaneously, information about the capacity margin of each ESS was also transferred to the grid operator. Using the parameters listed in Table III, the optimized frequency response parameters KIR and RESS could be calculated as shown in Table IV and Table V, respectively. Prior to setting the parameters according to the optimized values, the maximum frequency response values had to be compared with the ESS capacity margins.
To confirm the parameters shown in Tables IV and V, a contingency scenario that tripped the largest generator in the grid was considered. According to the curves shown in

Fig. 6 Frequency and ROCOF after STP #3 tripping in different conditions. (a) With six generators. (b) With five generators. (c) With four generators.
As mentioned previously, the parameters listed in Tables IV and V were calculated based on two criteria for frequency stability: the steady-state frequency of the system after the contingency scenario, and the maximum value of ROCOF during the event. In the case of each simulation, the steady-state frequency remained above 59.8 Hz, which was a deviation of 0.2 Hz from the nominal frequency of the system. During the entire simulation, the absolute value of ROCOF appeared to be less than 0.5 Hz/s, owing to the modification of parameters.
The required ESS capacity was minimized to promote the maximum utilization of ESS within its original purpose such as load shifting or smooth-out operation. It should be noted that when adopting a calculated parameter for the ESS frequency support feature, the maximum IR and droop response from the ESS must not exceed the ESS operation margin. If both ESSs in the system are in the discharging mode, the operation margin of the ESS can be expected to decrease under the maximum IR and droop response quantities. In this case, the grid operator should implement an ESS limit, or additional generators should be dispatched in the system to maintain the stability of the network.
This paper presented an ESS frequency stability assessment algorithm and operation strategy for a power system with high penetration of renewable generators. The contributions of the proposed method can be listed as follows:
1) An assessment of the system capability in the event of an operation parameter change in the ESS was conducted. The proposed method was found to be effective against large wind farm interconnection in grid with high penetration of renewable generators, as the change in the output caused by wind-source variability was small enough to be compensated for by the small ESS capacity in 15 to 30 minutes [
2) A method was proposed to determine the minimum requirement of the ESS operation point in any given situation. As shown in Table II, the operation limit of the ESS margin was calculated and compared with the actual margin value.
The assessment of system reliability is important in systems with high renewable penetration, because the participation of each generator varies as the renewable output changes. Using the proposed method, the TSO can assess the IR and primary frequency response reserve in a relatively short time. In the studied cases, the penetration levels of renewable generators varied from 60% to 74%. For each penetration level, the optimal ESS parameters were assessed and studied under the assumption of a worst-case scenario.
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