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It is expected worldwide that the increase in renewable penetration will worsen the voltage profile in contrast to the conventional synchronous generators. This is because it is not proper to authorize all independent power producers’ renewable sources to control grid voltages. Although there has been a bunch of research to improve voltage stability, each has the problem of huge computational effort, limited grid reflection, calibration, etc. Thus, the indirect voltage control by droop can be a substantial solution. Then, it is the very one of the critical issues to determine the proper droop ratio, which might be strongly non-linear due to the complexity of the power system. This paper proposes the online nonlinear P-Q droop estimation of distributed generations. It improves voltage stability, which might get worse after the connection of renewable energies or energy storage devices through inverters. First, power sensitivities between multiple P and Q are derived and used to determine the initial state of the linear P-Q droop required for the initial operation that gets data for precise estimation of the P-Q droop. Thus, it enhanced the estimation performance by reducing the required data yet without the extreme P-Q range. Then, the nonlinear P-Q droop estimation is conducted with the Kalman-filter algorithm. The performance is verified by applying it to the real distribution power system of an island in Korea. For the verification, the distribution power system is modeled at the EMT level and simulated using the power system computer-aided design and electromagnetic transient and DC (PSCAD/EMTDC $^{\mathrm {TM}}$ ). The voltage stability was improved by the proposed nonlinear P-Q droop estimation compared to the cases using fixed droop or Q of zero.

The separation of transmission and distribution systems raises a variety of questions concerning the integration of many distributed generators (DGs) into future grids. They are difficult to solve by using current energy management methods. Especially, the cooperative control between transmission system operator (TSO) and distribution system operator (DSO) has been increasingly emphasized to manage the line congestion. This paper proposes a new cooperative control of TSO-DSO based on the generation-load power sensitivity analysis. To minimize the required computational effort and data communication, the information of TSO and DSO is processed separately in the generation-load power sensitivity matrix of power system. The proposed cooperative control is implemented by three-step process, which is aggregation, specification, and local distribution. Firstly, the DSOs aggregate the flexibility area of DGs in their networks, and they inform the TSO of the feasible operation regions (FORs). Then, the TSO sends the power references at the boundary buses to many DSOs. Thereafter, the DSOs use these references as loads to calculate the detailed power references for their generators. As the result, the net power of DSOs satisfies the power references requested by the TSO. The case studies are carried out to verify the effectiveness of proposed control. The results show that when the load is increased by 20%, the overall average of line loading is decreased by 4.34% with the proposed control. Also, when the generator of 634 MW is disconnected, the frequency nadir is increased by 0.1 Hz compared with the $P$ - $f$ droop control.

The parallel inverter-based scaled-up slack bus includes multiple battery energy storage systems (BESSs) to support the slack operation without short of real power. The multiple BESSs require a well-designed state of the charge (SOC) balancing algorithm for maximally effective usage. Until now, however, the SOC balancing algorithms only focus on strong SOC balancing without careful consideration of BESS efficiency during its charging and discharging. This paper proposes a soft SOC balancing algorithm based on the Levenberg-Marquardt (L-M) algorithm considering BESS’s charging and discharging efficiency. The charges or discharges of the BESSs are primarily determined by their efficiencies that indirectly reflect the SOCs. As a result, the proposed method provides some margins to improve efficiency rather than tightly focusing on only the SOCs balancing between multiple BESSs (e.g., two BESSs with 0.7 and 0.4 pu of SOCs discharge rather than only one BESS with large SOC discharges when total output is 0.86 pus). The optimizations and balancing are conducted by MATLAB software. Their related converter interactions are simulated using a power system computer-aided design/electromagnetic transient including DC (PSCAD/EMTDC $^{\mathrm {TM}}$ ), to measure data for optimization.

The high penetration of converter-based distributed generations (DGs) to power system can give rise to the lack of rotational inertia while replacing the conventional synchronous generators (SGs), which provide the primary frequency reserve (PFR) in power systems. As the result, the decrease in PFR aggravates the frequency stability. To overcome this problem, the droop coefficients of governors in the remaining conventional SGs must be re-determined newly and properly. This paper proposes a new solution based on the grey wolf optimization (GWO) method to optimally select the droop coefficients of SG governors in the low-inertia large-scale power system due to the high penetration of renewables. The proposed solution is very effective for reducing the computational effort significantly, and it is able to recover not only the steady-state but also the transient frequency stability. To verify the effectiveness of proposed optimization solution based on the GWO method, several case studies are carried out on the practical Korea electric power system with the penetration of wind power plants of 4 GW.

The recent railway system is a huge microgrid assembling multiplex structure with distributed active loads, sources and storage devices. The active load represents the train. The sources are a microgrid based on renewable energies. The big problem of the most of electrical train is that they can’t recover energy recovery during regenerative breaking phases. another problem of the electrical train is the long charging time from the stations. This paper suggests a techno-economic process for the energy storage by using SCs in the train, with the aim to reduce the energy consumptions. The proposed design of railway station use PV and wind sources, and batteries for energy storage system (ESS). For the train, SCs are implemented to the ESS where they are alimented breaking phases and from stations by a pantograph installed in an air power line in each stop. SCs are distinguished by high characteristics power and a wide number of charge/discharge cycles, they provide low particular energy and a fast charging time. An energy management approach is suggested to control the DC bus by voltage and the buck-boost converter by current. The Sizing of PI controller used for the stabilization of the DC bus of train and station is given. The whole system is modeled in MATLAB-Simulink. Simulations for the train and station show the suitability of the suggested powertrain and control strategy.

It is expected that the penetration of renewable-energy-based generators will rapidly increase worldwide. These factors also lead to an increase in the portion of inverters in the power system. This paper proposes a method to scale up the slack generator size based on parallel inverters. All of the multiple parallel inverters except one share the measured power from the slack bus to the grid by a predetermined ratio based on their sizes. A single inverter that does not participate in power sharing is allocated to be in charge of clearing the small power gap caused by the control delay and the measurement error (i.e., the physical slack). As a result, the net size of the scaled-up slack generator is $n$ times greater than that of the physical slack when $n$ non-slack inverters are added to the physical slack inverter. The proposed method is validated based on a real island power system model in Korea using a power system computer-aided design/electromagnetic transient including DC (PSCAD/EMTDC™).

Recently, renewable energy has received considerable attention worldwide due to environmental problems such as climate change. However, there are still many problems to be solved to increase the penetration level of renewable energy sources in a power system. Above all, a power system with a high renewable penetration level requires an entirely novel operational strategy and control approach to achieve reliable operation by considering the intermittency of variable renewable energy sources. This paper proposes power-sensitivities-based indirect voltage control of renewable energy generators with power constraints. The proposed method realizes seamless control of the voltage by compensating with the reactive power if the real power of the distributed generator (DG) is insufficient due to its intermittency. To achieve the proposed method, the real and reactive power references of DGs are initially operated based on the power sensitivities between generation and loads if all the DGs operate within the power constraints. However, if at least one of the DGs reaches its constraint limit, the references can be modified using other sensitivities between real and reactive power generation enabling indirect voltage control of DGs. Therefore, a high level of renewable penetration in power systems can be accomplished with the proposed method. The proposed method is verified with several case studies that are based on a microgrid with practical data from a real island power system. Verification is carried out using the power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDC™) simulation software.

The implementation of distributed generation (DG) becomes more common while causing a challenge to maintain the voltage stability in a power system. The optimal placement of multiple DGs in a planning stage can effectively solve this problem. In many cases, the best placement has been determined by the small-signal study like the modal analysis. However, it is acceptable only if the linearization of large-scale nonlinear power system is accurate. To overcome this, this paper proposes the new optimal placement algorithm for multiple DGs based on the model-free Lyapunov exponent estimation to maintain the voltage of system stable. In other words, the individual Lyapunov exponents of all buses are firstly estimated to determine the candidates for optimal placement of DGs. Then, the maximum Lyapunov exponent for each candidate is calculated to decide which bus is the best place to improve the voltage stability of system. Several time-domain simulation studies are carried out to verify the effectiveness of proposed algorithm. In particular, its performance is compared with that of the conventional method. The results show that the optimal placement of multiple DGs determined by the proposed algorithm improves the voltage stability of system much more effectively.