Robust Frequency and Voltage Stability Control Strategy - Standalone AC/DC Hybrid Microgrid - Engineering Assignment Help

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Abstract: 
The microgrid (MG) concept is attracting considerable attention as a solution to energy deficiencies, especially in remote areas, but the intermittent nature of renewable sources and varying loads cause many control problems and thereby affect the quality of power within a microgrid operating in standalone mode. This might cause large frequency and voltage deviations in the system due to unpredictable output power fluctuations. Furthermore, without any main grid support, it is more complex to control and manage the system. In past, droop control and various other coordination control strategies have been presented to stabilize the microgrid frequency and voltages, but in order to utilize the available resources up to their maximum capacity in a positive way, new and robust control mechanisms are required. In this paper, a standalone microgrid is presented, which integrates renewable energy-based distributed generations and local loads. A fuzzy logic-based intelligent control technique is proposed to maintain the frequency and DC (direct current)-link voltage stability for sudden changes in load or generation power. Also from a frequency control perspective, a battery energy storage system (BESS) is suggested as a replacement for a synchronous generator to stabilize the nominal system frequency as a synchronous generator is unable to operate at its maximum efficiency while being controlled for stabilization purposes. Likewise, a super capacitor (SC) and BESS is used to stabilize DC bus voltages even though maximum possible energy is being extracted from renewable generated sources using maximum power point tracking. This newly proposed control method proves to be effective by reducing transient time, minimizing the frequency deviations, maintaining voltages even though maximum power point tracking is working and preventing generators from exceeding their power ratings during disturbances. However, due to the BESS limited capacity, load switching (load shedding scheme) as the last option is also introduced in this paper. Simulation results prove the effectiveness of the proposed control strategy from both frequency and voltage perspectives.

1. Introduction
Recently, remote microgrids (MGs) [1] have been widely developed, especially for rural and distant areas, where providing electric energy from the main utility grid is costly and has destructive environmental effects. With increasing environmental awareness, renewable generation penetration has increased gradually [2–6]. There are several MGs already installed for providing the electricity for rural areas [7–12]. However, microgrid control in standalone mood is more difficult than in grid connected mode because they are vulnerable to frequency and voltage deviations as they are isolated power systems that have smaller equivalent inertia, more dynamic complexity, and a weaker grid than conventional utility power grid and furthermore, the distributed energy resources have an intermittent nature. Therefore, if a mismatch between generation and load occurs, deviation of the microgrid frequency and voltage from the standard value is unavoidable. In standalone mode, some microgrid resources should compensate this fluctuation in generation (wind power and solar irradiation) and load.

Several research studies have examined methods to support frequency and voltage control. In [13], a frequency droop control was applied to photovoltaic (PV) power generation. In [14], techniques enabling wind power to operate in a manner similar to a conventional power plant were suggested to dispatch power at the operator’s request. However, frequency control strategies using intermittent renewable generation are not economically profitable as these systems are unable to maximize their free energy utilization ability.

Several other researchers [15–17] have focused on droop control techniques for frequency and voltage stabilization. Reference [18] involves an alternative droop control method for synchronous generators operating with voltage source converters (VSCs) by treating generators like VSCs, but the system stability and dynamics are highly affected by the selected droop gains. Furthermore, microgrids can face oscillations and instabilities due to high droop gains to achieve accurate power sharing, whereas, our newly proposed control method can be an economical replacement of the abovementioned droop control techniques used in different forms with diesel generators (DGs) as a droop controller regulates the generator output gains, which makes it unable to operate at maximum efficiency. Also, with droop control we have to neglect maximum power tracking to extract the maximum possible power from renewable sources to stabilize dc link voltages, but in our designed microgrid model, we can stabilize the voltage and frequency even after using maximum power point tracking (MPPT) and DGs at maximum efficiency. Furthermore, as the declining feature of the droop controller has only one regulatory parameter, so more than one control target cannot be reached.

Recently, some researchers have proposed methods to support the frequency and voltage in a multisource microgrid that includes renewable distributed generation and energy storage systems. In [19], an improved grid-forming control scheme is developed, which can keep the charging voltage of a battery bank under control without physical communication. In [20,21], a coordinated control architecture for power management of standalone microgrids is presented in which power control of each generation unit can be achieved with seamless mode changes. However, the results of these studies show that frequency deviation from the nominal value is very high. Moreover, the existing control strategies greatly increase the complexity of microgrid energy management. Therefore, it is necessary to develop an efficient and intelligent stability control approach with high applicability and reliability, where the operation modes and power supply scheduling can be regulated according to power sources, load condition and battery state of charge.

Keeping in mind the aforementioned difficulties, this paper introduces a new control strategy that comprises battery energy storage system (BESS)-based frequency and voltage stabilization along with supercapacitors and a fuzzy logic-based intelligent control system for energy management in generation and load imbalance conditions. The BESS will assist in preventing the system frequency from deviating far from its nominal value. The supercapacitor is used to maintain the DC bus voltage by consuming big current spikes generated due to intermittent nature of renewable sources and maximum power point tracking. Power stored in the supercapacitor is used to charge the BESS when required. The fuzzy logic-based supervisory controller will maintain the frequency and voltage at the nominal value and also control dump loads in a positive way and load shedding scenarios in severe cases to avoid microgrid collapse.

The main architecture of the proposed system is composed of: a PV-based distributed generation, small wind turbine system (WT), synchronous diesel generator (SG), battery energy storage system, DC heat generation system as dump load, fuzzy logic based supervisory control and management system, bidirectional buck-boost converter, supercapacitors, inverter between DC and AC (alternating current) bus. The whole system is then connected with the various primary and secondary loads as shown in Figure 1.

2.1. Diesel Generator System
A diesel generator (DG) is a reliable backup source in case of an emergency like a power outage to prevent discontinuity of daily work activities. It is a source that can supply power demand up to rated power at constant frequency [22]. A DG usually is composed of a synchronous generator, excitation system and a diesel engine with governor. A 70 kW synchronous generator with its basic controlling principles is discussed here.

The DG is modeled in Simulink using the SimScape toolbox. A DG is a system that converts mechanical energy obtained from external source into electrical energy. The generator uses mechanical energy to force the electrical charge movement in its windings through some electric circuit. This flow is the reason for the generator’s electric current output. Power produced by a diesel engine is small without any load whereas with load, the machine speed decreases for a short time and the mechanical power input to the generator terminals also decreases.

The system stabilizes itself after a short period of time due to the operation of a speed control system and the power input to the generator stabilizes to the required level. Due to the operation of the excitation system [23], voltages at the generator terminals are also maintained at the load demand. If the load demand is greater than the generated power, then it eventually results in a decrease in the frequency of the output. In diesel engine governor block, the reference rotor speed is already defined and using a feedback system, the rotor speed of the generator is also input to get the error value between two speeds to perform the function of controlling fuel combustion which results in power with the desired frequency. The internal configuration of a diesel generator governor is shown in Figure 2.

The PV generation system studied in this section is connected to an inverter through a boost converter, which is commonly used in a multi-source renewable energy systems. A boost converter [25] is used to regulate the voltages according to the supercapacitor requirements. The boost converter output is controlled by switching the IGBT’s (insulated-gate bipolar transistor) duty cycle.

As for PV maximum power point tracking (MPPT), over the years many MPPT techniques have been developed and implemented to extract the maximum available power from the solar photovoltaic system. These methods [26,27] vary in several aspects, including convergence speed, cost, effective range, hardware implementation and complexity. In this PV system, we use the incremental conductance technique to calculate the MPP. In this method, array terminal voltage is always adjusted according to the MPP voltage; it is based on the incremental and instantaneous conductance of the PV array.

3. Fuzzy Logic Based Frequency and Voltage Stability Controller
As microgrids operating in standalone mode don’t have a large amount of inertia like utility grids with synchronous rotating machines, renewable power generation or load variations can severely affect the frequency and DC link voltages in a microgrid. The main contribution of this research work is to show that droop control with a diesel generator can be replaced economically by a fuzzy logic and battery storage system to stabilize the system frequency. Droop control is normally used with diesel generators to help change their power references proportionally to frequency deviations from nominal value, but in this case, diesel generator cannot be operated at maximum capacity, whereas in our proposed system, we will supply or consume power causing frequency deviation and system degradation controlled by a fuzzy logic scheme. A flow diagram of the proposed control method is shown in Figure 7.

3.1. Frequency and Voltage Stabilization
In a standalone situation, if the output power of sources cannot satisfy its load demand, the security of the system might be deteriorated. Frequency and voltage stabilization are crucial in distributed generated system-based standalone microgrids. Active and reactive powers are fundamental factors to consider while designing a stable power distribution system. A slight deviation from the maximum active and reactive power limit will induce frequency and voltage stability issues.

Active power consumption is directly related to the frequency of a diesel generator. If the active power shoots beyond the maximum threshold limit, it increases the induced torque within the generator (T = P/w), which means that induced torque exceeds the rotor torque, causes the generator to slow down and decreases its output frequency. The output frequency must stay within permissible limits for safe operation. Similarly, excessive reactive power extraction causes over-excitation in the alternator of a diesel generator and the excitation voltage drops below the rated value which causes a terminal voltage drop. Therefore, various control schemes have been introduced by researchers to overcome these serious issues leading to degradation of the power grid. Droop control is a very common and old technique used in various configurations. The most common technique is master-slave frequency droop where multiple parallel units with the same droop characteristics can respond to falls in frequency by increasing their active power output.

Generally, two or more parallel diesel generators are connected as slaves. A master DG controls the slave output power and commands them to provide a predetermined amount of power for frequency stabilization. In this scenario, slave diesel generators work as standby power sources. This droop technique prevents the slaves from providing maximum power, which reduces the overall efficiency because of higher fuel consumption with respect to the output power of diesel generators. Droop control methods are not competent in renewable energy-based microgrid systems due to their limited power delivery capability. MPPT is an essential process to extract the peak power from renewable sources, whereas, droop control voltage stabilization restricts the system to a limited output power which causes enormous renewable energy losses. Therefore, a supercapacitor-based voltage and frequency stabilization technique is introduced, which offers MPPT with output voltage and frequency stabilization in hybrid standalone microgrids.

The proposed system consists of a supercapacitor-based DC-link voltage stabilizer and battery energy storage system (BESS)-based frequency stabilizer. Supercapacitors are connected is parallel with the DC-link via a DC/DC buck converter to maintain the DC-link voltage within the specified range while the system is operating at MPPT. The DC/DC buck converter is controlled by a standard PID controller, which is responsible for maintaining the DC-link voltage to 500 V DC. Further voltage increase will be consumed and stored in current form in the supercapacitors. Energy stored in supercapacitors is further used to charge the BESS that is attached to the DC link and supercapacitors via s bidirectional DC/DC buck-boost converter. In case, if the BESS is fully charged and the generated power is still more than the load demand, extra generated power will be supplied to the heat generating system used as dump load. This dump load system will keep consuming the extra power to generate heat until the system scenario changes. In general, any dump load system can be selected to meet the requirement, such as a water pump system, heat-generating system, etc.

The rest of the system works in normal condition until the DC-link voltage stays within the limits. Another case is an overload scenario, where the diesel generator and renewable energy sources fall short of holding the increased load demand either in form of active power or reactive power. As mentioned earlier, any increase in active power directly affects the frequency and causes a frequency drop, while reactive power drops the output voltage. A fuzzy logic-based supervisory controller is designed for voltage and frequency stabilization. If the active power demand is increased, the diesel generator output frequency will keep dropping below its nominal range. In that case, the fuzzy logic-based controller will detect the frequency drop and generate commands to the bidirectional DC/DC buck-boost converter to provide deficit power. Energy stored in the BESS provides the required power to maintain the output frequency. In the reactive power case, the diesel generator output voltage tends to drop, eventually causing the DC-link voltage to drop too. The fuzzy logic-based supervisory controller will sense this disruption and control the BESS system to provide the required power to maintain the DC-link voltage. If the BESS reaches its minimum capacity as discussed in the next section for a loadshedding case, the secondary loads can be disconnected from the main system to avoid overall system degradation and a complete blackout.

4. Simulation Studies
Frequency and voltage stabilization of an AC/DC hybrid standalone microgrid system is briefly described in Sections 2 and 3. Now the corresponding Matlab/Simulink simulation study is examined in this section. The whole configuration of the proposed system is shown in Figure 11.

The proposed system simulation study is carried out in different scenarios to verify system performance. The different cases mentioned are the following:

• Frequency and Voltage Stability with Battery Charging and Discharging
• System Overloading (Load Shedding)

These cases are discussed below with the output characteristics of the system response while operating in these specific modes.

5. Conclusions
In this research work, an efficient and reliable strategy for frequency and voltage stabilization of a standalone hybrid microgrid is presented. This study aims to solve the problem of stable operation of standalone microgrids, especially concerning the frequency and voltage stability. A BESS is used to stabilize the frequency and voltage in emergency cases. The rapid response of the BESS results in the frequency being maintained at a standard value and kept within permissible limits, even when it deviates. Furthermore, a supercapacitor is introduced to keep the DC bus voltage stabilized even though MPPT is used to extract the maximum possible power from renewable resources. Finally, a fuzzy logic supervisory controller based on 21 rules has demonstrated its advantages by ensuring a smart power management system, even when the system faces load-shedding scenarios.

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