14.1 Microgrid

As discussed in the earlier chapters, the concern for climate change has resulted in the use of various renewable energy sources (RESs) to reduce greenhouse gases. These renewable sources are generally connected at the distribution level and are known as distributed generators or distributed energy resources (DERs). Using DER in distribution system reduces the physical and electrical distance between generators and loads. The presence of energy sources near the load contributes to enhancement of voltage profile, reduction in losses and postponing the investment in new transmission systems and large-scale generation. However, the effect of penetration of a large number of DERs is to change the pattern of power flow in the electric distribution systems. Currently, the relatively low penetration levels of renewable systems cause few problems. As penetration becomes greater, the available wind and solar energy become a greater problem requiring central generation to provide the power backup. Since these sources are intermittent sources and can cause stability problems found with intermittent loads such as roiling mills and arc furnaces, central generation or distributed generation (DG) or storage is required to provide this backup energy. Without storage and/or local generation, there is a technical limit to the amount of wind generation that can be added to the grid system, perhaps as much as 20% of the peak demand is possible.

To realize the potential of integrating the distribution system with DER, a system approach which views generation, storage, protection and loads as an integral part of the distribution system is required. Such integration must not depend on fast, complex command and control systems. Each active component of the new distribution system must react to local information such as voltage, current and frequency to correctly change its operating point or faults. Distribution systems can efficiently handle disturbances using extended microgrid concepts. These concepts require that some of the generation, storage and corresponding loads need to separate from the distribution system to isolate sensitive loads from the disturbance (and thereby maintaining service) without harming the integrity of the remaining T&D system.

In order to improve the reliability of power supply, with better quality of power and increased stability margins, concept of smart grid and microgrid are being considered and implemented. The microgrid is defined as follows [1]:

• The term “Microgrid” has become popular nowadays and is used to describe the concepts of managing energy supply and demand using an isolated grid that can work in islanded mode or as connected to the utility's distribution grid.
• Another definition of microgrid presently being used: A microgrid is group of interconnected loads and DERs within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid and that connects and disconnects from such grid to enable it to operate in both grid-connected or island mode.

A microgrid is basically a cluster of interconnected distributed generators, loads and intermediate energy storage units that can be collectively treated by the grid as a controllable load or generator. This approach allows for local control of DG, thereby reducing or eliminating the need for central dispatch. During disturbances, the generation and corresponding loads along with storage can separate from the distribution system to isolate the critical loads from the disturbance. Therefore, dynamic islanding is a key feature of a microgrid. There are a number of benefits that can be obtained from this ability to island for events such as faults and voltage sags. Thus, with the availability of a large number of DER sources, a microgrid is an important part of the distribution system. However, the control of microgrid with the presence of distributed energy sources (DERs) requires a major change in the control strategy of grid control.

If the present energy management system (EMS) is considered as reference, the following changes may be required in monitoring and control of microgrids.

• Forecasting: It is very difficult to forecast the power generating capacity especially when large number of DERs such as solar and wind power are connected.
• The generation needs to respond to various events such as voltage drops, faults, blackouts and so on and switch to island operation using local information only. This will require an immediate change in the output power control of the microgenerators as they change from a dispatched power mode to one controlling frequency of the islanded section of network along with load control. Controlled islanding operation and resynchronization are special requirements for microgrids.

From the point of view of the control structure of microgrid EMS, microgrid can be divided into centralized control and decentralized control.

A centralized control system achieves intelligence from a particular central location, which depends on the network type and could be a switch, a server or a controller. It is easy to operate a centrally controlled network. However, the centralized control system requires a single control device that processes all measured data. This unique controller point could cause several communication problems.

In decentralized control, all devices are able to control themselves independently as opposed to a single controller. This kind of control strategy believes in providing full autonomy to the local controllers of the DERs as it trusts that they are intelligent and smart enough and can communicate with each other to form a larger intelligent, smart and efficient unit. In the decentralized control scheme, the main focus is on the improvement of the overall performance of the system. Environmental conditions, weather conditions and so on are the main deciding factors for the decentralized scheme, and hence, the multi-agent system is used in this control strategy.

14.2 Types of Microgrids

A microgrid can be built using ac or dc network. Despite the fact that the ac microgrid system has a benefit to utilize existing ac grid technologies, protections and standard, its application involves drawback of low efficiency due to the number of power conversions required within the crucial current path from the main grid to the loads. Since the RESs such as PV cells and fuel cells produce dc and wind energy system produces ac of variable frequency, dc-to-ac and ac-dc-ac conversion is required in ac microgrid. One solution of this problem is the application of a dc microgrid as an efficient method to combine high reliability and the possibility to reduce the losses [2]. It can eliminate dc/ac or ac/dc/ac power conversion stage and thus has advantages in terms of efficiency, cost and system size. dc microgrid also has an added advantage that it does not require reactive power.

In most of the countries, energy infrastructure is old and vulnerable to extreme weather events. During the last few years, power failures have increased. Because a microgrid is localized, it can mitigate power disruptions by continuing to operate – providing electricity to its local customers – when the main grid supply is not available to consumers.

Microgrids can meet the needs of a wide range of applications in commercial, industrial and institutional settings. Larger microgrid applications include communities ranging from large neighbourhood to small towns to military bases. Another application of microgrid is in isolated mode in areas where people live without regular access to electricity. These “off-grid” areas are currently served (if at all) by diesel generators or similar small-scale electricity generating equipment.

A microgrid operates in either grid-connected or autonomous mode depending on the main grid condition/existence. During disturbances, the generation and corresponding loads can separate from the distribution system to isolate the load serviced by microgrid from the disturbance and thereby maintaining service without harming the transmission grid integrity. Operation of the microgrid assumes that the power electronic controls of current DERs are modified to provide a set of key functions, which currently do not exist. These control functions include the ability to: regulate power flow on feeders; regulate the voltage at the interface of each DER; ensure that each DER rapidly picks up its share of the load when the system islands. In addition to these control functions, the ability of the system to island smoothly and automatically reconnect to the bulk power system is another important operational function. In addition, fast and flexible action of under-voltage and under-frequency relays is required in islanded mode for keeping the MG stable. The main challenge in operating an ac microgrid with different types of DER system is the coordination of the numerous generators for sharing the real and reactive power output and the control of power system frequency and voltage. Some of these problems are not faced in dc grid.

14.3 dc Microgrid

dc microgrids eliminate the waste of energy associated with the conversion of dc to ac and ac to dc to ac and then again to dc which is required for so many of the electrical loads found these days. LED lights, variable-speed motors, computers, televisions and countless other forms of consumer electronics are loads that account for a steadily increasing fraction of the electricity consumed requiring dc supply for their operation. Moreover, many distributed RESs (PV cells and fuel cells) produce dc power which is now converted to ac to connect it the ac system. This ac power is again converted to dc for many end applications described earlier. One possible solution is to use the dc power directly in a dc microgrid. At present, local dc grids are in use within data and telecommunications centres or, on a much smaller scale, within automobiles, ships and aeroplanes. But it is expected that more dc microgrids will be coming up soon, at least in certain settings. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular and distributed power systems, can be classified under dc MG framework. dc microgrids may also prove beneficial in many energy-intensive manufacturing operations. These include paper and pulp production and the smelting of aluminium, which now wastes more than 6% of the total energy consumed in the conversion of ac to dc.

The rapid development of power electronics technology which made dc voltage regulation a simple task, in addition to the increasing penetration of dc loads and sources, encouraged researchers to reconsider dc distribution for at least portions of today's power system to increase its overall efficiency. One of the great benefits of dc microgrids is their capability of static storage integration. Most of storage elements such as batteries and ultra-capacitors use dc supply. Moreover, flywheels, even though they are mechanical energy storage systems (ESS), are mostly coupled to a permanent magnet synchronous machine (PMSM) that is integrated to the distribution system through a dc link [3]. Adding dc storage to a dc microgrid is a comparatively simple compared to the complications of integrating dc storage in the ac domain where additional hardware is required.

A simple dc microgrid is schematically shown in Fig. 14.1. As for a typical dc microgrid, the main components of a microgrid are as follows: (i) DG sources such as photovoltaic panels, small wind turbines, fuel cells, diesel and gas microturbines; (ii) distributed energy storage devices such as batteries, supercapacitors, flywheels; and (iii) critical and non-critical loads. Energy storage devices are employed to compensate for the power shortage or surplus within the microgrid. They also prevent transient instability of the microgrid by providing power in transient. The transient power shortage in a microgrid can be compensated for by fast energy storage devices in the microgrid or by the utility grid through a bidirectional power converter when operating in grid-connected mode. A microgrid is connected into the utility grid through a bidirectional power converter that continuously monitors both sides and manages power flow between them. There is a single point of connection to the main distribution utility called point of common coupling (PCC). If there is a fault in the utility grid, the power converter will disconnect the microgrid from the grid, creating an islanded energy system. The microgrid can continue to operate in the islanded mode that is primarily intended to enhance system reliability and service continuity.

14.3.3 Operational Modes of dc Microgrid

dc microgrid should operate in both grid-connected and islanding states. In each of these two states, there are different operating modes, and the control and management system should regulate the dc bus voltage. The satisfactory operation of the dc microgrid during the variations of wind and solar generation, load and grid connection conditions results in different operating modes that need to be considered in order to ensure a secure and reliable power supply. The following four operation modes are considered [5].

14.3.3.1 Mode 1: Islanding Mode (Battery Discharge)

During utility grid disturbances, microgrid is transferred from the grid-connected to the islanded mode, and a reliable and uninterrupted supply of consumer loads is offered by local DERs. Mode 1 corresponds to islanding and subsequent island operation. Due to the disconnection to the external ac network, the dc microgrid becomes an island system, and the grid-tied power converter releases control of the dc-link voltage level, and one of the converters in the microgrid must take over that control. During the islanded mode, the battery plays the main role in regulating the dc-link voltage level, and the supercapacitor plays a secondary role in responding of the sudden power requirement as an auxiliary source/sag, that is, for peak shaving during transients. If the generated PV/wind power is less than local the load demand, dc bus voltage is regulated by battery discharging.

14.1

where is the power supplied by batteries, is local load and is local generation.

In case the power generated is much less than load demand, appropriate load shedding may be required. Similarly, if prolonged operation in island mode is required, it may result in low battery energy storage, and in order to guarantee power supply to the most critical loads, appropriate load shedding also becomes necessary. If the required power from the battery ES system is smaller than its maximum power rating, the dc voltage can be fully controlled, and no load shedding is required. However, there are various conditions where load shedding becomes necessary.

If the required power from the battery ES system exceeds its maximum power rating, the ES system operates at current/power limit, and consequently, the dc voltage cannot be fully controlled and will continue to decrease from the desired value. In order to prevent the total collapse of the dc microgrid, load shedding is implemented based on the dc voltage measurement without the need for communication. Two different voltage levels can be used for each load, and the loads are prioritized to ensure those with lower priority being tripped first. The load with the highest priority will have the disconnected only as last resort.

14.3.3.2 Mode 2: Islanding Mode (Excess Power Available)

In this mode, the dc microgrid operates in islanding mode and the dc bus voltage is regulated by the battery ES system through charging. In this mode, the dc bus voltage is regulated by dc/dc converters for PV and WECS. Since the maximum power generated by PV and wind in this mode is greater than the local load demands, the dc/dc converters for PV modules and WECS may not work with MPPT. Depending on the power generated by PV and wind, the power control mechanism will be applied. If the battery has been fully charged, the dc/dc converter for battery is disabled; otherwise, the dc/dc converter is enabled. In islanded mode, the grid-tied power converter releases control of the dc-link voltage level, and one of the converters in the microgrid must take over that control. Since each converter of DG sources is used for optimal control of its belonging source, only the converters of the energy storage elements are free to regulate the dc-link voltage level. During the islanded mode, the battery plays the main role in regulating the dc-link voltage level,

The operation of the grid in different modes is based on the dc grid voltage. Microgrid works in modes 1 and 4 when the reference voltage is set to 0.9 and 0.95 times of normal grid voltage, respectively. The microgrid enters in modes 3 and 2 when the reference voltage is 1.0 and 1.05 times the normal grid voltage, respectively. The reference voltage is decided on the following basis:

1. 1. The voltage difference between different modes must not be too small; otherwise, the malfunction of mode switching will occur due to the sampling inaccuracies and external disturbance.
2. 2. The voltage difference between different modes should not be too large; otherwise, the converters will be required to work at low voltage (LV) and high current.

14.3.3.3 Mode 3: Grid-Connected Mode (Power Taken from Grid)

In this mode, the dc microgrid operates with connection to ac grid through bidirectional dc/ac converter. In the grid-connected operation mode, the grid-tied power converter has control over the dc-link voltage level.

If the output sum of the power of the DG systems is sufficient to charge the storage devices, any excessive power is supplied to the utility grid. If the sum of the power of the DG and storage systems is less than the total load demand, the required power is supplied from the utility grid. In the grid-connected mode, power management is performed in a complementary manner between storage devices, and as a result, a dc microgrid can operate safely and efficiently. If the total load is more than the power available from wind and solar system and the grid is also not able to supplement it, the battery storage supplies the power. For the battery ES system, its dc voltage and charging level are monitored by its battery energy-management system whose aim is to maintain a certain amount of energy storage by providing appropriate levels of battery charge/discharge current. The control is achieved by regulating the duty ratio of the bidirectional dc-dc converter and various approaches, such as the predictive method which can be used.

14.3.3.4 Mode 4: Grid-Connected Mode (Power Supplied to Grid)

In this mode, the dc microgrid operates with connection to ac grid through grid-side converter. The dc bus voltage is regulated by the grid-side converter through inversion, which means the generated PV power and wind is greater than the local load. The grid converter works in constant power mode. The dc bus voltage is regulated by PV and wind interface converter based on droop control. The battery ESS can be charged with the constant power, and the surplus power is injected to the utility power grid. The converters for PV modules and WECS work with MPPT. If the battery has been fully charged, the dc/dc converter for battery is disabled. The wind turbine operates at the MPPT mode where its output power is set according to wind speed. The converter of WECS is directly connected to the wind generator which is PMSG. The converter controls the generator current to ensure that the output power is equal to reference value. The PV interface dc-to-dc converter also works on MPPT. The G-VSC controls the common dc voltage. A closed-loop PI-regulator-based dc voltage control system whose input is the difference between the desired and actual dc voltages is usually used. The output from the dc voltage controller is the active power order.

14.4 ac Microgrid

As discussed earlier in this chapter, the interconnection of small modular generation systems (PV, fuel cells, micro turbines, small wind generators) and storage devices to LV distribution grids leads to a new energy system [1, 6]. The main microgrid components include loads, DERs, master controller, smart switches, protective devices, as well as communication, control and automation systems. Microgrid loads are commonly categorized into two types: fixed and flexible (also known as adjustable or responsive). Fixed loads cannot be controlled and must be satisfied under normal operating conditions while flexible loads are controllable. DERs consist of DG units and distributed ESSs which could be installed at electric utility facilities and/or at consumers' premises. Microgrid DGs are either dispatchable or non-dispatchable. Dispatchable units can be controlled by the microgrid master controller and are subject to technical constraints depending on the type of unit, such as capacity limits. Non-dispatchable units, on the contrary, cannot be controlled by the microgrid master controller since the input source such as solar and wind is uncontrollable.

The connection of small generation units with power ratings less than a few tens of kilowatts to LV networks results in the following benefits.

• Ensures energy supply with power quality and reliability for critical loads.
• Promotes demand-side management and load levelling.
• Promotes community energy independence and allows for community involvement in electricity supply.

It also brings additional benefits for global system operation and planning, particularly regarding investment reduction for future grid expansion. The ac microgrid such as dc microgrid can operate in the following two different operating conditions.

14.4.1 Interconnected or Grid-Connected Mode

Figure 14.2 presents schematic diagram of a generic multiple-DER microgrid. In grid-connected mode, no direct voltage and frequency control is required. The utility grid provides a reference voltage and frequency source. The PCC voltage is dominantly determined by the main grid, and the main role of the microgrid is to accommodate the real and/or reactive power generated by the DG units and the load demand. In this mode, the main objective is to export a controlled amount of active and reactive power into an established voltage which is done through the control of active and reactive components of current. Interface converters are Voltage-Source Converters (VSCs) which are operated in current-controlled mode (CC-VSC) in synchronization with the grid voltage, according to P-Q control strategy. Thus, the inverter is operated in constant current control mode using d-q axis–based current control.

Under normal operation, each DG system in the microgrid usually works in a constant current control mode in order to provide a preset power to the main grid. All the power generating sources follow the value of active power reference generated by an internal control system such as a MPPT2 system. The overall control structure of interface converters in the grid-connected mode is the current control mode in stiff synchronization with the grid [7, 8].

14.5 Control of ac Microgrid in Grid-Connected Mode

As shown in Fig. 14.2, each generation unit is interfaced to the microgrid at its respective point of connection and the microgrid is connected to the main grid at the PCC. Integration of DER units, in general, introduces a number of operational challenges that need to be addressed in the design of control and protection of distribution systems. The most relevant challenges in microgrid protection and control are as follows: the interconnection system must be able to measure the relevant indicated voltages and frequencies at the PCC or the point of connection of DR and disconnect within a given allowed time all local power generating units in the microgrid when these measured voltages or frequencies fall within a specified range. In addition, because of connection of DERs, which are normally power electronics interfaced, the fault current can be bidirectional. It may also result in reduction in fault current capacity, disruption in fault detection and protection sensitivity.

The main variables used to control the operation of a microgrid are voltage, frequency and active and reactive power. Reactive power injection by a DG unit can be used for

1. 1. power factor correction,
2. 2. reactive power supply, or
3. 3. voltage control at the corresponding point of coupling.

In grid-connected mode, the main converter of MG operates in the PQ mode. The power is balanced by the utility grid. The battery is fully charged. ac bus voltage is maintained by the utility grid, and dc bus voltage is maintained by the main converter. Each DG system is usually operated to provide or inject preset power to the grid, which is the current control mode in synchronization with the grid. In this mode, the function of the overall control is thus to issue the real- and reactive-power commands for the DER systems.

The commands by overall control are based on a variety of criteria, such as economic operation of the microgrid; optimal operation and stability of the microgrid; and microgrid internal conditions and requirements.

An important aspect to consider in grid-connected operation is synchronization with the grid voltage. For unity power factor operation, it is essential that the grid current reference signal is in phase with the grid voltage. This grid synchronization can be carried out by using a PLL. There are two approaches for the overall control applied to power system: (i) centralized control or (ii) decentralized.

A fully centralized control is shown in Fig. 14.3. In a centrally controlled microgrid, the communication network is necessary to communicate control signals to the microgrid components. The central controller with the help of extensive communication between the central controller and controlled units performs the required calculations and determines the control actions for all the units at a single point. On the other hand, in a fully decentralized control, each unit is controlled by its local controller, which only receives local information and does not consider system-wide variables or other controller actions. In a microgrid with decentralized control, the communication network enables each component to talk with other components in the microgrid, decides on its operation and further reaches predeterminned objectives. Since interconnected power systems are spread over a large area, a fully centralized control is complicated as it requires extensive communication.

At the same time, a fully decentralized approach is also not possible due to the strong coupling between the operations of various units in the system, requiring a minimum level of coordination that cannot be achieved by using only local variables. It is therefore desired that the controllers of various distributed resources are also distributed to avoid the communication delay and the single point of failure. A compromise between fully centralized and fully decentralized control schemes can be achieved by means of a hierarchical control scheme consisting of three control levels: primary, secondary and tertiary as shown in Fig. 14.4. Although microgrids may not be spread over large area as conventional power systems, this type of control is now suggested for the microgrid, because of the large number of controllable resources and stringent performance requirements.

This three-level hierarchical control is organized as follows. The primary control deals with the inner control of the DG units by adding virtual inertias and controlling their output impedances. The secondary control is conceived to restore the frequency and amplitude deviations produced by the virtual inertias and output virtual impedances. The tertiary control regulates the power flows between the grid and the microgrid at the PCC. These control levels differ in their (i) speed of response and the time frame in which they operate and (ii) infrastructure requirements (e.g., communication requirements).

14.6 Autonomous Operation of Microgrid

During normal grid operation, the DG provides a constant power to the grid. The main grid is supplying or absorbing the power difference between the DGs and the local load. The islanding phenomenon that results in the formation of a microgrid can be due to either pre-planned or unplanned switching incidents. Islanding of the MG can take place by unplanned events such as faults in the MV network or by planned actions such as maintenance requirements called intentional islanding [10]. When the main power grid is out due to faults or switching, the DG delivering preset or available maximum power to the microgrid can create voltage and frequency transients which are dependent on the degree of the power difference. In the case of a pre-planned microgrid formation, appropriate sharing of the microgrid load amongst the DG units and the main grid may be scheduled prior to islanding. Thus, the islanding process results in minimal transients and the microgrid continues operation, as an autonomous system.

Prior to islanding, the operating conditions of microgrid could be widely varied, for example, the DG units can share load in various manners and the entire microgrid portion of the network may be delivering or importing power from the main grid. Furthermore, the disturbance can be initiated by any type of fault and line tripping may be followed up with single or even multiple reclosure actions. Thus, the severity of the transients experienced by the microgrid, subsequent to an unplanned islanding process, is highly dependent on (i) the pre-islanding operating conditions, (ii) the type and location of the fault that initiates the islanding process, (iii) the islanding detection time interval, (iv) the post-fault switching actions that are envisioned for the system and (v) the type of DG units within the microgrid. However, the switching transient will have great impact on MG dynamics. During islanding, the power balance between supply and demand does not match at the moment. As a result, the frequency and the voltage of the microgrid will fluctuate, and the system can experience a blackout unless there is an adequate power-balance matching process. A microgrid composed only of RESs and conventional CHP units, such as the diesel generator, gas engine and microturbine, is hard to make sure the good dynamic performance of microgrid during islanded operation due to the intrinsic characteristics of these generation systems. The RES has an intermittent nature since their power outputs depend on the availability of the primary source, wind, sun and so on, and therefore, they cannot guarantee by themselves the power supply required by loads. On the other hand, CHP systems are limited by their insufficient dynamic performance for load tracking especially, the frequency of the microgrid may change rapidly due to the low inertia present in the microgrid. Therefore, local frequency control is one of the main issues in islanded operation.

If there are no synchronous machines to balance demand and supply, through its frequency control scheme, the inverters should also be responsible for frequency control during islanded operation. In addition, a voltage regulation strategy is required; otherwise, the MG might experience voltage and/or reactive power oscillations. In grid-connected mode, the inverters may be operated in PQ control mode. But with large number of microsources connected to form a microgrid, PQ control cannot be applied because voltage regulation is necessary for local stability and reliability. However, by using a VSI to provide a reference for voltage and frequency, it is thus possible to operate the MG in islanded mode, and a smooth movement to islanded operation can be performed without changing the control mode of any inverter. The advantage of using VSI is that it can react to network disturbances based only on information available at its terminals. Thus, a VSI can provide a primary voltage and frequency regulation in the islanded MG. After identifying the key solution for MG islanded operation, two main control strategies are possible: (i) single-master operation (SMO) or (ii) multi-master operation (MMO). In both cases, a convenient secondary load-frequency control during islanded operation must be considered to be installed in controllable MS.

14.6.1 Islanding Detection

The main islanding detection techniques used may be broadly classified as remote and local techniques. Local techniques can be divided into passive and active detection methods. Remote islanding detection techniques are based on the communication between utilities and DGs. Supervisory control and data acquisition (SCADA) has been used to determine whether the distribution system is islanded or not. These methods are reliable but not economic to implement for small systems. As for the local techniques, the core of passive method is that some of the system parameters (voltage, frequency, etc.) change greatly with islanding but not much in normal running when connected with grid. This character can be helpful in islanding detection by continuously monitoring the parameters of the system without bringing disturbance. During the grid-connected operation, the DGs are operated to provide the optimum power to the grid according to many factors such as the availability of energy, energy cost. The main grid is supplying or absorbing the power difference between the DG and the local load demand. When the main power grid is out (power outage), the DG that continues to inject predetermined optimum power can cause voltage and frequency transients, depending on the degree of power difference. The power difference makes the voltage and frequency drift away from the nominal values. When the voltage and frequency drifts have reached certain levels, it is deemed that an islanding is occurring. This is the method that has been used to detect islanding. This methodology is enough for islanding detection. However, this method would cause large non-detection zones because islanding cannot be detected under a perfect match of generations and loads in the island system.

With active methods, islanding can be detected even under the perfect match of generation and load. Active methods directly interact with the power system operation by introducing perturbations. The idea of an active detection method is that this small perturbation will result in significant change in system parameters when the microgrid is in islanded mode, whereas the change will be negligible in grid-connected mode. Some of the active detection techniques are as follows [11].

14.6.1.1 Impedance Measurement Method

Impedance measurement method is similar to passive method which measures the change in impedance caused by islanding. In an active direct method, however, a shut inductor is momentarily connected across the supply voltage from time to time, and the short-circuit current and supply voltage reduction are used to calculate the power system source impedance. A large number of impedance detection methods have recently been proposed because of the belief that this method has no NDZ in the single-inverter case.

14.6.1.2 Slip-Mode Frequency Shift (SMS) Method

The method applies positive feedback to the phase of the voltage as a method in shifting the phase and, subsequently, the short-term frequency. The SMS issued to detect the islanding condition because of the easy implementation of the method is caused by the involvement of only a slight modification of a required component.

14.6.1.3 Active Frequency Drift Method

The principle of the active frequency drift (AFD) or frequency bias method is forcing variations in the inverter output using positive feedback to accelerate the frequency of the inverter current. The AFD uses the waveform of the inverter current, shown in Fig. 14.5, along with an undistorted sinewave for comparison. When this current waveform is applied to a resistive load in an island situation, its voltage response will follow the distorted current waveform and go to zero in a shorter time than it would have under purely sinusoidal excitation. This causes the rising zero-crossing of voltage at node (PCC) to occur sooner than expected, giving rise to a phase error between this voltage and PV inverter current. The PV inverter then increases the frequency of the current to attempt to eliminate the phase error. The voltage response of the resistive load again has its zero-crossing advanced in time with respect to where it was expected to be, and the PV inverter still detects a phase error and increases its frequency again. This process continues until the frequency has drifted far enough from 0 to be detected by the over/under-frequency protection. The waveform drift of the grid is not present as a stabilizing influence.

14.6.1.4 Sandia Frequency Shift (SFS)

Sandia frequency shift (SFS) is an extension of the frequency bias method and is another method that utilizes positive feedback to prevent islanding. In this method, it is the frequency of voltage at node (PCC) to which the positive feedback is applied.

There are two cases that must be considered. They are as follows: (i) the inverter is bidirectional and (ii) the inverter is unidirectional. To implement the positive feedback, the “chopping fraction” defined in Fig. 14.6 is made to be a function of the error.

The chopping function is given by

14.1

This is the third method that uses positive feedback to prevent islanding. Sandia voltage shift (SVS) applies positive feedback to the amplitude of voltage of PCC node. If there is a decrease in the amplitude of (usually it is the RMS value that is measured in practice), the PV inverter reduces its current output and thus its power output. If the utility is connected, there is little or no effect when the power is reduced. In the islanded mode, the microgrid operates as an independent entity and must provide voltage and frequency control, as well as real- and reactive-power balance.

For example, if the net load demand is less than the total generation, the microgrid central controller should decrease the net generated power. This is done by assigning new set points to the DER units. On the other hand, if the power generated within the microgrid cannot meet the load demand, either non-critical load shedding or activation of battery charging may be initiated.

14.7 Load Frequency Control in Microgrid

The goal of LFC in a microgrid system is to maintain the system's frequency within acceptable limits around nominal value under various conditions, such as fluctuating power demand, and/or contingency situation such as unexpected loss of one or more of the system's generating units, in order to ensure system's stable operation. In case of small and isolated microgrid systems, however, the stability of the microgrid system is an issue of much greater significance as there are no means of connecting to primary grid power. The transition to islanded operation mode and the operation of the network in islanded mode require micro generation sources to particulate in active power frequency control, so that the generation can match the load. During this transient period, the participation of the storage devices in system operation is indeed very vital, since the system has very low inertia, and some micro sources (micro turbine and fuel cell) have very slow response to the power generation increase. The power necessary to provide appropriate load increase is obtained from storage devices. Knowing the network characteristics, it is possible to define the maximum frequency droop. To maintain the frequency between acceptable limits, the V/f inverter connected to storage device will adjust the active power in the network. It will inject active power when frequency falls from the nominal value and will absorb active power if the frequency rises above its nominal value.

14.8 Combined ac/dc Microgrid

There were historical reasons that led ac to dominate over dc power systems. However, these are not valid any longer. In power generation, many DERs, such as photovoltaic systems, and fuel cells have emerged that generate power in dc form. Many of the modern electrical loads, as well as ESSs, are either internally dc or work with dc power and connect to the ac systems through converters. Thus, application of dc power would allow the elimination of many ac-dc and dc-ac conversion stages, which would in turn result in considerable decrease in component costs and power losses and increase in reliability. Therefore, dc microgrid is more suitable for the integration of distributed RESs. However, for a comprehensive microgrid, where the various sources as complementary should be integrated to overcome the environmental influence and reduce the interruption maintenance time, a pure dc grid would be deemed inappropriate. In addition, because of widespread use of ac appliances as well as ac distribution systems that prevail now, it is worthwhile to ingrate dc into existing ac system in microgrid.

Therefore, a hybrid ac/dc microgrid should be assumed to fully demonstrate the advantages of ac and dc distribution networks in view of easier renewable energy integration.

In hybrid ac/dc microgrid, ac sources and loads are connected to the ac network whereas dc sources and loads are tied to the dc network. ESSs can be connected to dc or ac links as shown in Fig. 14.6. The hybrid grid can operate in a grid-tied or autonomous mode.

14.9 Summary

The renewable sources are generally connected at the distribution level and are known as distributed generators or DERs. Using DER in distribution system reduces the physical and electrical distance between generators and loads. The effect of penetration of large number of DERs is to change the pattern of power flow in the electric proliferation of DER units in the form of DG, distributed storage (DS) or a hybrid of DG and DS units that has brought about the concept of microgrid. A microgrid is defined as a cluster of DER units and loads, serviced by a distribution system, and can operate in (i) the grid-connected mode, (ii) the islanded (autonomous) mode and (iii) ride-through between the two modes. In addition, a microgrid can be a dc microgrid or ac microgrid or hybrid ac/dc microgrid.

In this chapter, the operation and control of dc microgrid are first considered in grid-connected as well as isolated mode. It is followed by operation and control of ac microgrid in these modes, and finally, hybrid ac/dc microgrid is described.

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