Distributed Control System for Microgrid

Abstract

In this study an example of a microgrid composed of diesel generator and two uninterruptable power supply systems is considered. This microgrid installed in the three buildings of the Tallinn University of Technology. This paper deals with how to implement a distributed control and monitoring system based on the Ethernet network in the microgrid. The paper describes a control strategy to implement both grid connected and islanded operation modes of the microgrid.

Keywords - Control system, diesel generator, microgrid

Introduction

Distributed generation (DG) is becoming an increasingly attractive approach to reduce greenhouse gas emissions, to improve power system efficiency and reliability, and to relieve today's stress on power transmission and distribution infrastructure [1]. Distributed generation encompasses a wide range of prime mover technologies, such as internal combustion engines, gas turbines, microturbines, photovoltaic, fuel cells and windpower [32]. A better way to realize the emerging potential of DG is to take a system approach which views generation and associated loads as a microgrid [21].

Microgrid is a concept of defining the operation of distributed generation, in which different microsources operate as s single controllable system that provides power and heat to a cluster of loads in the local area [3], [8] - [9].

A well designed microgrid should appear as an independent power system meeting the power quality and reliability requirements [3]. The primary goal of microgrid architectures is to significantly improve energy production and delivery to load customers, while facilitating a more stable electrical infrastructure with a measurable reduction in environmental emissions [10]. The most positive features of microgrids are the relatively short distances between generation and loads and low generation and distribution voltage level. The main function of a microgrid is to ensure stable operation during faults and various network disturbances.

The microgrid is a promising concept in several fronts because it [18]:

• provides means to modernize today's power grids by making it more reliable, secure, efficient, and de-centralized;
• provides systematic approaches to utilize diverse and distributed energy sources for distributed generation;
• provides uninterruptible power supply functions;
• minimizes emissions and system losses.

Despite many advantages of microgrid there remain many technical challenges and difficulties in this new power industry area. One of them is the design, acceptance, and availability of low-cost technologies for installing and using microgrids [4]. The increased deployment of power electronic devices in alternative energy sources within microgrids requires effective monitoring and control systems for safe and stable operation while achieving optimal utilization of different energy sources [35]. Microgeneration suffers from lack of experience, regulations and norms. Because of specific characteristics of microgrids, such as high implication of control components, large number of microsources with power electronic interfaces remains many difficulties in controlling of microgrids. Realization of complicated controlling processes in microgrids requires specific communication infrastructure and protocols. During the process of microgrid organization many questions concerning the protection and safety aspects emerge. Also, it is required to organize free access to the network and efficient allocation of network costs.

The predominant existing distributed generation is based on an internal combustion engine driving an electric generator [36]. To investigate various aspects of integration of alternative energy sources such as conventional engine generators, this paper proposes a prototype of the microgrid for three academic buildings at the Tallinn University of Technology which consists of a diesel generator, and batteries storage with power electronic interface. The main goal of this work is to design an intelligent control system of the microgrid that is efficient enough to manage itself for power balance by making use of state of the art communication technology. Moreover, the aim of this paper is to describe the control strategy of the microgrid operation in both stagy state modes. This control system enables the microgrid system to balance the electric power demand and supply and to simultaneously control the state of power network.

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Microgrid Theoretical Background

A microgrid is described as a small (several MW or less in scale) power system with three primary components: distributed generators with optional storage capacity, autonomous load centers, and system capability to operate interconnected with or islanded from the larger utility electrical grid [10], [11]-[13]. According to [39], [22], multiple facility microgrids span multiple buildings or structures, with loads typically ranging between 2MW and 5MW. Examples include campuses (medical, academic, municipal, etc), military bases, industrial and commercial complexes, and building residential developments.

Microgrids include several basic components for operation [3], [4]. An example of a microgrid with is illustrated in Fig.1.

1. Distributed Generation
   Distributed generation units [1] are small sources of energy located at or near the point of use. There are two basic classes of microsources; one is a DC source (fuel cells, photovoltaic cells, etc.), the other is a high frequency AC source (microturbines, reciprocating engine generators, wind generators), which needs to be rectified. An AC microgrid can be a single-phase or a three-phase system. It can be connected to low voltage or medium voltage power distribution networks.
2. Storage Devices
   Distributed storage technologies are used in microgrid applications where the generation and loads of the microgrid cannot be exactly matched. Distributed storage provides a bridge in meeting the power and energy requirements of the microgrid. Distributed storage enhances microgrid systems overall performance in three ways. First, it stabilizes and permits DG units to run at a constant and stable output, despite load fluctuations. Second, it provides the ride through capability when there are dynamic variations of primary energy (such as those of sun, wind, and hydropower sources). Third, it permits DG to seamlessly operate as a dispatchable unit. Moreover, energy storage can benefit power systems by damping peak surges in electricity demand, countering momentary power disturbances, providing outage ridethrough while backup generators respond, and reserving energy for future demand. There are several forms of energy storage, such as the batteries, supercapacitors, and flywheels.
3. Interconnection Switch
   The interconnection switch is the point of connection between the microgrid and the rest of the distribution system. New technologies in this area consolidate the various power and switching functions (power switching, protective relaying, metering, and communications) traditionally provided by relays, hardware, and other components at the utility interface into a single system with a digital signal processor. The interconnection switches are designed to meet grid interconnection standards.
4. Control System
   The control system of a microgrid is designed to safely operate the system in grid-parallel and stand-alone modes. This system may be based on a central controller or imbedded as autonomous parts of each distributed generator. When the utility is disconnected, the control system must control the local voltage and frequency, provide (or absorb) the instantaneous real power difference between generation and loads, provide the difference between generated reactive power and the actual reactive power consumed by the load, and protect the internal microgrid.

Structure of the Proposed Microgrid

The microgrid installed in three buildings of the Tallinn University of Technology (TUT): Faculty of Power Engineering, TUT Library, School of Economics and Business Administration. Consequently, according to the classification given in [22], this power system can be defined as a multiple facility microgrid. Fig.2 illustrates the various components of the power system of the microgrid at TUT.

The structure of the microgtid for the campuses of the TUT is proposed. Fig.3 shows a schematic of the power system. Microgrid systems targeted in this study are autonomous areas having the power demand of several kilowatts including a diesel generator, two uninterruptable power supply (UPS) systems with batteries storage, and loads. They are connected to the power electronic interface forming local AC network with 230V, 50Hz.

The diesel generator is used as the main distributed energy resource in this microgrid. It has a nominal power of 176kW/220kVA, voltage of 240V/400V and maximum current of 318A. This generator is connected to the AC bus via the automatic relay logic (ARL2). The ARL2 is continuously observing it both sides: the main grid and the microgrid. If there is a fault in the general grid, the ARL2 will disconnect the microgrid, creating an energetic island.

The battery banks (E1 and E2) are used as the distributed energy storage devices in the microgrid to insure continuous supply of the local load. They are interfaced to the electrical network through the two UPS systems: UPS1 (160kVA), and UPS2 (240kVA). Hence, we can conclude that the microgrid has two main possible operation modes: grid-connected and islanded mode.

Main customers of the microgrid are the computers and servers located in the laboratories and office rooms in the three buildings of TUT. The clients in the Library Building (computers) are interfaced to the electrical network using ARL1. In addition, four experimental loads (Experimental loads 1..4) are used that can be connected to the distributed shield located in the Laboratory of Electrical Drives. The nine intelligent sensors (P1..P9) - assign these loads. Their task is to measure electrical power and energy parameters of the network, such as voltage, current, power, energy, power factor and transmit this information to the controller.

The microgrid is connected to the general city electricity grid using two two-section transformer substations (6000kV/400kV) located in the Faculty of Power Engineering and the School of Economics and Business Administration Buildings.

Description of the Control System

Taking into account the configuration and features of the power network of the Tallinn University of Technology, the control system structure for the microgrid is designed with the following specifications:

• the balance of electric power demand and supply of power network are provided;
• both the steady state modes and the transient performance of the microgrid are achieved.

A block diagram of the hierarchical control system which is based on the multiagent technology [40], [41], is demonstrated in Fig.4. The design of the control system can be divided into hardware and software.

The control structure of the microgrid has three levels:

1. Operator console and application server;
2. Central controller (CC);
3. Local controllers (LC) and measuring devices.

Operator console is a computerized workstation with special software which comprises of supply and demand calculation units, monitoring units, control schemes and dispatching units. The function block diagram of the software is shown in Fig.5. The operator console heads the hierarchical control system. Its main goals of are: to keep track of the whole system by monitoring the status of the communication nodes and generating units; to collect data from the measuring devices; to calculate supply and demand of power; to visualize information received; to display the basic modes of the microgrid; and to transfer control commands to the central controller. Application server is designed for archiving data received from the measuring devices.

The main interface between the operator console and others communication nodes of the microgrid control system is the central controller. It is the main responsible for the management of the microgrid. for the optimization of the microgrid operation. The central controller operates in real time. Its main functions are: connection and disconnection of the microgrid, the synchronization process, the detachment of loads. In addition, the aims of the central controller are: to collect information from the measuring devices; to transfer data from the operator console and the application server; to manage the power supply switches; and to transmit the control commands to the local controllers.

Group of the local controllers are related to the third hierarchical control level. They include microsource controller that located in the distributed resources of the microgrid. It manages active and reactive power production levels at the diesel generator. Moreover, the microsource controller is responsible for the maintaining desired steady-state and dynamic performance of the power network. The other local controllers are located in the two UPS systems. Their main goals are to provide management of charge of the batteries storage.

1. Measuring process

2. Information required by the proposed monitoring and control system is voltage, current, power, energy, and power factor measurements. Real-time information is acquired through the intelligent measuring devices located at the output of the energy source, at the input of each loads, and at the both UPS systems. In this system, Allen-Bradley Powermonitor 3000 [25] is used to measure these instantaneous values. It implements real-time power monitoring with 50 ms selectable update rate. Such operating information is displayed in real-time for monitoring and energy management purposes.

3. Communication network

4. A communication infrastructure is needed between the central controller and the local controllers [23]. The short geographical span of the microgrid may aid establishing a communication infrastructure using low-cost communications. The adoption of standard protocols and open technologies allows designing and developing modular solutions using off-the-shelf, low-cost, widely available, and fully supported hardware and software components.

At the present time, many low cost microcontrollers include at least an Ethernet controller, standalone cheap controllers are also available. The main advantages of using Ethernet are: the transition from a centralized control to a distributed control; wiring reduction - no need for point to point connections. This solution provides flexibility and scalability for low-cost implementations.

Taking these into account, the Ethernet industrial protocol has been chosen in this microgrid as communication network for data transfer for all those control units. The amount of data to be exchanged between network controllers includes mainly messages containing set-points to LC, information requests sent by the MGCC to LC about active and reactive powers, and voltage levels and messages to control microgrid switches. The LC is responsible of collecting local information from the attached energy resource and takes some real-time decisions based on the control algorithm. The communication network of the control system is illustrated in Fig.6. Every communication node has to get registered to the master server. The node sends its information to the master server through diverse communication channel. Furthermore, this topology provides an opportunity for immediate control center access via remote consoles and web based laptops for necessary actions to be taken.

To include new generation resources or storage devices in a flexible manner into the microgrid, multi-agent technologies [40] might be applied. The proposed hierarchical control scheme provides a flexible platform to make high level decisions.

Control Strategy of Operation of the Microgrid

A microgrid may operate either connected to the main grid or disconnected from it. There are two steady states of operation, grid-connected (Mode-G) and islanded (Mode-I). Furthermore, there are two transient modes of operation, transfer from Mode-G to Mode-I and transfer from Mode-I to Mode-G. The key issue of the control is how to maintain the voltage and frequency stability of the microgrid [20].

Grid-connected mode

In the grid-connected operation mode, the main function of a DG unit is to control the output real and reactive power. The real and reactive power generated by a DG can be controlled through current or voltage regulation, thus the DG output power control schemes can be generally categorized as current-based and voltage-based power flow control [43].

During Mode-G operation, the voltage and frequency of the microgrid is set by the main grid. The aim of the uninterruptible power supply systems is to obtain energy backup as much as possible, so during Mode-G operation, the main grid, the microgrid or both of them, will charge the batteries [20].

In grid-connected mode the balance between the generation and the consumption as well as the control of the parameters of the system is guaranteed by the utility grid. Thus, generators are regulated with the criterion of optimized economic exploitation of the installation [23]. Concerning the programmable generator, the objective of the control is to optimize the microgrid performance.

1. Islanded mode

2. The MG operates autonomously, in a similar way to physical islands, when the disconnection from the main grid occurs [37].

   When the grid is not present, the ARL2 disconnects the microgrid from the grid, starting the autonomous operation.

   The instant at which the intentional islanding occurs must be detected in order to the inverter changes between grid-connected to intentional island modes. The detection is achieved using an algorithm described in [23].

   When the main distribution network is faulted, the fault current will flow into the main grid from the microgrid continuously. At the same time, the circuit breaker of microgrid should detect the frequency and voltage-drop, and open in time, which makes the microgrid disconnect automatically from the main grid and change to islanded operation mode. Diesel generator should adopt the reasonable control strategies to ensure the stability of frequency and voltage in microgrid [42].

   While switched from Mode-G to Mode-I, the UPS system operates in voltage control mode, is setting the voltage and frequency of the microgrid through absorbing or releasing energy.

   In islanded mode, due to the unavailability of the utility grid, two requirements must be fulfilled: the power balance between the generation and the consumption and the control of the main parameters of the installation (voltage amplitude and frequency). In synchronous islanded mode this reference is the same as the grid voltage. This mode is also called synchronization mode and it is the mode that necessarily precedes a reconnection with the grid. The control system is responsible for assuring the power balance. In case of energy excess the management system can limit the output power of the diesel generator's power in order to avoid the operation in extremely inefficient low power generation modes. On the contrary, if all the available power is not enough to feed the local loads, the management system will detach non-critical loads. The control system is voltage controlled and it regulates the main parameters of the system.

   The UPS systems sets the voltage and frequency of the islanded microgrid and maintains them within acceptable limits by injecting or absorbing active power and reactive power as required. As soon as the presence of mains is detected, the microgrid control system uses feedback information from the mains voltage to adjust the energy storage unit voltage and frequency control loops to synchronize the microgrid voltage with the main voltage of the main grid.

3. Transition from Grid-Connected to Islanded Mode

4. There are various islanding detection methods proposed for DG systems [44].

   As mentioned above, there is a different control strategy when the laboratory-scale microgrid system operates in Mode-G or Mode-I. If there is a transition between these two modes, the control mode of the battery inverter will change. A switching circuit, as shown in Fig.7, is designed to realize this transition [20].

   A load-voltage control strategy proposed by [23] is employed to provide the operation of the microgrid.

   Disconnection of the microgrid from the grid can be provoked by many causes, like unsatisfactory grid voltage (in terms of amplitude or waveform) or even economic aspects related to power price. In order to monitor grid voltage characteristics a Voltage monitoring module is required. This module measures continuously the rms grid voltage comparing it with a preestablished threshold value. When any of the phase voltages goes down the threshold value (0.9 pu in this case) the detection signal is activated. If 20 ms after the first detection this signal is still activated the microgrid must be disconnected from the utility grid and it must pass to islanded operation mode, otherwise the microgrid will remain connected to the utility grid. This way unnecessary islandings are avoided and selectivity is respected. A 20 ms time window has been chosen after verifying through experimental tests and standards [47] that a personal computer (which is considered as the most critical residential load in this microgrid) is not affected by a 20 ms voltage interruption. As soon as the microgrid is disconnected from the grid, the programmable generator controller passes from a power control mode to a voltage control mode. Microgrid power consumption is also continuously measured in order to detach non-critical loads if there is no enough local available power. In addition if consumption or generation conditions are modified and it becomes possible to feed all the local loads, non-critical loads will be reconnected.

5. Transition from Islanded to Grid-Connected Mode

6. When the grid-disconnection cause disappears, the transition from islanded to grid-connected mode can be started. To avoid hard transients in the reconnection, the diesel generator has to be synchronized with the grid voltage [23]. The DG is operated in synchronous island mode until both systems are synchronized. Once the voltage in the DG is synchronized with the utility voltage, the DG is reconnected to the grid and the controller will pass from voltage control mode to current control mode.

   When the microgrid is working in islanded mode, and the ARL2 detects that the voltage outside the microgrid (in the grid) is stable and fault-free, we have to resynchronize the microgrid to the frequency, amplitude and phase of the grid, in order to reconnect seamlessly the microgrid.

   If the grid-disconnection cause disappears and the gridvoltage fulfills the desired requirements, the transition from islanded to grid-connected mode can be started. The grid voltage conditions will be again monitored by the Voltage monitoring module. This way if the grid voltage exceeds the threshold value the detection signal is deactivated. If 20 ms after the first detection the detection signal is still deactivated it means that utility grid has returned back to normal operating conditions and the microgrid can reconnect to the grid. However, before the reconnection, the microgrid has to be synchronized with the grid voltage in order to avoid hard transients in the reconnection. To do so, the microgrid operates in synchronous islanded mode during 100 ms with the aim of decoupling the reference variation and the physical grid reconnection transients. In this operating mode the voltage in the microgrid is set to the characteristics of the grid voltage, frequency and phase. Once the voltage in the microgrid is synchronized with the utility voltage the microgrid can be reconnected to the grid and the programmable generator controller will pass from a voltage control mode to a power control mode. In the same way if non-critical loads are detached they are also reconnected.

In the presence of unplanned events like faults, microgrid separation from the MV network must occur as fast as possible. However, the switching transient will have great impact on microgrid dynamics.

The microgrid functionalities as well as its control methods depend on the mode of operation [23]:

Islanding of the MG can take place by unplanned events like faults in the MVnetwork or by planned actions like maintenance requirements. In this case, the local generation profile of theMG can be modified in order to reduce the imbalance between local load and generation and reduce the disconnection transient [48].

Conclusions

In this paper the microgrid system installed at the Tallinn University of Technology, has been presented. The microgrid includes a diesel generator, batteries storage with power electronic interface.

The architecture of the microgrid for the Tallinn University of Technology and a control system structure for the microgrid were proposed. Design of a control and monitoring system for a microgrid is presented in this paper. A hierarchical control scheme is proposed.

This will enhance the reliability and stability of the microgrid on one end and will make microgrid an easy to use product on the other.

Acknowledgement

This paper was supported by the Project DAR8130 "Doctoral School of Energy and Geotechnology II".

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