Review of Microgrid Technology Hartono BS
Budiyanto
Rudy Setiabudy
Faculty of Engineering. Universitas Indonesia Depok 16424
[email protected]
Faculty of Engineering. Universitas Indonesia Depok 16424
[email protected]
Faculty of Engineering. Universitas Indonesia Depok 16424
[email protected]
From the electric utility provider implementation of distributed generation systems with the ability microgrid can reduce the power flow on transmission and distribution lines, so as to reduce losses and reduce costs for additional power. Moreover microgrid can also reduce the load on the network by eliminating the impasse in meeting electricity needs and help repair network in case of errors[3]. Implementation of microgrid system will also help improve the reduction of emissions and the threat of climate change.
Abstract—The use of renewable energy source (RES) in meet the demand of electrical energy is getting into attention as solution of the problem a deficit of electrical energy. Application of RES in electricity generation system is done in a variety of configurations, among others in microgrid system. Implementation of microgrid systems provide many advantages both from the user and from the electric utility provider. Many microgrid development carried out in several countries, because microgrid offers many advantages, including better power quality and more environmentally friendly. Microgrid development concern in technology generation, microgrid architecture, power electronics, control systems, protection systems. This paper reviewing various technological developments related to microgrid system and case study about microgrid system development using grid tie inverter (GTI). Microgrid system can implemented using GTI, power transfer can occur from GTI to grid when GTI has power excess and grid supplying power to GTI when GTI power shortage.
Microgrid development done by many countries since microgrid offers many advantages such as better power quality and more environmentally friendly. Moreover the economic potential that may still be used from this system is the opportunity to utilize the waste heat from the engine generator using a combined heat and power (CHP). Application of this system with RES as an alternative generation system in the future. Surely this system requires the operating mechanism and a sophisticated control system to make the finger with a reliable and efficient, and it can all be met by the microgrid[4].
Keyword : microgrid, microgrid architecture, microgrid control and managemen, grid tie inverter
II. ARCHITECTURE OF MICROGRID
I. INTRODUCTION
Microgrid system operate at a low voltage distribution, and has several distributed energy resources. Microgrid system also has the ability to operate connected to the grid (on grid) or disconnected to the grid (off grid/islanded) [5].
The use of renewable energy source (RES) in meet the needs of electrical energy is getting into attention as solution of the problem a deficit of electrical energy, particularly for areas that are difficult to reach existing power grids. A variety of development related to the use of RES continues. Start from optimizing the use of energy sources, the development of the power conversion system up to the electrical power system architecture. Application of RES in electricity generation systems performed in a variety of configurations. Starting from the most simple systems such as the utilization of PV in solar home systems (SHS) to the application of RES in the microgrid system.
The microgrid structure consists of several types of distributed energy sources (DER) such as solar panels, wind turbines, microturbin, thermal power plant each in the form of distributed generation (DG), including energy reserves from battery (Distributed Storage/DS).
Production of electric power from RES such as solar power generation varies greatly depending on the source of the sun received at the time. This raises concerns on quality of generated power, especially if it is connected to the grid system, where solar power would be seen as a negative expense by net system because it has characteristics associated with uncontrolled fluctuation from energy sources [1]. This problem can be addressed by adding another generation systems more controlled, such as, the addition of energy storage systems (batteries) or forming a hybrid system by adding diesel generators or micro turbines[2]. Implementation of microgrid systems provide many advantages both from the user and from the electric utility provider. From the user's application of the microgrid is connected to the grid, it can improve network quality, reduce emissions and can reduce the cost to be incurred by the user.
978-1-4673-5785-2/13/$31.00 ©2013 IEEE
(a)
127
Quality in Research 2013
Stabilization of the microgrid system in the face of fluctuating energy sources and load changes. Enables load sharing operation in microgrid system. Reduce the loads spikes and electrical interference Backup energy source
Switch interconnection technology in microgrid system utilizing digital technology using Digital Signal Processor (DSP) and equipped with communication devices, while meeting the IEEE 1574 standard network interconnection. To improve the response speed semiconductor switch used technologies such as thyristors and IGBT, Figure 3. Circuit Breaker DG CT
(b) PT
Figure 1. Architecture of Microgrid (a) AC microgrid (b) DC microgrid
3 PT
Load 3
3
Microgrid electrical connection points that connected to the low voltage network in the PCC (point of common coupling) that connected to the DG, DS and loads, which consist of several types of loads such as residential, commercial buildings, campuses and industrial complexes. As shown in Figure 1, architecture of microgrid organized as AC microgrid (AC bus) or DC microgrid (DC bus) or combine of both [6].
CT
DSP: Relay + Comm + Monitoring/ Diagnostic
Measurement Communication
Utility Grid
III. TECHNOLOGY OF MICROGRID
Figure 3. A schematic diagram of circuit breaker on connection to the grid
Operation of microgrid system can not be separated from technologies that support from each part that make up the microgrid system, as the source of energy (distributed generation), energy storage, interconnect switches and microgrid control system. Technologies in energy sources distributed generation include the utilization of renewable energy sources such as photovoltaic, wind turbines, and fuel cells. Several power systems improve efficiency by implementing the use of flue gas using CHP technology (combined heat and power) as microturbin, figure 2.
Control system technologies in microgrid can be grouped in two modes of operation are connected to the network mode and isolated mode (islanding). The control system is intended to regulate the stability of microgrid operation particularly in frequency and voltage to maintain stability in face of changes in load and interconnection with other networks. The control system is applied to power converter technology in regulating active and reactive power suply, applying droop control and frequency control [7]. IV. CONTROL AND MANAGEMENT OF MICROGRID Control system in microgrid contrast to conventional power systems, this is due to several reasons, among others: Steady state and dynamic characteristics of microgrid different from conventional plants microgrid possesses inherent unbalanced load due to one phase loads The supply of power from microgrid can come from uncontrolled sources such as wind The role of energy storage is very large in the control mechanisms used microgrid accommodate disconnection and connection mechanisms to maintain expenses during its operating microgrid requires initial requirements of power quality or service preferences for certain types of loads
Figure 2. Microturbin Technologies in energy storage microgrid systems which include battery, super capacitor and flywheels. Energy storage in microgrid system is used among others to:
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TABLE I. SEVERAL TYPES OF MECHANISMS CONTROL USED IN DER Main Energy Sources DG conventional
Interface/inversion
Long time storage (DS)
reciprocating engine small hydro Wind Turbin fixed speed Wind Turbin variabel speed Microturbine Solar Panel Fuel cell battery storage
Short time storage (DS)
Flywheel
DG Non conventional
Super Capacitor
synchronous generator induction generator Power electronics converters (conversion AC-DC-AC) Power electronics converters (conversion DC-DC-AC) Power electronics converters (conversion DC-DC-AC) Power electronics converters (conversion AC-DC-AC) Power electronics converters
As described previously microgrid consists of DER configuration, loads with classified characteristics and management control systems and microgrid. DER may include distributed generation (Distributed Generation/DG) or distributed storage systems (Distributed Storage/DS). Diagram of DG on the microgrid system consists of primary energy sources, media interface and interconnect switches. A DS can be a major energy source for the DG. Moreover main energy source can be generated using a rotary machine /spinning and generating device that consists of power electronics converters.Both provide concepts, strategies and characteristics of different controls. Moreover control strategy and operation of interconnect system, as well as energy/power management used largely determined the type of DER technologies in use, the type of load demand and the expected operating scenarios. Several types of control mechanisms used in DER described in Table I. PV
Grid-side Converter
AVR Control and Governor (+P,±Q) stall or pitch control of turbine (+P,-Q) Turbine speed control and DC link voltage control (+P,±Q) MPPT Control and DC link voltage control (+P,±Q) state of charge and / or control output voltage / frequency (±P/±Q) state of charge (±P,±Q) Speed control (±P,±Q)
In context of power flow control DER units can be grouped into unit dispatchable energy (power output can be regulated) and non-dispatchable (power output can not be adjusted). In dispatchable energy unit output power setting is set externally using supervisory control such as AVR, while for non-dispatchable energy unit output power settings based on the maximum power that can be generated using MPPT concept.A non-dispatchable energy units can be converted into dispatchable energy units using additional energy storage systems and power electronic circuit converter dc-dcac. In addition to provide faster response electronic converters also able to limit short circuit contribution not less than 200% from current capability and to prevent damage due to currents, figure 4. Stability of microgrid operation was also obtained by setting the loads connected to the network, especially on non-critical loads. Critical loads more attention than the other loads that are not critical. Settings done in several ways including termination control loads in order to maintain the stability of the voltage and frequency. The distinction between loads service, improved power quality and reliability for certain expenses.
PV Inverter PES-side Converter
Power Flow Control
VPC
...
VDC
PV
Control Method of microgrid
(a)
The aim control mechanisms of microgrid is to regulate voltage and frequency, as well as reactive and active power output, to fit the setting. Microgrid control strategies can be grouped into several alternative control as shown in table II.
PV Inverter + Battery Storage VPC VDC
PV
TABLE II. CLASSIFICATION OF DER UNIT CONTROL METHOD Battery Storage
Genset
Control method
(b)
Non-interactive control method Interactive control method
Hybrid Structure VPC VDC
Grid following control Power Export (with/without MPPT) Power dispatch real and reactive power support
Grid forming control Voltage and frequency control Load sharing (droop control)
In non-interactive control strategies, output power settings carried out independently while at interactive control strategies output power setting performed as command from control unit. Each control strategy is divided into gridfollowing and grid-forming control. In grid-following control, settings power output including voltage and frequency are determined by the microgrid. Setting output active power and reactive power can use a synchronization
Short-Term Storage (c)
Figure 4. Configuration of non dispatchable (a) and dispatchable (b) and (c) 129
control strategy framework "dq0". In this control strategy current in each phase is used as an input to determine PWM voltage that will regulate power output. Magnitude of input current is transformed into massive d axis and q axis which then transformed into Vabc voltage magnitude. Mechanisms of active and reactive power dispatch by an energy management system to perform optimization strategy based on potential energy profile. Moreover setting output power is also determined by load profile, Figure 5. d-Reference Controller
Va (ref)
id
abc
ib
d-q
ic
In decentralized supervisory control strategies each LC has ability to determine operating autonomy of energy production that will be generated by LC. The main purpose of control strategy in each LC is not aimed at increasing financial income but rather to overall performance of microgrid. So at each LC already has economic parameters, environmental conditions/ potential energy (weather) and the estimated load. One method of control that can be applied to this system is using Multi Agent System (MAS). MAS is a evolution form of classical control of distributed control systems with the ability to control large and complex entity. The main feature of MAS is the ability to incorporate elements of intelligence in each local control (LC). Configuration of MAS system on a microgrid as shown in figure 7.
id (ref) +
ia
determine LC operations include setting LC production capacity. The amount of loads to be served and amount of market price for energy optimization of LC in determining biding further production capacity to MCC.
d-q
Vd
current Control
iq
d-q
Vq
abc
Vb (ref)
PWM Signal
Vc (ref)
Q
+ q-Reference Controller
Q iq (ref)
Grid Level
Figure 5. dq control on DER unit inverter
DNO
In grid-forming control power settings, including output voltage and frequency, by DER units and will be followed by another DER units. DER units that implement grid-forming should have a greater energy potential. Droop control strategies made using voltage and frequency droop control. Determination of allowable output power according to droop characteristics given parameters, as shown in figure 6,
………...
Agent
Management Level Agent
Microgrid
Microgrid
Agent
MCC
V (p.u.)
LC
LC
LC
Field Level
Agent
Agent Agent
1 + Df 1.0
1.0
1 - Df
1 - Dv
1 + Dv
0
P0
Pn
P
(a)
-Qm
Agent
Microgrid
LC
f (p.u.)
MO
Agent
Figure 7. Multi Agent System configuration on microgrid 0
Q0
Qn
Q
(b)
Figure 6. DER unit droop diagram on grid-forming control Amount change in voltage and frequency of droop characteristics are used as input to dq transformation in order to determine amount of active and reactive power output. To ensure microgrid operation mechanism works as expected then the operation of each DER unit must be coordinated properly for it required method of supervisory control strategy microgrid operation. Microgrid operation supervisory control strategies can be centralized or decentralized [8].
In the centralized and decentralized systems both require reliable data communications facilities. Data communication network can be radio, telephone or power line carrier. Through this communication networks microgrid operation mechanism arranged between each DER unit or the main control system in form of energy management system applications. V. TECHNICAL CHALLENGES ON MICROGRID As a new paradigm of power systems, implementation of microgrid still face many obstacles. Less understanding about microgrid and unfavorable government policies become an obstacle in applying microgrid technology. In general, in addition can be applied as a solution to electricity in remote areas, microgrid technology can also be used as electrical solutions such as urban residential complexes, offices, schools and others. In which implementation of microgrid technology will provide advantage compared if have to build a new transmission and distribution network. Advantages and disadvantages in applying microgrid technology among others [9]:
Managing control of microgrid On centralized supervisory control strategy, amount of power output from each LC (Local Control) is determined by MCC (Microgrid Control Center) based on input (biding) production capacity owned by each LC. MCC based on inputs of LC and operating policy that covers current energy market prices, estimates needs and production as well as consideration of infrastructure conditions other microgrid, 130
Microgrid Advantages
In this experiment, PV source simulated with variable DC source. a set of Configuration tests looks like in figure 8
Microgrid, have ability, during a utility grid disturbance, to separate and isolate itself from the utility seamlessly with little or no disruption to the loads within the Microgrid. In peak load periods microgrid can prevents utility grid failure by reducing the load on the grid. Microgrid have environmental benefits made possible by use low or zero emission generators. In microgrid to increasing energy efficiency, the use of both electricity and heat is permitted to get closer the generator to user. Microgrid can act to mitigate the electricity costs to its users by generating some or all of its electricity needs.
A
GTI
A V
V
AC
A V
Microgrid system using GTI
In this trial used 2 pieces of GTI and a local load. Measurements were taken to see the distribution of power flow of each GTI and power flow from PLN in meeting needs of power required by load.
In microgrid, that must be considered and controlled voltage, frequency and power quality parameters to acceptable standards whilst the power and energy balance is maintained. Electrical energy needs to be stored in battery banks thus requiring more space and maintenance. The difficulty of resynchronization with the utility grid. Microgrid protection is one of the most important challenges facing the implementation of Microgrids. Issues such as standby charges and net metering may pose obstacles for Microgrid. Interconnection standards needs to be developed to ensure consistency. IEEE P1547, a standard proposed by Institute of Electrical and Electronics Engineersmay end up filling the void.
Gambar 9.
GTI output voltage on grid condition
The measurement results of GTI output voltage shown in Figure 9. The form of GTI output voltage in accordance with PLN. Power measurements performed on each output of inverter, load and line to PLN. The measurement begins by activating the GTI without being connected to the PLN, characterized by value of PLN power = 0W as shown in table III.
Future direction which require further investigation in the context of microgrid research are [10]:
V
Figure 8.
Future Direction On Microgrid Research
GTI
A
Microgrid Disadvantages
A V
To investigate full-scale development, field demonstration, experimental performance evaluation of frequency and voltage control methods under various operation modes Transition between grid connected and islanded modes on interaction phenomena between distribution generation and high penetration of distributed generation Analysis the issue of black starting in an unbalanced system on the control, protection and power quality Transformation of microgrid system today into the intelligent, robust energy delivery system in the future by providing significant reliability and security benefits.
TABLE III. POWER FLOW IN MICROGRID SYSTEM PGTI1 (W)
PGTI2 (W)
PL (W)
Ppln (W)
0 122 120 120 0
0 0 40 40 0
0 170 170 0 0
0 55 14 -158 0
From the data presented can be seen that when there is no supply from PLN then both the GTI does not generate power to the load. When GTI 1 (PGTI1) supplied power to load and GTI 2 (PGTI2) has not been issued, then power to load (PL) supplied by GTI 1 and PLN (PPLN). When GTI2 start generating power then the power of the PLN decreases proportional to the input power of GTI2. When there is no power supplied to load (load disconnected) power of both GTI supplied to PLN. Minus sign indicates direction of power flow towards PLN grid. When the source of PLN disconnected (isolated/islanding), as seen in last data table, PLN power = 0W, then both the GTI no output power again. The measurement results show that the number of power
VI. MICROGRID DEVELOPMENT USING GTI Microgrid systems can be implemented using grid tie inverter (GTI). The weakness of this system is when there is no grid, the system can not work because GTI will work if there is a power from grid that is used as a reference of GTI operation. microgrid trials using GTI have been conducted in laboratory. 131
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does not show balance of power, this can coused each power output measurements performed using different tools. VII. CONCLUSION Microgrid system is an alternative electricity network that can be used to meet the electricity needs of the future. In the microgrid system consists of multiple power sources, which can use renewable energy sources. Microgrid system works autonomously so it requires a complex control system to regulate the operation of microgrid. Implementation of microgrid can be done using the inverter GTI. Microgrid with GTI can transfering power to/from grid. Power transfer can occur from GTI to grid when GTI has power excess and grid supplying power to GTI when GTI power shortage. Power sharing between parallel GTI based on input power of each of GTI, if there is shortage of power will be supplied from grid. REFERENCES [1] [2]
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