Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE A REVIEW ON VECTOR CONTROL AND SVPWM APPROACH FOR GRID IN
POWER SYSTEM
1Anand, 2Govind Prasad Pandiya Bhopal Institute of Technology and Science 1. INTRODUCTION
In order to solve this problem, a secondary controller implemented in the microgrid central control can restore the frequency and amplitude in the microgrid.[1] The conventional secondary control approach relays on using a Micro Grid Central Controller (MGCC), which includes slow controls loops and low bandwidth communication systems in order to measure some parameters in certain points of the MG, and to send back the control output information to each DG unit[3] Microgrid control is designed to facilitate an intelligent network of autonomous units. Microgrids have an interfaces witch, DER units and loads. The interface switch has the ability to autonomously island the microgrid from disturbances such as faults, IEEE 1547 events or power quality events.
1.1 Distributed Secondary Control The distribution system provides major opportunities for smart grid concepts. The distribution system provides major opportunities for smart grid concepts.
There is need of MGCC for coordination of units during black start process and among other management functionalities of MG. The initial idea is to implement primary and secondary controllers together as a local controller Primary and secondary controls are implemented in each DG unit. The secondary control is placed between the communication system and the primary control.
Frequency control, voltage control and reactive power sharing will also be reviewed by using this control approach.[3]
For distribution systems to utilize the emerging diversity of DER technology at significant levels of penetration the basic distribution pyridine needs to be rethought. Managing such a wide and dynamic set of resources and control points can become over whelming. The best way to manage such a system is to break the distribution system down into small clusters or microgrids, with distributed optimizing controls
coordinating multi microgrids. The applications include power support at substations, deferral of T&D upgrades, high fuel efficiency through capturing waste heat, use of renewable energy, higher power quality and smarter distribution systems.
1.2 Der-Based Distribution
Basic issue for DER is the technical difficulties related to control of a significant number of distributed energy sources. Using DER in the distribution system reduces the physical and electrical distance between generations and loads.
For voltage drops, faults, blackouts etc.
the DER with local loads needs to switch to island operation. The two major benefits of DER-based distribution are
Increased efficiencies using waste heat
Reduction of line losses and enhanced customer reliability through islanding during a power system outage.
2. LITERATURE SURVEY AND BACKGROUND
2.1 Overview
In this section we discuss about an extended literature survey on the microgrid control model. We have studied various research and journal papers related to automatic microgrid control system. According to our research they have analyzed that many of the authors focuses on the problem of better controlling of microgrid and smart grid technique for it. Few review of summary described here and implicated with their respective author.
Josep M., MukulChandorkar, Tzung-Lin Lee, Poh Chiang Loh Et.al.
[1] authors discuss on advanced control techniques for microgrids. This paper covers decentralized, distributed, and hierarchical control of grid connected and islanded microgrids. A control scheme for regulating power flows in a hybrid ac-dc microgrid interlinked by power converters was explained. Through proper sensing of
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE the network information from localized
quantities and normalization, results show that the converters are capable of enforcing rated proportional active power sharing among all sources. This sharing is achieved with no dependence on the source natures (AC or DC) and their physical placements within the hybrid microgrid.
Future work is also expected in terms of cooperative control for power quality enhancement in microgrids, for instance in the area of electrical vehicles.
In these applications, huge charging current of electrical vehicle may deteriorate power quality. In this case, cooperative control can be integrated into the vehicle charger to assist improving voltage fluctuations and voltage harmonics in the low-voltage distributed system.
Another important issue is that PV or WT power converters, could used additional capacity power rating not only to inject or absorb reactive power, but also to improve the power quality in microgrids. A control scheme for coordinating DSs in microgrids, where DGs and localized loads, was reviewed. An alternative set of droop characteristics and technique for determining control references, that are different from those of DGs, are formulated for DS control.
Earlier results have already verified that the presented DS control can autonomously sense for excess generation capacity or supply-demand unbalance, before deciding on the appropriate amount and direction of active power flow.
Shafiee, Qobad, Guerrero, Josep M. Quintero, Juan Carlos Vasquez Et.
al. [3] describes in this paper a novel approach to conceive the secondary control in droop-controlled Micro Grids.
The conventional approach is based on restoring the frequency and amplitude deviations produced by the local droop controllers by using a Microgrid Central Controller (MGCC This paper has introduced a distributed control strategy for droop controlled MGs. In this method, a decentralized secondary control encompasses every DG unit local controller and the communication system.
Thus producing an appropriate control signal to be locally sent to the local primary controller. In this sense, the
failure of a DG unit will fail down only that individual unit and other DGs can work independent. Thus, adding more DG units is easy, making the system expandable.
However, still having a MGCC is mandatory to achieve some other purposes like coordination of the MG units in black start process or energy management. The concept is evaluated based on the system performance in a laboratory case study with the goal of regulating voltage and frequency, and at the same time properly sharing reactive power between DG units.
Furthermore, the impact of communication system delay as well as data drop-out over the MG has been compared between the proposed decentralized secondary control system and the conventional centralized one. The results experimental showed that the proposed control strategy has a good performance in removing frequency and voltage steady state errors and can share reactive power between DG units perfectly. Even though the proposed secondary control needs more information interchange capability, however, it shown higher robustness in front large communication latency delays and date drop-out.
Florian Dorfler, John W.
Simpson-Porco, FrancescoBullo Et. al.
[9] authors discuss on control strategies for three layers and illuminate some possibly-unexpected connections and dependencies among them. Building from a first-principle analysis of decentralized primary droop control; they study centralized, decentralized, and distributed architectures for secondary frequency regulation. They find that averaging-based distributed controllers using communication among the generation units offer the best combination of flexibility and performance.
They further leverage these results to study constrained AC economic dispatch in a tertiary control layer.
Surprisingly, they show that the minimizers of the economic dispatch problem are in one-to-one correspondence with the set of steady-states reachable by droop control. In other words, the adoption of droop control is necessary and sufficient to achieve economic optimization. This equivalence results in
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE simple guidelines to select the droop
coefficients, which include the known criteria for power sharing. They illustrate the performance and robustness of our designs through simulations. Modeled after the hierarchical control architecture of power transmission systems, a layering of primary, secondary, and tertiary control has become the standard operation paradigm for islanded microgrids.
Despite this superficial similarity, the control objectives in microgrids across these three layers are varied and ambitious, and they must be achieved while allowing for robust plug-and-play operation and maximal flexibility, without hierarchical decision making and time- scale separations. They studied decentralized and distributed primary, secondary, and tertiary control strategies in microgrids and illuminated some connections between them. Thereby, they relaxed some restrictions regarding the information structure and timescale separation of conventional hierarchical control strategies adapted from transmission-level networks to make them more applicable to microgrids and distribution-level applications. While this work is a first step towards an understanding of the interdependent control loops in hierarchical micro grids, several complicating factors have not been taken into account.
In particular, our analysis is only local and so far formally restricted to acyclic networks with constant resistance-to reactance ratios. Moreover, future work needs to consider more detailed models including reactive power flows, voltage dynamics, and ramping constraints on the inverter injections. In preliminary work they extend the present analysis to cyclic networks possibly with higher-order generator dynamics in transmission grid settings, and we provide some first guarantees on the region of attraction. Finally, another interesting direction for future work is to remove the idealistic communication assumptions and resort to sampled or event triggered schemes in presence of delays. Event- triggered or deadband-enforcing control could also be useful for relaxing frequency regulation by ignoring sufficiently small deviations.
Ritwik Majumder, Balarko Chaudhuri, Arindam Ghosh, Rajat Majumder, Gerard Ledwich, FiruzZare Et. al. [10] authors presents in this paper investigates the problem of appropriate load sharing in an autonomous microgrid.
High gain angle droop control ensures proper load sharing, especially under weak system conditions. However it has a negative impact on overall stability.
Frequency domain modeling, Eigen value analysis and time domain simulations are used to demonstrate this conflict. A supplementary loop is proposed around a conventional droop control of each DG converter to stabilize the system while using high angle droop gains. Control loops are based on local power measurement and modulation of the d- axis voltage reference of each converter.
Coordinated design of supplementary control loops for each DG is formulated as a parameter optimization problem and solved using an evolutionary technique.
The supplementary droop control loop is shown to stabilize the system for a range of operating conditions while ensuring satisfactory load sharing. Load sharing in an autonomous microgrid through angled roop control, instead of commonly used frequency droop, is investigated in this paper. High gain angle droop control ensures proper load sharing, especially under weak system conditions, but has a negative impact on the overall stability. This is illustrated through frequency domain modeling, Eigen value analysis and time domain simulations.
A supplementary loop is proposed around the primary droop control loop of each DG converter to stabilize the system despite having high gains that are required for better load sharing. The control loops are based on local power measurement and modulation of the axis voltage reference of each converter. The coordinated design of supplementary control loops for each DG is formulated as a parameter optimization problem and is solved using an evolutionary technique.
The use of the supplementary droop control loop is shown to stabilize the system for a range of operating conditions while ensuring satisfactory load sharing.
Ritwik Majumder, Arindam Ghosh, Gerard Ledwich, FiruzZare Et.al. [11] authors in this paper
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE describes control methods for proper load
sharing between parallel converters connected in a microgrid and supplied by distributed generators (DGs). It is assumed that the microgrid spans a large area and it supplies loads in both in grid connected and islanded modes. A control strategy is proposed to improve power quality and proper load sharing in both islanded and grid connected modes. It is assumed that each of the DGs has a local load connected to it which can be unbalanced and/or nonlinear. The DGs compensate the effects of unbalance and nonlinearity of the local loads. Common loads are also connected to the microgrid, which are supplied by the utility grid under normal conditions.
However during islanding, each of the DGs supplies its local load and shares the common load through droop characteristics. Both impedance and motor loads are considered to verify the system response. The efficacy of the controller has been validated through simulation for various operating conditions using PSCAD. It has been found through simulation that the total Harmonic Distortion (THD) of the of the microgrid voltage is about 10% and the negative and zero sequence component are around 20% of the positive sequence component before compensation. After compensation, the THD remain below 0.5%, whereas, negative and zero sequence components of the voltages remain below 0.02% of the positive sequence component. In this paper a local load sharing technique is proposed for a distributed microgrid. The controllers are capable of compensating the local unbalanced and non linear loads.
The local loads can be shared with utility in any desired ratio. The common loads which are normally supplied by the utility in grid connected mode, shared among the DGs proportional to their rating in the islanded mode. A smooth transfer between the islanded and grid connected mode assures as table operation of the system. The controller efficacy is checked both with impedance and motor loads. The application is mainly aimed at rural area where unbalanced load is common and wireless communication is always desirable due to the large network size. Similar to any droop control method, the distance among
the load and DG determine the line impedance between them and that impedance has impact on the load sharing. However load sharing can be made more accurate by incorporating the line impedance values in the power reference calculation.
Yun Wei Li, Ching-Nan Kao Et.
al. [12] authors define, a power control strategy is proposed for a low-voltage microgrid, where the mainly resistive line impedance, the unequal impedance among distributed generation (DG) units, and the microgrid load locations make the conventional frequency and voltage droop method unpractical. The proposed power control strategy contains a virtual inductor at the interfacing inverter output and an accurate power control and sharing algorithm with consideration of both impedance voltage drop effect and DG local, load effect. Specifically, the virtual inductance can effectively prevent the coupling between the real and reactive powers by introducing predominantly inductive impedance even in a low voltage network with resistive line impedances.
On the other hand, based on the predominantly inductive impedance, the proposed accurate reactive power sharing algorithm functions by estimating the impedance voltage drops and significantly improves the reactive power control and sharing accuracy. Finally, considering the different locations of loads in a multi bus microgrid, the reactive power control accuracy is further improved by employing an online estimated reactive power offset to compensate the effects of DG local load power demands. The proposed power control strategy has been tested in simulation and experimentally on a low-voltage microgrid prototype.
When implemented in a low-voltage microgrid system, this method is subject to a few particular problems, which areas follows.1) the method is developed based on the predominantly inductive line impedance.
In a low-voltage microgrid, as the distribution feeder is mainly resistive, this droop method is subject to poor transient (or even poor stability) due to the real and reactive power coupling among DG units when no additional inductance is present.2) The unequal line impedances and DG output impedances significantly affect the accuracy of reactive power
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE control during grid-connected operation
mode and the reactive power sharing during islanding mode due to the unequal voltage drops.3) The reactive power sharing accuracy is further deteriorated if there are local loads at DG output. To avoid the power control coupling, the virtual real and reactive power frame transformation was recently proposed.
In this paper, a power control and sharing strategy was proposed for power- electronics-interfaced DG units in a low voltage multi bus microgrid. The proposed power control strategy contains a virtual inductor at the interfacing inverter output for real and reactive power decoupling and an accurate reactive power control and sharing algorithm with online impedance voltage drop effect estimation and local load demand effects compensation. The proposed strategy can accurately control the DG output real and reactive powers in both grid-connected mode and islanding mode without physical communications among DG units. The performance comparison between the proposed power control strategy and the traditional droop control method. Both simulation and experimental results are provided to verify the effectiveness of the proposed control strategy.
Shivkumar V. Iyer, Madhu N.
Belur, Mukul C. Chandorkar Et. al. [13]
discuss in this paper a practical microgrid, where the number of inverters may be large or the capacity of the units may differ, it becomes essential to develop a method by which stability can be examined without much computational burden. The system of differential algebraic equations has been simplified using justifiable assumptions to result in a final expression that allows the stability of the microgrid to be examined separately with respect to the droop control laws of each inverter transformed into an equivalent network. Moreover, the procedure allows taking into consideration the R/X ratio of the interconnecting cables.
Analysis of final expressions validate the stability results reported in literature. Experimental results on hardware show the stable operation of the microgrid 1). The dynamics of the inner voltage controller of the inverter has been neglected and the inverter output voltages
have been assumed to be equal to the references generated by the droop control strategy. Since the inner voltage controller is a fast controller with a high bandwidth, while the droop controller is a slow controller with a low bandwidth. 2) The load impedances in the microgrid will not be significantly less than 1 p.u., as smaller values would imply inverter overloads. In an inverter-based microgrid, the overload capacity is limited, as the inverters are likely to be damaged even if overloaded for short durations.
However, the impedances of the interconnecting cables is much smaller in the range of 0.01–0.05 p.u., since the microgrid covers a small area 3) The small values of impedances of the interconnecting cables require small phase angle differences and voltage magnitude differences between inverters to produce power flow between inverters.
As a result, the output voltage phasors of the inverters will be very close to each other in phase angle and magnitude.
Hence, while linear zing the microgrid in the d and q components of the inverter output voltages at the equilibrium point have been assumed to be equal. This assumption is valid for a large class of microgrids with small cable impedances.
4)
The deviation in the voltage frequency of the microgrid is limited to 1%–2% from the nominal frequency (50–
60 Hz), while deviation in voltage magnitude is limited to 2%–4% from the nominal distribution level voltage (230 or 110 V line neutral). The p–ω and q–V droop control gains are therefore maintained to be small of the earlier mentioned orders. The product of two or more droop control gains will be negligible, and therefore, a term multiplied to such a product of more than one gain can be neglected. Simulation results have been presented to show the stability of the microgrid for varying droop control gains. The boundaries of stability are obtained using Scilab.
Savaghebi, Mehdi, Jalilian, Alireza, Vasquez, Juan C., Guerrero, Josep M., Lee, Tzung-Lin Et. al. [14]
Describe in this paper, a method for voltage harmonic compensation in a microgrid operating in islanded and grid connected modes is presented. Harmonic compensation is done through proper
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE control of distributed generators (DGs)
interface converters. In order to achieve proper sharing of the compensation effort among the DGs, a power named Harmonic Distortion Power (HDP) is defined. In the proposed method, the active and reactive power control loops are considered to control the powers injected by the DGs. Also, a virtual impedance loop and voltage and current proportional-resonant controllers are included. Simulation results show the effectiveness of the proposed method for compensation of voltage harmonics to an acceptable level.
In this paper, an approach for compensation of voltage harmonics in a microgrid operating in islanded and grid- connected modes is presented. In order to improve sharing of harmonic compensation effort, a new definition for harmonic distortion power is used. The results show that by using the proposed control approach fundamental and harmonic distortion powers are properly shared between DGs and also the output voltage waveforms are improved.
Applicable for both grid-connected and islanded modes of microgrid operation.
On the other hand, the method of compensation effort sharing is improved.
In the proposed method, the overall control system is designed in the stationary (αβ) reference frame.
Savaghebi, Mehdi Guerrero, Josep M. Jalilian, Alireza, Vasquez, Juan C. Et. al. [15] authors In this paper define, a method for voltage unbalance compensation in an islanded microgrid based on the proper control of distributed generators (DGs) interface converter is proposed. In this method, active and reactive power control loops are considered to control the power sharing among the DGs. Also, a virtual impedance loop and voltage and current proportional-resonant controllers are included. Experimental results show the effectiveness of the proposed method for compensating voltage unbalance to an acceptable level. The approach presented in is based on controlling the DG as a negative sequence conductance in order to compensate the voltage unbalance.
This is done by using the negative sequence reactive power to generate the reference conductance. Then, this conductance is multiplied by the negative
sequence voltage to produce the compensation reference current. In this way, the effort of unbalance compensation can be shared between the DGs. The compensation reference is added to the output of the voltage control loop.
However, such compensation is considered as a disturbance to be rejected by the voltage control loop. Hence, there is a trade-off between unbalance compensation and voltage regulation, which will limit the unbalance compensation capability. To cope with this, the present paper proposes the direct change of voltage reference to compensate voltage unbalance in a microgrid. In this method, the overall control system is designed in stationary (αβ) reference frame. The control structure consists of the following loops:
•
Voltage and current controllers•
Virtual impedance loop•
Active and reactive power controllers•
Voltage unbalance compensator In this paper a novel control approach to compensate voltage unbalance in a microgrid through proper control of DGs interface converter is proposed. The method of extracting positive and negative sequence components of voltage and current is described. These values are used to calculate positive sequence active and reactive powers and negative sequence reactive power. The positive sequence powers are used by the power controllers and negative sequence reactive power is applied for the generation of voltage unbalance compensation reference. The experimental results show that by implementing this method voltage unbalance is well compensated and also the compensation effort is properly shared between the DGs.Jin-Hong Jeon, Jong-YulKim, Hak-Man Kim, Seul-Ki Kim, Changhee Cho, Jang-Mok Kim, Jong-Bo Ahn, Kee- Young Nam Et. al. [16] authors discuss on hardware in-the-loop simulation (HILS) system as a new method to develop and test control algorithms and operation strategies for a microgrid. The HILS system is composed of a real-time digital simulator (RTDS) for real-time simulation of the microgrid, a prototype microgrid management system (MMS) under test, and a communication emulator for interface between the prototype MMS and
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE the RTDS. The prototype MMS is designed
to operate micro sources of microgrid and to control power flow at the point of common coupling (PCC) in the grid- connected mode, and voltages and frequency in the islanded mode of the microgrid. The MMS is tested in the grid- connected mode and in the islanded mode, respectively, to show the validation of the proposed HILS system.
This paper proposes an HILS system as a new method to test functions of control and operation of microgrid. In particular, a developed prototype microgrid management system (MMS) is tested on the environment of the proposed HILS system to show the validation of the HILS system. The proposed HILS system consists of the RTDS, the MMS under test and a developed communication emulator. The RTDS performs the real- time simulation of a specific microgrid which is designed based on a small-scale microgrid pilot plant in the Korea Electro technology Research Institute (KERI). The communication module emulates communication functions between the MMS and components in the microgrid.
The MMS is designed to control power flow at the point of common coupling (PCC), voltages and frequency of the microgrid.
This paper proposed an HILS system as a new method to develop and test control algorithms and operation strategies for microgrid. The proposed HILS test system was composed of the RTDS, a prototype MMS under test, and a communication emulator. The RTDS performed real-time simulation of component models of the microgrid and the communication emulator simulated communication functions of components of the microgrid. The prototype MMS was designed to manage a 50-kVA microgrid pilot plant with a PV/wind hybrid system, a diesel generator, and a BESS. Constant power flow control in the grid connected mode, and frequency and voltage control in the islanded mode by a cooperative control scheme were tested to validate the proposed HILS system.
Joerg Dannehl, Marco Liserre, Friedrich Wilhelm Fuchs Et. al. [17]
discuss in this paper issues related to the reduction of PWM harmonics injection in the power grid are becoming more relevant. The use of high-order filters like
LCL filters is a standard solution to provide the proper attenuation of PWM carrier and sideband voltage harmonics.
However, those grid filters introduce potentially unstable dynamics that should be properly damped either passively or actively. The second solution suffers from control and system complexity (a high number of sensors and a high-order controller), even if it is more attractive due to the absence of losses in the damping resistors and due to its flexibility.
An interesting and straightforward active damping solution consists in plugging in, in cascade to the main controller, a filter that should damp the unstable dynamics. No more sensors are needed, but there are open issues such as preserving the bandwidth, robustness, and limited complexity. This paper provides a systematic approach to the design of filter-based active damping methods. The tuning procedures, performance, robustness, and limitations of the different solutions are discussed with theoretical analysis, selected simulation, and experimental results.
This paper has offered a systematic study of the filter-based active damping with reference to the bandwidth, robustness, and complexity. Different approaches are compared, highlighting their performance and limitations in case of different positions of the current sensors and different resonance frequencies. The tuning procedures are discussed, and the impact on low- and high-frequency harmonic contents is investigated through simulations and experimental results. The notch-filter solution is proven to be the most flexible and effective active damping method even if it is not needed in all the possible depicted scenarios. In fact, the low-pass filter can be enough in case of line- current feedback and a medium–low resonance frequency that is frequently found in medium-power distributed generation systems.
By Robert H. Lasseter Et. al. [23]
authors describes managing significant levels of distributed energy resources (DERs) with a wide and dynamic set of resources and control points can become overwhelming. The best way to manage such a system is to break the distribution system down into small clusters or microgrids, with distributed optimizing
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE controls coordinating multi microgrids.
The Consortium for Electric Reliability Technology Solutions (CERTSs) concept views clustered generation and associated loads as a grid resource. The clustered sources and loads can operate in parallel to the grid or as an island. This grid resource can disconnect from the utility during events (i.e., faults, voltage collapses), but may also intentionally disconnect when the quality of power from the grid falls below certain standards.
This paper focuses on DER-based distribution, the basics of microgrids, possibility of smart distribution systems using coupled microgrid and the current state of autonomous microgrid technology. This paper focuses on DER- based distribution, the basics of microgrids, possibility of smart distribution systems using coupled microgrid and the current state of microgrid technology. An alternative to smart grid concepts using many DER units with a sophisticated command and control systems is to build on microgrid concepts.
Using many microgrids in the distribution system is shown to be straightforward. Many BSmart Grid functions such as improved reliability, high penetration of renewable sources, self-healing, active load control and improved generation efficiencies through the use of waste heat can be implements using coupled microgrids. Microgrid technology has maturing to the point that it is possible to design a full range of microgrid functions from high power quality to utilizing PV sources. Work is need on how to dispatch many microgrids in a distribution system to achieve the desired BSmart Grid objectives.
3. PROBLEM FORMULATION&
LIMITATION
3.1 Smart Metering
In the smart grid, reliable and real-time information becomes the key factor for reliable delivery of power from the generating units to the end-users. The impact of equipment failures, capacity constraints, and natural accidents and catastrophes, which cause power disturbances and outages, can be largely avoided by online power system condition monitoring, diagnostics and protection. To
this end, the intelligent monitoring and control enabled by modern information and communication technologies have become essential to realize the envisioned smart grid.
The Advanced Metering Infrastructure (AMI) is a key factor in the smart grid which is the architecture for automated, two-way communications between a smart utility meter and a utility company. A smart meter is an advanced meter which identifies power consumption in much more detail than a conventional meter and communicates the collected information back to the utility for load monitoring and billing purposes.
Consumers can be informed of how much power they are using so that they could control their power consumption and the consequent carbon dioxide emission.
By managing the peak load through consumer participation, the utility will likely provide electricity at lower and even rates for all. Smart meters enable two-way communication between energy suppliers and domestic loads and the sending of price information, energy measurements and DLC commands. IEEE Standard 1646 specifies the maximum communication time delay for under- frequency load shedding as 10 ms. With many data routing devices in the communication path between the system operator and smart meters, it is unlikely that DLC commands would reach smart meters within 10 ms. The company that provided demand response services in took 2-5 minutes to control loads using DLC, after receipt of the system operators request. Therefore due to the communication and processing delays, DLC may not be able to provide primary response.
3.2 Optimization Control
The primary functionality of the system is to control the domestic generation and buffering technologies in such a way that they are used properly. Furthermore, the required heat and electricity supply and the comfort for the residents should be guaranteed. Some devices have some scheduling freedom in how to meet these requirements. This scheduling freedom of the domestic devices is limited by the comfort and technical constraints and can be used for optimizations.
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE More scheduling freedom can be
gained when residents are willing to decrease their comfort level leading to less restrictive constraints for the scheduling.
This (small) decrease in comfort should lead to benefits for the residents, e.g. a reduced electricity bill. Next to different objectives, control methodologies can have different scopes for optimization: a local scope (within the house), a scope of a group of houses e.g. a neighborhood (microgrid) or a global scope (Virtual Power Plant). Every scope again might result in different optimization objectives.
4. PROPOSED METHODOLOGY AND ARCHITECTURE
Nonlinear current controllers can produce the switching signals required to control grid connected converters directly. They are employed in applications where the PWM modulators are negligible.
Examples of non-linear current controllers are hysteresis controller, neural network controllers and fuzzy logic controllers. Non-linear current controllers have excellent control response. With the exception of hysteresis controller, these controllers are difficult to implement in practical systems.
Hysteresis control is simple and robust. The output of the hysteresis comparator is the state of the switches in the power converter. In the case of a two- level three phase grid connected converter, three hysteresis controllers are required (one for each leg of the converter). Three-phase output currents
of the inverter are detected and compared with a corresponding phase current references individually. The switching signals are produced when the error exceeds an assigned tolerance band.
4.1 Statcom
STATCOM is a member of the Flexible AC transmission systems (FACTS) family that is connected in shunt with ac power systems. STATCOM has played an important role in the power industry since the1980s. STATCOM provides many advantages, in particular the fast response time and superior voltage support capability. STATCOM is used for dynamic voltage control to suppress short term voltage fluctuations because its dynamic performance far exceeds other VAr compensators.
STATCOM is a DC-AC voltage source converter with an energy storage unit, usually a DC capacitor. Power electronic switches are used to derive an approximately sinusoidal output voltage from a DC source. The power circuit diagram of a VSC-based STATCOM is illustrated in where six IGBTs with its anti- parallel diodes and a DC-link capacitor are used to produce the three-phase voltage. The STATCOM is coupled to the ac power grid via coupling inductors Lc. The coupling inductors are also used to filter out the current harmonic components that are generated by the pulsating output voltage of the power converter.
Figure 4.1 Power circuit diagram of a STATCOM
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE 4.2 Current Control Design
The plant for the current control loop design can be derived from the dynamic equations of the STATCOM. The dynamic equations are defined as follows
V Sc dt di c c L c Ri V
V Sb dt di b b L b Ri V
V Sa dt di a a L a Ri V
4.1
Applying the d-q transformation described in appendix A to Equation (4.1), gives:
V Sq Li d dt e
di q q L q Ri V
V Sd Li q dt e
di d d L d Ri V
4.2
Where:-
Vd= Supply voltage: d-axis component
Vq= Supply voltage: q-axis component
id= Line current: d-axis component
iq= Line current: q-axis component
Vsd=STATCOM output voltage: d-axis component
Vsq=STATCOM output voltage: q-axis component
R=Total resistance per phase
L=Total inductance per phase
ω=Angular frequency of the rotating reference frame
If the d-axis of the reference frame is aligned to the grid voltage vector, Equation (4.2) can be written as:-
V Sq Li d dt e
di q q L Ri
V Sd Li q dt e
di d d L d Ri V
0
4.3
To control the d and q axis currents, the STATCOM has to produce associated output voltages according to the reference voltage signals determined from Equation (4.3). The associated output voltages are given as:
Li d e V q
V Sq
V d Li q e V d
V Sd
*
*
4.4 Nothing that,
dt di q q L q Ri V
dt di d d L d Ri V
4.5
Vol.04, Issue 09, September 2019, Available Online: www.ajeee.co.in/index.php/AJEEE REFERENCES
1. Josep M., Mukul Chandorkar, Tzung-Lin Lee, Poh Chiang Loh Et.al. [1] authors discuss on advanced control techniques for microgrids.
2. Shafiee, Qobad, Guerrero, Josep M.
Quintero, Juan Carlos Vasquez Et. al. [3]
describes in this paper a novel approach to conceive the secondary control in droop- controlled Micro Grids.
3. Florian Dorfler, John W. Simpson-Porco, Francesco Bullo Et. al. [9] authors discuss on control strategies for three layers and illuminate some possibly-unexpected connections and dependencies among them.
4. Ritwik Majumder, Balarko Chaudhuri , Arindam Ghosh, Rajat Majumder , Gerard Ledwich, Firuz Zare Et. al. [10] authors presents in this paper investigates the problem of appropriate load sharing in an autonomous microgrid.
5. Ritwik Majumder, Arindam Ghosh, Gerard Ledwich, Firuz Zare Et.al. [11] authors in this paper describes control methods for proper load sharing between parallel converters connected in a microgrid and supplied by distributed generators (DGs).
6. Yun Wei Li, Ching-Nan Kao Et. al. [12]
authors define, a power control strategy is proposed for a low-voltage microgrid, where the mainly resistive line impedance, the unequal impedance among distributed generation (DG) units, and the microgrid load locations make the conventional frequency and voltage droop method unpractical.
7. Shivkumar V. Iyer, Madhu N. Belur, Mukul C. Chandorkar Et. al. [13] discuss in this paper a practical microgrid, where the number of inverters may be large or the capacity of the units may differ, it becomes essential to develop a method by which stability can be examined without much computational burden.
8. Savaghebi, Mehdi , Jalilian, Alireza , Vasquez, Juan C. , Guerrero, Josep M., Lee, Tzung-Lin Et. al. [14] Describe In this paper, a method for voltage harmonic compensation in a microgrid operating in islanded and grid connected modes is presented.
9. Savaghebi, Mehdi Guerrero, Josep M.
Jalilian, Alireza , Vasquez, Juan C. Et. al.
[15] authors In this paper define, a method for voltage unbalance compensation in an islanded microgrid based on the proper control of distributed generators (DGs) interface converter is proposed.
10. Jin-Hong Jeon, Jong-Yul Kim, Hak-Man Kim, Seul-Ki Kim, Changhee Cho, Jang- Mok Kim, Jong-Bo Ahn, Kee-Young Nam Et. al. [16] authors discuss on hardware in-the-loop simulation (HILS) system as a new method to develop and test control algorithms and operation strategies for a microgrid.
11. Joerg Dannehl, Marco Liserre, Friedrich Wilhelm Fuchs Et. al. [17] discuss in this paper issues related to the reduction of PWM harmonics injection in the power grid are becoming more relevant.
12. By Robert H. Lasseter Et. al. [23] authors describes Managing significant levels of distributed energy resources (DERs) with a wide and dynamic set of resources and control points can become overwhelming.
