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1. Field of invention
This invention relates to the indigenous development of control and monitoring
system, without using a dedicated computer, for a standalone micro-grid to manage
power balance between load and generation for the first time in this field.
2. Description of the Related Art
The tapering of fossil fuels and ever-increasing load demand has prompted the
operators of power system to utilize the renewable energy sources (photovoltaic
modules, wind turbine, fuel cells, etc.). Nevertheless, the lower energy density and
the volatility of individual energy sources cannot offer the reliable power supply.
The micro-grid, a conglomerate of several renewable energy sources, has emerged
as a reliable power supply in remote locations. However, there is a requirement of
controller for managing the loads and also for effective utilization of available
energy sources. Hence, FPGA Based Micro-grid Control and Monitoring System
with separate source and load control features is developed. The controller
facilitates load management (switching ON/ OFF non-critical loads) based on the
generation availability, energy storage capacity, etc. Also, the sources will be
switched OFF if there exists, a light loading condition. Thus, the controller
facilitates source optimization.
Objects of the Invention;
It is an object of the present invention to develop an algorithm for controlling a
stand-alone micro-grid to balance the power generation and load demand at any
instant.
It is another object to implement the developed algorithm as an embedded system
using two Field Programmable Gate Arrays (FPGAs) with separate load and source
control features communicated through Ethernet for load management and source
optimization.
It is yet another object to build a monitoring system without using a dedicated
computer to observe the amount of power generation and consumption, status of
loads and charge/discharge status of battery storage unit.
Further, other objects of the present invention will become apparent from the
description contained herein.
Summary of the Invention
The integration of multiple renewable energy resources to meet the energy needs
of the consumers in remote locations with reliability is a major challenge due to
inherent characteristics of available renewable sources viz. unpredictability &
variability. This requires development of micro-grid controllers to manage balance
between load and generation. In this work, a micro-grid controller integrating the
output from multiple types of renewable energy conversion systems, namely, wind
and solar along with diesel generator as well as battery storage has been
indigenously developed with source and load control features using Field
Programmable Gate Arrays (FPGAs) for the first time in this field. The load
controller facilitates load management (switching ON/ OFF non-critical loads)
based on the generation availability and energy storage capacity. The source
controller facilitates source management. The various parameters of different
power generation sources and loads are monitored and displayed using FPGA. It
also includes the communication between source and load controllers using
Ethernet for exchange of monitored data for a suitable control action. This
innovative work has been developed in such a manner that it will be replicable in
the field to bring it on the large scale.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG.l is an overall block diagram of the implemented prototype of standalone
micro-grid;
FIG.2 is the photograph of hardware setup implemented in the laboratory;
FIG.3 is the block diagram of signal acquisition and controller;
FIG.4 is the photograph of signal acquisition and conditioning circuits;
FIG. 5 is the photograph of FPGA controllers (source and load) with Ethernet
connectivity;
FIG.6 is the flowchart of source and load control algorithm;
FIG.7(a) and (b) are the flowcharts of programs running in source and load FPGA
controllers respectively;
FIG. 8 is the block diagram representation of initial condition of the prototype
during experimentation;
FIG.9 is the photograph of the display unit showing the initial condition;
FIG. 10 is the snapshot of the display unit when the load demand increases;
FIG. 11 is the block diagram representation of status of the prototype with Noncritical
load of least priority, NCL II disconnected;
FIG. 12 is the snapshot of the display unit when NCL II disconnected;
FIG. 13 is the snapshot of the display unit when Diesel Generator is OFF;
FIG. 14 is the block diagram representation of status of the prototype when Diesel
generator is connected;
FIG. 15 is the snapshot of the display unit when Diesel Generator is ON;
FIG. 16 is the snapshot of the display unit when the load is decreased with solar,
wind and batteries are in ON condition.
FIG. 17 is the block diagram representation of the prototype indicating the status of
disconnection of wind energy conversion system (WECS) during light
loading condition;
FIG. 18 is the snapshot of the display unit when WECS is switched OFF after
sensing the light loading and fiilly-charged battery condition.
DETAILED DESCRIPTION
The overall schematic of the prototype implemented is shown in FIG.l. The power
conditioning unit is Liebert.ESU module from M/s Emerson Network power,
which has inbuilt DC-DC converter employed for solar Maximum Power Point
Tracking (MPPT). The Self Excited Induction Generator (SEIG) based Wind
Energy Conversion System (WECS) consists of an induction generator (squirrel
cage induction machine coupled with dc motor as prime mover) provided with a
capacitor bank for self-excitation. The AC voltage generated by the induction
machine is of variable voltage and variable frequency, since the speed of prime
mover is varied. This voltage is converted into DC with the help of Diode Bridge
Rectifier (DBR). A buck converter, which is coupled with the DBR, is employed to
extract maximum power. In this system, the output current is considered for
tracking Maximum Power Point (MPP), since the output voltage (DC bus) is held
constant.
The hardware setup of the prototype implemented is shown in FIG.2. The
schematic of the signal acquisition, signal conditioning, controller block and the
photographs of the same are shown in FIG.3 and FIG.4 respectively. The signal
acquisition system consists of voltage and current sensors (LEM make LV 25P and
LA 55P), which sense the DC voltages and currents of Photovoltaic (PV), WECS,
and Battery; the AC voltages and currents of Diesel Generator and load. The
signal-conditioning unit developed in the laboratory consists of peak detector
circuits for AC voltage and current signals, power factor detectors and signal
multiplexer for feeding all the analog signals through Analog to Digital Converter
(ADC) to the source FPGA controller.
The peak detector circuit, designed using 1N4007 along with a capacitor, gives the
peak value of the AC signals obtained from signal acquisition system. The peak
values are used for the calculation of r.m.s. values of the respective sensed signals.
In order to calculate the true AC power, the power factor has to be computed. A
digital pulse, whose ON duration representing the phase difference between the
voltage and current of a particular phase is obtained from the pulse detector circuit
designed using IC TL084 and XOR gate. The ON duration of the pulse is
calculated using the embedded module developed in FPGA and is converted into
angle and then cosine of the angle is computed to obtain power factor. The DC
signals from sensors and peak values of AC signals are fed to the ADC of source
FPGA controller through ADC interface card. The photographs of signal
acquisition and signal conditioning circuits are depicted in FIG. 4.
The photograph of FPGA controllers (source and load) with Ethernet connectivity
is shown in FIG.5. The source FPGA controller acquires the voltage and current
signals from various energy sources and computes the generated power. Further, it
senses the three-phase voltage and current at the output of power conditioning unit,
and computes total power supplied to the loads (critical (CL) and non-critical
(NCL)). Based on the available sources (wind and solar) and demand, the source
controller decides upon switching ON/OFF the non-critical loads and the decision
is communicated to the load FPGA controller through Ethernet interface.
The load FPGA controller controls different non-critical loads as per the command
given by the source controller and priority assigned to the loads. Based on the
control activity by the load controller and the power available, the source controller
effectively utilises the energy resources by activating/ de-activating the relevant
contactors. The electrical quantities, the status of the sources and the loads are
displayed using monitoring system for visualization. An algorithm has been
developed to realize the above logic of the controllers and the flow chart of the
developed source and load control algorithm is presented in FIG.6.
As per the flowchart, the control logic is as follows:
Initial State:
PV Power (Ppv): ON Wind Power (Pw): ON Diesel Generator (PDG) : OFF
PT ~ Ppv + Pw
Load Power (PL): Contactor 1: Non- Critical Load I (NCL I) - ON
Contactor 2: Non-Critical Load 2 (NCL II) - ON
Contactor 3: Critical Load (CL) - ON
Load Control:
If PL > PT , then SWITCH OFF NCL II
If still PL > PT , then SWITCH OFF NCL I
The generated power and battery will supply the Critical Load alone.
If the PT increases, and if Pj > PL, then Switch ON NCL I
Still if PT > PL, then Switch ON NCL II
Source Control:
If PT < PL and (PLI and PLI) are OFF and Battery Voltage < 114 V,
then Switch ON Diesel Generator.
If Battery charges above a Voltage > 136 V,
then Switch OFF Diesel Generator.
The Ethernet communication between the controllers is established based on
Fig. 7(a) and (b). The communication algorithm that runs in the source controller
starts with the initialisation of related Application Programming Interface (API),
creation of a socket and binding to it. This step is followed by accepting the
connection from the load controller. The statuses of the sources (amount of power
generated, ON/ OFF condition) are sensed, digitized using ADC, updated and
displayed in the display unit. The control packet based on the statuses is sent to the
load controller. Based on the control packet information, the load controller will
switch ON/ OFF the non-critical loads as per priority to equal the total power
generated with the load power. Similarly, based on the packet received from the
load controller, the control signals will be sent by the source controller to switch
ON/ OFF the sources to match the total power generated with the load power. The
status information will be updated time to time (periodically) in the display unit
connected with the source controller. The act of source and load controllers will be
in such a way that source controller will act as master/ server and the load
controller as slave/ client.
Test Cases of Micro-grid Controller:
The algorithm implemented in FPGA has been tested on the prototype of microgrid
built in the laboratory under various operating conditions as described below.
The initial state of the prototype and its display are shown in FIG.8 and 9
respectively.
Initial State:
PV Power (Ppv): ON Wind Power (Pw): ON Diesel Generator (PDG): OFF
Total power generation from renewable energy sources (Pj) = Ppv + Pw
Load Power (PL): Contactor 1: Non-Critical Load 1 (NCL I) - ON
Contactor 2: Non-Critical Load 2 (NCL II) - ON
Contactor 3: Critical Load (CL) - ON
Test Case -1: Increase in load demand
In this case, the load demand has been increased more than the available power
generation from wind and solar (i.e. PL > PT)- The snapshot of the display bearing
the corresponding electrical parameters is shown in FIG. 10. Following the increase
in load, the source controller decides to disconnect the non-critical loads and
communicates it to the load controller, which disconnects NCL II as per priority.
The status of the prototype and its corresponding displays are enumerated in
FIG.l 1 and 12 respectively.
Test case — 2: Decrement in power generation (both solar and wind)
The test case 2 analyses the condition of micro-grid when there is a vast reduction
in both Wind and Solar power. In response to the present situation, source
controller decides to disconnect all non-critical loads and communicates with the
load controller to do the same. The demand of the critical loads is met by the
available renewable power generation and the energy stored in the battery. Once
the battery voltage drops down to 114 V, the source controller acts to connect the
Diesel generator with the setup. The Diesel generator feeds the critical loads and
charges the battery along with the available renewable power generation. This
situation can be visualised in FIG. 13 to 15.
Test Case 3: Disconnection of sources
When there is no sufficient load to consume the available power generated by the
sources and if the battery is also fully charged (Battery Voltage exceeds 136 V),
the source controller decides to disconnect the Diesel generator and subsequently
the Wind generator, if the same condition persists. This situation can be visualised
inFIG.16tol8.

5. CLAIMS
1. We claim that an algorithm for FPGA based Micro-grid Control and
Monitoring System (MCMS) is developed for a standalone micro-grid
topology.
2. We claim that of claim 1, includes the implementation of source and load
controllers using two FPGAs.
3. We claim that of claim 2; includes the establishment of communication
between source and load FPGA controllers using Ethernet.
4. We claim that of claim 1, includes the embedded monitoring system
developed using FPGA and displayed using LED Monitor.
5. We claim that the FPGA based Micro-grid Control and Monitoring System
(MCMS) of claim 2, 3 and 4 does not need any dedicated computer system.