In the present day, its nearly impossible to imagine a single day without access to the electricity. The power sector is instrumental in ensuring uninterrupted power supply to meet our daily needs. Within this sector, grid synchronization stands as an essential requirement for maintaining the seamless distribution of power. A key element in achieving grid synchronization is the man-agement of electrical frequency (Biswash et al., 2022).

This article delves into the concept of grid synchro-nization, shedding light on its significance in the realm of power distribution. We will explore the intricacies of parallel operation within power systems, the condi-tions necessary for such operations, and the advantages that grid synchronization brings to the table. Addi-tionally, we will also touch upon the role of micro-controllers in this context, highlighting their relevance and impact in modern power management systems.

Grid Synchronization 

An Overview

Grid synchronization is a critical concept in the field of electrical power systems. It refers to the process of ensuring that a distributed power generation system (such as renewable energy sources or microgrids) ope-rates in the harmony with the larger utility grid. When multiple power sources are connected to the grid, they must synchronize their voltage, frequency, and phase angle to maintain stability and prevent disruptions. Here are some key points about grid synchronization:

Parallel Operation

Imagine a scenario where solar panels, wind turbines, or other distributed energy resources feed power into the grid. For efficient utilization, these sources need to operate in parallel with the existing grid. Grid synchro-nization ensures that their output aligns seamlessly with the grids voltage waveform.

Frequency and Phase Alignment

The frequency (measured in hertz) and phase angle (measured in degrees) of the generated power must match those of the grid. If they dont, it can lead to power quality issues, voltage fluctuations, and even the damage to equipment.

Conditions for Synchronization

Phase Sequence 

The phase sequence (order of phase voltages) must match between the generator and the grid.

Voltage Magnitude

The generators voltage magnitude should closely match the grid voltage.

Frequency

The generators frequency (Hz) should be synchroni-zed with the grid frequency.

Phase Angle

The phase angle difference between the generator and grid should be minimal.

Advantages of Grid Synchronization

Stability

Proper synchronization ensures stable operation of the entire power system.

Efficiency

Synchronized generators contribute efficiently to the grid without causing disturbances.

Safety

Grid synchronization prevents sudden power surges or drops during connection.

Role of Microcontrollers in Power Management

Microcontrollers play a crucial role in achieving grid synchronization and the efficient power management. Heres how

Dynamic Clock Speed Control

High-speed microcontrollers can dynamically switch between clock management modes (such as stop mode, idle mode, and optimal clock frequency). By adjusting the clock speed based on system requirements, micro-controllers optimize performance while minimizing power consumption.

Integrated Power Management Units (PMUs)

Some microcontrollers incorporate the PMUs, which handle tasks like voltage regulation, power monitoring, & energy harvesting. PMUs ensure efficient utilization of available power sources, especially in the battery-backed systems.

Peripheral Control

Microcontrollers manage external peripherals (such as sensors, actuators, & communication interfaces) while minimizing power usage. They can selectively activate or deactivate peripherals based on real-time needs. In summary, grid synchronization and microcontroller-based power management are essential for maintaining reliable and efficient electrical systems. As technology advances, microcontrollers continue to play a pivotal role in optimizing power consumption and ensuring seamless integration with the grid. If youd like to explore further, I recommend checking out the IEEE article on grid-forming converters and their control approaches4. It provides the valuable insights into the topic.

Objectives of the Study

Precision in Measurement, Enhanced Security, and Expedited Operations

1) The primary objective of digital frequency relays is to deliver precise measurements, ensuring the accuracy of the power systems data.

2) They also bolster security measures, safeguarding critical grid operations against potential threats or faults.

3) Furthermore, these relays are designed for speed, facilitating faster decision-making and response times in managing grid conditions.

Grid Power Assurance

1) Another pivotal goal of digital frequency relays is to ensure the stability and continuity of the power within the grid.

2) By monitoring and controlling frequency devia-tions, these relays play a key role in preventing grid disruptions and blackouts.

Accurate and Secure Protection for Frequency Anomalies

1) Digital frequency relays are engineered to provide accurate and dependable protection in scenarios involving under-frequency and over-frequency system conditions.

2) They act as a critical safety net, swiftly identi-fying and mitigating frequency-related issues to maintain grid integrity and reliability.

Grid Synchronization

In the context of an alternating current electric power system, grid synchronization entails the critical proce-dure of aligning the speed and frequency of a power source, be it a generator or another supplier, with that of the existing network. Notably, it is essential to recognize that an alternators ability to supply power to an electrical grid hinge upon it operating at the iden-tical frequency as the network itself. This synchro-nization process serves as the foundational link for the effective integration of power sources into the opera-tional grid.

Why Parallel Operation

Continuity of Service

One of the primary reasons for implementing parallel operation in power systems is to ensure uninterrupted service. By connecting multiple power sources or generators in parallel, if one unit encounters a problem or needs maintenance, the others can seamlessly pick up the load, preventing disruptions in power supply. This redundancy is vital for critical applications where downtime is not acceptable.

Efficiency

Parallel operation enhances the overall efficiency of a power generation system. When multiple generators operate in parallel, they can collectively the handle varying load demands more efficiently. This means that generators can operate closer to their optimal load levels, reducing fuel consumption and operational costs. Improved efficiency is crucial for both economic and environmental reasons.

Maintenance and Repair

Parallel operation simplifies maintenance and repair procedures. If one generator needs servicing, it can be taken the offline without interrupting the supply. This flexibility minimizes downtime & allows for scheduled maintenance activities to be carried out without affec-ting the power grids reliability. 

Increase Plant Capacity

Parallel operation also enables the expansion of plant capacity when needed. Additional generators can be added to the system, increasing the overall capacity of the power plant. This scalability ensures that the power generation facility can adapt to growing demand or unexpected spikes in the load, maintaining a balance between supply and demand in the grid.

Conditions for Parallel Operation

Four essential conditions must be met to facilitate parallel operation within a power system: 

Voltage

Voltage compatibility is paramount for parallel opera-tion. All power sources or generators involved must produce voltages within a specified range and maintain a similar voltage magnitude. Ensuring voltage con-formity prevents imbalances that could lead to undesir-able current flows and potential equipment damage. 

Frequency

Frequency synchronization is the equally critical. All generators operating in parallel must generate electri-city at the same frequency. Maintaining consistent frequency levels is essential for avoiding phase misma-tches & maintaining the integrity of the power supply. 

Phase Sequence

Phase sequence refers to the order in which the alter-nating current waveforms of different generators reach their peak values. In parallel operation, the phase seq-uences of all sources must align correctly to ensure that the combined output remains synchronized and that the loads receive power uniformly. 

Phase Angle

The phase angle represents the angular displacement between the alternating current waveforms of different generators. Parallel operation requires that these phase angles be coordinated to the prevent issues like phase interference and potential short circuits. Proper phase angle alignment the ensures that the generators work harmoniously together in providing a stable power supply to the grid.

Advantages of Synchronization

Increased Reliability

Synchronization in power systems contributes signi-ficantly to enhanced reliability. By ensuring that mul-tiple power sources or generators operate in harmony, the likelihood of service interruptions due to individual unit failures is minimized. This reliability is especially crucial for critical facilities, industries, and essential services that require a continuous power supply to function without disruptions.

Increased Flexibility

Synchronization provides a notable boost to the flexi-bility of power generation and distribution. It allows for seamless integration of various power sources, such as renewable energy systems, backup generators, or multiple utility grids. This flexibility enables grid operators to adapt quickly to changing load demands, incorporate diverse energy sources, and maintain grid stability.

Reduced Maintenance Costs

Implementing synchronization protocols can lead to reduced maintenance costs. By allowing generators to share the load efficiently, individual units are subject to less wear and tear. Additionally, scheduled mainte-nance can be performed without affecting power supply, minimizing downtime and associated expenses. Overall, this leads to cost savings for power system operators and end-users.

Economy

The economic benefits of synchronization are subs-tantial. Efficiently synchronized power systems can optimize the use of available resources, improving energy utilization. This, in turn, leads to cost savings for both power providers and consumers. Moreover, synchronization enables the integration of renewable energy sources, which can reduce dependence on fossil fuels and promote a more sustainable and economical energy landscape.

Synchronization Limits 

Phase Angle (+/- 20 degrees)

Within a synchronized power system, maintaining a phase angle within the specified range of +/- 20 degrees is the crucial. Deviations beyond this limit can disrupt the synchronization of generators and lead to phase mismatches, potentially causing electrical inst-ability and equipment damage. Adhering to this phase angle constraint ensures harmonious operation. 

Maximum Voltage Difference (-7%)

The maximum allowable voltage difference, typically limited to -7%, is a critical parameter for synchro-nization. Keeping voltage variations within this range is essential to prevent imbalances in the power distri-bution system. A voltage difference exceeding this limit can lead to overloading, voltage instability, and undesirable current flows.

Maximum Frequency (-0.44%)

The maximum permissible frequency deviation, often restricted to -0.44%, is another pivotal factor in syn-chronization. Consistency in the frequency is vital for maintaining the uniform operation of generators and load equipment. A frequency exceeding this threshold can disrupt the synchronization process, potentially causing equipment damage and instability in the power grid. Staying within this frequency limit is crucial for grid reliability.

Alternator frequency & Grid frequency

The frequency of the sinusoidal voltage produced by the generator must be equal to the frequency of the sinusoidal voltage produced by the grid.

Alignment of Frequency

A fundamental requirement in power generation and distribution is the synchronization of frequency. The frequency of the alternating current (AC) voltage gene-rated by an alternator or the generator must precisely match the frequency of the AC voltage produced by the electrical grid it connects to. This alignment ensures that the generated power seamlessly integrates with the existing grid supply.

Grid-Generator Harmony

Ensuring the equality of frequencies between the alternator and the grid is essential for the harmonious operation of the entire power system. When these frequencies are not in sync, it can lead to issues like phase mismatches, voltage instability, and ultimately, disruptions in the power supply.

Grid Frequency as a Reference

Typically, the grid frequency serves as the reference    

point for generators. Generators are the designed and controlled to produce electricity at the same frequency as the grid they are connected to. This coordination is critical for maintaining grid stability and the reliable delivery of electricity to consumers.

Consequences of Synchronization Failure

Circulating Current and Power Surges

When synchronization between power sources is unsu-ccessful, it can result in circulating currents and power surges within the electrical system. These unwanted electrical phenomena can lead to inefficiencies, over-heating, potential damage to equipment & components.

Undesirable Rotor Oscillations

Synchronization failure can cause undesirable rotor oscillations in the generators. These oscillations can compromise the mechanical integrity of the rotating components, affecting the generators performance and longevity. 

System Imbalance

A lack of synchronization can introduce imbalances within the power system. These imbalances may mani-fest as uneven distribution of loads & voltages, leading to unstable and unreliable power supply.

Circuit Breaker Tripping

To prevent further complications and protect the grid, circuit breakers may trip in response to synchroniz-ation failure. This action isolates the faulty section of the electrical system, safeguarding the rest of the grid from potential damage or instability.

Alternator Shutdown

Ultimately, if synchronization cannot be restored or maintained, the alternator or generator may shut down automatically to the prevent further complications and protect the equipment from damage. This ensures the safety and integrity of the power generation system.

Why We Measure Frequency in the Power Grid

Maintaining Generator Speed Consistency

Frequency measurement is crucial for ensuring the consistent speed of generators. According to the for-mula Ns = 120f/p, where the Ns represents the syn-chronous speed, f is the frequency, and p is the number of poles, any change in frequency directly impacts the generators speed. To maintain a specific speed for generators, it is the imperative to keep the frequency constant.

Synchronization Requirement

Frequency measurement serves a critical role in power system synchronization. Both the grid frequency and the generator frequency must align perfectly for successful synchronization. When these frequencies deviate from each other, it can lead to issues, potentially cau-sing generators to disconnect from the grid. To restore synchronization, adjustments are made to ensure that both frequencies match, allowing the generators to reconnect and supply power seamlessly to the grid.

When the Alternator is Out of Synchronism with the Grid

Alternator Slower than the Grid

If the alternators rotational speed is slower than that of the grid its attempting to connect to, it can lead to problematic scenarios. In this situation, the alternator effectively operates as a motor, and the grid exerts an effort to accelerate it to the required speed. This forced synchronization can subject the alternator to mecha-nical stress and potential damage, undermining its operational integrity.

Alternator Faster than the Grid

Conversely, if the alternator operates at a speed faster than the grid, similar issues can arise. The grid, in this case, would endeavor to slow down the alternator to match its own frequency. This mismatch can result in excessive mechanical strains and adverse effects on the alternators components, potentially leading to opera-tional problems or even failure. Ensuring the precise synchronization of alternators with the grid is essential to prevent these scenarios and maintain the smooth and reliable operation of power generation systems.

Effects of the Load Changes on a Synchronous Alternator

Increased Load - Decreased Frequency and Speed

When the electrical load on a synchronous alternator is increased, it experiences a decrease in both frequency and rotational speed. This phenomenon occurs because the alternator must work harder to meet the heightened demand for electrical power, leading to a natural reduction in speed and frequency.

Decreased Load - Increased Frequency and Speed

Conversely, when the electrical load on the alternator is decreased, it results in an increase in both frequency and speed. With a reduced load, the alternator operates more efficiently, requiring less effort to generate the power. 

As a result, the rotational speed and frequency tend to rise in response to the diminished load. Its essential to note that while these tendencies occur when the load changes, synchronous alternators are designed to maintain a relatively constant speed and frequency for a specific, rated load. However, deviations from this rated load will lead to the speed and the frequency adjustments as the alternator adapts to the varying electrical demand.

Digital Frequency Relay and Its Applications

Digital frequency relay is a modern protection system, driven by microcontroller technology and equipped with software-based algorithms designed to detect electrical faults swiftly and accurately. Digital frequ-ency relay is a MCU-based system with software-based protection algorithms for the detection of electrical faults.

Applications of Digital Frequency Relay

Protection of the Power System

A primary application of the digital frequency relay is to safeguard the power system from potential disrupt-tions and damage. By continuously monitoring the frequency of the electrical grid, it can detect abnor-malities and respond rapidly to protect the system from faults, short circuits, or other irregularities that could lead to power outages or equipment damage.

Preventing Blackouts

One of the critical roles of the digital frequency relay is to prevent grid blackout events. In the event of a major generation or load loss, this relay acts as an essential safety net. It can detect significant deviations in frequency and initiate protective actions to stabilize the grid, helping to avert blackouts and ensure uninterrupted power supply to consumers.

Islanding Detection

The digital frequency relay is also instrumental in identifying islanding operations. Islanding typically occurs when a section of the power system becomes electrically isolated from the main grid due to distributed generation or other factors. Detecting islanding is crucial for safety and grid stability. The relay can sense the loss of mains and respond by disconnecting or reconfiguring the isolated section to the maintain synchronization with the main grid, ensuring a coordinated and secure power supply.

Modeling of Digital Frequency Relay

The Digital frequency relay consists of two parts

1) Frequency Measuring Unit (FMU) 

2) Under-Over Frequency Detection Element (FDE). 

FMU is used to measure the digital value of frequency from single phase line while FDE takes actions on Over-Under frequency limit. 

Frequency Measure Unit (FMU)

1) Hit Crossing block is used to detect the zero crossing. 

2) The block passes the input signal at its zero crossings to ‘if block, which in starts sending ramp signal to the output. 

                             Fig. 1: Block diagram for implementing over-under frequency relay.

Implementation of Digital Frequency Relay and Grid Synchronization using MATLAB SIMULINK

3) The time duration of generated ramp is measured and saved to a variable ‘A. 

4) The variable A is stored in another variable B using the ‘Transport Delay block and the time of the next zero crossing is measured. 

5) Subtracting B from A at any instant will give half the time period whose the value is held by the ‘Sample and Hold the block, till the next zero crossing. 

6) The output (measured frequency) from FMU sends to the FDE for necessary tripping action, in case of fault. 

Frequency Detection Element (FDE)

The digital relay has superiority over electromecha-nical relay in terms of the accuracy and speed. This dissertation presents the design and various data con-version steps of a digital frequency relay. The designed relay will cover both over and under the frequency conditions.

Fig. 2: Over and under frequency detection block on SIMULINK.

Flow Chart

Case-1 

All alternators are running at same speed (Normal condition).

Fig. 3: Alternator 01, 02 & 03; (a) Relay status; (b) 3 phase voltage, and (c) 3 phase current.

Case 2

Alternator 02 speed fall (Under frequency condition) and other alternators are running at the same speed (Normal condition).

Fig. 4: Alternator 02; (a) Relay status, (b) 3 phase voltage, and (c) 3 phase current.

Fig. 5: Alternator 01 & 03; (a) Relay status, (b) 3 phase voltage, and (c) 3 phase current.

Case 3

Alternator 02 speed increase (over frequency condi-tion) and other alternators are running at same speed (normal condition).

Fig. 6: Alternator 02; (a) Relay status, (b) 3 phase voltage, and (c) 3 phase current.

Fig. 7: Alternator 01 & 03; (a) Relay status, (b) 3 phase voltage, and (c) 3 phase current.

Case 4

All alternators are running at same speed with over load.

Fig. 8: Alternator 01, 02 & 03; (a) Relay status, (b) 3 phase voltage, and (c) 3 phase current.

Hardware Implementations

Hardware Components for Implementation

In the setup of a digital frequency relay system, vari-ous hardware components play vital roles in its opera-tion & functionality. These components include:

Power Supply Block 

Provides the necessary electrical power to drive the entire system, ensuring proper functioning.

Micro-controller (Atmega32) 

Serves as the central processing unit, executing control algorithms and coordinating the relays response to frequency variations.

Relays

Act as switches or control elements to open or close electrical circuits, the enabling protective actions when frequency deviations are detected

LCD (Liquid Crystal Display)

Offers a visual interface for monitoring and displaying relevant information about the relays status and detec-ted faults.

LED (Light Emitting Diode)

Provides visual indicators to the signal specific system states or alarms, aiding in the system monitoring and diagnostics.

Resistors and Capacitors

Play essential roles in circuit design, controlling cur-rents and voltages, ensuring proper signal processing.

Diode (IN4007)

A diode component that can rectify and control the flow of electrical current within the system.

Optocoupler

Used to isolate and protect sensitive components by electrically separating input and output circuits.

Timer (NE555)

Provides the precise timing functions within the relay system, aiding in the accurate detection and response to frequency deviations.

BJT (Bipolar Junction Transistor, PN2222)

A transistor that may be used for signal amplification or switching functions within the relay circuitry. These hardware components collectively form the infrastruc-ture of a digital frequency relay system, allowing it to monitor, detect, and respond to frequency anomalies effectively in power systems. 

Power Supply Block

Microcontroller

A microcontroller (MCU) is a compact computing device integrated onto a single chip, purpose-built to manage & oversee specific functions within electronic 

systems. This miniature computer unit combines the essential elements of a central processing unit (CPU), memory storage, & input/output interfaces, efficiently handling tasks tailored to its designated application

Pin Description of ATmega32A MCU

The ATmega32A microcontroller is equipped with a range of pins, each serving distinct functions within the devices operation. Understanding these pin des-criptions is crucial for harnessing the capabilities of the MCU and interfacing it with external components and systems.

Relay Overview

A relay serves as an electrical switch that operates via an electromagnetic mechanism. It comprises two main sets of terminals: input terminals for receiving one or more control signals and operating contact terminals. 

This versatile switch can feature varying numbers of contacts, available in different forms, including make contacts (which close when activated), break contacts (which open when activated), or combinations thereof. Relays play a pivotal role in controlling the electrical circuits by enabling or interrupting the flow of current based on the input signals they receive.

Working Principle

In this project, weve created a straightforward frequ-ency relay using the ATmega32A microcontroller and the CODEVISION AVR compiler. This uncomplicated frequency relay is capable of measuring frequencies up to 8 MHz, thanks to the utilization of a 16 MHz clock for the ATmega32A microcontroller. The underlying concept behind a frequency meter is quite simple: the frequency, measured in Hertz (Hz), represents the number of cycles or pulses that occur in one second. The microcontroller (MCU) counts these pulses within a one-second interval to determine the frequency of the incoming signal. Heres a more detailed explanation:

Pulse Counting with Timer1

We employ Timer1 on the ATmega32A to count the pulses of the incoming signal, which we want to the measure for its frequency. These pulses are applied externally to the T1 pin2 (PB1) of the microcontroller. Timer1 operates in normal mode and begins counting the pulses when initiated.

Delay of One Second

After starting the pulse count, the MCU introduces a delay of precisely one second.

Stopping Timer and Reading TCNT1

At the end of the one-second interval, the MCU halts Timer1 and reads the value stored in the register TCNT1. This register contains the count of pulses that Timer1 accumulated during the one-second period.

Handling Timer Overflow

In cases where Timer1 overflows, the MCU enables the overflow interrupt of Timer1. An overflow event signifies that Timer1 has completed 2^16 counts (as its a 16-bit register). Therefore, the MCU counts the num-ber of such overflows occurring within one second.

Calculating Frequency

With the number of overflows (denoted as i) counted within one second and the value stored in TCNT1, we can calculate the frequency using the following equ-ation:

This equation accounts for the total number of pulses observed in one second.

Updating Frequency Reading

The reading of the frequency meter is refreshed every one second, allowing for continuous monitoring and measurement of the input signals frequency. This pro-ject showcases a basic yet effective method for mea-suring frequency using an ATmega32A microcontro-ller, enabling applications where accurate frequency monitoring is essential.

Block Diagram

Proteus Simulation 

Under Frequency State

                 Normal Frequency State                                                     Over Frequency State

Sinusoidal signal, Pulsating DC signal, and

Zero crossing Puls

Frequency Relay in Bus System

                     Hardware Setup                                                                    Top Part

Bottom Part                                           Various Signals Analysis & Initial Frequency

 Sinusoidal Signal                                                             Pulsating DC Signal

Zero-Crossing Pulse                                                          Initial Frequency

Measurement of Frequency & Rotational Speed

Under Frequency State

Normal Frequency State

Over Frequency State

Benefits of Grid Power Security

One of the primary advantages of this project is its ability to enhance the security of the power grid. It ensures that power plants primarily supply electricity to the grid rather than drawing power from it, thus bolstering grid stability and reliability.

Accurate and Secure Protection

The project offers precise and dependable protection mechanisms for under-frequency and over-frequency system conditions. This protection is crucial for pre-venting grid instability and blackouts due to frequency anomalies.

Enhanced Measurement and Security

Additionally, the project provides accurate measure-ment capabilities, improving the precision of data coll-ection. It also contributes to better security, dependabi-lity, faster response times in managing grid operations.

Limitations

Limited Alternator Integration

In the hardware setup, the project doesnt incorporate more than one alternator, whereas in the Simulink simulation, three alternators are used. This limitation could affect the systems representation in the hard-ware setup compared to the simulation.

RPM Measurement Inaccuracy

The accuracy of RPM (rotations per minute) measure-ment is compromised due to the inherent speed in-stability of the machine. This limitation may affect the precision of certain measurements & control processes.

Need for Parasitic Resistive Load

When employing synchronous machine blocks in the Simulink, a small parasitic resistive load is necessary, connecting to the machine terminals. This load is the required to mitigate numerical oscillations and ensure the stability of simulation, introducing a minor com-plexity to the model.

Future Goals

Implementation of a Feedback System

A key objective for the future is to the incorporate a feedback system into this project. Feedback mecha-nisms can enhance the systems performance by con-tinuously monitoring and adjusting its parameters in response to changing conditions. This will contribute to greater control and stability in managing power grid synchronization.

Integrated Multi-Measurement Device

Another compelling future goal is the development of a comprehensive device capable of measuring multiple parameters simultaneously. This device would be desi-gned to measure not only frequency but also voltage magnitude, phase sequence, and phase angle, all within the single integrated system. Such a multifunctional solution can provide a more holistic view of power system health & performance, offering valuable in-sights for grid operators and engineers.

In conclusion, this research paper underscores the vital role of digital frequency relays in maintaining grid synchronization, emphasizing their importance in pre-venting power system disruptions. The study outlines the objectives of digital relays, explores the intricacies of grid synchronization, and discusses the consequences of synchronization failure. The paper highlights the benefits of the digital relays, including enhanced grid power security & accurate protection against frequency anomalies. It also acknowledges limitations such as limited alternator integration and RPM measurement inaccuracy. The future goals include implementing a feedback system and developing a multi-measurement device for the comprehensive monitoring. Overall, this research contributes to the understanding of grid syn-chronization and sets the stage for advancements in digital frequency relay technology, aiming to improve the resilience and reliability of power grids in the face of evolving challenges.

We extend our heartfelt gratitude to the cosmic forces governing the UniversePG and express our sincere appreciation to the anonymous reviewers for their in-valuable comments that have significantly enriched the content of this manuscript. Our profound thanks also go to all those who have contributed to refining and preparing the paper for publication, as well as the engaging in insightful discussions.

The authors affirm that there are no conflicts of interest associated with the content of this article.