Integrated Nanogrid for the Impressed Current Cathodic Protection System in Desalination Plant

Abstract

The impressed current cathodic protection (ICCP) scheme is a more reliable and efficient method of corrosion prevention mechanism than the sacrificial method. Currently, the grid connected transformer rectifier units supported with a battery banks are used for the ICCP-based corrosion protection system in the desalination plant. This conventional method is entirely grid-dependent, more expensive, and suffers during prolonged grid failure. The present trend of industrialization is the application of multi-renewable energy sources based on a nanogrid to power the station’s auxiliary power supply. This paper introduces a concept of distributed energy resources (DERs) operated integrated nanogrid (ING) system to provide a stable power supply solution to the ICCP scheme. A 100-million-litter per day capacity-based seawater desalination plant (SWDP) in India has been chosen as the test station. The conceptual hardware design and operational logic details for smooth integration of the integrated nanogrid module into the ICCP scheme of the Desalination plant is proposed. This research aims to investigate the behaviour of DERs during on-grid, off-grid and switching over from one mode of operation to another and vice-versa by using the accelerated Gauss–Seidel method in ETAP software (version 16.0.0). The simulation results confirm that the ING suffers from a high-frequency change rate during islanded operation, and in some cases, a complete blackout occurs. A PLC-based Smart Versatile ING Controller has been suggested to overcome the blackout issue. Finally, it has been proven that the stability of an industrial power system can be improved further by introducing the ING module into it.

1. Introduction

The multiple distributed energy resources (DERs)-based microgrid and nanogrid into different industries is booming in order to reduce the carbon footprint and CO₂ emission. The integrated nanogrid (ING) concept has come to address the power supply issues in remote villages, industrial power supply and specified sectors. In the last couple of decades, there has been a substantial increase in renewable energy-based multiple distributed generation systems and integration models of smart grids and microgrid along with storage mechanisms in different industries [1,2,3,4,5,6,7,8]. The growing interest in research on DERs-based hybrid power system consists of renewable sources such as solar and wind along with battery storage has been implemented from small scale to large scale. Based on the requirements, these practical installations are going on in different fields, including the domestic, commercial and industrial fields. The integration of different micro sources such as solar, wind, hydrogen energy, tidal energy, etc., are categories based on their installation capacity and applications as Pico grid, nanogrid and microgrid [9,10]. Overall, all these autonomous power systems are known as smart grids. Numerous research activities are currently being undertaken to mitigate the fluctuation and oscillation of the delivered power to provide stable power supply solutions through these smart grids [10,11,12]. The authors in [13,14] have discussed the concept of nanogrid and their integration technologies. Further authors also defined nanogrid as small modular load blocks corresponding to low-power management within a given microsystem and in managing a set or a point load [15,16].

In various industries, from auxiliary power supply to critical power supplies such as emergency station blackouts in nuclear power stations, solar-based offshore power supply in hydrocarbon sectors, microgrid systems have gradually been integrated. With the rapid technology enhancement era, as well as with the aim of reducing the carbon footprint that affects the atmosphere, renewable energy is the only solution. Similarly, the application of integrated nanogrid systems has been achieved in corrosion protection industries. To protect the industrial installations, the industries are used to implement either a sacrificial or impressed current method of cathodic protection (ICCP) schemes. In this part of the research, a detailed study to implement an ING concept which may be a suitable and stable power supply to the ICCP system to ensure that complete corrosion prevention is possible through this mechanism. A seawater desalination plant (SWDP) industry near the seashore area has been considered a case study to implement the ING-operated ICCP system to protect its structural equipment from corrosion. The specified industry located near the seashore area has been specifically selected due to its geographical location will support wind and solar power station establishment purposes.

Corrosion is an oxidation and natural phenomenon that plays a significant role in deteriorating (ultimately damaging) metal structures, especially in various industries. Due to this corrosion process, the metal structures used to suffer from deterioration or degradation of its properties due to an interaction with its environment [17]. A lot of money is being invested in the cathodic protection scheme to add a coating to maximize corrosion prevention in underground and submerged metal structures [18]. Corrosion is a natural oxidation process in which the metal tends to return to its lowest possible energy states from the refined steel state, i.e., hydrated iron oxides. The hydrated iron oxides, commonly known as rust, are similar in chemical composition to the original iron oxide [19]. Electrochemical reactions between the metal and the electrolyte in which it is immersed, i.e., chemicals present either in the soil or water, form corrosion cells. The corrosion cells formed are responsible for the degradation of the metallic surface [20].

The term cathodic protection scheme applies an external power source to the underground or submerged metal structures to shift its potential to mitigate corrosion; hence, it is termed an ICCP scheme [21]. In general, a certain amount of direct current (DC) is injected from an external source through an external anode to protect the structures. The injected current flows from the anode terminal to the cathode (structures to be protected act as the cathode) through some ionized media such as seawater, soil, etc. The impressed current to the structures must be injected and controlled into the metal structures to maintain uniform current density throughout the metal [22]. The ICCP method of a cathodic protection scheme is a widely industrial acceptable method of corrosion prevention scheme. Here, an external DC power source injects the impressed current into the ionized water to develop the necessary polarized potential, which will saturate the negative ions and ultimately protect the metal structures. The ICCP method is a more effective and long-term solution to prevent corrosion. However, the drawback of the ICCP method is that it needs a dedicated continuous DC power supply of rated capacity to prevent corrosion successfully. Here, zero interruption power supply is required to protect the structure effectively.

To fulfil the same requirements, it needs huge investment to provide a suitable and dedicated power supply arrangement.

The authors have conducted a substantial literature review to learn about different industrial power supply solutions for the ICCP method of corrosion prevention schemes. Additionally, the recent development, as well as implementation methodologies of various ICCP methods, are studied. In the recent era of industrialization, the ICCP method of corrosion prevention is one of the best-preferred models due to its performance. Generally, a uniform current density must be maintained throughout the structure by injecting a pre-calculated amount of current (i.e., constant impressed current). These are the main conditions to be satisfied by an ICCP system. The current amount must be calculated based on the structure area size, chemical properties and the surrounding medium [23]. Various methods of power supply arrangements and implementations of ICCP technologies in different industries, such as nuclear power plants, hydrocarbon sectors, SWDPs and large capacity seawater pumping stations, etc., have been studied to understand the recent developments.

The literature review shows that most of the ICCP method of cathodic protection system is dependent on a transformer rectifier unit (TRU) to obtain the low voltage and high current DC supply to inject the metal structure. In general, the TRUs take input alternating current (AC)-based power supply from the utility grid and converter it into DC to feed the metal structure. Though this is a year-old traditional technology, it is still used in various industries worldwide. Some countries, including China and USA, have already implemented solar and grid-based hybrid power supply to this ICCP method of the cathodic protection system. However, authors in [24] enunciate that using TRU is not economical for powering ICCP systems in remote areas such as deserts and mountains. This is due to AC power lines (grid) being commonly unavailable. In [25], hybrid renewable energy has been used to supply corrosion protection schemes in metallic structures. In [26], studies have been conducted to use solar power to run corrosion protection systems for buried pipelines. However, no studies were carried out to ensure continuity of supply during the night and when the weather conditions are unsuitable for solar energy. It is clear from [27] that more research must be conducted on cathodic protection systems supplied by solar power or hybrid power via a DC-to-DC controller such as DC choppers. In [18], the paper investigates the corrosion of metallic structures and issues design, including using solar, wind and solar–wind hybrid systems to feed corrosion protection systems. On the other hand, research conducted in [28] indicates that AC currents affect the CP systems badly and cause the corrosion rate to rise.

However, the application of DERs-based microgrid and nanogrid systems is yet to be implemented in this area. In practice, even today, the ICCP method of protection schemes is entirely dependent on utility grid power supply backed up by battery banks. Therefore, during the grid power interruption, the battery bank will only extend the backup for a limited time. It may be noted that the ICCP system needs a continuous power supply. Even losing the continuity in the impressed current will lead to starting corrosion in the metal structures. This power interruption period is more dangerous for the structure to start corrosion. In the present power system trend, some industries have already adopted the standard grid power backed up by an uninterrupted power supply (UPS) with battery banks as the primary power source for ICCP. In this case, there may be more viable options than capital investment and maintenance costs concerning reliability. In the ICCP method of corrosion prevention scheme, any form of electricity, such as a utility grid supply or solar or battery bank, can be controlled to ensure its terminal voltage and current are within the limit. Authors in [29,30,31] have suggested that the power electronics control systems such as DC-DC buck and boost–buck converters can be used for utilizing solar and wind-generated power for the ICCP-based corrosion protection scheme purpose. There are substantial research works already carried out and also is in progress in worldwide to address various aspects of AC and DC microgrid issues. The stability issues including equal load sharing by different DERs are still under unaddressed stage only. Authors in [32,33] have suggested a novel concept of adaptive droop control mechanism for the DERs-operated microgrid system is best suitable for equal distribution of load current among the micro sources. This concept is mostly applicable for DC microgrid with battery banks to ensure the equilibrium loading pattern. Even though the renewable energy-based integrated power system may be suitable to provide an alternate power supply source, specific issues such as small signal stability, transient stability, voltage stability, and black start operational difficulties persist [10,11]. Among these stability issues, transient stability and black start operational issues are major operational concerns for most DER-operated microgrid and nanogrid installations [12]. From this point of view, it is understood that the transient analysis for the newly designed micro power system is urgently needed.

1.1. Existing Research Gap and Motivation

The in-depth literature review shows that the TRUs in ICCP can take only alternating current as input power supply and regulates the electrical parameters based on the structure chemistry. It is also found that some researchers are discussing on-grid or off-grid solar-powered ICCP methods to reduce the per-unit consumption of grid power supply. In this method, during the failure of the utility grid, the on-grid solar system will not work, and the off-grid solar system only will work in standalone mode along with the battery bank. However, the prolonged failure of grid supply or utility shadow areas is not safe for the battery banks to obtain a complete recharge cycle; therefore, the life of the battery banks will be under threat, as suggested by authors in [34,35]. Furthermore, some industrial consultants are keeping their option to run diesel generators to provide power supply to ICCP during the prolonged interruption. The sectors including (but not limited to these only) thermal/nuclear power plants, hydrocarbon, and direct sea water implication-based industries are used to spending a considerable amount to protect their metal structures from corrosion in terms of electricity bills. And sometimes, the situation worsens during a complete blackout due to prolonged grid supply failure. Though much research is underway, many improvements are anticipated to better implement the ICCP corrosion prevention method in industries. Therefore, authors in this research work have felt that the study of techno-economical viable power supply arrangement as well as its method of implementation in ICCP, is the most essential. Neither researcher nor industrial consultants have discussed the application of the ING concept to the ICCP corrosion prevention method. Therefore, some technological gap exists in the industries, including implementing the ING system into the ICCP method of the cathodic protection scheme. This concept is novel, and the identified research gap (implementation of ING system into ICCP method of corrosion protection scheme) is yet to be filled out in industrial, technological advancement. The proposed ING system is a multi-source-based hybrid power system proven to be a stable supply compared to the conventional grid and ups system used in the corrosion protection scheme. Since this hybrid power system prioritized renewable energy sources such as solar and wind and the least priority to DG set, this concept is one of the best techno-economical models. The conventional method of the ICCP scheme entirely depends on the grid and UPS power supply. However, the proposed scheme operated through four different power sources, including solar and wind systems, apart from the grid and UPS supply. Since this integrated power system consists of multiple power sources, it is necessary to study the stability and load flow analysis at its design stage to understand its performance.

1.2. Research Contribution

A 100 million litter per day (MLD) capacity-based SWDP plant has been considered in this research work as a test station to experiment with the ICCP method of cathodic protection scheme and discuss the existing power supply arrangement. The traditional way of power supply arrangement consists of standard grid power supply converting to low voltage high current DC through a standard transformer rectifier unit along with backup battery banks. The existing mechanism of cathodic power supply arrangement used to suffer during prolonged grid failure when the battery could not sustain a long time. The power interruption causes loss of injection of impressed current to the metal structures to be protected in the industry, which ultimately causes corrosion. Hence, it has been proven that the foremost cause of corrosion is the uninterrupted power supply solutions in industries. To address this power supply issue, authors in this research work have contributed as following:

(a)

The detailed engineering works as part of design activities for the conceptualization of an integrated micro power system nomenclature ING module suitable for ICCP method of carrion prevention scheme for the SWDP industry (Test Station).

(b)

To validate the ING concept, the stability and load flow analysis carried out using ETAP simulation software and results are discussed.

(c)

The various failure analysis of DERs operated ING module including black start operation mechanisms are also carried out to make the system robust.

(d)

Authors also worked out a conceptual hardware design of a PLC-based intelligent ING controller which will be suitable to handle the black-start operational issues and load restoration process.

These are the overall novel contributions of this research work.

An architectural view of the proposed test station has been designed and it is shown in Figure 1.

Figure 1 shows there are four micro sources, solar photovoltaic (SPV) system, wind turbine generator (WTG), diesel generator (DG) set and uninterrupted power supply (UPS) with battery bank, which formed a nanogrid which is also integrated with the utility grid to deliver a stable power supply to the SWDP industry. The typical parameters of the micro sources used for the proposed ING system configuration is shown in

Table 1. Typical ratings of DERs used in ING.

Sl. No.	Type of DERs	Electrical Parameters	Qty.
1	SPV system along with MPPT and Inverter module	125 kVA, 415 V, 50 Hz system	1 set
2	Permanent magnet-type WTG system	125 kVA, 415 V, 3-Ph, 50 Hz system	1 set
3	DG set	125 kVA, 415 V, 3-Ph, 50 Hz system	1 set
4	UPS along with suitable battery bank	125 kVA, 415 V, 3-Ph, 50 Hz system	1 set

.

A substantial literature review has been carried out, and multiple field visits have been carried out for a feasibility study and implementation of the ING system in the proposed test station. (1). A complete design of the ING system and sizing of DERs was carried out. (2). The load flow and transient study for the proposed ING system has been simulated using Electrical Transient and Analysis Program (ETAP) software (version 16.0.0), and the results have been discussed. Furthermore, (3). a PLC-based smart nanogrid control (SNC) conceptual design was also created for the smooth operation of the ING system for the ICCP application purpose.

1.3. Organization of the Paper

There are a total of five sections in this article. In this section, a brief introductory note is presented. The remaining sections are organized as follows. In Section 2, the identification of the problem and its proposed resolutions are discussed. A brief note on the hardware design configuration and mathematical modelling of the ING system suitable for ICCP power supply in a 100 MLD SWDP industry is presented in Section 3. Section 4 discusses the necessary stimulation for load flow and transient analysis through different case studies with the help of ETAP simulation software. Finally, the work is concluded in Section 5 while focusing on future research directions.

2. Problem Identification and Proposed Resolutions

The ICCP method of cathodic protection scheme required a continuous low voltage DC power supply to inject the required amount of impressed current to the metallic structures kept under prevention from corrosion. If the continuity of the injected impressed current is lost at any point, the oxidation process will affect the metallic structures, and the corrosion will start. Hence, most industries are used to ensure stable power supply solutions through backup power supply solutions. Therefore, there will always be uncertainties with the utility grid power supply; therefore, all the industries are providing dedicated UPS and battery banks to keep a backup power supply for the ICCP system. If the main utility mains suffer frequent power failure, the batteries charged by mains-powered battery chargers may be used instead of transformer/rectifiers. Batteries may also be charged with a wind-powered generator or solar cells. The batteries should be charged on a regular basis to provide a continuous power supply to the cathodic protection system [29]. Authors in [30,31] have strongly recommended that renewable energy-based power sources such as solar and wind-based hybrid power supply will be economical and reliable for the ICCP method of corrosion prevention scheme for the remote pipelines in hydrocarbon sectors.

In this context, various research projects are currently being undertaken to provide renewable energy-based integrated power supply solutions along with grid power supply may be a better and more economical solution for large-scale ICCP installations in industries. Hence, in this research work, authors have decided to design a multiple DERs-based nanogrid, which will integrate along with the primary utility grid to operate both on-grid and off-grid modes. To design it, a detailed study has been carried out by conducting various field studies at the 100 MLD capacity-based SWDP industry, which is located in the southern part of India. A typical single-line diagram (SLD) for the auxiliary power distribution scheme of the proposed SWDP industry has been considered for the detailed analysis. All the loads belonging to the ICCP system have been segregated and connected to two buses named ICCP Bus-1 and -2. The SLD has been designed by integrating the DER-based ING into it and is shown in Figure 2.

As per the SLD, the SWDP is connected to the utility grid (Tamil Nadu Electricity Board, India) with two independent 132 kV feeders. There is a 132 kV switchyard having a “double main bus with single circuit breaker (CB)” scheme. The total connected load of the plant is 20 MVA. Only one three-winding station transfer (ST) of capacity 20 MVA can step down the grid voltage and feed to the plant. The ST has two secondary windings which step from 132 kV to 6.6 kV each and feed two different primary high voltage (HV) buses at the plant for the primary distribution purpose. These two 6.6 kV main power distribution buses (HV buses) feed the auxiliary loads connected to them in the proposed SWDP industry. They are interconnected through a bus coupler with a 2/3 interlock, i.e., at any point, only two CBs out of three can be closed to avoid unnecessary source parallel. The plant got two independent emergency diesel generators (EDG) set with a capacity of 3.6 MW and are connected to the 6.6 kV HV buses as an emergency backup power supply for chemicals storage and refrigeration purpose only. In the SWDP unit, the grid supply has been designated as a class-IV, and the EDGs power has been considered a class-III emergency power supply. Even though EDGs are directly connected to the HV bus-1 and -2, there is a power setting relay to ensure a certain load current only to avoid unnecessary DG power consumption during the unavailability of class-IV grid supply. However, these EDGs will not operate for normal day-to-day operational activities. Hence, the plant depends only on the class-IV utility grid supply. However, a scheduled load shedding in the utility grid as well as grid disturbance, etc., used to hit the plant’s production target. Furthermore, from the cathodic protection system point of view, it has been provided with UPS backed up by dedicated battery banks for continuous power supply purposes. Therefore, to address this power interruption issue, a suitable design capacity of alternating current (AC)-based ING system is proposed for the plant.

Post-design of the ING system, stability analysis has been carried out to validate the concept. ETAP software has simulated mathematical modelling of the proposed tiny grid (ING system), followed by load flow and transient analysis. Furthermore, the hardware design concept of the nanogrid control mechanism has been discussed.

3. Design Configuration of ING System

In the recent era of green electrifications, mini, micro, Pico, and nanogrids based on renewable energy sources are becoming quite popular methods of providing energy access to remote/rural areas/specified industries of developing and least developing nations [10]. Solar-based nanogrid systems are especially suitable for rough topographies such as hilly areas and scattered households such as limited areas [36]. The proposed ING system is an integration platform for micro power generation, storage units and loads located in a local distribution grid in a controllable manner. In this research, the main focus of ING is to supply AC power to some restricted loads (ICCP buses only) within the specific boundary limit of the SWDP.

The proposed renewable energy-based and distributed energy resources (DERs) operated ING system has been integrated into the SWDP industry. The SLD shown in Figure 2 has been illustrated in the same way. It may be noted from the SLD of the proposed test station, as shown in Figure 2, that there are two ING buses, i.e., ING bus-1 and -2, and each ING bus consists of four sets of DER. Each ING bus is connected to four different nano sources: solar of 125 kVA, wind turbine of 125 kVA, diesel generator set 125 kVA, and the ups with a battery bank capacity of 125 kVA in an integrated manner. Both these ING buses are independently connected to ICCP bus-1 and -2, respectively.

Furthermore, the ICCP buses are already interconnected with the HV bus-1 and -2, which is further connected to SWDP’s class-IV utility grid switchyard buses with backup by class-III EDG power supply. All these tiny sources generate power and synchronise at 415 V, 50 Hz AC level. Furthermore, the ING bus is connected to the ICCP bus, which is already connected to the utility grid at 415 V, 50 Hz level through the auxiliary station transformer. Hence, these ING systems are independently connected to the utility grid through ICCP bus-1 and -2. The ING systems are designed to operate in on-grid and off-grid modes per the requirements. However, at any point, no direct parallel operation is allowed among the ING buses. It may be noted that each ICCP bus is connected with four numbers of 100 kVA loads. However, out of these four loads, only two will be operational, and rest two will be on standby. i.e., each ICCP load bus is designed for 50% load operation purposes only. Similarly, each ING system has its installed capacity of four different sources of 125 kVA ratings and will always be loaded with 50% of the rated capacity. The rest 50% of generated power can be traded to the utility grid. Furthermore, each ING bus and its connected sources are designed to feed the power to its connected load buses (ICCP bus-1 and -2) together, even if the utility grid and anyone ING are unavailable.

3.1. Modeling of ING System

Mathematical Modeling of ING System

In this section, the proposed ING power system has been modelled mathematically based on the Quasi Steady State (QSS) approach through Eigenvalue analysis. Even the proposed ING system contains four DERs: solar, wind, DG set and UPS with battery banks, etc. However, only two tiny sources are modelled as a sample basis due to space constraints. Figure 3 shows the analytical modelling for two source models for representation purposes only.

An assumption was made that each DER is modelled as a controlled voltage sources (V₁ and V₂) and is connected to the load ZL through a line impedances (Z₁ and Z₂). Furthermore, it is also considered that each DER is regulated based on the droop control scheme as the method already discussed by in [37]. The application of QSS approach of mathematical model for the ING system can be derived with respect to the model used in [38].

It may be noted from Figure 3 that the steady state currents ${\underset{\_}{I}}_1$ and ${\underset{\_}{I}}_2$ are given by Equation (1).

$$\left[\frac{ {\underset{\_}{I}}_1}{{\underset{\_}{I}}_2}\right]=\left[\begin{array}{cc}{\underset{\_}{Z}}_L+{\underset{\_}{Z}}_2& -{\underset{\_}{Z}}_L\\ -{\underset{\_}{Z}}_L& {\underset{\_}{Z}}_L+{\underset{\_}{Z}}_1\end{array}\right]\left[\begin{array}{c}{\underset{\_}{V}}_1\\ {\underset{\_}{V}}_2\end{array}\right]. \frac{1}{{\underset{\_}{Z}}_1 {\underset{\_}{Z}}_L+{\underset{\_}{Z}}_2 {\underset{\_}{Z}}_L+{\underset{\_}{Z}}_1 {\underset{\_}{Z}}_2}$$

(1)

In general, complex rms voltage (line-to-neutral) and line current of the $n\mathrm{th}$ DG are given as

$${\underset{\_}{V}}_n=V_{nx}+j{V}_{ny}$$

(2)

$${\underset{\_}{I}}_n=I_{nx}+j{I}_{ny}$$

(3)

Steady state apparent power of nth DG is given as

$${\underset{\_}{S}}_n=3.{\underset{\_}{V}}_n\ast {\underset{\_}{I}}_n^{*}$$

(4)

From Equations (2)–(4), it follows that

$$P_n=3.V_{nx }. I_{nx}+3.V_{ny }. I_{ny}$$

(5)

$$Q_n=-3.V_{nx }. I_{nx}+3.V_{ny }. I_{ny}$$

(6)

The total change in active and reactive power $\left(\Delta P_n, \Delta Q_n\right)$ using small signal analysis is described by

$$\left[\begin{array}{c}\Delta P_n \\ \Delta Q_n\end{array}\right]=3. \underset{Operating pt}{\underbrace{\left[\begin{array}{cccc}{I}_{nx,0}& I_{ny,0}& V_{nx,0}& V_{ny,0}\\ -I_{ny,0}& I_{nx,0}& V_{ny,0}& -V_{nx,0}\end{array}\right]}} \left[\begin{array}{c}\begin{array}{c}\Delta V_{nx} \\ \Delta V_{ny}\end{array}\\ \begin{array}{c}\Delta I_{nx}\\ \Delta I_{ny}\end{array}\end{array}\right]$$

(7)

The magnitude $\left(V_n\right)$ and phase $({\varnothing }_n)$ of the voltage ${\underset{\_}{V}}_n$ in Equations (2) and (3) is a non-linear function of $V_{nx}$ and $V_{ny}$, respectively, and is given by

$$V_n=\sqrt{V_{nx}^2+V_{ny}^2}$$

(8)

$${\varnothing }_n=\arctan \frac{V_{ny}}{V_{nx}}$$

(9)

$${\omega }_n={\varnothing }_n$$

(10)

Rahmoun et al. in [37] have further discussed that the change in frequency and voltage for the nth DER is given as

$$non smoothed \left\{\begin{array}{c}\Delta {\tilde{\omega }}_n=-K_{Pn}.\Delta P_n\\ \Delta {\tilde{V}}_n=-K_{Qn}.\Delta Q_n\end{array}\right.$$

(11)

$$smoothed \left\{\begin{array}{c}\Delta {\omega }_n=\frac{1}{1+s{T}_{c,n}}.\Delta {\tilde{\omega }}_n\\ \Delta V_n=\frac{1}{1+s{T}_{c,n}}.\Delta {\tilde{V}}_n\end{array}\right.$$

(12)

The linearization of Equations (8) and (9) for $V_n$ and $Q_n$ around the operation point ($V_{nx,0}, V_{ny,0},{\varnothing }_{n,0}$) and using Equation (11) in Equation (12) results in a state space representation as in [39]

$$\left[\begin{array}{c}\dot{\Delta {\omega }_n}\\ \dot{\Delta V_{nx}}\\ \dot{\Delta V_{ny}}\end{array}\right]={\underset{\_}{A}}_n. \left[\begin{array}{c}\Delta {\omega }_n\\ \Delta V_{nx}\\ \Delta V_{ny}\end{array}\right]+{\underset{\_}{B}}_n.\left[\begin{array}{c}\Delta P_n\\ \Delta Q_n\end{array}\right]$$

(13)

where ${\underset{\_}{A}}_n$ and ${\underset{\_}{B}}_n$ are the system and input co-efficient matrices respectively. Using Equation (7) in Equation (13) and rearranging results in

$$\left[\begin{array}{c}\dot{\Delta {\omega }_n}\\ \dot{\Delta V_{nx}}\\ \dot{\Delta V_{ny}}\end{array}\right]={\underset{\_}{A}}_{int, n }. \left[\begin{array}{c}\Delta {\omega }_n\\ \Delta V_{nx}\\ \Delta V_{ny}\end{array}\right]+{\underset{\_}{B}}_{int, n }.\left[\begin{array}{c}\Delta I_{nx}\\ \Delta I_{ny}\end{array}\right]$$

(14)

The Equation (14) can be extended for two DERs. Using Equations (1)–(3), in the resulting system gives the closed loop system as

$$\left[\begin{array}{c}\begin{array}{c}\Delta \dot{{\omega }_n}\\ \Delta {\dot{V}}_{1x}\end{array}\\ \begin{array}{c}\Delta {\dot{V}}_{1y}\\ \Delta \dot{{\omega }_2}\end{array}\\ \begin{array}{c}\Delta {\dot{V}}_{2x}\\ \Delta {\dot{V}}_{2y}\end{array}\end{array}\right]={\underset{\_}{A}}_{closed}. \left[\begin{array}{c}\begin{array}{c}\Delta {\omega }_n\\ \Delta V_{1x}\end{array}\\ \begin{array}{c}\Delta V_{1y}\\ \Delta {\omega }_2\end{array}\\ \begin{array}{c}\Delta V_{2x}\\ \Delta V_{2y}\end{array}\end{array}\right]$$

(15)

Kundur et al. in [40] has discussed that the poles of the closed loop “system matrix” ${\underset{\_}{A}}_{closed}$ give the Eigen values for the system in Figure 3. The “system matrix” is a function of the operating point values of the electrical and control parameters given as

$${\underset{\_}{A}}_{closed}=f \left({\underset{\_}{Z}}_1, {\underset{\_}{Z}}_2, {\underset{\_}{V}}_{1,0}, {\underset{\_}{V}}_{2,0}, {\underset{\_}{I}}_{1,0}, {\underset{\_}{I}}_{2,0}, {\omega }_{1, 0, }{K}_{P1, }{K}_{P2, }{K}_{Q1, }{K}_{Q2, }{T}_{C1, }{T}_{C2, }\right)$$

(16)

3.2. Hardware Design of ING System

The proposed ING system for the 100 MLD capacity-based SWDP industry has been conceptualized. The proposed hardware module design concept will be suitable for providing uninterrupted power supply solutions to the ICCP loads. The smooth power switching over the mechanism of the proposed ING modules from on-grid mode to off-grid mode and vice-versa is one of the main features of the proposed hardware design. Furthermore, if by any means all the sources trip during the switch over from on-grid mode to islanded mode due to an overload issue, that case, restarting of sources and sequential load restoration process required to be carried out to bring the power supply back to the service. Therefore, keeping all these requirements in mind, the authors in this research have suggested a PLC-based Smart Versatile ING Controller (SVIC) module and described its operational mechanism. Figure 4 shows the architectural design of PLC-based SVIC module, which is a hardware-based control and automation panel. This panel is specifically designed for the smart grid application, which deals with islanding, black start operation and sequentially load restoration.

It may be noted from Figure 4 that the PLC-based SVIC module is an integrated control module and is designed for the smooth operation of the ING system along with its connected loads (i.e., ICCP loads). Due to space constraints, only one set of ING modules and one set of ICCP buses are shown as typical hardware design. However, in the actual case, it needs two sets of SVIC control modules to meet the requirement. The SVIC module consists of components such as an ING control unit (ICU), relay module, PLC unit, switchgear modules and local controller, which are significant components or sub-systems as indicated in the architectural design block diagram. All these subcomponents are integrated into a single control platform to build up the complete SVIC panel, which will be suitable for the ING-operated ICCP bus’s power supply purpose. All the parts or sub-components are standard industrial-grade items. For example, the PLC contains its central processing unit (CPU) with inbuilt memory, power supply unit, communication module, input/output (I/O) device, human–machine interface (HMI), etc. The necessary real-time field parameters (from the DERs as well as the grid) are used to communicate to the CPU of the PLC module via its dedicated analogue input (AI) and digital input (DI) channels. The CPU is used to analyse the field received data and generate suitable analogue outputs (AOs) and digital outputs (DOs). In addition to the above, four different interfacing hardware modules exist, which will lead to the convenient operation and control of DERs (i.e., SPV, WTG, DG and UPS) along with ICU. One of the significant components of this design is the relay panel, which contains all the protection and control relay devices. The operation DI and DO commands from the PLC to the field and vice-versa have been routed through these control relays to avoid direct interfacing of high voltage applications. A standard industrial-grade control auxiliary supply (i.e., 220 V DC) has been allotted for the operation and control of these relays. The function of DI and AI is to take field feedback/ command status in the form of either analogue or digital and deliver the necessary DO and AOs for the system’s smooth operation. However, protection relays play a different role here and are dedicatedly installed to sense the fault from the power circuit, and necessary trip command initiates to protect the system. Since all these protection relays are of a microprocessor-based programmable type, they work based on the predefined setting parameters of relays. The operational concept of the PLC-based SVIC module is described in the flow chart in Figure 5.

Figure 5 illustrates the micro sources’ operational sequence during on-grid and off-grid operational modes. Since the operational concept of the flow chart is given in a very self-explanatory manner, it needs to be mentioned in detail in this script due to space constraints. It may be noted that if, beyond our control logic, the complete blackout of the power system occurred due to simultaneous tripping of all the DERs during the changeover from the on-grid operational stage to off-grid mode, then there will be a requirement for black start operation. The proposed PLC-based SVIC got its feature to initiate the black start operations followed by a sequential load restoration process. If the black start mechanism operates, then the SVIC controller starts checking the status of connected sources and loads automatically and ensures that all the CBs should be in open condition only. Then the controller initiates DG started command at first, followed by closing its outgoing CB to the bus. Once the DG is ready to take up to 50% of its rated capacity, it closes some of the load CBs (30% load only). Now, the controller gives the close command to the respective CBs belonging to UPS, SPV and WTG sequentially. Once all these micro sources are in service, the controller acts fast to close the rest of the load CBs one after another while maintaining the stability of bus voltage and frequency.

4. Stability Issue and Simulation Case Study

Microgrid and nanogrid stability issues are very similar to the immense power system, and the same has been categorized as voltage stability, transient stability and small signal stability. These stability issues are further discussed in detail in [41]. The continuous load switching, feedback PID controller and the micro sources’ power limit, etc., played a significant role in the small signal stability of the microgrid or nanogrid.

The transient fault during on-grid or off-grid operation causes stability issues in a multi-DERs-operated power system. It is required that the bus voltage stability of a remote system should be maintained at a steady state level for smooth operation purposes. From this point of view, the authors in this research feel that the transient stability analysis is most important for the ING system. Therefore, the load flow and transient stability analysis have been carried out using ETAP software (version 16.0.0).

4.1. Load Flow Study

In this research work, the Figure 2 showing the SLD has been taken for the load flow analysis. There are four different case study scenarios considered for the proposed load flow analysis in this section. This analysis has been carried out based on the scenarios given in

Table 2. Typical parameter ratings of DERs used in ING.

Case Study No	Micro Sources								Large Sources				200 kVAr Capacitor Bank
	Solar PV		UPS with Battery		WTG		DG		Class-III Supply		Class-IV Supply
	ING1	ING2	ING1	ING2	ING1	ING2	ING1	ING2	EDG1	EDG2	Grid1	Grid2
1	√	√	√	√	√	√	×	×	×	×	√	√	OFF
2	√	√	√	√	√	√	√	√	√	√	×	×	ON
3	√	√	√	√	×	×	√	√	√	√	×	×	ON
4	×	×	√	√	√	√	√	√	√	√	×	×	ON

considering the on-grid and off-grid operational mode of ING operated ICCP buses-1 and -2.

It may be noted from Table 2 that four different source combinations are considered for the ING operational performance through load flow analysis with and without the presence of the SVIC module in the ING system. It may be noted from case 1 that the utility grid is available. Both the ICCP buses are getting power from both INGs and the utility grid in a paralleled way; however, both the DG units belonging to INGs are not operating. In this case, the on-grid mode of the operational scenario of the INGs is analysed. Due to on-grid operation, capacitive loading was not required; therefore, the same bank was kept isolated. However, the same capacitor banks are returned to service during an off-grid operation mode. The second case has been considered with the off-grid operation of ING-1 and ING-2. The EDG-1 and -2 in Class-III systems are in service. In case 3, both EDGs and DGs are available; however, the WTG-1 and -2 are absent when the class-IV grid supply is not there. Therefore, the ING system depends only on DGs, solar and UPS systems to operate in islanded mode. In case 4, the ING system contains only UPS, WTG and DGs to operate along with EDGs in islanded mode. Only during the availability of utility grid supply will the 200 kVAr capacitor bank be kept OFF. If the grid is unavailable, the exact needs to be kept ON condition. For these four scenarios, the load flow analysis was carried out using ETAP simulation software, and the results are given in Figure 6. Due to space constraint, only the flow of current in different buses are shown in Figure 6 for illustration purpose. However, the detailed current flow in all feeders is not shown here.

4.2. Transient Analysis

In general, the transient stability issue of any power system is treated as a nonlinear problem. This preliminary study is one of the essential aspects to consider during the planning, design, and implementation of a new power system installation. The dynamic behaviour of the power system can be analysed through transient study reports of DERs operated microgrid/nanogrid system. This transient behaviour is subjected to real-time fault scenarios and starting inrush current as applicable. A system is said to possess transient stability if, after the fault, it can maintain synchronous operation and return to the initial state or close to it [42].

In this research work, using ETAP simulation software, Figure 2 SLD is taken for the transient study. The proposed SLD is a simplified 7-bus scheme of the proposed SWDP industry, specifically worked out for ING-operated ICCP power supply. In this analysis the accelerated Gauss–Seidel method has been adopted. There are four case studies with different scenarios are chosen for the detailed analysis with and without considering the presence of PLC-based SVIC module in the ING system. Since the role of the PLC-based SVIC module is restricted to smooth islanding, black start operation and sequential load restoration process, there will not be much impact on the load flow and transient analysis results. The on-grid and off-grid modes of operation of the ING system and ICCP buses-1 and -2 are considered for preparing the case studies mentioned in

Table 3. Case study for transient analysis.

Case No	ING Operated ICCP System at SWDP Industry	Action Summery
1	Utility grid (Class-IV) power supply of SWDP is available (ING on-grid): DG-1 and -2 OFF, EDG-1 and -2 OFF; The capacitor bank is OFF and bus coupler between ICCP bus-1 and -2 is kept open. ICCP buses obtaining power from grid and ING bus (WTG, solar and UPS in operation)	Event-1: At 0.5 s, a 3-phase (L-L-L) fault occurred at SWDP 132 kV switchyard bus. Event-2: At 0.6 s, fault cleared at SWDP 132 kV switchyard bus.
2	Utility grid power supply is not available and EDGs is in operating condition (offgrid mode of operation of ING)—Both the EDG-1 and -2 are in operation, DG-1 and -2 are also in ON condition, and its capacitor bank is also ON. Here, the bus coupler among ICCP bus-1 and -2 is open and ICCP buses are obtaining power from ING buses (DG, WTG, solar and UPS in service) which is already in operation along with EDGs.	Event-1: At 0.5 s, EDG-1 tripped and stopped. Event-2: At 0.6 s, Class-III LT CB-8 breaker open and ING-1 becomes completely isolated.
3	Grid failed and EDG-1 and -2 are in operation (ING in Islanding mode)—EDG-1 and -2 as well as DG-1 and -2 are in ON condition, the bus coupler of ICCP bus-1 and -2 is open and its capacitor bank ON; ICCP bus obtaining power from EDG and ING buses (WTG, solar UPS and DG are in operation)	Event-1: At 0.5 s, 3-phase (L-L-L) fault occurred at HV bus-1. Event-2: At 0.6 s, fault cleared at HV bus-1.
4	Utility grid and EDG-1 and -2 are not available in extreme scenario (ING in fully Islanded mode of operation)—DG-1 and DG-2, WTG-1 and -2 and solar-1 and -2 are in operation. The bus coupler of ICCP bus-1 and -2 is in open condition. The capacitor bank is also ON. (i.e., ING-1 and ING-2 buses are operating in islanded mode independently)	Event-1: A 3-phase (L-L-L) fault occurred at time 0.5 s, at ING-1 bus. Event-2: At 0.6 s, fault cleared at ING-1 bus.

. The transient analysis was based on case studies with different transient fault conditions. This analysis examined four conditions related to transient faults (i.e., 0.0 s, 0.5 s, 0.6 s and 5.0 s) on ING-operated ICCP power systems. Since the load flow and transient analysis is a multiple iteration process with different combinations, it is difficult to show all the simulated results in this manuscript due to space constraints. Hence, only one typical simulated SLD is shown in Figure 6 for understanding purposes, i.e., only four case studies are shown in this manuscript out of a total of 16 configurations/ iterations as simulated. Results are generated from different combinations, as the cases are mentioned in Table 3.

Case 1 is contemplated for the ING-1, and -2 operated ICCP systems during the availability of utility grid power supply (i.e., in the on-grid mode of operation). In this mode of operation, all the micro sources of INGs such as SPV, WTG, and UPS are in service and operating in grid synchronization mode. In this scenario, the SWDP station’s EDGs are not operational. Since the grid is available, the 2 × 100 kVAr capacitor banks are not kept in service. In this scenario, a transient 3-phase (L-L-L) fault scenario was made at the 132 kV bus at 0.5 s and got it cleared at 0.6 s to analyse the case. The ING system’s transient response and other buses are shown in Figure 7.

In Case 2ING-1 and -2 operate in islanded mode, and the utility grid supply is unavailable. However, both the EDGs belonging to the class-III supply of SWDP are in service. All four micro sources are available, along with the capacitor banks are in operation. Furthermore, all these micro sources operate in synchronization with EDGs. In this scenario, the EDG-1 tripped at 0.5 s, and the corresponding outgoing breaker CB 4 opened at 0.6 s and isolated the ING-1 completely from the EDG-1. In this scenario, the ICCP bus-1 will only obtain power supply from the ING bus-1 without closing the bus coupler breaker CB 12. Therefore, the ING bus’s transient response and others are shown in Figure 8.

Case 3 has been considered with an assumption that the utility grid supply failed condition. However, both EDGs are in service. In this case, both the ING systems are operating in islanded mode along with EDGs. In this scenario, a transient fault (i.e., 3-phase (L-L-L)) scenario was simulated at HV bus-1 at 0.5 s, and the fault was removed at 0.6 s to analyse the transient scenario. The simulation results from the ETAP software are shown in Figure 9.

Case 4 has been considered for the transient analysis, where the 3-phase (L-L-L) fault was created in the ING bus-1 at 0.5 s and got it cleared at 0.6 s. This fault scenario was taken when both ING-1 and -2 modules were operating in islanded mode, and both utility grid (class- IV) supply and class- III EDGs were not in service in the SWDP industry. However, all these micro sources are available, including SPV, WTG, DGs and UPS, with battery banks for both the ING-1 and -2. The simulated results for this case are shown in Figure 10.

4.3. Simulation and Discussion on Results

The simulation results for the transient analysis of case study-1 are discussed below. In this study, the 132 kV SWDP switchyard was taken as a sample piece, and the 3-phase (L-L-L) fault has been simulated for a fraction of time (i.e., transient fault with initiation time t = 0.5 s and cleared time t = 0.6 s), and the following results are noted. It is observed that both the utility grid-1 and grid-2 are feeding to the fault centre at a rate of 17 kA fault current (as it is recorded in the load flow simulation study). The fault level was 3562.86 kVAr at the fault point of the SWDP switchyard bus. The frequency, power factor and %voltage have been recorded as 50 Hz, 26% and 7% during this transient fault and the same is shown in Figure 7a. However, at the same time, all other buses are measured at 0% voltage in the fault period. However, post fault scenario, i.e., at time t = 0.6+ s onwards, the ICCP bus-1 and -2 voltage could normalize back to its rated value.

Furthermore, it is noted from Figure 7a that all other buses’ frequency and voltage responses could restore to the expected value after clearing the transient fault. This indicates that, even with any transient faults, the ICCP bus-1 and -2 terminal voltages will not become unstable due to a backed-up power supply. The actual power loading pattern can be seen in Figure 7b. Even though there is a considerable fluctuation in the output current of the WTG-1 and -2 during a transient fault, as shown in Figure 7c, the terminal voltage could maintain constant, as seen in Figure 7d.

Case study-2 stands for the off-grid operational scenario of the SWDP industry when the utility grid supply fails, and the emergency chemical safety-related power supply is feeding from the EDG-1 and -2. Here, the capacitor bank is ON, and the ICCP bus-1 and -2 are getting supply from both ING-1 and -2, which are already synchronised with the station EDGs.

The transient fault scenario here is considered such that the EDG-1 tripped at time t = 0.5 s. Therefore, the corresponding downstream breaker CB (i.e., CB-8) immediately became automatically open at time t = 0.6 s due to breaker inter-tripping features. In this case, the ING-1 becomes completely isolated to feed the ICCP bus-1. The transient response for all the SWDP auxiliary buses, including ING buses, is shown in Figure 8 for illustration purposes. The huge voltage dip occurred (i.e., 50% of the rated value) in the ING bus-1 and the ICCP bus-1 during this sub-transient period, as shown in Figure 8d. Post fault scenario, the voltage could regain 98.25% of the rated value, and the frequency also reached back to 50 Hz. Hence, it has been proved that the ING module can maintain its voltage and frequency even during simultaneous failure of EDG and grid power supply.

Furthermore, it may be noted from Figure 8a that the power angles for both the DGs and EDGs are almost standard. The real power loading pattern of all the buses, including ING buses-1 and -2, is shown in Figure 8b. It may be noted that during a transient fault, there is a significant voltage dipping happening in the ICCP bus-1 and -2; however, the same has been regained just after clearance of the transient fault. The generated percentage of the terminal voltage output of both WTG-1 and WTG-2 is also recorded from the simulation results, as shown in Figure 8c, and the transient response of all the bus voltage has been shown in Figure 8d.

Case-3 has been selected when the utility grid of the SWDP industry is unavailable, and the ING systems are operating in semi-islanded mode (which means INGs are in operation along with EDGs). In this case, the DG-1 and DG-2 are operating with both EDGs. Here, the 200 kVA capacitor bank is ON. In this scenario, a 3-phase (L-L-L) transient fault case has been examined in HV bus-1 for 0.1 sec of time fault duration (i.e., created at t = 0.5 s and removed at 0.6 s). The simulation results for the transient fault analysis for this case are shown in Figure 9. It may be noted from Figure 9b that a complete power interruption or blackout is seen during this transient fault. However, the voltage and frequency could increase and become stable in ICCP bus-1 and -2 at time t = 0.6+ s onwards.

Another scenario, as shown in case-4, is that the ICCP bus-1 and -2 are feeding from the ING bus-1 and -2, which synchronize with EDG-1 and -2, respectively. A 3-phase (L-L-L) transient fault was created at the ING bus-1 at time t = 0.5 s. The fault has been removed and normalized at time t = 0.6 s. Figure 10 shows the transient analysis results for the affected SWDP buses, including ING bus-1 and -2. It may be noted in Figure 10a that the rate of change in frequency in ICCP bus-1 during the islanded operation of ING is very high; this occurs due to the sudden loading of the bus during islanded operation. Figure 10c,d confirms that the power output of WTG-1 and DG-1 heavily fluctuates during the brief period (i.e., sudden overloading occurred). It may be noted that the DG-1 suffered from momentary overloading near 200% of its rated capacity, i.e., 200 kW was recorded, shown in Figure 10c. However, the DG-1 install capacity is only 125 kW. This short-term overloading of DG could be sustained, and all the voltage and current profiles of ADG-1 and WTG-1 have become stable after t = 0.6 s and onwards. In addition to the above, there is a specific observation that both ICCP bus-1 and -2 terminal voltages and frequencies become stable after 0.6 s. No further disturbances were observed in these two buses (see Figure 10d).

The above case study reports are compared with the prior transient analysis results already generated by different authors, and the summary is presented here. The researchers in [43] reported that, while the grid-connected smart grid system switches between the grid-connected modes to islanded mode, the frequency falls immediately from 50 Hz to 49.5 Hz and then continues to decrease slowly up to 49.3 Hz. This value depends on the demand, the fuel cell production and the wind generator production at the time of islanding. If demand is lower than the production of the distributed generators, the frequency will rise after islanding. In the case examined in this section, the active power of the fuel cells, batteries and wind turbine was less than the demand. For longer simulation times, the microgrid frequency would increase and reach 50 Hz again since the slower responding fuel cells will gradually increase their active output over time and regulate frequency.

5. Conclusions and Future Scope of Works

This research presents the power supply arrangement of a 100 MLD capacity-based SWDP industry, which belongs to the chemical factory selected to study and implement multiple DERs-based ING systems for providing suitable and independent power supply solutions to the ICCP method of the cathodic protection system. The detailed work carried out has been summarized below.

(a)

Two sets of multi-micro sources-based ING modules with an install capacity each of 500 kW have been proposed, and their integration with the existing power system in the SWDP industry is proposed.

(b)

The strategic operation and control mechanism of both these ING systems has been discussed along with necessary mathematical modelling, and the concept of PLC-based SNC has been discussed with the elaboration of its operation philosophy.

(c)

To check the stability of the proposed ING system, a detailed simulation on load flow and transient analysis was carried out with the help of ETAP software.

(d)

Finally, the results obtained from the simulation analysis have been discussed and summarized.

The key outcomes are as follows:

(a)

The connected loads to the ICCP bus-1 and -2 are very sensitive power electronics and rectifier circuitries, which are used for corrosion prevention of metal structure purposes. These need a stable and uninterrupted power supply solution. Therefore, the proposed ING concept is one of the best solutions as it has proven its stability from the load flow and transient stability point of view.

(b)

It may be noted from Figure 10a that the frequency change rate in ICCP bus-1 during the islanded operation of ING is very high and varies from 0–150%. Therefore, it is suggested to implement the proposed design of PLC-based SVIC module, which will help to sequentially load shedding and restoration operation during islanded operation of the ING system.

(c)

A well-developed renewable energy-based ING concept will not only help to establish a stable power system in the SWDP industry but it also will help to address the significant techno-economic issue in industrial auxiliary supply purposes. The extra generated renewable energy from the ING module may be traded to the grid to generate revenue.

(d)

This research is expected to bring new light to the field of multiple micro sources based on unique power systems, which will support the corrosion prevention technology to safeguard the large machinery and structures in the proposed SWDP and other similar industries.

After this part of the work, the authors feel it is necessary to widen this research area to add value to the ING system’s application in ICCP-operated cathodic protection systems. Hence, it is suggested to carry out further research on the ING protection schemes and black start operational mechanism as a future topic.