*Proceedings* **Considerations Regarding the Negative Prices on the Electricity Market** †

## **Pavel Atănăsoae \* , Radu Dumitru Pentiuc and Eugen Hopulele**

Faculty of Electrical Engineering and Computer Science, Stefan cel Mare University of Suceava, Universitatii 13, 720229 Suceava, Romania; radup@eed.usv.ro (R.D.P.); eugenh@eed.usv.ro (E.H.)

**\*** Correspondence: atanasoae@eed.usv.ro; Tel.: +40-721-246-229

† Presented at the 14th International Conference INTER-ENG 2020 Interdisciplinarity in Engineering, Mures, , Romania, 8–9 October 2020.

Published: 16 December 2020

**Abstract:** Increasing of intermittent production from renewable energy sources significantly affects the distribution of electricity prices. In this paper, we analyze the impact of renewable energy sources on the formation of electricity prices on the Day-Ahead Market (DAM). The case of the 4M Market Coupling Project is analyzed: Czech-Slovak-Hungarian-Romanian market areas. As a result of the coupling of electricity markets and the increasing share of renewable energy sources, different situations have been identified in which prices are very volatile.

**Keywords:** electricity market; renewable energy; negative prices; price coupling of regions; day ahead market

## **1. Introduction**

Electricity cannot be stored economically on a large scale and in appreciable quantities that would influence the operation of the power systems. Therefore, at all times, the supply injected into the network must be strictly equal to the demand and energy losses in the transmission and distribution networks. This explains why in the spot market, where prices are negotiated hourly, the price of electricity is very volatile [1].

Most European countries have chosen to accelerate the penetration of renewable energies into the electricity mix (20% by 2020). The production of electricity from renewable energy sources is supported by various support schemes [2–4]. To encourage the development of renewable energy sources, European countries have adopted a priority injection system that guarantees them access to the grid as soon as they produce electricity. This injection priority changes the way different means of production are used.

Increasing the production of electricity from renewable energy sources creates new challenges. The electricity generated by wind and solar energy is intermittent and difficult to predict, as it strongly depends on weather conditions [5]. Decision makers face various economic and technological challenges because renewable energy support instruments have a distortionary impact on electricity prices [6]. More specialized studies try through different models to anticipate the variation of prices on the energy markets in conditions of uncertainty. In the paper [7], the behavior of wind power producers adopting two different bidding modes in day-ahead electricity market is modeled and experimentally compared. The merit order effect for the Hellenic electricity market is analyzed in the paper [8]. Minimizing market risks and increasing profits are the main objectives of the market participants. Therefore, forecasting market prices is a challenge for all stakeholders [9–11]. As the supply and demand of electricity must be constantly balanced, the price varies depending on consumer behavior, climatic conditions or even compliance with the production schedules of

power plants. Prices fall in the event of a decrease in demand or a surplus of production compared to forecasts and an increase in the opposite case [12–14]. The issue of forecasting consumption and production levels is therefore crucial. Supply cannot always be adjusted to demand a few hours earlier, especially because electricity is difficult to store on a large scale. The increasing integration of intermittent production capacities, such as wind turbines and photovoltaic systems, makes this adjustment even more difficult to control.

## **2. Coupling of the Electricity Markets**

Price Coupling of Regions (PCR) is based on a single price coupling solution to be used for the calculation of electricity prices between coupled energy markets. The Day Ahead Market (DAM) from Romania operates in a regime coupled with spot markets in Hungary (HU), Slovakia (SK) and the Czech Republic (CZ) inside the 4M Market Coupling project starting with 2014 (Figure 1a). As well, starting with 19 November 2019, the Intra-Day Market (IDM) from Romania operates in conjunction with the markets in the other 20 countries (Figure 1b) participating in the European project SIDC (Single Intra-Day Coupling).

**Figure 1.** Coupling of the Electricity Markets: (**a**) The Day Ahead Market (DAM); (**b**) The Intra-Day Market (IDM); (Source: http://www.opcom.ro).

On the Day-Ahead Market are concluded on each trading day, firm transactions with electricity for each trading hour of the next delivery day, based on the offers submitted by the DAM participants. Transactions on the Intra-Day Market start with the day before delivery day, after the Day-Ahead Market trading has finished, and ends with an hour before starting the delivery.

#### **3. Analysis of the Electricity Prices on DAM: Case of the 4M Market Coupling**

Electrical network interconnections are a key element in regulating the European energy market, both for import and export. They allow for mutual assistance between the Member States of the Union, depending on the profiles of the respective consumers and their production capacities. This system makes it possible to strengthen the security of supply of the territory. At the same time, suppliers get the best price for energy at any time on the wholesale market.

The objective of the market coupling mechanism is to ensure the use of interconnections in the right direction, that is, from the market where energy is the cheapest to the most expensive. The only limit to the coupling of markets is given the import and export capacities at the borders. Network administrators set a limit to ensure security of supply. This limit cannot be exceeded by exporters.

Thus, based on flows, it makes it possible to provide the most useful exchanges in the service of the network, by finding the best solutions between the countries concerned. The establishment of this system, however, required the harmonization of national rules governing the various integrated networks [15–17].

Even after the coupling of the energy markets, a variation in very large limits of the electricity prices is observed. Thus, several situations were identified with very high prices (Figures 2 and 3), with very low prices (Figures 4 and 5) and even negative (Figures 6 and 7). These situations were analyzed in comparison with the electricity production of Romania and with the energy exchanges and available transmission capacities existing on the border (Tables 1–3).

**Figure 2.** High prices on coupled electricity markets at the RO-HU border (data processed from website: http://www.opcom.ro).

**Figure 3.** Electricity production in Romania on 19 September 2019 (data processed from website: http://www.transelectrica.ro).

**Figure 4.** Low prices on coupled electricity markets at the RO-HU border (data processed from website: http://www.opcom.ro).

**Figure 5.** Electricity production in Romania on 5 April 2020 (data processed from website: http: //www.transelectrica.ro).

**Figure 6.** Negative prices on coupled electricity markets at the RO-HU border (data processed from website: http://www.opcom.ro).

**Figure 7.** Electricity production in Romania on 24 May 2020 (data processed from website: http: //www.transelectrica.ro).


**Table 1.** Characteristic values for coupling of the electricity markets on 19 September 2019.

**Table 2.** Characteristic values for coupling of the electricity markets on 5 April 2020.


Electricity generation and consumption must be balanced so that the market is balanced and does not collapse. When there is too much production, short-term markets tend to lower their prices or even have negative prices to discourage producers from generating.

Prices on the wholesale electricity market result from the meeting of supply (production) and demand (consumption). Basically, the higher the supply compared to the demand, the lower the price of electricity.

In these periods of overproduction, producers who cannot shut down their power plants have to pay to sell their production, while some large buyers are encouraged to consume more. These short-term movements have little influence on the bill of small consumers, as most individuals have fixed-term contracts for long periods of time. On the other hand, for large industrial consumers who have short-term contracts, this is an advantage, because they can buy electricity very cheaply, even at a negative price.


**Table 3.** Characteristic values for coupling of the electricity markets on 24 May 2020.

Cross-border electricity exchanges use the available transmission capacity (ATC) on the border to reduce price differences between coupled electricity markets. Supply and demand are traded on the stock exchange until cross-border transmission capacity is exhausted or market prices are the same in both countries. These situations can be easily identified in Figure 2-Table 1 (on the SK-CZ border in time intervals 7–21); Figure 4-Table 2 (on the RO-HU-SK-CZ borders in the time intervals 10–13 and 17–18; on the HU-SK-CZ borders in the time intervals 7–9, 14–16 and 19–21); Figure 6-Table 3 (on the RO-HU border in time intervals 5–17; on SK-CZ borders in time intervals 5–21).

If the available transmission capacity on the border is reached before the price alignment, then electricity prices remain different: The other situations in Figure 2-Table 1; Figure 4-Table 2; Figure 6-Table 3.

Exports through interconnections and flexible consumption are not necessarily enough to reduce falling prices. If, at some point, electricity demand rises sharply and supply is difficult to maintain due to a lack of production supply, prices are rising because some suppliers are willing to pay a high price to avoid a blackout. If, on the other hand, demand is weak in the face of abundant supply, prices will fall.

Situations with negative prices certainly correspond to low marginal costs at a given time, but this does not mean that these prices make it possible to cover the total production costs of the installations concerned. This phenomenon can be amplified by the structure of certain support mechanisms for renewable production. Currently a large part of the production of electricity from renewable sources is supported by various schemes. Thus, support payments depend on the production of electricity achieved. Therefore, even when prices are negative it can be profitable for renewable producers to continue to operate.

#### **4. Conclusions**

Radical changes have taken place in the energy sector in recent years, with a clear trend of increasing the production of electricity from renewable energy sources and new challenges for the market participants. Thus, some wholesale electricity markets have faced episodes of negative prices. In these single market situations, it is the producers who pay the suppliers. Negative prices occur especially in periods of abundant production of electricity from renewable sources and low demand, situations in which certain conventional energy sources cannot operate below a technical minimum. Negative values of electricity prices were recorded in the markets of Hungary, the Czech Republic and Slovakia (a record negative value of –65 EUR/MWh being registered on the SK-CZ border).

As well, the lack of the participants skills in developing adequate bidding strategies was one of the causes of the price increase.

The electricity prices on the Romanian Day Ahead Market are very variable between a minimum registered value (0 EUR/MWh) and a maximum registered value (158 EUR/MWh). No negative prices have been registered so far in Romania, however, the energy markets with which Romania operates in coupled regime register frequent situations with negative electricity prices.

However, negative wholesale prices are not good news. An increase in overproduction episodes creates uncertainty in the markets and reflects an imbalance. The main undesirable effect of these negative prices is that the market no longer sends the right signals to investors. This is equivalent to destroying value, as producers' revenues from the electricity market no longer cover real production costs. Electricity producers are no longer encouraged to invest in the renewal and expansion of new production capacity, and this can lead to a risk of shutdown during periods of high energy demand.

Coupling regional energy markets can partially solve these situations. Therefore, the imbalance of a system can be resolved by a neighboring system if the interconnections are not already used at their maximum capacity level.

After a strong period of development, it becomes necessary to review the mechanisms to support the development of electricity production from renewable energy sources. In the longer term and when renewable energy production systems reach a certain level of maturity, we can imagine that their development will depend much more on market mechanisms. For the time being, and because European targets are particularly ambitious, a first step towards avoiding these situations could be to stop paying support schemes in the event of negative wholesale prices or when renewable energies cause network congestion.

However, a positive effect of situations with lower or even negative prices in the electricity markets must be highlighted. The development of large-scale storage capacity will be able to be encouraged by such behavior of the electricity market. As well, more levers to increase the flexibility of electricity consumption will need to be activated. Some uses are easy to control, such as the thermal use of electricity (heating or cooling by heat pumps) or the recharging of electric vehicles.

Continuous improvement of electricity demand and supply forecasts, thanks to weather models, will make it possible to better predict imbalances in the electricity system and anticipate the use of levers to increase the flexibility of final energy consumption.

**Author Contributions:** Conceptualization, P.A. and R.D.P.; writing—review and editing, E.H. All authors have read and agreed to the published version of the manuscript.

**Acknowledgments:** This work was supported by a grant of the Romanian Ministry of Research and Innovation, CCCDI—UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0404/31PCCDI/ 2018, within PNCDI III.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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## *Proceedings* **Overview on E**ffi**cient Naval Power Architecture** †

## **Mariana Dumitrescu**

Department of Automation and Electrical Engineering, "Dunarea de Jos" University of Galati, 80008 Galati, Romania; mariana.dumitrescu@ugal.ro

† Presented at the 14th International Conference INTER-ENG 2020 Interdisciplinarity in Engineering, Mure¸s, Romania, 8–9 October 2020.

Published: 14 December 2020

**Abstract:** Possible configurations for propulsion and electric power generation on vessels are an important field for the researcher in the naval area. The most important is electric power generation. Nowadays, diesel engines are the main prime movers, although all sorts of other electric power generating types are coming more and more. Apart from that, there are some specific high voltage/power applications that use steam or gas turbines. As well as this, wind power, either on a traditional sailing vessel or in a wind electric power generator, and also solar power, is gaining interest. The modern configuration is developing until the goal of zero carbon emissions is reached, using the electric power system configuration, introducing fuel cells as prime movers, and using batteries as energy storage devices, to increase the system's safety.

**Keywords:** design; electric power generation; electric installation; ship; propulsion

## **1. Introduction**

The future of ships is the electric propulsion, so more and more the ship designers are developing eco-friendly solutions (safe for the environment), with the goal of zero carbon emissions [1–4]. An electric ship is more than an electric drive system; it includes power generation, delivery, automation, and control. More and more naval applications require a larger amount of electric power generated and delivered (much more than the commercial ships), which can be used by high power consumers, high-power military loads, energy conversion systems, and a bigger delivery system. Other loads may include an electro-magnetic assistance launch system, communication, radar, sonar systems, and hospitality and service loads, as is shown in Figure 1.

**Figure 1.** Power electric warship system: 1—advanced motor and propulsor, 2—motor drive, 3—advanced generators, 4—pulse forming network, 5—energy storage, 6—actuators and auxiliaries, 7—fuel cell stacks, 8—sensors, 9—distribution, 10—integrated power system, 11—integrated thermal and power management systems, 12 and 13—radar systems, 14—electromagnetic vertical launching system, 15—electromagnetic gun.

The synchronous electric generator driven by a gas turbine or diesel engines electrically connected with an electric propulsion motor is nowadays used in a large variety of vessels. According to [1],

the total installed electric propulsion power in marine vessels was in 2002 in the range of 6 to 7 GW. Azimuth thrusters and podded thrust units brought important maneuvering capabilities, control of the dynamic positioning, and intelligent applications [5–7]. Now, electric propulsion is applied mainly in the following types of marine vessels: cruise vessels, ferries, dynamic positioning drilling vessels, cargo vessels, moored floating production facilities equipped with thrusters, shuttle tankers, cable layers, pipe layers, icebreakers and other ice going vessels, supply vessels, submarines, combatant surface ships, and unmanned underwater vehicles.

The scientific literature [1,8–10] mentions that the expected compound annual increasing, for the electric engines and electric generators for ship propulsion systems, is expected to be around 20%. Today, almost all the cruise ships and a lot of cargo ships changed to electric motor propulsion systems. For example, the largest marine electric motors are installed on cruise vessel passenger liner Queen Elizabeth 2, which has a redundant system of propulsion—44 MW, 144 RPM, 60 Hz salient pole synchronous motors—driving the propeller shafts. The dimensions of the motors are 9 m in diameter, weighing more than 400 t each. The installed electric power is 95 MW, produced by nine three-phase 10.5 MW, 10 kV, 60 Hz salient pole synchronous generators—electric driven by diesel engines. The vessel is one of the largest, longest, tallest, widest, and most expensive passenger cruise vessels. Its power plant includes gas turbines and diesel engines that produce 118 MW of electricity, enough to power a city of 300,000 people. Most of the produced power is used for the propulsion system; each of the electric motors draws 21.5 MW during full power and it has Rolls Royce Mermaid pod propulsors, two fixed and two azimuths rotating 360◦.

## **2. Propulsion and Electric Power Network**

The naval system configurations for ship propulsion are presented below [4–6,11].

• Direct diesel propulsion, as shown in Figure 2a, uses electric power generated and distributed by a separated auxiliary system, gearbox reduction and fixed/controllable pitch propellers, and steerable thrusters with fixed/controllable pitch propellers.

The direct propulsion is the most common and basic configuration used still for many operating profiles because when the auxiliary power is a part of the propulsion power needed, there is no benefit in the combination of the propulsion and energy. Vessels like bulk, container carriers, or multipurpose have this direct configuration.

• Hybrid diesel electrical, as shown in Figure 2b,e, uses direct propulsion, integrated electric power generation and distribution, gearbox reduction and fixed/controllable pitch propellers, and steerable thrusters with fixed/controllable pitch propellers.

The hybrid configuration combines the advantages of combined propulsion and energy at a lower sailing speed, improving the carbon emissions goal and the efficiency (less power losses) of a diesel direct drive at higher speed.

• Electric diesel propulsion, as shown in Figure 2c,d, uses integrated electric power generation and distribution, gearbox reduction and fixed/controllable pitch propellers, and steerable thrusters with fixed/controllable pitch propellers.

The main switchboard power delivery system uses AC voltage, as shown in Figure 2c, or CC voltage, as shown in Figure 2d; in the second case, the power system is safer using the advantage of the energy storage system. There are many reasons for this type of system—better ship design, better power efficiency and reduction of emissions, better comfort from reduction of vibrations and noise, reduction of the maintenance of mechanical components, and more flexible operability.

• Direct diesel propulsion by shaft uses electric power, which can be also generated by driven shaft generators, as shown in Figure 2b,e, and a separated or integrated auxiliary system, with gearbox reduction and controllable pitch propellers, and steerable thrusters with controllable pitch propellers.

The propulsion with shaft configuration is mainly applied on vessels with an important power consuming process. When the process is not operated during sailing at maximum speed, then this mechanical combination of propulsion and power is more efficient for investment, fuel consumption, and emissions for container vessels or heavy lift ships. For separated or integrated main and auxiliary electrical power systems, the goal is the efficiency of the prime movers and the availability of the ship's electric network. The shaft generator is mostly operated at synchronous speed, but permanent magnet machines and power electronic converters allow generation at variable speed, so a hybrid diesel electrical configuration can be examined, allowing electric drive as well.

**Figure 2.** Naval configurations of the propulsion and electric power system, Direct diesel (**a**), Hybrid diesel electrical (**b**,**e**), Electric diesel (**c**,**d**).

Tables 1 and 2 give a synthetic overview of the naval system configurations for ship propulsion [12]. Presented are the types of ships that match with different types of propulsions, advantages and disadvantages of the propulsion and power systems on vessels, and installed power and parameters.


**Table 1.** Trends for configuration of propulsion and power supply architecture on vessel types.

**Table 2.** Advantages and disadvantages of propulsion and power supply technologies on vessels [12].


#### **3. Aspects of the Air Emission Configuration Reduction**

Only for the electric propulsion configuration the goal of zero carbon emissions is achievable if the diesel engine is replaced with the renewable power generation, like the fuel cell [12,13]. Hybrid fuel cell propulsion, and integrated electrical energy generation and distribution, with steerable thrusters with fixed pitch propellers, uses as the prime movers the fuel cells instead of diesel engines. There is a limited operating time, so for now it is used in the inland vessels.

The configuration is hybrid because of the direct online connected power battery. The advantages to the fuel cells are: zero emission for the power generation, all electric ship, and high redundancy because of the multi-functional power battery.

The air emissions coming from the combustion motors can be of different types. Substances, like NOx, SOx, and particulate matter (PM 2.5 and 10 μm), are regulated by the Maritime Environmental Pollution Committee—MEPC of the International Maritime Organization—IMO, the European Union, the Environmental Pollution Agency—EPA of the United States of America, and by local governments. The design is influencing only the PM type. For greenhouse gases, the IMO is involved in the reductions of CO2 emissions. The issues are also economical, like improvement of fuel efficiency, future fuel prices may also include additional carbon prices, such as future capitalization of CO2 emissions at a current approximate rate of EUR 22/t.

The ways to achieve the green ship power system goal, from a design point of view, are several. The traditional configurations for electric power generation, distribution, and consumption can be improved by the reduction of consumption, improving efficiency for the auxiliary equipment, LED lighting and intelligent consumption/control systems, and power management systems. For the electric propulsion design, changes involve the complete drive train and changing the hull shape, which are going to see the efficiency growing up to 40%. The potential for improvement of energy efficiency is clear from the efficiency diagram for the example of a container vessel sailing at 24 knots, as shown in Figure 3 [5,13]. We can consider that the prime mover can partially or completely be replaced by green renewable sources, wind or sun power, or by energy carriers like H2. When applying H2, the local energy conversion will be done with fuel cells.

**Figure 3.** Efficiency diagram (Sankey diagram) for a container vessel, according to [13].

Electric diesel, hybrid electric diesel, and hybrid fuel cell power systems have a big power delivery system for the transportation and distribution of the main power [13,14]. The main delivery can use AC or DC voltage, low or medium voltage, with a maximum power rating; for example, an application of low voltage has 5 MWe. Selecting AC or DC is mainly a matter of energy losses; for example, if many frequency control power consumers are needed for gaining efficiency in power consumption, a DC voltage power system implementation will prevent the double conversion AC-DC and DC-AC, saving up to 5% in efficiency. Because DC voltage power systems do not have reactive power, they can be a smaller size. They do not need phase synchronization during parallel operation of generators. As fuel cells and batteries produce DC voltage, it seems logical to select DC for the main power delivery of a fuel cell propulsion system. We also have to take into account that DC voltage systems are less easy to protect for overload and short-circuit, so ultra-rapid fuses are needed. Furthermore, DC voltage power systems need more care than AC voltage power systems, which concerns safety for humans and the machine.

The developing of hybrid diesel electric propulsion, power saving devices in the auxiliary grid like frequency converters, AC voltage switching power supplies, increasing application of LED lighting, and the electric network suffers more and more from current and voltage distortion and from increase of the zero sequence currents. Because the power system is an isolated one, it is more sensitive to the non-linear and the unbalanced loads, and for transient regimes caused by switching, so there is a need for extra attention to electric power systems design, to electromagnetic compatibility (EMC) and earthing. Figure 4 shows a simulation for switching on 950 kW in the electrical network of a mega yacht, presenting the total harmonic distortion on the system voltage, which is going to 9% and is too high. For preventing such a distortion, adding 12-pulse transformers supplying the frequency converter is a solution.

**Figure 4.** The total harmonic distortion on the system voltage by simulating the switching on of the auxiliary power system of a mega yacht, according to [13].

#### **4. Conclusions**

The overview of the ship design architecture, for the isolated power system of a vessel, is pointing to the architecture with less air pollution effects, which is a feasible solution offered by the electric diesel with the total or partial fuel cell power supply. Fuel cell power generation can be considered from a shipbuilding point of view if the ship's design process can provide an optimized system solution with a DC voltage electric network and a minimum of negative effects on the protection and safety on the network. Operating profiles have to give answers to the following questions: Does the operating speed profile justify the selection of an electrical diesel power system and propulsion? Can the electric power and energy system be supplied by fuel cells? The proposed solution depends also on the operating fuel consumption and the load characteristic of the prime movers with respect to the functional safety of the ship, but also to the maximal environmental pollution reduction.

**Conflicts of Interest:** The author declares no conflict of interest.

## **References**


**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Proceedings* **Isolated Power System Safety Analysis** †

## **Mariana Dumitrescu**

Department of Automation and Electrical Engineering, "Dunarea de Jos" University of Galati, 80008 Galat,i, Romania; mariana.dumitrescu@ugal.ro

† Presented at the 14th International Conference INTER-ENG 2020 Interdisciplinarity in Engineering, Mures, , Romania, 8–9 October 2020.

Published: 14 December 2020

**Abstract:** For a specific vessel, the safety of the isolated power system is analyzed. Concerning the safety, the choice of propulsion system and the classification regulations have a major role for the power system design. The rules of SOLAS (International Convention for the Safety of Life at Sea), the flag state, and the harbor authorities are pointing to the basic level of safety, while the classification societies are pointing to the basic navigational regulations. A case study of the main switchboard and emergency switchboard safety, while taking into account different short circuits for the specific operational configuration of the electrical network, makes a comparative study possible.

**Keywords:** safety; design; power system; electric installation; ship

## **1. Introduction**

Electric installations design of the ships is a very complete part of electrical engineering. Their safety is one of the main concerns of the specialists, as they are isolated power systems and they have to cover the complete spectrum from power generation to distribution and switchgear to all consumers [1,2]. The protections, the automations, the communication, and the nautical/navigation systems have to be included in the design. The electrical power for ships comes mainly from the synchronous generators [3,4]. The most common prime mover for an electric generator is often the diesel engine. Other smaller engines can be installed on a common base generator frame, but for diesel-electric important applications, it is possible that both have their own base frame and are connected with a flexible coupling [5,6]. For safety reasons an emergency diesel generator (EG) is always used to give main electrical power if the main source is in failure/blackout [7]. The emergency generator/source must be self-contained and built independent of the main engine room systems. It is completed with its own independent systems for starting, fuel oil, lubrication oil, cooling, and preheating, for an independent working purpose. The consumers supplied by the EG, as the regulations require, are the main consumers like: emergency lighting, navigation/communication devices, steering gear, fire and sprinkler pumps, bilge pump, water tight doors, and lifts.

When the emergency source of electrical network is an electric generator, it has to be provided with a transitional emergency electrical power source. The ship uses the selected uninterruptible power supply (UPS) application. A battery or an uninterruptible power supply must be used as a standby power supply with a capacity of 30 min. Navigation and safety aspects, required by the ship class, point to the use of the UPS, such as: automation system, navigation system, radio/safety announcement equipment, emergency lighting, watertight doors, etc.

## **2. Structure of the Electrical Network**

Taking into account the isolated configuration nature of electrical power system onboard ship, several means are used to assure its safety and continuous availability. The main switchboard is divided into two or more sections power supplied by the main diesel generators (DG) [8,9]. The switchboard

supplied by an emergency generator as well as the uninterrupted (battery secured) power supply as Figure 1 shows is very important, both for reasons of safety and to ensure fault tolerant, redundant configuration. The EG must also be able to start automatically if the main source of electrical power fails to supply the emergency switchboard. In this case, the EG is automatically started and connected to the emergency switchboard by its automation power management system [9,10].

**Figure 1.** Low-voltage 690V electric network for an isolated power system, for a vessel with diesel-mechanical propulsion [10].

In addition to auxiliary generators an electrical network often has shaft generators (SG) driven by the main engine. Sometimes shaft generators are used only for driving the thrusters during maneuvers, while in other applications they are able also to supply the ship's network. For this reason, some interconnections are required to avoid overload or damage to the network. The following feeder combinations, for example, are prohibited [5,10]:


When the SG is connected and is supplying the ship electrical installation, a constant speed mode for the main engine has to be chosen, meaning the network frequency. In constant speed mode, the propulsion will be controlled with the help of the pitch adjustments, using the controllable pitch propeller (CPP). The main engine can drive the SG when this is disconnected from the main propulsion line. In this case, the main engine runs as an auxiliary engine, for example to supply the big consumers in special regime like loading or unloading in the harbor.

First, the propulsion type has to be chosen, diesel-mechanic or diesel-electric, then other decisions are made, like the following [10,11]:


## **3. Designing the Electrical Network According to the Safety Rules**

In the design stage, the first step is to define the ship type. The ship type, size, and purpose are key factors when dimensioning the electrical network. The following are expressing the different electrical needs of different types of ship [4]:


The choice of propulsion system and the classification regulations have an impact on the design of the power electrical network. The rules of SOLAS (International Convention for the Safety of Life at Sea), the flag state, and the harbor authorities specify the basic level of safety, while the classification societies specify the basic navigational regulations, like a redundant propulsion, an unmanned engine room, and a green power system.

For the study case configuration of the power system in Figure 2, the generators were already selected, and the short circuit safety analysis is presented, taking into account the functional alternatives:

**Figure 2.** Configuration of the main switchboard and emergency switchboard for the power system study case.

Case 1—Generator 1 is in running mode, the rest of the generators are stopped. Generator 1 is supplying the thruster engine and the electrical system. Q1 is closed, Q2 and Q3 opened.

Case 2—Generators 2 and 3 are in running mode, the rest of the generators are stopped. Generators 2 and 3 are supplying the thruster engine and the electrical system. Q1 is closed, Q2 and Q3 opened.

Case 3—Generator 3 is in running mode, the rest of the generators are stopped. Generator 3 is supplying the electrical system. Q1, Q2, and Q3 are opened.

Case 4—Generator 2 is in running mode, the rest of the generators are stopped. Generator 3 is supplying the electrical system. Q1 is closed, Q2 and Q3 are opened.

Case 5—Emergency generator (EG) is in running mode, the rest of the generators are stopped. EG is supplying only the essential consumers from the electrical system, but not the thruster. Q1 is opened, Q2 and Q3 are closed.

The short circuit computation, with the help of the Germanischer Lloyd, Short Circuit Calculation Program and using its design safety rules [12,13], takes into account the short circuit locations A, B, C, and D. The calculation obtained the short circuit current values for the a.c. three phase system based on IEC 61363 standard. The short circuit current comprises three components, a.c component (Iac), d.c. component (Idc), and the peak current (Ip). The results of the short circuit currents (in Ampere), for the location C on the main switchboard, are presented in Table 1.

**Table 1.** Alternatives comparation for the main switchboard (C point) short circuit computing, taking into account the functional variants in cases 1, 2, 3, and 4.


These results give the overview of the needed information concerning the system safety and point to the protection equipment selection used to achieve the system safety goal. For comparison, of the system safety in all 4 analyzed cases, the peak current Iptot is used, which has the biggest computed value in the case 4. This is a result of adding the generators' total current and the engines/motors total current, taking into account the equivalent reactance Xd", the nominal generator current Ign, the nominal motor current Imn, and the multiplication factors 6 for the motors case and 2.3 for the Iptot computing.

As Table 1 shows, the most dangerous case, from short circuit safety point of view, is case 4. Because case 4 is detected as the heaviest situation the system can deal with in the circuits design and equipment selection, the numerical results from case 4 will be used in design and equipment sizing. In Table 2 the nominal data computed for the electric circuits connected to the main switchboard, generator 1, generator 2, generator 3, emergency generator, and equivalent engine are presented. Table 2 also gives the selected circuit breakers data needed to achieve the operational security of the analyzed system for all the above-mentioned circuits.


**Table 2.** Electric circuits nominal data and circuit breakers selection for the heaviest operational state, represented by the case 4.

#### **4. Conclusions**

This paper presents a study case analysis of the operational safety for an isolated power system, in the situation of a vessel. Because the safety of the power system is the most important goal, especially for the ship's electrical network, it is important to identify the design steps which can give the most suitable design architecture for the circuits. The marine special rules, which are mandatory, give more pressure when it comes to proposing the optimal design for the ship power system from a safety point of view. The main contribution of the paper is the proposed algorithm finding the most dangerous study case, from a short circuit safety point of view. The novelty is the procedure to select the study cases configuration which are subject for analysis by the Germanischer Lloyd, Short Circuit Calculation Program. The interpretation of the results and the conclusions of safety analysis are also a subject of the research proposed methodology. The alternatives analyzed in the paper for the operational power system specific situations help the specialist to find the most dangerous operational situation and the safest alternative possible. This gives the best confidence in the system response in case of the most dangerous short circuit possible, which can appear in the ship power electric network.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**


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## *Article*
