Graduation internship Dennis Hermus
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Foreword
In the last year of the study Electrical Engineering at the Avans University of Professional Education in Breda a graduation internship is needed to prove the requirements of an Electrical Engineer. Because I have also studied Offshore Engineering & Automation at Avans University of Professional Education in Den Bosch I searched for an company in the offshore industry. This resulted in HFG Engineering. HFG Engineering is a part of the Heerema Fabrication Group(HFG) which is specialized in the engineering and fabricating of jackets, topsides and structurals for the oil, gas and wind industry. HFG has facilities to design and build large constructions in controlled conditions around the North Sea.
The main part of this report is a transformer substation platform for offshore wind farms in the North Sea. This substation increases the generated voltage of the wind turbines. In this report I investigated in detail the utility power supply part, which take care of the electrical power of components in different situations, and makes a guide for the engineering of such a transformer substation. This utility power supply was a subject of many discussions, problems and adjustments during engineering and building of previous projects. The structure, welding, testing, ground investigations and the size and pricing of components are not treated in this report. Because HFG Engineering is an international company this report is written in English.
By this way I would like to thank everyone who has made my graduation internship a success. This are especially the company mentors of HFG Engineering Mr. Van der Heijden & Mr. Tillema, all the employees of HFG Engineering and my mentor of Avans University of Professional Education Mr. Voermans.
Zwijndrecht, June 2011
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Contents
Foreword ... 2 Summary ... 4 1. Introduction ... 5 1.1 The company ... 51.2 Description of the problem ... 5
1.3 Structure of report ... 5
1.4 The origin and the future of offshore wind energy ... 6
2. Standards and guidelines ... 8
2.1 Standards performing company ... 9
2.2 Standards Substation ... 10
2.2.1 General Standards (design, implementation, operation) ... 10
2.2.2 Electrical installation ... 11
3. Reducing energy losses during transport by increase voltage. ... 12
4. The installation of an offshore transformer substation platform ... 13
4.1 The transformer substation ... 13
4.2 Electrical components of the utility power supply ... 14
4.2.1. High power components ... 16
4.2.2. Control and uninterruptible power supply ... 21
4.3 Worst-case scenario’s ... 25
Conclusion ... 28
Recommendation ... 29
List of used parameters ... 30
Glossary ... 31
References ... 33
Annex 1: Project Management Document(in Dutch) ... 34
Annex 2: standards ... 42
Detailed standards ... 42
Less important electrical standards ... 45
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Summary
In the last year of the study Electrical Engineering at the Avans University of Professional Education in Breda a graduation internship is needed to prove the requirements of an graduated Electrical Engineer. This graduation internship is completed at HFG Engineering, which is specialized in the engineering of jackets, topsides and structures for the offshore oil, gas and wind industry. In this report the utility power supply part of a transformer substation for offshore wind farms is investigated in detail and a guide for the engineering of such a transformer substation is made. The construction, calculation and redundancy of components of the utility power supply, which takes care of the electrical power of components in different situations, were subjects of many discussions, problems and adjustments during engineering and building of previous substation projects. This report functions as a guideline for these discussions, problems and adjustments.
Before the utility power supply will investigated and recommended a few studies are done. The conclusions of these studies are:
increasing the voltage will reduce losses. The cable losses(=resistance) will increase quadratically by doubling the current. This resistance depends on many factors, like temperature and isolation, but that influence is limited.
an enumeration of the legislation of an offshore substation is known. There are several standards for the company, general standards for offshore substations and guidelines for the electrical installation which are very important. The most important electrical standards describes the requirements of the whole substation installation.
The advantages and disadvantages of HVDC compared to HVAC and the break-even point of costs for AC and DC transmission, when varying the length of the connection, are known. HVDC will be cheaper compared to HVAC after 90km between the substations.
The components of such a transformer substation could be chosen after these studies. The choice of these components depends of many factors, such as performance, quantity and redundancy. The choice of redundancy depends of several (quantity) analyses: HAZID, HAZOP and EMTP. The recommended structure of the offshore transformer substation is made with using these analyses, conclusions and legislations.
This guide for the utility power supply part of a transformer substation for offshore wind farms will result to: a good explanation to determine the various voltage levels.
an explanation, calculation and determination of the power of the different components. which influences on components take into account.
different own power supply opportunities, like auxiliary generator, emergency generator and UPS, are known and could be calculated.
the various worst-case scenarios, which a substation has to deal with, are known and a scenario could be determined.
different outside power supply opportunities, like power taken from grid and power taken from wind turbines, are known and could be determined.
The recommendation is to investigate a clean own power supply, the competitive advantage of the recommended structure, the utility power supply of a convertor substation and other properties of components of different fabricators.
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1. Introduction
It is good to know something about the company HFG Engineering, the definition of the problem, why HFG Engineering think it is important to solve this problem and why offshore wind is an important industry for the future and for The Heerema Group.
1.1 The company
The Heerema Group is an international company and employ more than 2,000 people around the world. The Heerema Group exists of Heerema Fabrication Group(HFG) and Heerema Marine Contractors(HMC). HFG is specialized in the engineering and fabrication of jackets, topsides and structures for the offshore oil, gas and wind industry. HMC transports, installs and removes all types of offshore facilities. These include fixed structures, complex infrastructures and floating facilities, in shallow water, deep water and ultra-deep water. Heerema Fabrication Group has facilities to design and build large constructions in controlled conditions. They have three large production facilities situated around the North Sea, a workshop facility in Poland and the world-wide operating multi-disciplined engineering company HFG Engineering in the Netherlands. HFG engineering provide the engineering of the jackets, topsides and structures but also FEED studies, detailed design, procurement support and engineering project management. HFG Engineering Europe was established in 2009 to mainly serve the European and Scandinavian market and today has approximately 30 engineers, designers and project managers.
1.2 Description of the problem
The main part of this report is a transformer substation platform for offshore wind farms in the North Sea. In this report I investigated in detail the utility power supply part and made a guide for the engineering of such a transformer substation. The utility power supply, which takes care of the electrical power of components in different situations, was a subject of many discussions, problems and adjustments during engineering and building of previous substation projects. With using this report it is possible for HFG Engineering to solve these problems at the beginning of a project.
1.3 Structure of report
After this section you will find an introduction and explanation about the newest source of sustainable energy: the grown wind industry. The following chapter will treat the standards for engineering and building of offshore substations. To keep this report readable the most referenced standards for engineering and building are summarized in the annex and the most technical expressions are explained in the glossary. After the explanations about standards and laws the technical part of this report will follow.
In chapter 3 you will find the explanation about the purpose of an offshore transformer substation and the explanation of increasing voltage. The last chapter will treat the intern components and the recommended structure of the transformer substation. The jacket, welding, testing, ground investigations, thermography and the size, choice and pricing of components are not treated in this report.
Because The Heerema Group, and so HFG Engineering, is an international company this whole report is written in English.
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1.4 The origin and the future of offshore wind energy
Today, green or sustainable energy is a hot item in the news, because today’s way of generating energy has a negative impact on the environment. Our energy will be more expensive in the future if we keep using fossil fuels. There are a lot of different ways of generating sustainable energy, but wind is the most popular one because wind is everywhere. The development of wind energy has started on land but nowadays offshore wind farms are increasing because the wind on the sea blows stronger and the yield is higher here. In the past offshore wind energy was impossible but it is slowly becoming reality. An offshore wind farm is usually situated a few miles from the shoreline, in order to prevent so-called visual pollution and because the wind is more constant here. Offshore wind contains a higher capacity factor (35% to 40%) than onshore wind (23% to 28%) so the efficiency is greater. Because power is proportional to the cube of the speed, this amounts to an offshore wind yield of about 60% more revenue compared to onshore wind.
The choice of placing a wind farm depends on many factors, like ports and branches of the high voltage network (grid). The government looks at these factors and decides the location of an offshore wind farm. For example, in the Netherlands it is not desirable to place a wind farm within the 12 miles zone and the ports of Rotterdam and Antwerp should be taken into account. The fact that only a branch of the high voltage network is in Beverwijk, the Maasvlakte and Eemshaven gives only a few places to build. These problems are less in Germany and the United Kingdom where the offshore wind industry increases faster. Once the location of the park is chosen, there is calculated how the wind turbine generators should be placed and where a substation will be situated.
So far, Denmark, the Netherlands and the United Kingdom are leaders in offshore wind energy production (February 2011) but in the next decade this will change significantly. Japan, Germany and the USA haves a lot of future plans for offshore energy production. The largest wind farm is planned in Japan for 2025. This will have to generate 250000MW, while the Netherlands has set a target of 6000MW for 2020. Also, the traditional wind turbine will make way for new models. The traditional wind turbine has three blades, a horizontal shaft, a gearbox and a generator in the nacelle, as shown in figure 1. There are now newer models being designed which have a vertical axis, multiple blades, no gearbox or no old-fashioned ball bearings.
Page: 7 of 46 Date: 8/6/2011 Especially Germany, USA and Japan are developing new models for offshore wind farm projects. Wind energy
projects are developed by energy companies and public assistance shall be covered. This development is also an increasing demand for structural wind turbines and substations. Because the development of offshore wind energy has only just begun, there are only a few specialized companies. The offshore wind turbines are mostly land turbines and are suitable for offshore use after a little modification. But developments go quickly in this industry. If the applications for the award of offshore wind farms continue to increase, industry will change and will specialize it. However, it is launched by the government by providing subsidies.
The use and placement of offshore wind farms is thus highly dependent on many factors but it is for sure that growing numbers around the world will be posted. We are now living in the sustainable era where wind turbine generators are an important, if not the most important, part in sustainable power generation. However, this industry is still at an early stage and must therefore be taken up by subsidies. The knowledge is there, environment polluting energy is more taxed by most governments and there is enough space at sea so it is only a matter of time before the offshore wind industry will not need subsidies anymore. The more installed capacity, the less investment per kW, as shown in the expected trend of Figure 2.
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2. Standards and guidelines
All industrial (offshore) projects are confronted with many laws and standards. There are rules concerning construction, transport and electrical energy supply. There are laws to protect the environment and surroundings and there are rules for designing and building a system and overall security. This legislation is not made to make projects difficult and expensive but to be sure that projects are good, safe and clearly performed. But there is also legislation to promote wind energy, because wind energy is important for the goals of climate and renewable energy.
To ensure that projects are this good, safe and clear, there are many guidelines drafted by national institutes like the NEN (Dutch Standards Institute) in the Netherlands, BSI (British Standards Institution) in Great Britain and DIN (German Standards Institute) in Germany. The national guidelines of these institutes are seen by international institutes such as the CEN (European Committee for Normalization) and ETSI (European Telecommunications Standards Institute) but also by global institutes such as ISO (International Organization for Standardization) and IEC (International Electro technical Commission). By comparing and summarizing these national standards, they become international standards. This is done to get international clarification, because many products and companies come from abroad or work there. These standards are also prepared in case of accidents or uncertainties; it is possible to fall back on the person who is responsible for the mistake. Guidelines are set to include quality management, design, documenting, performance, safety and use of materials. The guidelines are written in such a way that they are all equal to each other but safety is still top priority. In this chapter, the most important laws, rules and standards will be explained but only concerns the electro technical parts of substations for offshore wind farms. Laws, rules and standards for offshore WTG are disregarded.
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2.1 Standards performing company
Because offshore is work at sea, safety has the highest priority. To be sure that safety is not compromised, a lot of standards are made. These standards are discussed when a company signs a contract for a project; to be sure that the customer and the company performing it are aware of both. Today, the customer also requires that the company (and employers) own this certification. The international standards that are used usually contain multiple national standards. If the international standard is met, the company also includes the knowledge of the national standards. This chapter will only refer to the standards; the summarized version is given in annex 2.
ISO-EN 9001
ISO-EN 9001 is an international standard for quality. This standard is used not only in the offshore industry but generally as a significant meaning for performance of quality companies. This standard contains requirements which a company must meet so that it is capable to deliver quality. This standard does not mean that the company or organization actually delivers quality, but it proves that it can deliver this quality. ISO9001 can be used as a client to assess whether the company or organization shall be able to meet customer requirements, the requirements of the company itself and the laws and regulations that have been made for that product. There are more than 1 million issued certificates and most manufacturing companies have it in possession.
ISO-EN 3834
For offshore construction companies it is important that their products match good tensions and forces. By the ISO9001 standard, the customer is more confident that a product meets all requirements, but ISO9001 does not refer to the requirements for welding. In ISO9001 welding is described only as a special process. This is the reason why customers of welding products are often demanding for the ISO3834 certification requirements. The ISO3834 ensures that a company of welding operates on a clearly more advanced level and is regularly tested through audits. Following the correct steps before, during and after production will obtain a product with the right quality, ISO 3834 describes this.
ISO-EN 14001
ISO- EN14001 is an international standard which indicates the demands an environmental management must meet. If an environmental management system is desired, it must be certified to this standard but with most companies it will be a part of the daily management during the ISO9001 audits. This standard focuses on controlling and improving environmental performance and environmental work at a company. Because this is normally a standard part of ISO9001, it contains fewer assumptions.
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2.2 Standards Substation
Chapter 3 defines which electronic components and systems the substation platform contains. All these devices will be tested based on standards before they can be placed in the substation. Because the substation is finally placed at sea, processes very high voltages and run independent the standards for this energy conversion facilities are more strict than the substations on land. The most important standards for engineering are given in this paragraph; extensive parts and less important standards are given in annex 2.
2.2.1 General Standards (design, implementation, operation)
These standards are made for design, implementation and operation for offshore substations for wind farms. These standards do not cover oil and gas installations, wind turbines, subsea cables, procedures for construction, operation or decommissioning of the offshore substations and subsea installations.
DNV-OS-J201
It has been developed primarily to assist in the development of new offshore transformer substations, HVDC substations and associated accommodation platforms. Locally applicable legislation may include requirements in excess of the provisions in this standard depending on type, size, location and intended service of the installation. Regional guidance is included throughout this standard by example only. The standard focuses on fixed, bottom-mounted installations. It may also be applied to floating installations if additional requirements are being taken into account. The principles, requirements and guidance shall be applied to all stages in the lifecycle of the installation, beginning at the concept design stage. Updates shall be made throughout the detailed design phase. The principles shall also be applied during the construction, operation and decommissioning phases and whenever modifications are made.
BSH Standard Design of Offshore Wind Turbines
This standard is intended to provide legal and planning security for development, design, implementation, operation and decommissioning of offshore wind farms within the Marine Facilities Ordinance. It is dynamic and integrative, so that it will be possible to take account of new knowledge and developments. A range of representatives from expert bodies and institutions have been involved in developing this standard, and have played a constructive role in its development. The representatives of the classification societies Det Norske Veritas (DNV) and Germanischer Lloyd (GL) provided expert accompaniment for the process. Representatives of the business and scientific communities made valuable contributions, so that overall it has been possible to create a solid basis for constructive cooperation in terms of system security in order to protect the marine environment and maintain the safety and efficiency of navigation.
IEC EN 61400-3
This part of IEC 61400 specifies additional requirements for assessment of the external conditions at an offshore wind turbine site and it specifies essential design requirements to ensure the engineering integrity of offshore wind turbines. Its purpose is to provide an appropriate level of protection against damage from all hazards during the planned lifetime. This standard focuses on the engineering integrity of the structural components of an offshore wind turbine but is also concerned with subsystems such as control and protection mechanisms, internal electrical systems and mechanical systems. A wind turbine shall be considered as an offshore wind turbine if the support structure is subject to hydrodynamic loading. The design requirements specified in this standard are not necessarily sufficient to ensure the engineering integrity of floating offshore wind turbines.
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2.2.2 Electrical installation
These standards are made for engineering of offshore electrical systems and installations for substations of wind farms. The most complete important standards for electrical installation are given in this paragraph.
IEC 61892: Mobile and fixed offshore units - Electrical installations
The IEC 61892 series contains provisions for electrical installations in mobile and fixed offshore units including pipeline, pumping or 'pigging' stations, compressor stations and exposed location single buoy moorings, used in the offshore petroleum industry for drilling, processing and storage purposes. This international standard applies to all installations, whether permanent, temporary, transportable or hand-held, to AC installations up to and including 35000 V and DC installations up to and including 1500 V (AC and DC voltages are nominal values).
DNV-OS-D201: Electrical installations
This offshore standard provides principles, technical requirements and guidance for design, manufacturing and installation of electrical installations on mobile offshore units and floating offshore installations. The requirements of this standard are in compliance with relevant parts of SOLAS Ch.II-1 and the IMO MODU Code. SOLAS references are as quoted in MODU Code 1989 and fulfill class requirements. Note that for compliance with flag state requirements, later amendments may be applicable.
The standard has been written for general world-wide application. Governmental regulations may include requirements in excess of the provisions by this standard depending on the size, type, location and intended service of the offshore unit/installation.
The objectives of this standard are to:
provide an internationally acceptable standard of safety.
defining minimum requirements for offshore electrical installations.
serve as a contractual reference document between suppliers and purchasers.
serve as a guideline for designers, suppliers, purchasers and regulators.
Specify procedures and requirements for offshore units or installations subject to DNV certification and classification.
NORSOK E-001: Electrical systems
This NORSOK standard contains provisions for electrical installations at all voltages. The standard provides safety in the design of electrical systems, selection and use of electrical equipment for generation, storage, distribution and utilization of electrical energy for all purposes in offshore units which are being used for the purpose of exploration or exploitation of petroleum resources.
This standard does not apply for the electrical installations in rooms used for medical purposes or in tankers. This NORSOK standard applies to all electrical installations. The installation may be permanent, temporary, transportable or hand-held, to AC installations up to and including 35000V and DC installations up to and including 1500V. This standard is also applicable for these voltages, even if a different voltage limit may be given in some of the parts in the IEC 61892 series of standards. It is expected that the voltage levels in the IEC 61892 series of standards will be corrected as part of the maintenance cycle of this IEC standard. Where this standard does not give guidelines for systems, equipment and installation for higher voltage level than 11 kV, reference is made to relevant IEC standards given in annex 2.
The other extensive and important standards for electrical installation, safety, communication and components are given in annex 2.
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3. Reducing energy losses during transport by increase voltage.
The electrical power that is generated by the wind turbine generators (WTG) will be increased to a higher voltage by an internal transformer. From these transformers the electrical power will be transported to the transformer substation through high quality power cables in the seabed. Multiple WTG are linked together, usually about six to seven, to stabilize the electrical energy and reduce costs of unnecessary cables. Because of these links only a few cables will arrive in the transformer substation platform. In this substation the voltage of the WTG, usually about 33kV-36kV, will be controlled and increased further, to about 150kV-250 kV. The output, hundreds of MW, can transport to a DC converter substation or transported directly to shore. This chapter will explain why increasing the voltage reduces losses and when using HVDC instead of HVAC.
The total electrical power of a few WTG would be received by the transformer substation platform by one cable. In the substation the voltage would increase to a higher voltage. By increasing the voltage the losses are less and the possibility to connect on the grid of many European countries is bigger. The following calculation is to explain the difference in losses of electrical power by increasing the voltage. The power losses are calculated for two different single phase voltages over the same cable with the same distance and the same electrical power. For this calculation the Law of Pouillet would be used, which considers that electrical resistance of a conductor is constant. In reality the resistance depends on many factors, like temperature and isolation, but that influence is limited. This calculation proves that the losses are higher when a low voltage is used instead of a high voltage.
U=33kV, Ptot=200MW
Plosses = Rcable * I 2
= ((* l) /A) * (P/U)2 Plosses = Ccable * (P/U)
2 Plosses = 36,731*Ccable MW U=155kV, Ptot=200MW Plosses = Rcable * I 2 = ((* l) /A) * (P/U)2 Plosses = Ccable * (P/U)
2
Plosses = 1,665*Ccable MW
It is clear to see that the losses are lower by increasing the voltage. The power is constant, the cable resistance is constant and the voltage decreases, so the electrical current will increase. The cable losses will increase quadratically by doubling the current. If the voltage of the example above would be increased from 33kV to 155kV, the losses would be 22 times less. It is easy to say that the voltage has to be as high as possible, but it is not that simple. It is impossible to increase the voltage as high as possible because components are made for specified voltages and cables have to be made much stronger. But HVAC is not the only way to reduce losses.
With HVAC it is possible to transport a maximum power of several hundred megawatt, because HVAC arises reactive power. HVDC does not arise reactive power because the current direction is constant. HVDC can transport a power of a few thousands megawatt because of this advantage. In 2008 a group of four people of IEEE presented a technical and economic analysis to evaluate the benefits and drawbacks of grid connecting offshore wind farms through a DC link. It concerned a 100MW wind farm. The main part of this article was the sensitivity analysis, as shown in figure 3. There appears a break-even point of costs for AC and DC transmission when varying the length of the connection. This break-even point is at a distance of about 90km from offshore substation to onshore substation, where the onshore substation is a several km out of shore. This break-even-point may fluctuate by commodity prices, by distance between offshore wind farm to shore and from the distance between shore to onshore substation. More information about HVDC and this analysis can be found in annex 3.
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4. The installation of an offshore transformer substation platform
Offshore transformer substations are used to reduce electrical losses by increasing the voltage and exporting the power to a convertor substation or directly to shore. Generally a transformer substation does not need to be installed if the costs of a substation are higher than the costs of the losses. This is in case of a small project, less than 100MW, or if the connection to the grid is at the collection voltage of 30kV-36kV. Most future offshore wind farms will be large and/or located far from shore, so the field will require one or more offshore transformer substations and/or offshore convertor substations. The convertor substation is not situated in every wind farm because it depends on the distance to shore, so it will not be treated in this chapter. The transformer substation will contain not only the transformers but also the switchboards of the WTG field, the controlling, cooling pumps, compensation coils and emergency and utility power supply. In this chapter you will find an analysis of the offshore transformer substation, which can be used for future projects.
4.1 The transformer substation
The substation platform of today mostly exists of 3 floors: the cable deck, the main deck and the weather deck, as shown in figure 4.
Figure 4: the transformer substation platform
The cable deck is the lowest deck and contains the smallest components, such as the fuel tanks for emergency generators and fuel for the helicopter, the switchboard for the WTG-field, batteries/uninterruptible power supply, most ducts, emergency transformers, workshop container and cooling. This deck is totally closed.
The main deck is situated above the cable deck and is the room where bigger component are placed, such as main transformers, emergency generators, high voltage switchboards and the air-conditioning/ventilation system. An emergency accommodation can be found here too. This deck is also totally closed.
The weather deck is the highest deck of the substation. This deck is exposed to the outside and it contains various sensors and a place for landing of helicopters, because maintenance staff has to be deposited safely. There is also a possibility to refuel the helicopter.
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4.2 Electrical components of the utility power supply
Offshore transformer substations are used to reduce electrical losses by increasing the WTG-field voltage to transport voltage. A part of the power from the WTG-field is used for control of the utility power supply (switches, emergency generators, UPS, transformers etc.). The transformer substation has many (control) systems for working completely independently and assist in case of short circuit, malfunction component or another defect in the WTG field. The recommended schematic structure of the utility power supply of a transformer substation is shown in Figure 5. Which high power systems and components the transformer substation includes, their functions and how the power have to calculated is explained in 4.2.1. In 4.2.2 the low power components are explained.
Figure 5: recommended structure transformer substation
The various powers, the various voltage levels and the possible requirements must be known or be selected before the recommended components could be determine.
The various voltage levels
Voltage Description Common voltage
U1 Voltage of the WTG-field, specified by the builder of the WTG-field 33-36kVAC
U2 Transport voltage. This voltage depends of the destination of the transport voltage. If the
transport cable is connected to a prospective platform, the voltage is free selectable. If connection is directly to an existing platform or to the grid, the transport voltage is specified by the existing platform or grid operator.
150kVAC-350kVAC
U3 AC voltage for own use. This voltage is selectable by the substation engineer. It depends of
the working voltage of the large components(pumps, h.v.a.c., crane etc.). and of the total power of the own use. There is a possibility to use higher voltages like 690VAC/1000VAC to decrease the current, because some components are limited to a particular current.
400VAC
U4 Voltage for DC components. This voltage depends of the loading voltage of the UPS and
asked voltage of other DC components(communication, security etc.)
220VDC U5 DC voltage for control. This voltage depends of the working voltage for component control 24VDC/48VDC
Page: 15 of 46 Date: 8/6/2011 every transformer is different, so every voltage of a tertiary winding is different. paragraph 4.2.1.3.2 the various power
A document which includes all possible requirements and worst-case scenarios should be drawn. An explanation and examples of these analyses is given in 4.3.
Power: Description: Calculation & detailing in
paragraph:
Ptotalwtg the total maximum possible power generates by the WTG-field None, known by engineer
of WTG-field Qreactor the total maximum reactive power that is generating by the shunt reactors 4.2.1.1
Paux. the total maximum possible auxiliary power. 4.2.1.7
Paux. gen. the total maximum power that has to generate by auxiliary generators
(Paux + Plosses aux. trans.).
4.2.1.7 Pown use total minimum and maximum possible power needs for all components of
own use
4.3 Pemer total maximum possible power needs for emergency power 4.2.2.11
Pemer. gen total maximum possible power needs that have to generate by emergency
generator(Pemer + Plosses emer. trans.)
4.2.2.11 Pcomponent AC total maximum possible power needs for AC components. This power
depends of the situation(components which are switched on)
4.3 Pcontrol the total maximum possible power needs for power control. This power
depends of the different situations
4.2.2.13 Pups total maximum power given by UPS after inverter(=Pups supply - Plosses inverter) 4.2.2.9
Pupssupply total maximum power given by UPS before inverter 4.2.2.7
Pupsasked total maximum power asked by UPS to load it 4.2.2.7
Ptrans total maximum possible power for transport before primary transformer
(Ptotalwtg - Pown use)
4.3 Poutput total maximum possible power for transport after primary transformer
(Ptrans - Plosses prim. trans).
Page: 16 of 46 Date: 8/6/2011 4.2.1. High power components
The high power components, which are shown in figure 6, are normally not chosen by HFG Engineering but mostly by a company specialized in high power components, like ABB, Siemens or Toshiba or another subcontractor. Because these components are very important for good increase of the power and to prevent important problems(earth fault, short circuit, capacitive reactive power etc.) they are all performed redundant. It is necessary to understand how the components are provided and designed before the other part of the utility power supply could be engineered. It is important to know that al the following calculations are bases on details after different analysis, without analysis it is impossible to get the wright power. Before the components could be chosen, an EMTP(Electro Magnetic Transient Program) study is required to explain the complex behaviours of the power system.
Figure 6: recommended structure high power components
1. Shunt reactors/conductors
With using (HV)AC capacitive reactive power is generated in the electric wires, because its length and its high voltage works as a capacitor. Because a transformer substation uses a lot of inductive components, like pumps and generators, an inductive reactive power is also generated. The capacitive and inductive power will partly neutralize each other, but an part of a reactive power stay exist because the components never neutralize each other exactly. During low loads (low effective power) and other different situations the reactive power could be a larger percentage of the effective power (the power factor is lower). This means a very high phase shift which increases the voltage, current and the losses. To prevent these fluctuations and the power failures that can result, this reactive power must be compensated and kept in balance.
The function has always been performed by passive elements such as reactors or capacitors, as well as combinations of the two, that supply inductive or capacitive reactive power. Mostly a substations have to neutralize capacitive reactive power, so it have to use shunt reactors, the inductive reactive power of the internal components never neutralize the capacitive reactive power which is generated by the electric wires. These reactors provide a very high inductive reactive power which is completely counter phase with the residual capacitive reactive power.
Page: 17 of 46 Date: 8/6/2011 The shunt reactors have to spread over the WTG-field and the transport wires. For different voltages there are different shunt reactor systems. The majority of shunt reactors for three phase system voltages of 72,5 kV or above are in the 30-300 MVAr range and they are normally connected directly to high voltage bus bars or transmission line ending. For these voltage levels, reactors are most commonly oil-filled type. Future reactors in the range 72,5-145 kV will tend to be air-core dry-type coil units. Shunt reactors rated below 72,5 kV are either oil-filled or air-core dry-type units and they are normally connected to the tertiary winding of large power transformers.
Most fabricators build their own complex shunt reactor system, but most of all are based on 2 different systems. The winding connection of three-phase reactors or a bank of three single-phase units can be either wye (most common configuration) or delta, as shown in figure 7. Typically, for system voltages of 72,5 kV or above, the reactors are wye connected with the neutral grounded directly or through a neutral reactor. For system voltages below 72,5 kV, the reactors are wye connected with the neutral ungrounded.
Figure 7: Wye & Delta connection of the shunt reactors
For neutralizing reactive power, it is important to know if the reactive power is capacitive or inductive, this should be known after the EMTP analyse. Shunt reactors are need for neutralizing capacitive reactive power, conductors for neutralizing inductive reactive power. Once the model has been developed, many scenarios can be simulated and detailed statistical studies can be performed. In most cases the reactive power that is generated can be estimated with the formula:
Qreactor = Pmax. gen.* (tan (φdesired) - tan (φsystem) ) var
Pmax. gen. = Maximum generated power (W) = Ptotal wtg or Poutput
φsystem = phi of the current system without neutralising reactive power.
φdesired = phi of the desired system
2. Earth fault protection
This security uses a tertiary winding to ensure that no voltage will pass the neutral point of a transformer or a generator in case of an earth fault or short circuit in the system. This system limits the fault voltage for a minimum damage to the switchboards, generators, transformers and control systems. An earth fault protection limits the current to 108% of the maximum current and can be enabled several times before it needs replacing. Every transformer in the WTG-field needs one security, so the error is always immediately deleted. The size of the earth fault protection depends of the voltage, the total power and the maximum current.
Imax = ( Pmax. trans. / (U*cos φ*√3) ) A
Page: 18 of 46 Date: 8/6/2011 3. High power transformers
3.1 High voltage step-up transformers for WTG-field voltage to transport voltage
These high power transformers are placed to convert the generated voltage from the WTG-field(U1) to the
transport voltage(U2), to minimize losses. The power demand is distributed over several step-up transformers;
the size of one transformer depends of the total power and the total transformers.
It should be possible that one transformer can be shut down and that the other transformer(s) will take over the power. It is not desired for the transformers to work at 100%, but at a normal working percentage of 80%. The power of this three phase transformer can be calculated with the formula
Strans = ( ( (Ptotal wtg –Pown use) / (cos φ) ) / (t-1) )* 1,25 VA
3.2 Tertiary winding or separate step-down transformers for own power supply
The engineer has two choices for own power supply from generated power of WTG-field: - a tertiary winding from the high power transformers.
- separate transformers directly from the WTG-field voltage.
The choice depends of the requirements of the customer, the total asked power of the (control)systems, the delivery time of components and the available room. The method of tertiary winding is most used compared to separate transformers because it is more efficient, cheaper, saving room and suppress harmonics. But a tertiary winding gives a limited power(maximum of 33% of Ptrans), has a long delivery time, because the primary
transformer has to be prepared, and needs a good early observation of the total asked power of the whole system. Separate transformers are more expensive, ask more room and induce more losses but have a short delivery time and selectable power.
- Tertiary winding for own power supply
The voltage of one tertiary winding depends of the total windings of the primary and secondary coil; every transformer is different, so every voltage of one tertiary winding is different. The best way to select the total tertiary windings(the more windings the more voltage) is to measure the voltage from one tertiary winding and compare this with the desired voltage for own use(U3). The voltage from the tertiary winding has to be as close
as possible to the desired voltage to minimize losses. The voltage of the winding(s) has to transform to the workable and desired voltage(U3) by custom transformers, because the voltage of a tertiary winding is not a clear
voltage.
- Separate step-down transformers for own power supply
These step-down transformers are placed to decrease the voltage from the WTG-field(U1) to the voltage for own
use(U3). The power demand is distributed over several step-down transformers; the size of one transformer
depends of the total power and the total transformers. It should be possible that one transformer can be shut down and that the other transformer(s) will take over the power. It is not desired for the transformers to work at 100%, but at a normal working percentage of 80%.
The power of a three phase transformer can be calculated with the formula: Strans = ( ( Pown use / (cos φ) ) / (t-1) ) * 1,25 VA
Page: 19 of 46 Date: 8/6/2011 3.3 Electrical losses
In both cases there are electrical losses, but a tertiary winding is more efficient. The losses of the separate step-down transformers are much higher, because the difference between voltage of the WTG-field(U1) and the
workable voltage(U3) is higher compared to the difference between the voltage of the tertiary windings(U6) and
the workable voltage(U3). Summarized the separate transformers has to transform a higher voltage compared to
the tertiary winding transformers, this will result in more losses. These losses are produced in the way of heat, sound and magnetic dispersion.
4. rectifiers
These rectifiers, using a diode bridge or igbt technology, are placed to convert the own use AC voltage(U3) to
DC voltage for own use(U4). The power demand is distributed over several rectifiers; the size of one rectifier
depends of the total power and the total rectifiers. It should be possible that one rectifier can be shut down and that the other rectifier(s) will take over the power.
The power of one rectifier can be calculated with the formula Prect. = (Pcontrol+Pcomponent DC) / (t-1) W
Note: There should always be placed a diode-bridge between different DC networks(like UPS-stations) to suppress harmonics interferences.
5. DC-DC Transformers low voltage (48V)
These transformers are placed to convert the voltage for control to voltage for small component control. The power demand is distributed over several transformers; the size of one transformer depends of the total power and the total transformers. It should be possible that one transformer can be shut down and that the other transformer(s) will take over the power.
The power of one transformer can be calculated with the formula Ptrans = Pcontrol / (t-1) W
Note: There should always be placed a diode-bridge between different DC networks(like DC transformers) to suppress harmonics interferences.
6. High voltage switchboards transport voltage and WTG-field voltage
The switchboards (not shown in figure 6) are used for control of the connections between WTG-field to transformer substation, the connections between the internal components and systems, the connections between substation to land or from substation to another substation. It is recommended to perform important components like switchboards redundant with using components of two different series or different producers to be sure that each connection would disconnect(extra safety). Every connection to the substation has to have two different connections, namely a normally closed and a normally open, to avoid any possibility of construction defects. If the power is too high, the current will exceed the current limit of the switchboard. If the current on the switchboard is too high, a component or switchboard could be damaged. In this case the switchboard will have to be replaced by a bigger one, which has a higher current limit, or the voltage has to increase which occurs to a change of voltage level U3. The maximum current depends of the size of the switchboards, how larger the
switchboard how higher the maximum possible current but how higher the price is. The different fabricators of switchboards have scalable types, but 5000A is the limit for control switchboards. The choice depends of the total power, current and voltage:
Page: 20 of 46 Date: 8/6/2011 7. Auxiliary generators WTG-field
If the WTG-field does not generate power(because there is too much/too little wind or there is a short circuit/maintenance on the transport line) several diesel driven auxiliary generators generate power for substation power use. This power is need for the WTG internal electrical systems (U1) and the control systems of the
substation (U3). It should be possible that one generator can be shut down and that the other generator(s) will
take over the power.
Buying the best or cheapest available generator without any other consideration is clearly not the best approach. If the appropriate generator is chosen there will be no unexpected system failures, no shutdowns due to capacity overload, increased longevity of the generator, guaranteed performance, smoother hassle-free maintenance, increased system life span and assured personal safety. To get these benefits you have to delve deep into your power generation requirements before making a choice. It is possible with the questions and operations on the next page.
- What are the items that need to be powered by the auxiliary generator? For substations this are most of the time the switchboards, pumps, instrumentation, control, radar, radiotelephone, intercom, camera system, alarm system, lighting etc. But it is possible that more systems have to be constant power or that other systems can be shut down. This is specified in the worst case scenarios (paragraph 4.3).
- Enumerate the starting and running wattage of all the respective items. Getting the right starting and running wattage of the devices you intend to power is crucial for calculating the accurate power requirements. Normally, you will find these data in the identification plate or the owner's manual in the buyer's kit of each respective device, tool, appliance or other electrical equipment. The total power of all components and systems to control the substation and WTG-field is also known as Pown use. Because there are different situations the needed power is
different. An example, at worst case, that means no power from WTG-field to substation, Paux=Pown use. This
worst case is also the minimum power requirement for the auxiliary generators, but this is explained in 4.3. - Define the total generators and calculate the minimum power requirements and with formula
Saux. gen. = ( ( (Paux + Plosses aux. trans.)/ cos φ) / (t-1) VA
Plosses aux. trans = known by generator fabricator
8. Step-up transformers for auxiliary generators
These transformers are placed to increase the voltage from the auxiliary generators to the WTG-field voltage(U1)
for internal use. The power of the auxiliary generators have to be distributed over several step-up transformers; the size of one transformer depends of the total power of the generators and the total transformers. It should be possible that one transformer can be shut down and that the other transformer(s) will take over the power of all auxiliary generators. It it is not desired for the transformers to work at 100%, but at a normal working percentage of 80%.
The power of one auxiliary step-up transformer for a auxiliary generator can be calculated with the formula: Saux. trans =( Saux. gen. / (t-1) ) * 1,25 VA
Page: 21 of 46 Date: 8/6/2011
4.2.2. Control and uninterruptible power supply
The low power components are recommended and engineered by HFG Engineering, as shown in figure 8. Some of these components are that important to prevent the most important problems on a substation, namely a black-out. The installation below provide emergency power supply, uninterruptible power supply and power control. As opposed to the high power components these are not always performed redundant.
Figure 8: control and uninterruptible power supply 9. UPS
There are several UPS-stations placed for intercepting voltage spikes, voltage drops, brown outs and black outs. At this way the power supply for the essential consumers will never be disturbed or interrupted. The UPS will also fill up the power gap between no power generating of the WTG (no wind/too much wind or ) and power generating by the generators.
The size of the UPS depends of the total power of the systems and the total time of a voltage gap. The desired power have to be supplied by several UPS stations; it should be possible that one UPS-station can be shut down or fail and that the other UPS station(s) will supply the power. There should be placed diode links between the different UPS-stations to reduce harmonic distortion because an UPS-station will be generating DC. You have to delve deep into your power generation requirements before making a choice of the size of the UPS station. The power of one UPS-station will be calculating with the following actions.
- What are the items that need to be powered by the UPS? What is the maximum power that is needed?
For substations these are most of the time the switchboards, instrumentation, control, radar, radiotelephone, intercom, camera system, light security, alarm system and instrumentation. But it is possible that more systems have to be constant powered. The choice of switch off systems in certain situations are explained in the worst case scenarios(paragraph 4.3).
- What is the minimum time that an UPS-station has to produce full power?
This depends of the total power(Pown use)that is need to fill up the gap between no power from WTG-field and
power generated by emergency generators. An example, if the total power for the essential control systems is 1kW and the gap is 1min, one UPS station needs a minimum power of 500 W/min(in case of two UPS stations). - Calculate the total power requirements and define the total UPS-stations with formula:
Page: 22 of 46 Date: 8/6/2011 10. inverters
There are only inverters, using igbt-technology, placed for converting the DC voltage from the UPS to the AC voltage for own use(U3). There are at least two UPS-stations and two transformers placed for excluding every
possibility of power failure. The power demand is distributed over several inverters; the size of one inverter depends of the total power and the total inverters. It should be possible that one inverter can be shut down and that the other inverter(s) will take over the power.
The power of one inverter can be calculated with the formula Sinv. = (Pups supply / cos φ) / (t-1) VA
Note: There should always be placed a diode-bridge between different DC networks(UPS-stations) to suppress harmonics interferences.
11. Emergency generators
If there is no power from the WTG-field, the auxiliary generator supply fails and no power is available from the UPS the emergency power must be provided to maintain all essential loads. Because the probability of failure of the both auxiliary generators is almost 0%, one diesel driven emergency generator have to be installed to supply the emergency switch board. This generator is not redundant.
Buying the best or cheapest available generator without any other consideration is clearly not the best approach. If the appropriate generator is chosen there will be no unexpected system failures, no shutdowns due to capacity overload, increased longevity of the generator, guaranteed performance, smoother hassle-free maintenance, increased system life span, much smaller chance of asset damage and assured personal safety. To get these benefits you have to delve deep into your power generation requirements before making a choice with using the ESD. It is possible with the following questions and operations.
- What are the components that need to be powered by the emergency generator? For substations these are most of the time the same components powered by the UPS, like switchboards, instrumentation, control, radar, radiotelephone, intercom, camera system, lighting, security, alarm system and instrumentation. But it is possible that less or more systems have to be powered by the emergency generator.
- Enumerate the starting and running wattage of the respective items. Getting the right starting and running wattage of the devices you intend to power is crucial for calculating the accurate power requirements. Normally, you will find these data in the identification plate or the owner's manual in the buyer's kit of each respective device, tool, appliance or other electrical equipment.
- Calculate the total power requirements and define the total generators with formula Sgen = ((Pemer+ Plosses emer. trans)/ cos φ) / (t-1) VA
Page: 23 of 46 Date: 8/6/2011 12. Transformers for emergency generators
If the voltage of the emergency generator is not the same as the voltage for own use(U3) a transformer for the
emergency generator is needed. This transformer is placed to convert the voltage from the emergency generator to the own use voltage(U3). Because there is only one emergency generator (a second emergency generator will
never be used)the power demand have to be distributed over a single transformer. Normally it is not desired for the transformers to work at 100%, but at a normal working percentage of 80%.
The power of one transformer can be calculated with the formula Strans = (Pemer / cos φ) * 1,25 VA
13. Control systems (fire protection, communication, radar, intercom, camera system, lighting security, alarm system, instrumentation etc.)
The group named ―utility power supply control systems‖ is very important for the constant power of the substation, but also for constant power of communication and safety systems. Some systems have to be constant powered while other systems can be shut down in case of an emergency. Mostly this choice depends of the legislation and safety laws of four different platform possibilities:
Not dependent manned platform
This platform is manned and not dependent of other platforms, so the communication from shore, vessels and aircraft to the substation is directly without the intervention of another substation.
Not dependent unmanned platform
This platform is unmanned and not dependent of other platforms, so the communication from shore, vessels and aircraft to the substation is directly.
Dependent unmanned platform
This platform is unmanned and dependent of other platforms, so the communication from shore, vessels and aircraft to the substation is indirectly.
Dependent manned platform
This platform is manned and dependent of other platforms, so the communication from shore, vessels and aircraft to the substation is indirectly.
Most offshore windfarm transformer substations will be specified as a not dependent unmanned platform, but it is possible that this will change in the future(more substations in one WTG-field). For these kind of platforms there are specified (control) systems that have to be installed. A lot of these systems are described in standards and laws, as treated in chapter 2.
There are different systems needs on the different platforms. Because most substations are specified as a not dependent unmanned platform, only systems for this type will specified. The power Pcontrol is the total power of
all the systems that are specified in this paragraph, because in case of maintenance this
Cooling
Cooling systems are one of the most important systems of a not-dependent unmanned platform. The different systems and components generating a lot of heat and will damage if there is no cooling. Seawater will be used to cool these systems and components, so seawater cooling pumps, seawater lift pumps and filtering systems will be need. For some components air-cooling is sufficient.
Page: 24 of 46 Date: 8/6/2011 (Tele)communication and radio equipment
Because most offshore wind farm substations are specified as an unmanned platform, most communication and radio systems have to be enabled if someone is on the platform for inspection and maintenance. The systems that have to be installed on a unmanned platform are different radio and communication systems.
Radio systems
- Maritime VHF with DSC, it is used for a wide variety of purposes, including summoning rescue services and communicating with shore.
- Aeronautical VHF communication system, for communication with helicopters. - GMDSS handheld, portable GMDSS communication for the whole substation. Communication equipment
- Radar - Intercom
- Camera system(cctv)
- Private automatic branch exchange(PABX) - Telephones
Fire and emergency protection
There are several systems installed in a substation to protect it to fire and in case of an emergency, thinking of fuel pumps, alarm systems(PA/GA) and fire fighting systems. These systems have to be 24/7 control and standby for fire or emergency’s.
Meteorological equipment Navigation Aids System
A not dependent substation have to be visible at all weather conditions. At least the following systems have to be installed on the platform and enabled if the situation needs it. A control panel will control all these components. - Fog horn
- Lantern
- Heli-deck lighting
- Crane cabin and boom lanterns - Several floodlight
Instrumentation
The instrumentation of a substation will control all the previous components and systems but also small power, pumps, h.v.a.c. and lighting for the accommodation of a substation. The control of these components depends on its quantity. An analysis(like electrical load analyse, HAZID & HAZOP) should be made, where the failure rate, mean time between failure & repair, downtime, probability of failure on demand, common cause failure, quantitative risk analysis and other component properties should be included. These should give a good view of the quantity of different components; redundancy could be considered if the quantity is too bad. After this risk-analysis the Emergency Shutdown System(ESD) system for instrumentation is known. The ESD is designed to minimise the consequences of emergency situations. In case of a fire, an accident, a changing of the situation, as mentioned in paragraph 4.3, or a total black-out this system makes the choice to shut down particular systems to avert different (dangerous) situations. The ESD does not include the operating philosophy, which describes the control of components
Page: 25 of 46 Date: 8/6/2011
4.3 Worst-case scenario’s
There are several situations where the substation should remain function. Some systems have to be constant powered while other systems can be shut down in case of emergency. This is specified in an ESD system. The ESD system ensures which systems may fall out in different cases. So in case of fire or explosion, parts of the plant may fall out.
There are four situations where the substation should remain function This four different weather and component situations are:
Normal situation
Wind situation
Maintenance or short circuit situation
Malfunction component situation
A general overview to determine when a substation(unmanned not-dependent platform) is in a particular situation and its consequences, is given in figure 9.
Page: 26 of 46 Date: 8/6/2011 Every situation has other priorities and other consequences for different systems and components. In the
following explanations the differences between the situations becomes clear.
Normal situation
Enough wind— WTG-lines will start up one by one.
— Power for starting up WTG-field and power supply for substation is taken from the grid. — All WTG-lines are generating power.
— Substation will use a part of the generated power from the WTG-lines for internal power use. The rest of the power will be distributed to the onshore grid.
Wind situation
Too much wind/storm— All WTG-lines are in idling mode or standstill. At this situation they are not generating power.
— The power for internal control systems of WTG and substation comes from power take from the onshore grid.
Not enough wind
— As long as the generated power is enough (Ptotal wtg
≥ P
own use) for internal systems of WTG and substation, the installation still works as normal situation.— If generated power is not enough (Ptotal wtg
≤ P
own use) for internal control systems for WTG and substation, power supply comes from power take from the onshore grid.Maintenance or short-circuit situation
Maintenance or short circuit transport line (from offshore substation to onshore substation)
All WTG-lines in idling mode or standstill, controlled by substation. Substation has to compensate the voltage for the internal control systems.
— Directly after close transport line UPS switch on and auxiliary and/or emergency generators will start, as given in figure 10(next page).
— If a generator is started and enough power is generated, shut down fully power generating of UPS. Maintenance or short circuit WTG-field
— The WTG-lines which contains short-circuit and/or need maintenance have to set in idling mode or standstill, controlled by substation.
— If not enough power is generated by other WTG-lines, auxiliary or emergency generators will generate power for substation as in figure 10(next page). If other WTG-lines are still generating power, the substation works as in normal situation.
Page: 27 of 46 Date: 8/6/2011
Figure 10: recommended flow chart for start generators
Malfunction component situation
Malfunction component on WTG-line— The WTG-lines which have malfunction of a component have to be set in idling mode or standstill, controlled by substation. Repair by technicians is directly needed.
— If not enough power is generated by other WTG-lines, auxiliary or emergency generators will generate power for substation as in figure 10. If other WTG-lines are still generating power, the substation works as in normal situation.
Malfunction component on Substation Component is redundant
— Because the most important components of the substation are executed redundant, as mentioned in paragraph 4.2, the system will keep working in case of a malfunction of a component. If a component is damaged, malfunctioning or defect the redundant component is taken over the whole power. Repair of the component is needed but the substation will still working.
Component is not redundant
— Some of the utility power supply parts are not executed redundant because they have a probability of failure of nearly 0% or it is not that important to execute redundant(like hot water pump for accommodation). It is recommended to analyse the parts as mentioned in paragraph 4.2, with risk-analysis like HAZID, HAZOP etc. These analyses shows components and/or systems that have to be performing redundant because they are very important or have a high probability of failure.
Page: 28 of 46 Date: 8/6/2011
Conclusion
The construction, calculation and redundancy of components of the utility power supply were subjects of many discussions, problems and adjustments during engineering and building of previous substation projects. For these discussions, problems and adjustments this report is a guideline because in this report the utility power supply part of a transformer substation for offshore wind farms is investigated in detail. In this guideline you will find the recommended structure of a substation and was made after a few studies. The conclusions of these studies are:
increasing the voltage will reduce losses. The cable losses(=resistance) will increase quadratically by doubling the current. This resistance depends on many factors, like temperature and isolation, but that influence is limited.
an enumeration of the legislation of an offshore substation is known. There are several standards for the company, general standards for offshore substations and guidelines for the electrical installation which are very important. The most important standards describes the requirements of the whole substation. The advantages and disadvantages of HVDC compared to HVAC and the break-even point of costs for AC
and DC transmission when varying the length of the connection are known. HVDC will be cheaper compared to HVAC after 90km between the substations.
After these studies the components of such a transformer substation could be chosen. The choice of these components depends of many factors, such as performance, quantity and redundancy. The choice of redundancy depends of several analyses: HAZID, HAZOP and EMTP. The recommended structure of the offshore transformer substation is made with using these analyses and conclusions.
This guide for the utility power supply part of a transformer substation for offshore wind farms will result to: a good explanation to determine the various voltage levels.
an explanation, calculation and determination of the power of the different components. which influences on components take into account.
the various worst-case scenarios, which a substation has to deal with, are known and a scenario could be determined.
different outside power supply opportunities, like power taken from grid and power taken from wind turbines, are known and could be determined.
different own power supply opportunities, like auxiliary generator, emergency generator and UPS, are known and could be calculated.
