1. The distribution of the heat load of consumers of thermal energy in the heat supply system between the sources of thermal energy supplying thermal energy in this heat supply system is carried out by the body authorized in accordance with this Federal Law to approve the heat supply scheme, by making annual changes to the heat supply scheme.
2. To distribute the heat load of consumers of heat energy, all heat supply organizations that own sources of heat energy in this heat supply system are required to submit to the body authorized in accordance with this Federal Law to approve the heat supply scheme, an application containing information:
1) on the amount of heat energy that the heat supply organization undertakes to supply to consumers and heat supply organizations in this heat supply system;
2) on the amount of capacity of thermal energy sources, which the heat supply organization undertakes to support;
3) on current tariffs in the field of heat supply and predicted specific variable costs for the production of thermal energy, heat carrier and power maintenance.
3. In the heat supply scheme, conditions must be determined under which it is possible to supply thermal energy to consumers from various sources of thermal energy while maintaining the reliability of heat supply. In the presence of such conditions, the distribution of heat load between sources of heat energy is carried out on a competitive basis in accordance with the criterion of minimum specific variable costs for the production of heat energy by sources of heat energy, determined in the manner established by the pricing principles in the field of heat supply, approved by the Government of the Russian Federation, on the basis of applications organizations that own sources of thermal energy, and standards taken into account when regulating tariffs in the field of heat supply for the corresponding period of regulation.
4. If the heat supply organization does not agree with the distribution of the heat load carried out in the heat supply scheme, it has the right to appeal against the decision on such distribution, taken by the body authorized in accordance with this Federal Law to approve the heat supply scheme, to the federal executive body authorized by the Government of the Russian Federation.
5. Heat supply organizations and heat network organizations operating in the same heat supply system, annually before the start of the heating period, are required to conclude an agreement between themselves on the management of the heat supply system in accordance with the rules for organizing heat supply, approved by the Government of the Russian Federation.
6. The subject of the agreement specified in part 5 of this article is the procedure for mutual actions to ensure the functioning of the heat supply system in accordance with the requirements of this Federal Law. The obligatory conditions of this agreement are:
1) determining the subordination of dispatching services of heat supply organizations and heat network organizations, the procedure for their interaction;
2) the procedure for organizing the adjustment of heat networks and regulating the operation of the heat supply system;
3) the procedure for ensuring access of the parties to the agreement or, by mutual agreement of the parties to the agreement, to another organization to heat networks for the adjustment of heat networks and regulation of the operation of the heat supply system;
4) the procedure for interaction between heat supply organizations and heat network organizations in emergency situations and emergencies.
7. If the heat supply organizations and heat network organizations have not concluded the agreement specified in this article, the procedure for managing the heat supply system is determined by the agreement concluded for the previous heating period, and if such an agreement has not been concluded earlier, the specified procedure is established by the body authorized in accordance with this Federal law for approval of the heat supply scheme.
As part of the supply of switchboard equipment, power cabinets and control cabinets for two buildings (ITP) were supplied. For the reception and distribution of electricity in heating points, input-distributing devices are used, consisting of five panels each (10 panels in total). Switching switches, surge arresters, ammeters and voltmeters are installed in the input panels. ATS panels in ITP1 and ITP2 are implemented on the basis of automatic transfer units. In the distribution panels of the ASU, protection and switching devices (contactors, soft starters, buttons and lamps) are installed for the technological equipment of heating points. All circuit breakers are equipped with status contacts signaling an emergency shutdown. This information is transmitted to the controllers installed in the automation cabinets.
To control and manage the equipment, OWEN PLC110 controllers are used. They are connected to the input / output modules ARIES MV110-224.16DN, MV110-224.8A, MU110-224.6U, as well as operator touch panels.
The coolant is introduced directly into the ITP room. Water supply for hot water supply, heating and heat supply of air heaters of air ventilation systems is carried out with a correction according to the outside air temperature.
The display of technological parameters, accidents, equipment status and dispatch control of the ITP is carried out from the workstation of dispatchers in the integrated central control room of the building. On the dispatching server, the archive of technological parameters, accidents, and the state of the ITP equipment is stored.
Automation of heat points provides for:
- maintaining the temperature of the coolant supplied to the heating and ventilation systems in accordance with the temperature schedule;
- maintaining the temperature of the water in the DHW system at the supply to consumers;
- programming of various temperature regimes by hours of the day, days of the week and holidays;
- control of compliance with the values of parameters determined by the technological algorithm, support of technological and emergency parameters limits;
- temperature control of the heat carrier returned to the heating network of the heat supply system, according to a given temperature schedule;
- outside air temperature measurement;
- maintaining a given pressure drop between the supply and return pipelines of ventilation and heating systems;
- control of circulation pumps according to a given algorithm:
- on/off;
- control of pumping equipment with frequency drives according to signals from PLC installed in automation cabinets;
- periodic switching main / reserve to ensure the same operating time;
- automatic emergency transfer to the standby pump according to the control of the differential pressure sensor;
- automatic maintenance of a given differential pressure in heat consumption systems.
- control of heat carrier control valves in primary consumer circuits;
- control of pumps and valves for feeding circuits of heating and ventilation;
- setting the values of technological and emergency parameters through the dispatching system;
- control of drainage pumps;
- control of the state of electrical inputs by phases;
- synchronization of the controller time with the common time of the dispatching system (SOEV);
- start-up of equipment after restoration of power supply in accordance with a given algorithm;
- sending emergency messages to the dispatching system.
Information exchange between automation controllers and the upper level (workstation with specialized MasterSCADA dispatching software) is carried out using the Modbus/TCP protocol.
Article 18. Distribution of heat load and management of heat supply systems
1. The distribution of the heat load of consumers of thermal energy in the heat supply system between those supplying thermal energy in this heat supply system is carried out by the body authorized in accordance with this Federal Law to approve the heat supply scheme by making annual changes to the heat supply scheme.
2. To distribute the heat load of consumers of heat energy, all heat supply organizations that own sources of heat energy in this heat supply system are required to submit to the body authorized in accordance with this Federal Law to approve the heat supply scheme, an application containing information:
1) on the amount of heat energy that the heat supply organization undertakes to supply to consumers and heat supply organizations in this heat supply system;
2) on the amount of capacity of thermal energy sources, which the heat supply organization undertakes to support;
3) on current tariffs in the field of heat supply and predicted specific variable costs for the production of thermal energy, heat carrier and power maintenance.
3. In the heat supply scheme, conditions must be determined under which it is possible to supply thermal energy to consumers from various sources of thermal energy while maintaining the reliability of heat supply. In the presence of such conditions, the distribution of heat load between sources of heat energy is carried out on a competitive basis in accordance with the criterion of minimum specific variable costs for the production of heat energy by sources of heat energy, determined in the manner established by the pricing principles in the field of heat supply, approved by the Government of the Russian Federation, on the basis of applications organizations that own sources of thermal energy, and standards taken into account when regulating tariffs in the field of heat supply for the corresponding period of regulation.
4. If the heat supply organization does not agree with the distribution of the heat load carried out in the heat supply scheme, it has the right to appeal against the decision on such distribution, taken by the body authorized in accordance with this Federal Law to approve the heat supply scheme, to the federal executive body authorized by the Government of the Russian Federation.
5. Heat supply organizations and heat network organizations operating in the same heat supply system, annually before the start of the heating period, are required to conclude an agreement between themselves on the management of the heat supply system in accordance with the rules for organizing heat supply, approved by the Government of the Russian Federation.
6. The subject of the agreement specified in part 5 of this article is the procedure for mutual actions to ensure the functioning of the heat supply system in accordance with the requirements of this Federal Law. The obligatory conditions of this agreement are:
1) determining the subordination of dispatching services of heat supply organizations and heat network organizations, the procedure for their interaction;
3) the procedure for ensuring access of the parties to the agreement or, by mutual agreement of the parties to the agreement, to another organization to heat networks for the adjustment of heat networks and regulation of the operation of the heat supply system;
4) the procedure for interaction between heat supply organizations and heat network organizations in emergency situations and emergencies.
7. If the heat supply organizations and heat network organizations have not concluded the agreement specified in this article, the procedure for managing the heat supply system is determined by the agreement concluded for the previous heating period, and if such an agreement has not been concluded earlier, the specified procedure is established by the body authorized in accordance with this Federal law for approval of the heat supply scheme.
Modernization and Automation of Heat Supply System Minsk experiencce
V.A. Sednin, Scientific Consultant, Doctor of Engineering, Professor,
A.A. Gutkovskiy, Chief Engineer, Belorussian National Technicl University, Scientific Research and Innovations Center of Automated Control Systems in heat power industry
keywords: heat supply system, automated control systems, reliability and quality improvement, heat delivery regulation, data archiving
Heat supply of large cities in Belorussia, as in Russia, is provided by cogeneration and district heat supply systems (hereinafter - DHSS), where facilities are combined into a single system. However, often the decisions made on individual elements of complex heat supply systems do not meet the systematic criteria, reliability, controllability and environment protection requirements. Therefore modernization of the heat supply systems and creation of automated process control systems is the most relevant task.
Description:
V.A. Sednin, A.A. Gutkovsky
The heat supply of large cities of Belarus, as in Russia, is provided by heating and district heating systems (hereinafter referred to as DH), the facilities of which are linked into a single scheme. However, decisions made on individual elements of complex heat supply systems often do not meet system criteria, reliability, manageability and environmental friendliness requirements. Therefore, the modernization of heat supply systems and the creation of automated process control systems is the most urgent task.
V. A. Sednin, scientific consultant, doctor of tech. sciences, professor
A. A. Gutkovsky, Chief Engineer, Belarusian National Technical University, Research and Innovation Center for Automated Control Systems in Heat Power and Industry
Heat supply to large cities of Belarus, as in Russia, is provided by district heating and district heating systems (DH) whose facilities are linked into a single scheme. However, decisions made on individual elements of complex heat supply systems often do not meet system criteria, reliability, manageability and environmental friendliness requirements. Therefore, the modernization of heat supply systems and the creation of automated process control systems is the most urgent task.
Features of district heating systems
Considering the main features of the SDT of Belarus, it can be noted that they are characterized by:
- continuity and inertia of its development;
- territorial distribution, hierarchy, variety of technical means used;
- dynamic production processes and stochastic energy consumption;
- incompleteness and low degree of reliability of information about the parameters and modes of their functioning.
It is important to note that in the district heating network, unlike other pipeline systems, they serve to transport not the product, but the energy of the coolant, the parameters of which must meet the requirements of various consumer systems.
These features emphasize the essential need for the creation of automated process control systems (hereinafter referred to as APCS), the implementation of which makes it possible to increase energy and environmental efficiency, reliability and quality of functioning of heat supply systems. The introduction of automated process control systems today is not a tribute to fashion, but follows from the basic laws of the development of technology and is economically justified at the present stage of development of the technosphere.
| REFERENCE |
The district heating system of Minsk is a structurally complex complex. In terms of production and transport of thermal energy, it includes the facilities of Minskenergo RUE (Minsk Heat Networks, heating complexes of CHPP-3 and CHPP-4) and the facilities of Minskkommunteploset Unitary Enterprise - boiler houses, heat networks and central heating points. Creation of APCS UE "Minskkommunteploset" was started in 1999, and now it is functioning, covering almost all heat sources (over 20) and a number of districts of heat networks. The development of the APCS project for the Minsk Heat Networks was launched in 2010, the project implementation began in 2012 and is currently ongoing. |
Development of an automated process control system for the heat supply system in Minsk
On the example of Minsk, we present the main approaches that have been implemented in a number of cities in Belarus and Russia in the design and development of process control systems for heat supply systems.
Taking into account the vastness of issues covering the subject area of heat supply, and the accumulated experience in the field of automation of heat supply systems at the pre-project stage of creating an automated control system for Minsk heat networks, a concept was developed. The concept defines the fundamental foundations of the organization of automated process control systems for heat supply in Minsk (see reference) as a process of creating a computer network (system) focused on automating technological processes of a topologically distributed district heating enterprise.
Technological information tasks of process control systems
The implemented automated control system primarily provides for increasing the reliability and quality of operational control of the modes of operation of individual elements and the heat supply system as a whole. Therefore, this process control system is designed to solve the following technological information problems:
- provision of centralized functional-group control of hydraulic regimes of heat sources, main heat networks and pumping stations, taking into account daily and seasonal changes in circulation costs with adjustment (feedback) according to actual hydraulic regimes in the distribution heat networks of the city;
- implementation of the method of dynamic central control of heat supply with optimization of heat carrier temperatures in the supply and return pipelines of heating mains;
- ensuring the collection and archiving of data on the thermal and hydraulic modes of operation of heat sources, main heating networks, a pumping station and distribution heating networks of the city for monitoring, operational management and analysis of the functioning of the Minsk heating networks' central heating system;
- creation of an effective system for protecting equipment of heat sources and heating networks in emergency situations;
creation of an information base for solving optimization problems arising in the course of operation and modernization of objects of the Minsk heat supply system.
| REFERENCE 1 |
The structure of the Minsk thermal networks includes 8 network districts (RTS), 1 thermal power plant, 9 boiler houses with a capacity of several hundred to a thousand megawatts. In addition, 12 step-down pumping stations and 209 central heating stations are serviced by the Minsk Heat Networks. Organizational and production structure of the Minsk heat networks according to the "bottom-up" scheme:
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The structure of the automated process control system of Minsk heating networks
In accordance with the production and organizational structure of the Minsk Heat Networks (see reference 1), a four-level structure of the APCS of the Minsk Heat Networks was chosen:
- the first (upper) level is the central control room of the enterprise;
- the second level - operator stations of districts of thermal networks;
- third level - operator stations of heat sources (operator stations of workshop sections of heating networks);
- fourth (lower) level - stations for automatic control of installations (boiler units) and processes of transport and distribution of thermal energy (technological scheme of a heat source, heating points, heating networks, etc.).
The development (creation of an automated process control system for heat supply of the entire city of Minsk) involves the inclusion in the system at the second structural level of operator stations of heating complexes of Minsk CHPP-2, CHPP-3, CHPP-4 and an operator station (central dispatching room) of UE "Minskkommunteploset". All management levels are planned to be combined into a single computer network.
The architecture of the process control system for the heat supply system of Minsk
The analysis of the control object as a whole and the state of its individual elements, as well as the prospects for the development of the control system, made it possible to propose the architecture of a distributed automated process control system for the Minsk heat supply system within the facilities of RUE "Minskenergo". The corporate network integrates the computing resources of the central office and remote structural subdivisions, including automatic control stations (ACS) of objects in network areas. All ACS (TsTP, ITP, PNS) and scanning stations are connected directly to the operator stations of the respective network areas, presumably installed at master sites.
The following stations are installed at a remote structural subdivision (for example, RTS-6) (Fig. 1): RTS-6 operator station (RTS-6 OPS) - it is the control center of the network area and is installed on the RTS-6 master site. For operational personnel, RTS-6 provides access to all information and control resources of ACS of all types without exception, as well as access to authorized information resources of the central office. OpS RTS-6 provide regular scanning of all slave control stations.
The operational and commercial information collected from all central heating centers is sent for storage to a dedicated database server (installed in the immediate vicinity of the RTS-6 OpS).
Thus, taking into account the scale and topology of the control object and the existing organizational and production structure of the enterprise, the APCS of the Minsk Heat Networks is built according to a multi-link scheme using a hierarchical structure of software and hardware and computer networks that solve various control tasks at each level.
Management system levels
At the lower level, the control system performs:
- preliminary processing and transmission of information;
- regulation of the main technological parameters, functions of control optimization, protection of technological equipment.
Higher reliability requirements are imposed on lower-level hardware, including the possibility of autonomous operation in case of loss of connection with the upper-level computer network.
The subsequent levels of the control system are built according to the hierarchy of the heat supply system and solve the tasks of the corresponding level, as well as provide an operator interface.
Control devices installed at facilities, in addition to their direct duties, should also provide for the possibility of aggregating them into distributed control systems. The control device must ensure the operability and safety of the information of objective primary accounting during long interruptions in communication.
The main elements of such a scheme are technological and operator stations interconnected by communication channels. The core of the technological station should be an industrial computer equipped with means of communication with the control object and channel adapters for organizing interprocessor communication. The main purpose of the technological station is the implementation of direct digital control algorithms. In technically justified cases, some functions can be performed in supervisory mode: the process station processor can control remote intelligent controllers or software logic modules using modern field interface protocols.
Informational aspect of building an automated process control system for heat supply
Particular attention during the development was paid to the informational aspect of building an automated process control system for heat supply. The completeness of the description of the production technology and the perfection of the information conversion algorithms are the most important part of the information support of the APCS, built on the technology of direct digital control. The information capabilities of the automated process control system for heat supply provide the ability to solve a set of engineering problems that classify:
- by stages of the main technology (production, transport and consumption of thermal energy);
- by purpose (identification, forecasting and diagnostics, optimization and management).
When creating an automated process control system for Minsk heat networks, it is planned to form an information field that allows you to quickly solve the entire complex of the above tasks of identification, forecasting, diagnostics, optimization and management. At the same time, information provides the possibility of solving system problems of the upper level of management with the further development and expansion of the automated process control system as the relevant technical services for the main technological process are included.
In particular, this applies to optimization tasks, i.e., optimization of the production of thermal and electrical energy, modes of supply of thermal energy, flow distribution in thermal networks, operating modes of the main technological equipment of heat sources, as well as calculating the rationing of fuel and energy resources, energy accounting and operation, planning and forecasting the development of the heat supply system. In practice, the solution of some problems of this type is carried out within the framework of the enterprise automated control system. In any case, they must take into account the information obtained in the course of solving the problems of directly managing the process, and the information created by the process control system must be integrated with other information systems of the enterprise.
Methodology of software-object programming
The construction of the control system software, which is an original development of the center's team, is based on the methodology of program-object programming: software objects are created in the memory of control and operator stations that display real processes, units and measuring channels of an automated technological object. The interaction of these software objects (processes, aggregates and channels) with each other, as well as with operational personnel and with technological equipment, in fact, ensures the functioning of the elements of heat networks according to predefined rules or algorithms. Thus, the description of algorithms is reduced to the description of the most essential properties of these program objects and the ways of their interaction.
The synthesis of the structure of the control system of technical objects is based on the analysis of the technological scheme of the control object and a detailed description of the technology of the main processes and functioning inherent in this object as a whole.
A convenient tool for compiling this type of description for heat supply facilities is the methodology of mathematical modeling at the macro level. In the course of compiling a description of technological processes, a mathematical model is compiled, a parametric analysis is performed, and a list of adjustable and controlled parameters and regulatory bodies is determined.
The regime requirements of technological processes are specified, on the basis of which the boundaries of the permissible ranges of change of regulated and controlled parameters and the requirements for the choice of actuators and regulatory bodies are determined. Based on the generalized information, the synthesis of an automated object control system is carried out, which, when using the direct digital control method, is built according to a hierarchical principle in accordance with the hierarchy of the control object.
ACS of the district boiler house
So, for a district boiler house (Fig. 2), an automated control system is built on the basis of two classes.
The upper level is the operator station "Boiler" (OPS "Boiler") - the main station that coordinates and controls the subordinate stations. Fire station “Boiler reserve” is a hot standby station, which is constantly in the mode of listening and registering the traffic of the main fire station and its subordinate ACS. Its database contains up-to-date parameters and complete historical data on the functioning of the working control system. At any time, a backup station can be assigned as the main station with full traffic transfer to it and the permission of supervisory control functions.
The lower level is a complex of automatic control stations united together with the operator station in a computer network:
- ACS "Boiler unit" provides control of the boiler unit. As a rule, it is not reserved, since the reservation of the thermal power of the boiler house is carried out at the level of boiler units.
- ACS "Grid Group" is responsible for the thermal-hydraulic mode of operation of the boiler house (control of a group of network pumps, bypass line at the outlet of the boiler room, bypass line, inlet and outlet valves of boilers, individual boiler recirculation pumps, etc.).
- SAU "Vodopodgotovka" provides control of all auxiliary equipment of the boiler house, necessary for feeding the network.
For simpler objects of the heat supply system, for example, heat points and block boiler houses, the control system is built as a single-level one based on an automatic control station (SAU TsTP, SAU BMK). In accordance with the structure of heat networks, the control stations of heat points are combined into a local area network of the heat network area and are connected to the operator station of the heat network area, which, in turn, has an information connection with the operator station of a higher level of integration.
Operator stations
The software of the operator station provides a friendly interface for the operating personnel controlling the operation of the automated technological complex. Operator stations have advanced means of operational dispatch control, as well as mass memory devices for organizing short-term and long-term archives of the state of the parameters of the technological control object and the actions of operational personnel.
In cases of large information flows that are closed to operational personnel, it is advisable to organize several operator stations with the allocation of a separate database server and, possibly, a communication server.
The operator station, as a rule, does not directly affect the control object itself - it receives information from technological stations and also transmits directives to the operating personnel or tasks (settings) of supervisory control, generated automatically or semi-automatically. It forms the workplace of the operator of a complex object, such as a boiler room.
The automated control system being created provides for the construction of an intelligent superstructure, which should not only track disturbances that occur in the system and respond to them, but also predict the occurrence of emergency situations and block their occurrence. When changing the topology of the heat supply network and the dynamics of its processes, it is possible to adequately change the structure of the distributed control system by adding new control stations and (or) changing software objects without changing the equipment configuration of existing stations.
Efficiency of APCS of the heat supply system
An analysis of the operating experience of automated process control systems for heat supply enterprises 1 in a number of cities in Belarus and Russia, conducted over the past twenty years, has shown their economic efficiency and confirmed the viability of the decisions made on architecture, software and hardware.
In terms of their properties and characteristics, these systems meet the requirements of the ideology of smart grids. Nevertheless, work is constantly underway to improve and develop the developed automated control systems. The introduction of automated process control systems for heat supply increases the reliability and efficiency of the DH operation. The main saving of fuel and energy resources is determined by the optimization of the thermal-hydraulic modes of heating networks, the operating modes of the main and auxiliary equipment of heat sources, pumping stations and heating points.
Literature
- Gromov N.K. Urban heating systems. M. : Energy, 1974. 256 p.
- Popyrin L. S. Research of heat supply systems. M. : Nauka, 1989. 215 p.
- Ionin A. A. Reliability of systems of thermal networks. Moscow: Stroyizdat, 1989. 302 p.
- Monakhov G. V. Modeling of control modes of heat networks. M.: Energoatomizdat, 1995. 224 p.
- Sednin VA Theory and practice of creating automated heat supply control systems. Minsk: BNTU, 2005. 192 p.
- Sednin V. A. Implementation of automated process control systems as a fundamental factor in improving the reliability and efficiency of heat supply systems // Technology, equipment, quality. Sat. mater. Belarusian Industrial Forum 2007, Minsk, May 15–18, 2007 / Expoforum – Minsk, 2007, pp. 121–122.
- Sednin V. A. Optimization of the parameters of the temperature graph of heat supply in heating systems // Energetika. News of higher educational institutions and energy associations of the CIS. 2009. No. 4. S. 55–61.
- Sednin V. A. The concept of creating an automated process control system for the Minsk heat networks / V. A. Sednin , A. V. Sednin, E. O. Voronov // Improving the efficiency of power equipment: Proceedings of the scientific and practical conference, in 2 v. T. 2. 2012. S. 481–500.
1 Created by the team of the Research and Innovation Center for Automated Control Systems in Heat Power and Industry of the Belarusian National Technical University.
The automatic heat supply control system consists of the following modules, each of which performs its own task:
- Main control controller. The main part of the controller is a microprocessor with the possibility of programming. In other words, you can enter data in accordance with which the automatic system will operate. The temperature can change in accordance with the time of day, for example, at the end of the working day, the devices will switch to the minimum power, and before it starts, on the contrary, they will go to the maximum in order to warm up the premises before the shift arrives. The controller can perform adjustment of thermal installations in automatic mode, based on data collected by other modules;
- Thermal sensors. The sensors perceive the temperature of the system coolant, as well as the environment, send appropriate commands to the controller. The most modern models of this automation send signals via wireless communication channels, so laying complex systems of wires and cables is not needed, which simplifies and speeds up installation;
- Manual control panel. The main keys and switches are concentrated here, allowing you to manually control the SART. Human intervention is necessary when conducting test runs, connecting new modules, and upgrading the system. To achieve maximum convenience, the panel provides a liquid crystal display that allows you to monitor all indicators in real time, monitor their compliance with standards, take timely actions if they go beyond the established limits;
- temperature regulators. These are executive devices that determine the current performance of the SART. Regulators can be mechanical or electronic, but their task is the same - adjusting the cross section of pipes in accordance with current external conditions and needs. Changing the capacity of the channels makes it possible to reduce or, conversely, increase the volume of coolant supplied to the radiators, due to which the temperature will increase or decrease;
- Pump equipment. SART with automation assumes that the circulation of the coolant is provided by pumps that create the necessary pressure, which is necessary for a certain water flow rate. The natural scheme significantly limits the adjustment possibilities.
