Network control technology

The network control technology comprises the measurement, control and regulation technology of networks. The network control technology is mainly used in line-based networks such as electricity networks , but also in pipe networks such as gas , water ( drinking water / waste water ) and heating networks ( district heating / local heating ). The network control technology is operated by network operators and supply companies . Network control technology is a specialty of process control technology ; it belongs to the applied engineering sciences.

Network control technology tasks

The task of the network control system is to support the network operators in the management of their networks, for example power networks, i.e. to give the operating personnel (the people are also called operators, switching technicians, switching engineers) decision-making aids and to relieve them as much as possible of routine work . In addition to network monitoring, the resulting disturbance situations must be reacted to and control interventions in the network activity.

One of the essential basic tasks of network control technology is to transmit process information such as metered values, measured values and messages to a central control center , where it is processed and displayed in a user-friendly manner. In the opposite direction, the task is to issue control and setting commands to the process.

In the early days of power supply, network control technology mostly consisted of many decentralized control stations with simple remote control and remote monitoring devices. The progress made by these facilities essentially consisted of the correct chronological recording of process events and their ongoing logging.

In keeping with the development of computer technology since the 1980s, network control technology has grown in importance. In addition to the core tasks of network control and network monitoring, computer technology today offers a wide range of options for fault analysis, the simulation of network conditions and network faults, including for training and instruction purposes for operating personnel, as well as the determination and setting of optimal network conditions.

Secondary technology

The term secondary technology in electrical energy supply includes the facilities that are only indirectly involved in the process. However, this includes functions and facilities necessary for the operation of the networks, such as B .:

Local control / local control (control of the individual switchgear panels on site, usually via a field control cabinet)
Station control technology (If the local control / local control is implemented with computer technology for the purpose of preprocessing and information presentation and combined with network protection , then this is referred to as station control technology.)
Voltage regulation (keeping the mains voltage constant under different load conditions)
Grid protection (monitoring of the lines for short circuits or earth faults with disconnection of the line concerned)
Energy metering
Remote measurement (remote transmission of measured values to a remote network control center)
Own requirements (secured direct and alternating voltage supply, battery systems, rectifiers and inverters) Maintaining the function of the system even in the event of a power failure or network disruption.
Ripple control system (control of suitable consumers in the network, e.g. night storage heaters)
Telecontrol technology
Network control center

Components of network control technology

Telecontrol technology

The task of telecontrol technology is to transmit network information between the switchgear and a network control center. In the case of telecontrol systems, depending on the location, one speaks of a telecontrol substation , ie the telecontrol system is located in the system building on site, usually in a substation , or of a telecontrol center if it is located in a remote, central control center. To bridge the distances between the systems in the network and the central control center, the operated remote control technology of transmission equipment (digital transmission networks (for example, PDH , SDH ), Wechselstromtelegrafie (WT), etc.).

Telecontrol node

From the telecontrol node, the network information from the telecontrol systems connected to a control center is bundled and transferred to the control center's computer system. Today concentrators are mostly used here, which combine the serial channels from the telecontrol systems that come up with low bandwidth and transfer them to the network control system via LAN or WAN .

Control center

The central component of the network control system is the control center. Synonyms for this are switching line , main switching line, network control center and control room. The different terms mostly refer to the voltage level in which the network to be monitored is located. The task of the control center is the processing, preparation and presentation of the information for network operation in the control room.

equipment

While network control systems used to be equipped with one or two process computers , today the tasks at hand are carried out by multiple computer systems. Different functional complexes are distributed over several computers. The computers give each other multiple redundancy, this guarantees a high level of availability.

Instead of mosaic boards, as can be found in older control centers for network display, large-format display, the so-called rear projection technology, is now also installed. Process images are projected from the rear on a modular, transparent projection screen with the help of a special lighting system. Either LCD panels or DLP projectors are used to display the monitor content on the projection wall using X-Window technology . The resolution of the individual wall modules is comparable to that of conventional PC graphics. The projection screen as an overview medium is not only used to display a network overview, as in the past, but also serves larger, modern control rooms with a mouse and keyboard as an independent, interactive operating and display medium for large network displays.

The fulfillment of these tasks can result in considerable effort. The operator staff (switch technicians, switch engineers) are provided with various functionalities / tools to fulfill these diverse and responsible tasks.

Functions of the network control center

The functionality of a network control center is mainly implemented by the user software . Usually this is divided into the areas SCADA and HEO. For some time now, the network control technology has also been integrated more deeply into operational process planning. Couplings to the supply company's materials management as well as tools for personnel deployment planning are increasingly used. Information functions about the location and location of substations and lines in the geography, internal company data evaluations and information about current weather events are implemented and graphically presented to the operator.

SCADA functions for electrical energy supply, gas, water and district heating networks

Supervisory Control and Data Acquisition (SCADA) is the monitoring and control of technical processes using a computer system.

MMI / man-machine interface

The networks to be monitored are usually processed and monitored using images that are displayed and operated at workplaces with high-resolution color vision systems on monitors with a diagonal of up to 29 inches. Large-screen projection systems are often also used to display larger network interrelationships. Other peripheral devices are printers, hardcopy devices and, in some cases, plotters or data recorders.

A control room usually consists of a workstation computer with up to six color monitors, a keyboard and a mouse. If several monitors are used at the workstation, the cursor can be moved continuously across the monitors, whereby inputs via the keyboard and mouse then become effective on the monitor selected by the cursor (multi-monitoring).

Basic functions of the MMI:

Pull-down menus or fixed selection marks on the monitor
Enlarging / reducing the display scale (zooming)
Enlarging / reducing the display scale with suppression or output of additional previously defined information (decluttering)
horizontal or vertical shifting of the monitor image as a section over a world view (panning or scrolling)
Overlaying image levels to display or display different information at the same positions in an image (e.g. measured values for voltage / current / power, etc.)
Multi-window technology with drag & drop and navigator
X11 and JAVA standards
To protect against unauthorized access, privilege levels and responsibilities can be introduced, with which the operating personnel is authorized to work in various areas of responsibility via password or coded ID (for example for normal control room operation, for data maintenance, for system maintenance, for access to certain subnetworks, for training functions etc.).

Curve graphics

In curve graphics , the time course of currents, voltages, water levels, flow rates or other measured variables can be displayed.

Message processing / signaling / alerting

Processing of status information, including with real-time stamp
Evaluation of the acquisition status
- All relevant events are documented in event logs
- Alarms are listed in alarm lists and their processing status is set through acknowledgment processes

Measured value processing

Checking the plausibility of the measured values
Conversion of raw values via encoder characteristic via linear or polygonal scaling into finished values
Check for limit value violations and impermissible rates of change
temperature-dependent limit value tests (at low temperatures, cables can withstand higher loads, since they expand less with the same load and thus the sag is less)
Alerting if limit violations occur
automatic substitute value generation via substitute measuring points or manually specified values
Renewal control
mathematical processing
Archive processing
Archiving of the defined values with status and time, taking into account archive periods, grid cycles, compression levels
Compression of the actual values to mean, minimum or maximum values and sums
cyclical archiving in grid archives in minute / hour / day / month cycles
Event-triggered archiving in dynamic archives
Correction option with identification of the corrected values
automatic redensification in the compression stages after manual corrections
Long-term archiving (also on removable data carriers)

Count processing

Conversion of raw values
cyclical processing as absolute or differential value
Comparison with the maximum and minimum number of pulses per cycle
Monitoring for constant or variable limit values - hysteresis
Adding up the on / off cycles and the duration of all on states of equipment

Command processing / controls

Switching operations are checked using rule-based interlocks in order to prevent human switching errors.

Consideration of interlocking conditions for commands. Command outputs must not be issued if, for example, a processing block (for example a disposition authorization or switching block ) is set or the remote / local switch (F / O) is in the "Local" position.
Control of command runtimes
Output of setpoints

Complex switching operations are carried out using switching programs / automatic switching sequences / program switching for the purpose of technological / temporal process optimization and increasing safety, instead of being carried out in many individual switching operations .

Control system interlocks

There are extensive electrical and mechanical interlocks in the switchgear . The control system software carries out additional interlocking tests for all switching operations.

The following interlocking findings can occur:

Switching allowed
undefined resource: Switching on prohibited because the voltage state is undefined on at least one side of the switching element (at least one undefined item of equipment was found). In this case, switching off is permitted for load switches and circuit breakers.
faulty voltage condition: Switching on prohibited because the voltage state of the switching element is faulty on at least one side. In this case, switching off is permitted for load switches and circuit breakers. The faulty stress state arises when topologically impermissible combinations of network element states are present.
disturbed equipment: Switching on prohibited because the voltage state of the switching element is disturbed on at least one side (at least one disturbed device was found during the topological search). In this case, switching off is permitted for load switches and circuit breakers.
Risk of short circuit: Switching on prohibited because there is a risk of short circuit. This means that any live state and the "earthed" state were found. Switching off is only permitted for circuit breakers in order to remove a short circuit.
Do not operate the isolator under load: Switching prohibited because the states "live" and "no voltage (load)" were determined at the poles of the equipment. Lines are also assumed to be loads because their capacities and inductances allow a current to flow.
2 Do not connect feeds with isolators: Switching is prohibited because different infeeds may not be switched by isolators.
unauthorized multiple feed: Switching on is prohibited because an "unauthorized multiple feed" (several controlled feeds) can occur. Subnetworks may either have only one regulated feed or one or more manually controlled feeds. Unauthorized multiple supplies may be canceled by load switches and circuit breakers.
Load shedding possible: Switching prohibited because consumers can be switched to dark.
Short circuit present: Switching prohibited because a short circuit cannot be switched by load switches or disconnectors.
Isolation distance condition violated: A circuit breaker must not connect the "live" and "earthed" potentials. The isolating distance condition is checked if the circuit is permitted and there is no parallel topological path.
Do not operate the isolator when it is live: Switching is prohibited because the states "live" and "dead" were determined at the poles of the equipment.
Parameterization error: Switching prohibited because there is a parameterization error.

In addition, results from the network security calculation, for example (n-1) security, can be taken into account when locking.

More functions

Diagrams, logs, archives, balance sheets
automatic earth fault search and shutdown
Switching requests / switching orders
Message screen control
electronic needle pattern set
Power plant management
Fault locator
Functions related to renewable energies
- Feed-in management
- Smart grid

Coupling to external systems

Coupling to company information technology via LAN
Coupling to network planning systems
Coupling to geographic information systems (GIS)
Coupling to other network control systems (implementation often via TASE.2 )

Higher decision-making and optimization functions (HEO)

The complex of higher decision-making and optimization functions includes various calculation methods that support the operations management personnel in managing the network, i.e. an extended range of functions that goes beyond SCADA.

Topology recognition and coloring
Higher-level interlocking tests, for example switching off the last feed
Load control
Purchase cost optimization
Power frequency control
State Estimation (network status detection)
integrated load flow calculation, short-circuit current calculation
Failure variant calculation ((n-1) security)
Grid losses
Stress profile
Energy use
Energy data management
Forecasts / analyzes

Description of some important HEO functions

State estimation and nodal load adjustment: The state estimation is used to estimate the real current network state by evaluating various information and correcting the measured value errors. While the high-voltage network can be estimated with the existing measured values, the unmeasured loads of the transformer stations (UST) in the medium-voltage network must first be roughly estimated by means of a nodal load adjustment and daily curve analysis before they can be estimated. As a result, among other things, the current loads and feeds are delivered. They form the basis for most other calculations.
State simulation (load flow calculation in the high voltage network): In the simulation mode (short-term operational planning), the current measured values are missing. The state simulation as a preliminary stage of the load flow calculation serves as a substitute for the non-feasible estimation due to the lack of process information. The network status is the simulation basis for a change in the topology that can be specified by the operator and thus the load situation.
Load flow calculation (in the medium-voltage network): The medium voltage load flow calculation is used in the 10/20 kV network. It calculates (provided that the electrical four-pole data of the medium-voltage networks and consumption data are available in the network nodes), the load flow situation and the voltage distribution in the medium-voltage network. For this purpose, a measured value P / Q or a daily load profile and a load current base value are required for each transformer station.
Short-circuit calculation (in the high-voltage network): The short-circuit calculation is used to monitor the stress on the equipment in the event of a 3/1-pole short circuit KS. Furthermore, by monitoring the lower limit of a 1-pole KS, the fault current necessary for tripping the protection is checked.
Short-circuit current calculation (in the medium-voltage network): For medium voltage, the short-circuit current calculation is usually carried out as part of the switching status check. Here it is checked whether the expected three-pole short-circuit current at the electrically most distant end of the line reaches the protection trip value.
The failure variant calculation: The failure variant calculation is used to monitor the (n-1) security of the network. Usually only the high-voltage network is considered here. The failure variants to be calculated are automatically determined by the control system according to specified criteria or also by the operator in special network considerations.
The load flow optimization (OPF - Optimal Power Flow): The load flow optimization determines transformer step settings and reactive power operation of generators, which lead to a minimization of the active power losses in the high voltage network. Additional conditions (restrictions) are specified for the invoice. In addition to the voltage range at network nodes, these also include active and reactive flows or currents via branches. A distinction is made between hard and soft constraints. Hard constraints are network or operational restrictions that must always be complied with, while soft ones are treated in such a way that they are not violated as far as possible.
Compensation current calculation when switching in medium-voltage networks: When interconnecting medium-voltage lines or interconnecting medium-voltage network groups, an equalizing current calculation is carried out before the circuit is carried out. Before switching equipment, this is used to check whether the switching means (mostly load switches in substations) can switch on the current without destroying it.

HEO functions for gas networks

Consumption forecast: some methods are based on an artificial neural network .
- Short-term forecast (1 hour)
- Daily / long-term forecast
- Consideration of day types, temperatures
- Consideration of operating hours, storage contracts, network buffers
- Consideration of future environmental influences such as. Online weather data
Purchase monitoring and control / purchase and storage management
- Determination of total consumption from the reference stations
- Extrapolation of current reference, for example on the basis of linear regression to the current hour
- Comparison with contractually agreed reference dates
- Allocation per station and consumption customer
- Consideration of balance adjustment, network buffer, storage use, generator, spot quantities, disconnectable consumers
- Monitoring of limit values, storage and producer contracts
Purchase cost optimization: Based on a forecast consumption, a deployment plan for the gas storage and switching suggestions for consumers that can be influenced is determined. The result is shown as a "timetable". Only when the quantities to cover peaks, taking into account the consumer switch-off options, are not sufficient to maintain the target reference value, a corresponding increase in the target value is proposed through the optimization.
- Short-term optimization (1 hour)
- Optimization of the hourly reference (optimal distribution of gas consumption to the individual reference stations)
- Optimization of daily reference (determination of the optimal hourly reference target values based on the consumption forecast)
- Consideration of results (consumption forecast, network buffer and nominations from contracts)
Pipe network simulation
Network capacity management
- Objective: The gas volume made available by the shipper at the entry point should be made available at the exit point in accordance with the relevant transport regulations. This means that the network operator is responsible for carrying out the agreed transports in his supply network and for coordinating with the other network operators involved in the transport. All provisions of the EnWG and GNZV apply.

HEO functions for water networks

Consumption forecast: see above.
Water leak analysis: The leak analysis in the supply network is carried out at night at low-consumption times by checking defined idle consumption values of individual supply areas. For the leak analysis in the pipelines, quantity comparisons are carried out, provided that the quantities at the beginning and end of the pipes can be measured.
Well lowering: For the statistical evaluation of the effects of pumping processes on the groundwater level, the water levels of all pump shafts are recorded cyclically. From this, possible influences on the groundwater level, in particular its lowering due to pumping processes, can be recognized.
Pump optimization: As a supplement to load optimization, the system also takes over the pump control in the waterworks. The pumps of the wells and tanks are switched depending on the electricity supply situation and the measured tank levels.

HEO functions for district heating networks

The district heating prognosis is used for optimal deployment planning of "HKW" ( thermal power stations ). Due to similar consumer behavior as in gas networks, the same mathematical methods can be used. Optimizations are not necessary, as there are no consumers that can be switched off.

Control room technology / MMI / visualization / operating level

The control room technology forms the top level in a network control system. The process is now visualized via control stations that are equipped with 4 to 6 monitors and possibly also with large-screen projections .

In older control rooms you can also find the so-called mosaic picture (feedback panel) as a hard-wired display unit (only rarely as a control unit).

In the case of large-screen projection , several screens / graphic modules are attached to one another without any visible separating joints, if possible.

Today, the printers in the control rooms are usually only used to print out specific information. In the past, long-term documentation was guaranteed by constantly printing out all events.

System technology / control system / hardware

The control system provides the necessary central infrastructure to perform all tasks:

Client / server computer systems (mostly redundant) with associated mass storage devices
Networks
further access options

The computers in the control system have been Unix-based or Windows-based computers almost without exception since the mid-1990s. For security reasons, it is customary to design the computer technology redundantly , ie two computers with the same task are operated in parallel and monitor each other. If one of the two computers fails, the other takes over its tasks so that the operation of the network is not restricted. Workstations or standard PCs in workstation / server technology are used as computers.

The redundancy is usually extended to the local area network ( LAN ).

In many cases the telecontrol centers (FWZ) are also located directly at the location of the master computer. They are then usually also viewed as a component of the network control level. The FWZ and the control system computer are usually connected via LAN, using either manufacturer-specific protocols or standard protocols such as TASE.2 or IEC 60870-5-104 .

Analogous to the connection of the electrical networks of individual operators to a large European network ( UCTE network ), the information from control systems must also be exchanged within the individual companies, but now also across companies or even internationally. TASE.2 is also used as the protocol for this.

System monitoring / system availability

In addition to the SCADA functions and HEO modules, the scope of functions of the network control system includes numerous system functions, such as continuous self-monitoring, with which possible errors are detected and displayed at an early stage before they cause malfunctions.

The scope of the general system functions includes:

Monitoring for communication errors between the process interface and substations
Monitoring for failure of telecontrol lines
Monitoring for failure of control system components
Online diagnosis and remote diagnosis
Data backup functions for system software, data model and archives
Safety measures for redundancy configuration
System message processing
Operating and influencing the system components

The monitoring functions not only extend to hardware, but also include software modules, communication links and telecontrol devices. For example, the failure of a single telecontrol channel connection is compensated for by automatically switching to a redundant connection in another device. This significantly increases the overall availability of a control system.

Telecontrol coupling / telecontrol transmission

The information to be transmitted (messages, measured values, counter values, ...) is made available to the telecontrol system on site in parallel at a distribution board. The information is available via individual signal wires on one side of the distribution board and is routed on the other side of the distribution board in such a way that a standardized contact assignment results and is then connected to the input modules of the telecontrol device via plug connections. Information, such as control commands coming in the opposite direction from the control center, must then be extracted from the incoming telegram and output to the control circuits of the secondary technology via the output modules.

The telecontrol coupling takes place via telecontrol centers (FWZ). It is also called a process coupling system, telecontrol gateway, etc. The FWZ is connected to the substations (FWU, RTU ) via line couplers of various types . This telecontrol link can extend over very large distances. In fact, some routes are routed via satellite .

Normally, one FWZ is connected to many FWU. The languages used, so-called telecontrol protocols, are quite numerous. However, only certain protocols are used regularly in the area of telecontrol .

Since (redundant) WAN connections between the locations of the network control center and important points in the process (transfer stations, substations, sewage treatment plants, waterworks) increasingly exist in many areas today, the FWZ are increasingly being relocated from the network control center to decentralized locations in the network area. This generally increases the availability of the process connection , while higher data transmission rates become available at the same time.

Station control level / field control level

The station control technology often coincides with the field control technology . In the past it was mostly a telecontrol substation (FWU, English RTU). It is the link between the process and the network control level. A telecontrol substation has signal inputs and outputs. Here, for example, a measured value from the process is read in as an analog value and forwarded to the telecontrol center. Modern station automation systems (SAS) have a decentralized structure, mostly correspond to the IEC 61850 standard and are defined by four main components:

Intelligent electronic devices (IED) such as field control devices, protective devices, voltage regulators, etc.
Field and station bus or communication network consisting of network switches and preferably fiber optic connections
Local remote operator station (HMI) for protected operation of the entire station
Telecontrol gateway as a defined interface to the network control system

Local control is possible in many station automation systems (SAS) , ie the operator can switch directly on the IED or a remote operator station (HMI). Commands from the network control level are ignored after the operator control has been switched to local control.

Ripple control / load control

The ripple control , and audio-frequency ripple control ( TRA called), is part of the network control system and the large-area control of tariffs and charges used (such as storage heater) with the current customer. Here, coded audio frequency pulses (e.g. 190 Hz) are fed into the network via powerful transmission systems and superimposed on the 50 Hz voltage. At any point in the distribution network, these pulses can be decoded using simple receivers and can thus be used on site for switching commands for any number of customers.

Recently, radio ripple control systems using long-wave transmitters have been used instead of network-connected audio frequency ripple control technology . used.

Data model

The database required for network modeling can contain well over a million data points. For this purpose, data management systems and databases are used that allow the provision and manipulation of the required information and data.

Data model creation and maintenance

The effort involved in modeling technical networks is constantly increasing, as growing operational requirements require more and more information and complex display methods increase the effort involved in image construction. Consistent data modeling at just one central point, if possible, reduces the effort involved in initial data entry and maintenance. Network control systems mostly offer object-oriented data management software .

Object-oriented data model input: The experiences from many projects of the suppliers and users are reflected in object libraries for the supply areas (electricity, gas, water and district heating). At the beginning of the project, they provide the starting platform from which project-specific requirements can be made by varying the existing types. The advantage for the user lies in a tried and tested starting point for the data model and in the flexibility to be able to adapt it as required.
Typing: Objects suitable for typing in supply networks are, for example: detectors, measured values, commands, ... disconnectors, switches, slides, pump fields, busbar systems, substations, FW lines, networks (electricity, gas, ...), telecontrol systems, components of the control system
Adoption of existing data models: Another contribution to rationalization is the possibility of partially adopting existing data models - as far as this is technically sensible and possible. When replacing existing control systems, savings can be achieved by taking over checked old data, especially when entering data for the first time. Another, often even more important aspect is that of security, since the old data already belong to a checked database and can be transferred to the new system as a checked component.

process

To transmit electrical energy from the power plants to the customers, a widely branched power transmission and power distribution network with different voltage levels is required.

The extra high voltage network (in Western Europe a 380 kV network with an underlying 220 kV network) is interconnected as an interconnected network from Gibraltar to Poland and is operated with the same network frequency of 50 Hz. This network is managed in accordance with national responsibilities in close international cooperation via national high-voltage network control centers or, as in Germany, via the corresponding network control centers of the transmission system operators .

Subordinate to this extra high voltage network are galvanically isolated 110 kV high voltage networks with various feed points (transformer stations) from the extra high voltage network. The separation of this network level into 110 kV network groups with only a few feed-in points is necessary for reasons of the maximum short-circuit power in the network and in networks with earth fault compensation to limit the maximum permissible fault currents. The 110 kV high-voltage network groups mostly have a supraregional character.

The 110/10 kV or 110/20 kV feed-in substations for the local medium-voltage networks are connected to the meshed 110 kV high-voltage networks. The medium-voltage networks are used for the local supply of the network stations (20 / 0.4 kV or 10 kV / 0.4 kV) and the medium-sized industrial plants. Although these networks have a meshed structure, they are usually operated in individual circuits; Circuits operated in this way are also referred to as "open rings". The demarcation of the circuits from one another is carried out by open switching points in network stations , so-called standard separation points. A reserve can be provided by switching over these switching points.

While the switchgear , substation and feed-in and feed-out systems (transfer points) in the maximum and high-voltage network, including the substation switchgear for feeding into the medium-voltage network (20 or 10 kV), can be remotely controlled and monitored, the many network stations and industrial feed-in points are in the Medium-voltage network, apart from a few special main stations, cannot be remotely controlled. The switches of these systems must be "manually operated" on site. The entire low-voltage network can only be operated on site. These networks are operated in accordance with the medium-voltage network management in individual, galvanically separated low-voltage networks with one feed. The management of the low-voltage network takes place exclusively via a radio or telephone-supported operation between the operations manager in a network control center and the on-site switching staff.

Sensors / transducers provide information on current , voltage , active power and reactive power in the substation . Furthermore, messages , measured values and / or counter values are transmitted and commands or setpoints are executed.

Other processes include gas networks, sewage treatment plants, waterworks networks and district heating networks.