Data acquisition at the substation level
This technical article reviews the level of substation data acquisition architecture and communication protocol described in IEC 61850. IEC 61850 is the current international standard for substation automation (SA).
Substation level data acquisition architecture and communication protocol described in IEC 61850 (pictured: TransCo Laoag substation at night; credit: MikeHansZach via Flickr)
IEC 61850 aims to provide an interoperable standard for the communication of multi-vendor substation equipment. To date, a proprietary communication protocol has limited the use of a heterogeneous mix of substation equipment.
The description of IEC 61850 is a frame of reference from which three data acquisition architectures offered at a substation level can be compared.
Here, the substation level data acquisition architecture is used to describe the physical connection of devices and the flow of data within the system; while communication protocol is the language used to communicate information over the network. Both of these issues affect the overall data acquisition at the substation level. when it comes to fidelity, latency, and reliability.
The data acquisition system at the substation transmits the data from the UGPSSM (Universal GPS synchronized meters) to the control center. This functionality can be achieved through several architectures.
Three of these architectures will be described in terms of data flow in a typical substation. The three architectures described are point-to-point, networked, and wireless. In the proposed future substation, all data collected is initially processed via satellite synchronized universal time counter (GPS) (UGPSSM).
The UGPSSM device is similar to the IEC 61850 fuser. The processing of each UGPSSM involves sampling, digitizing, and GPS time-stamping of all substation data.
This article describes the functionality and proposed hardware of the UGPSSM.
1. Data acquisition at IEC 61850 substation
1. Data flow
2. Communication protocol
2. Data acquisition architectures at the substation level
1. Point to point
4. Communication protocol
3. Overview of data acquisition architecture at the substation level
4. Universal GPS time synchronized counters
1. Overview of data acquisition at the IEC 61850 substation
1.1 Data flow
A conceptual diagram of the data acquisition system at the substation level described in Standard IEC 61850 “Networks and communication systems for the automation of electrical networks”
GE HardFiber technology is an example of currently available technology using this approach.
Fusing Units (MUs) are analog to digital data collection devices that sample and digitizes electrical quantities. Electrical quantities are analog or digital signals that are of interest.
Analog quantities include:
· Voltage and current signals from potential transformers and current transformers,
· Transformer temperature signals from resistance temperature detectors (RTDs),
· Transformer turn reports from potentiometers,
Numerical quantities include auxiliary contact outputs, etc. MUs are physically placed near the signals they are monitoring. This indictment minimizes the risk of signal corruption. In the GE HardFiber System MUs are called Bricks.
GE bricks feature a weather-resistant exterior suitable for the extreme exterior and physical conditions of substations.
In Figure 2 above, communication from each MU to the process bus is via point-to-point communication. The data transmission rate in this part of the system is very high. This requirement requires a means of point-to-point communication.
In the GE HardFiber system prefabricated fiber optic cabling is used between the bricks and the fiber optic cable termination points of the substation, the Cross-Connect panels
Cross Connect panels are located in the substation control station (see Figure 3) and are used to connect bricks to protection relays, meters, and other intelligent electronic devices (IEDs).
The Cross Connect panels, as suggested by the name, allow the cabling of fiber optic cables between the ports of the substation station.
The configuration creates a dedicated fiber-optic communication channel between each brick and the corresponding IED.
Figure 4 – Cross-connection of bricks and IEDs
In Figure 2, the process bus block represents the interconnection of the MU data path to individual substation IEDs. Substation IEDs use the digital data provided by the MUs to generate additional data.
In the HardFiber system, prefabricated fiber optic patch cords are used in the Cross Connect panels to create a continuous fiber-optic channel between the bricks and the IEDs.
In Figure 2, each of the relays and PMUs have additional data on the station bus, including the magnitude of voltage and current, mean square, etc., based on MU data. This additional data requires multiple sampled data points – for example, calculating magnitude from sampled data requires the full period of samples.
The station bus, therefore, transmits data much slower than the process bus. This allows a network architecture at the station bus level.
In Figure 2 above, the station bus facilitates a data flow between all substation IEDs, substation control computers, and GATE hardware. This enables inter-DEI messaging, a human-machine interface, and communication with external stakeholders.
Key benefits of the HardFiber system include:
1. Standardized fiber optic cabling;
2. Prefabricated commercial components;
3. Engineering, installation, commissioning, and operation using existing skills;
4. GE GE series relays and other 61850 compatible IEDs can be used; and
5. Different IEDs can record data sampled at an independent sample rates.
The IEC 61850 standard allows existing equipment to operate in the same substation with newer equipment. This is shown in Figure 2 where Relay A and PMU A monitors voltages and currents through analog instrumentation channels.
1.2 Communication protocol
The IEC 61850 standard is more than a typical communication protocol. IEC 61850 includes specifications on how to communicate data. It also specifies which data should be communicated in an object-oriented manner.
The primary goal of the creators of the IEC 61850 standard is to create a communication protocol. which enables interoperable performance between all substation equipment suppliers.
2. Data acquisition architectures at the substation level
In all substations, the data is used locally and transmitted to external stakeholders. The architecture used to collect local data varies considerably from substation to substation.
Three possible data collection architectures for the substation of the future are described below.
2.1 Point to point
The conceptual diagram of the point-to-point architecture is shown in Figure 5.
Figure 5 – Design of point-to-point data collection architecture
In Figure 5, the UGPSSMs communicate over a fiber optic or copper data link. Periodic data is provided by each UGPSSM.
The advantages of point-to-point communication include: Higher speed
The disadvantages of point-to-point communications include:
· Requires raw material for communication channels
· Requires the largest communications infrastructure for rights of way
The second method used to collect substation data is the network architecture, described below.
However, this type of network communication requires an additional component impacting the reliability of the communication system.
In addition, switching communication increases the latency of the data stream.
Network architecture has a considerable market share of industrial and commercial communication infrastructure.
The use of the network communication infrastructure in the station environment is limited. The reliability and latency of this form of communication are in doubt for the hard real-time systems used in power system automation.
The advantages of network architecture include:
1. Lower requirements for communication channel equipment,
2. Lower requirement on the communication infrastructure.
The disadvantages of network communications include: Communication collisions cause delays.
A third method used to collect substation data is the network architecture, described below.
The wireless architecture has similarities and differences from the last two data acquisition architectures at the substation level. To date, the data acquisition methodology at the wireless substation has not been used. Wireless data transfer is used in other applications for the operation of power systems and in many other technical fields.
wireless modems are used to send UGPSSM data to the control center.
Wireless communication only requires modems placed at each measurement location. Thus, limited investments in infrastructure are needed. The typical data transmission distances of the stations allow highly reliable point-to-point data transfers.
The use of wireless modems creates a measurable reliability problem that can be continuously monitored through the existence of transmitted data.
The advantages of wireless architecture include:
1. Lowest requirement on communication channel material,
2. Minimum requirement on the communication infrastructure.
The disadvantages of wireless communications include:
1. Cybersecurity issues,
2. Speed (not a disadvantage with newer systems)
2.4 Communication protocol
There are several communication protocols applicable to data acquisition at the substation level.
A partial list of existing standards is provided below.
3. IEC 60890-5-103
4. IEEE C37.118
5. SEL Quick Message Protocol
The challenge for the substation of the future is to demand fast and reliable digital communications in the demanding high voltage substation environment.
3. Overview of the data acquisition architecture at the substation level
Of the three substation-level data acquisition systems examined (point-to-point, network and wireless) the advantages of the wireless architecture are significant.
4. Universal GPS time synchronized counters
UGPSSMs provide a common interface for all input and output data, between switching park equipment and control center equipment, within the proposed substation automation structure.
In general, the UGPSSM is similar to the IEC61850 fuser. UGPSSMs process all-analog measurements, digital measurements, and control signals. This processing for analog measurements includes sampling, digitization and GPS time stamping. This processing for digital measurements includes compressing/upsampling the appropriate sample rate and GPS timestamp.
Figure 8 shows a block diagram of the analog measurement channel in the proposed UGPSSM hardware.
Digital measurement channels include optical isolation, microprocessor (µP), phase-locked loop (PLL), and GPS clock signal. Control channels only include optical isolation and µP.
Figure 8 – Functional diagram of the analog input channel in the proposed UGPSSM
The blocks in figure 8 ensure the operation of the UGPSSM. Digitization (A / D) is provided by a 16-bit sigma/delta modulated analog-to-digital converter. The GPS timestamp is added to each measurement using a GPS clock signal. UGPSSMs also provide optical isolation between all low voltage equipment and switching station equipment.
Typically, UGPSSMs are physically placed near the switching equipment they monitor to minimize any corruption of the low energy analog signal.
The SuperCalibrator feedback signal in Figure 8 is used to automatically calibrate the measurement channels – leading to a self-correcting measurement channel within the proposed station automation structure.
The SuperCalibrator provides an error quantification measurement channel, monitoring the variance of the measurement channels leads to the quantification of the health of the measurement channels. This quantization can be used to derive a feedback signal to automatically increase the accuracy of all measurement channels.
By increasing the precision of the measuring channels, more precise local processing is achieved in the substation.
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