Technical Articles - From Traditional Substations to Smart Substations

Substations interconnect different voltage levels and form a critical link between transmission, distribution and consumption. Primary equipment such as power transformers, circuit breakers and disconnect switches located in the substation switchyard protect and manage the grid power supply. Auxiliary devices such as protection relays and terminal devices are usually located away from the switchyard in the control room panel to protect, control and monitor the primary devices.

Measuring electrical parameters in conventional substations

Conventional instrument transformers such as voltage transformers (PT) and current transformers (CT) measure the high voltage and current flowing through the primary device. Copper wires connect the analog output of the transformer to the auxiliary device, and the number of copper wires increases depending on the application.

Figure 1 shows that separate CTs and PTs for protection, control, and monitoring lead to installation and maintenance complexity due to the large amount of copper wires, and increase potential failures that lead to higher costs. In addition, the use of multiple transformers results in different primary current and voltage digital values ​​within the device, limiting system performance and reliability.

Digital Substation

The digital substation is part of the secondary system and includes all protection, control, measurement, condition monitoring, recording and supervision systems related to the primary process.

Digital substations replace hundreds (sometimes thousands) of meters of copper wires between switchyards and intelligent electronic devices (IEDs) with a few fiber optic cables. Digital substations that use fiber optic cables for communication use conventional or non-conventional instrument transformers (NCITs) and merging units to digitize data related to the process parameters being measured. Using less copper makes digital substations simpler, more compact, and more efficient.

Digital Substation Architecture

According to the definition of the International Electrotechnical Commission (IEC) 61850 standard, the digital substation architecture includes three levels: process level, bracket level and station level, as shown in Figure 2.

Each level performs a specific function and the applications work together to perform the digital substation functions. The process level includes power transformers, instrument transformers, and switchgear.

The bay level includes auxiliary devices or IEDs such as bay controllers, protection relays, fault recorders, and meters. IEDs no longer have analog inputs because data acquisition occurs at the process level. Consolidated inputs can also reduce or eliminate the need for binary inputs, enabling compact devices that typically occupy only half the traditional footprint. IEDs handle protection and control algorithms and logic, make trip/no-trip decisions, and provide IEC 61850-based communication capabilities for lower (process) and upper (station) level Ethernet networks. Communication network redundancy is a typical requirement to ensure the highest availability and reliability. Two IEC 62439 standards - High Availability Seamless Redundancy (HSR) and Parallel Redundancy Protocol (PRP) - facilitate IED interoperability and integration from different vendors into substation networks.

The station level includes substation computers, Ethernet switches, and gateways. In addition to the traditional supervisory control and data acquisition (SCADA) bus, the substation bus provides additional communication capabilities because it allows multiple clients to exchange data; supports point-to-point device communication; and links to gateways for inter-substation and wide-area communication. Station-level devices may include substation human-machine interfaces (HMIs), engineering workstations accessed by IEDs or local centralization and archiving of power system data, SCADA gateways, proxy server links to remote HMIs, or controllers.

Measuring electrical parameters using merging units

The merging unit converts the instrument transformer output to a standardized Ethernet-based data output and implements IEC 61850.

In a digital substation, instead of connecting the sensor outputs to protection and control devices at the bay level, merging units are placed near the sensors that are connected to the master devices at the process level.

The merging unit converts analog signals (voltage, current) into sampled values ​​based on IEC 61850-9-2 for protection, measurement and control, and communicates with the IEDs in the substation via digital communication, as shown in Figure 3. Some key merging unit functions include analog-to-digital conversion, resampling, synchronization with a global time base, sample conversion to IEC 61850-9-2 protocol, and communication with the IEDs using a fiber-optic Ethernet interface.

The merging unit performs the necessary processing to generate an accurate, time-aligned output data stream of sampled values ​​in accordance with the IEC 61850-9-2 standard. This processing includes sampling of analog values; accurate real-time referencing; message formatting into sampled values; and publishing of a single data source to measurement, protection, and control devices.

Key challenges in designing a merging unit

There are multiple challenges when designing a merge unit. Some of the key challenges that impact architecture and performance include:

• Select an ADC that can scale the sampling rate and synchronize the sampling with a precise global timing reference.

• Increase the number of analog input channels by connecting multiple ADCs to the host processor and capturing data in real time.

• Capture sampling in real time to meet protection and measurement sampling requirements.

• Use Ethernet communications with fiber optic interfaces.

• Implements communication protocols according to IEC 61850-9-2 and enables sampled data to be communicated to multiple users without packet loss.

• Enables the protocol stack to be used to implement redundancy protocols, including HSR, PRP, and time synchronization based on the Institute of Electrical and Electronics Engineers (IEEE) 1588 Precision Time Protocol (PTP).

• Implement multiple I/Os, including binary inputs (16 or more inputs), covering wide AC and DC inputs and DC sensor inputs and outputs, with expansion options.

• Reliable operation in harsh switchyard environments with high transients, higher ambient temperatures and magnetic fields.

Solving Merge Cell Design Challenges

Integrated circuits and reference designs from Texas Instruments (TI) can help designers meet these challenges. Figure 4 shows the functional blocks in a merging unit.

The merging unit consists of multiple subsystems described below that are interconnected to perform signal scaling/capture, processing, and communication functions. The unique features and capabilities (in brackets) of the TI recommended devices simplify the selection of key components and minimize the design effort.

• The processor module (using AM3359 or AM4372 or AM5706 or AM6548) is connected to the ADC using the Programmable Real-Time Unit Industrial Communication Subsystem (PRU-ICSS) and includes a Digital Signal Processor (DSP) core for processing electrical parameters and algorithms and an Arm® Cortex®-A15 microprocessor subsystem for external communication, user interface, and execution of substation communication protocols.

• Ethernet interface (DP83822, DP83840) over fiber optic cable or copper wire to connect to a host using Media Independent Interface (MII) or Simplified MII and hardware-assisted IEEE 1588 PTP-based time for communication synchronization at 100 Mbps.

• AC/DC (using UCC28600, UCC28740, UCC24630) wide input, high efficiency, synchronous rectifier based power supplies.

• A DC/DC power tree (using LMZM33604, TPS82085) that includes high-efficiency power modules with small size, integrated inductor, and > 2-A load current, with fast transient response and reduced electromagnetic interference (EMI) due to integration of the controller, high-side and low-side FETs, and inductor in one package.

• Memory termination (using TPS51200, TPS51116) using JEDEC-compliant source or sink double data rate (DDR) termination LDOs or an all-in-one DDR power management device with a synchronous buck controller, LDO, and buffered reference.

• AC analog input modules (using OPA4188, THS4541, ADS8588S, ADS8688, AMC1306x) include AC voltage and current inputs for protection, monitoring, and measurement. Gain amplifiers scale the sensor output to the ADC input range. 16-, 18-, or 24-bit precision successive approximation registers or delta-sigma ADCs acquire samples at 80 or 256 (or higher) per cycle, synchronized to a global time reference using pulses per second or inter-range instrument clusters.

• DC analog input or RTD modules (ADS1248, ADS124S08) for bidirectional or unidirectional DC voltage or current control operation for remote communication between devices. 24-bit precision delta-sigma ADC improves measurement range and accuracy.

• Binary input modules (ADS7957, ISO7741, ISOW7841) for monitoring batteries, providing interlocks between devices and indicating configuration changes and status. ADC and digital isolator-based architectures improve measurement accuracy and reduce circuit complexity by using fewer components compared to optocoupler- and Zener-diode-based designs.

• Relay or high-speed type digital output module (using TPS7407, DRV8803) for alarm and external circuit breaker operation.

• On-board protection of analog inputs against transients (using TVS3300 or TVS3301) and board-level diagnostics (using HDC2010, TMP423, and TMP235) for measuring ambient temperature/humidity for measurement drift compensation.

The merging unit connects to different types of transformers for measurement, including traditional instrument transformers, NCITs such as optical current transformers, or Rogowski for current and resistor-capacitor voltage transformers (RCVTs) for voltage. NCITs connected to a merging unit, as shown in Figure 5, provide the option of metering, protection, and control accuracy in a single device. NCIT technology reduces transformer size and weight, saving space and cost.