Grid Health Rides on Smart Protection Relays
Contributed By Electronic Products
Located at central stations and substations, protection relays have long served as the final stage for shielding the grid and substations from faults. Today, the combination of the smart grid and distributed power generation systems has driven requirements for smart protection relays able to bring intelligent fault protection closer to power consumers and individual pieces of equipment. Well beyond conventional mechanical or even solid-state relays, smart-protection relays are sophisticated processor-based systems designed to identify faults and safely disconnect from the grid—and reliably reconnect when service is restored. For engineers, the complexity of building cost-effective smart-protection relay systems has been significantly reduced thanks to a broad range of available devices from manufacturers including Analog Devices, Atmel, Linear Technology, Maxim Integrated, Micrel, Microchip Technology, TE Connectivity, and Texas Instruments.
Smart-protection relays, also called numerical-protection relays, are complex systems in their own right (Figure 1). In contrast to traditional mechanical or static solid-state relays, these systems provide a unique combination of precision performance, flexibility, and ease of use. At a minimum, these systems monitor line current and voltage for faults, using a multichannel analog front end (AFE) to sense characteristics on each wire.
Figure 1: Processor-based smart-protection relays combine line monitoring with relay control, enabling both independent operation and remote control of connection and disconnection events. (Courtesy of Texas Instruments)
Depending on specific application performance requirements, the AFE signal chain typically starts with a precision instrumentation amplifier stage designed to optimize the AFE’s dynamic range while reducing noise and drift errors. In turn, the amplifiers drive a conversion stage—built with analog-to-digital converters (ADCs) at resolutions ranging from 10 bits to 16 bits to meet application accuracy requirements ranging from about 1% to 0.05%, respectively.
Specific conversion requirements dictate the choice of converter architecture. For example, delta-sigma (ΔΣ) ADCs offer high resolution and signal-to-noise ratios, while successive-approximation register (SAR) ADCs feature zero latency and the high linearity often needed in these applications. For oversampling ΔΣ ADCs, the signal chain includes a final filtering stage to smooth ADC output.
AFE ICs integrate all the stages of a suitable signal chain, offering resolution up to 24 bits and providing a near drop-in solution for these applications. For example, the Analog Devices AD73360 ADC offers six 16-bit channels, each with a complete signal chain comprising signal conditioner, programmable gain amplifier (PGA), Σ-Δ converter, and digital decimation filter.
On the output side, smart-protection relays drive high-voltage relays or circuit breakers to open or close connection with the grid. Here, the processor enables or disables relay drivers such as the Linear Technology LTC4441, Maxim Integrated MAX4820/4821 (Figure 2), and Micrel MIC4420. In turn, these drivers open or close high-voltage relays such as the TE Connectivity LEV200A4ANA , part of the company’s LEV200 KILOVAC family of high-voltage relays.
Figure 2: Smart-protection relays use processor-based control of relay drivers such as the Maxim Integrated MAX4820 (serial interface) and MAX4821 (parallel interface) to open or close high-voltage relays or breakers during line fault episodes. (Courtesy of Maxim Integrated)
At the heart of smart-protection relay systems, microcontrollers and DSPs execute algorithms designed to detect faults and control line relays. The availability of processors with integrated analog peripherals helps simplify design while providing horsepower and resolution required to implement sophisticated protection-relay functionality.
In fact, designers can draw on 32-bit MCUs boasting integrated multichannel ADCs with resolution up to 24 bits for applications requiring the best possible accuracy. For example, the Analog Devices ADUCM360 in Analog’s ADuCM family combines an ARM Cortex-M3 32-bit core with two channels providing separate paths with dedicated signal conditioning, 24-bit Σ-Δ converter, and digital filter. Each channel is further connected to an input mux for handling multiple sources. For applications with less stringent accuracy requirements, designers can find 10- and 12-bit ADCs in integrated MCUs such as those in the Atmel SAM D20 family, Microchip Technology PIC 32MX family, and Texas Instruments Concerto family. Atmel’s ATSAMD20J18, for example, provides a 12-bit ADC but can be configured for 8- and 10-bit results to reduce conversion time.
Among their capabilities, smart-protection relays augment their own capabilities to independently monitor and disconnect from the grid with features designed to support remote operation by humans and automated grid-management software. Here, these smart-relay designs implement a variety of connectivity options needed to ensure low-latency communication of grid events to remote operators and fast response to commands for disconnection issued by remote operators.
As a result, smart relays typically need to support a variety of flexible TCP/IP and Ethernet-based protocols such as EtherCAT, Profinet, Profibus, and EtherNet/IP designed for low-latency real-time control and industrial applications. In addition, these relay systems are expected to support more specialized protocols for grid connectivity. In particular, IEC 61850 over Ethernet has emerged as the preferred grid communications protocol, providing the necessary speed and efficiency required to meet standards requirements such as millisecond response times for protection relays.
To support these growing connectivity requirements, semiconductor manufacturers are enhancing MCUs with direct, integrated support for industrial connectivity requirements. For example, the Texas Instruments AM335x Sitara family of 32-bit processors includes a dedicated communications block, called the Programmable Real-Time Unit Subsystem and Industrial Communication Subsystem (PRU-ICSS) (Figure 3).
Figure 3: Integrated MCUs such as the Texas Instruments AM335x Sitara family combine 12-bit ADC and common peripherals with a specialized Programmable Real-Time Unit Subsystem and an Industrial Communication Subsystem (PRU-ICSS) designed to support industrial protocols needed for grid communications. (Courtesy of Texas Instruments)
Designed to offload real-time communications processing tasks from the ARM core, the PRU-ICSS directly supports real-time industrial communications protocols such as EtherCAT, PROFINET, EtherNet/IP, PROFIBUS, EtherNet Powerlink, Sercos, and others. Furthermore, engineers can take advantage of the PRU-ICSS’s programmability to support more complex protocols or custom interfaces—all without impacting the processing load on the device’s integrated ARM Cortex-A8 host processor.
Smart-protection relays form the last line of defense in the grid, preventing faults from propagating to substations, local subnets, or even across individual pieces of equipment. Accordingly, as the need for these complex systems extends deeper into the grid periphery, designers require more cost-effective solutions for building complex systems tuned to the specialized needs of each application and operating environment. For engineers, a correspondingly rich array of ICs allows efficient design of smart-protection relay systems able to meet a broadening range of requirements for accuracy, rapid response, and sophisticated functional capabilities.
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