A centralized protection and control system using a well proven transmission class protection relay

Chee-Pinp Teoh and Graeme Lloyd, GE Grid Solutions, UK, Rich Hunt, GE Grid Solutions, USA, and Gerardo Rebollar, GE Grid Solutions, France

Importantly, a centralized protection device must be a technologically mature solution, robust and self-supervised platform, using proven protection algorithms, scalable for any substation size, on hardware capable of the expected 20-year service life.
This article describes one specific actual implementation of such a system for distribution substations, based on an existing transmission class protection relay. This centralized distribution system takes advantages of features already developed for the transmission protection platform, including:

  • Proven protection algorithms for feeder, bus, and transformer protection
  • A robust cyber security implementation
  • Support for multiple SCADA protocols
  • Support for IEC 61850, including sampled values subscription
  • Numerous communications ports to support separate connections to station bus, process bus, and engineering networks simultaneously

A growing requirement for distribution substations is the ability to integrate into wide area monitoring, protection and control schemes. As the device is based on a transmission-based relay platform, this ability is quickly met by including multiple Phasor Measurement Units, and the use of Routable GOOSE to provide inter-station events and control indications.

Centralized Protection and Control System (CPC)
Centralized Protection and Control is a concept to combine the function of several IEDs into one single hardware. This allows more efficient use of the processing power and achieves cost saving by reducing the number of hardware used. While achieving the cost saving is important, CPC shall not compromise the stability and neglect the reliability offered by the conventional solution. Therefore, it is important to find a balance of saving without overlooking the criticality of power system operation.
Centralized Protection and Control is not just about a single centralized protection device, because this device alone will not bring benefit to the user. CPC is a system involving process level digitization and a substation HMI, along with interfaces to wide area monitoring and control as in the Figure on page 38 (on the left). Having all functions running in one centralized hardware opens up some new requirement too, such as the need of robust cyber security in a CPC unit, redundant system to prevent maloperation due to a faulty device, and time synchronization.

Process Level Digitization
The process level is the input and output interface for the primary equipment. Digitization takes place in rugged I/O devices to convert the analogue measurements and statuses into digital messages using the data models and message formats of the IEC 61850 Standard. IEC 61850-9-2LE defines two distinct sampling rates for the SV-based process bus:

  • 80 samples per nominal system frequency cycle
  • 256 samples per nominal system frequency cycle

80 samples per cycle is sufficient to satisfy most of the common protection functions, whereas 256 samples per cycle is used for high-speed functions such as power quality monitoring and digital fault recorder.
Process bus should facilitate flexibility and adaptability by providing all basic data for state, status, and control of the substation to the station level for protection, automation and control application integration. This digitization and process bus turn field wiring of primary equipment into a one-time exercise to interface to input/output devices and eliminates the need for a wiring interface to control devices, protective relays, and especially the centralized protection device.

Merging Unit (MU): To digitize process bus, Merging Unit is used to convert the analogue measurement to IEC 61850-9-2 Digital sampled values (SV). IEC 61850-9-2 SV represents primary current and voltage measurements as instantaneous digital samples.

Remote Input Output unit (RIO): As for status and control function, the Remote Input Output unit  is equipped with a high number of contact inputs and outputs to monitor the status and control of circuit breakers and switches and to respond to control and tripping indications from other devices. A RIO can be equipped with High Speed High Break (HSHB) contact outputs for protection tripping purpose, to compensate for the delay of publishing and subscribing by GOOSE traffic.

Process interface unit (PIU):  Process interface unit is a term this article refers as to a multi-functional merging unit. A PIU is a single device that combines both MU and RIO functions, extending to cover all functions required at the bay level. The limits to PIU functionality are defined by suppliers. A well-defined PIU will have multiple sets of CT and VT inputs to connect to both protection class current transformers (CT), and measurement class CTs. A PIU should also support multiple SV streams with different data rates. A PIU can also have HSHB contacts for tripping and control purposes
A well-defined PIU should support all the required inputs and outputs for a single bay in a single unit, as in Figure 1. The proposed goal of the PIU is illustrated in Figure 2: the SV and GOOSE messages from the PIU interface to any device in the substation, supplying the appropriate data at the appropriate data rates for the applications. CPCs are the centralized protection and control scheme. The revenue meter normally is an independent approved meter device used by the utility for the consumer revenue purpose. The Digital Fault Recorder and Power Quality Meter can work on the same process bus network for monitoring purposes.

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