Monthly Archives: January 2018

What are Numerical Protection Relays?

Numerical protection relays protect power transformers and distribution systems from various types of faults. For power transformers, these faults include protection from distance, line differential, pilot wire, low-impedance busbar, high-impedance differential, frequency, voltage, failure of circuit breaker, auto reclosing, and synchronism faults. For power distribution systems, these faults include protection from overcurrent, under or overvoltage, directional overcurrent’s, and feeder manager relay faults.

Numerical protection relays are digital systems in constant communication with substation automation systems through menu-driven interfaces. They have configurable binary inputs, outputs, and programmable logic. They monitor, measure, and record electrical values, fault and disturbances, and events. Numerical protection relays feature high-speed operation and multi-functionality, offering improved selectivity and stability. As they detect faults with automatic supervision, they bring high reliability to power systems, while at the same time being compact in size and consuming very low power.

Numerical protection relays have a multiple microprocessor design. Each microprocessor within the relay performs software functions such as executing protection algorithms and scheme logic, processing signals from sensors, controlling output relays, and handling the human interface.

The relay handles several analog inputs such as phase control inputs, phase voltage inputs, and residual current inputs. Depending on the type of relay, the number of analog inputs may vary.

Internal auxiliary transformers isolate the electronics from the high voltage on the system—isolating, generating step down voltages, and conditioning the inputs from the voltage and current transformers. Analog to digital converters transform these analog signals into digital data, which the microprocessors can process further.

The front panel of a numerical protection relay is a liquid crystal display (LCD) along with pushbutton keys providing local access to the relay menu. Light emitting diodes (LEDs) on the panel visually indicate the present status of the relay.

Three types of communication ports are available on a typical numerical protection relay-an RS232C port for locally connecting to a PC, an RS485 port for connecting to a remote PC, and an IRIC-B port for connecting an external clock.

The LCD exhibits information the relay is measuring continuously and simultaneously displays the same on the local PC, and the remote PC when connected. For instance, this information shows several voltages and currents such as phase, phase-to-phase, their symmetrical components, frequency, and active and reactive power. The type of relay defines the parameters it will measure and display. Users can monitor locally as well as remotely the element output of the relay and input/out binary values.

Within the relay, the software program records several events such as tripping operations, alarms, change of relay settings, change of state of each binary input/output, and failure detected by automatic supervision. Typically, the relay stores each time-tagged event with a 1 ms resolution. The user may define additional events for the system to record.

Apart from providing a date and time for tagging of records, a numerical protection relay records faults initiated by a relay trip and logs data such as date and time of the trip operation, operating phase, protection scheme that triggered the trip, and measured current data. The relay stores the eight most recent faults, time-tagged to 1 ms resolution.

How are Transformers Protected in the Field?

For maintaining a power grid in continuous working order, power transformers play a critical part. As repair and/or replacement of components in a power grid typically has a long lead time, protection from faults has to limit the damage to a faulted transformer. Moreover, transformer faults need quick prevention, and certain protection features identify operating conditions that could cause a failure of the transformer. This includes over-excitation protection and temperature-based protection.

Classification of transformer failure is as follows:

  • Failure in windings due to short circuits—this includes turn-to-turn shorts, phase-to-phase shorts, phase-to-ground shorts, and open windings
  • Faults in the Core—this includes failure of core insulation, and lamination shorts
  • Failure of Terminals—this includes open leads, short circuits, and loose connections
  • Failures of On-Load Tap Changer—this includes electrical and mechanical failures, short circuits, and overheating

Utility and industry power distribution networks utilizing power transformers typically install protection relays for the supervision, protection, control, and measurement of different parameters of power transformers, step-up and unit transformers, and power generator-transformers as well.

Transformer relays provide a flexible protection scheme for power transformers with two windings. They limit the damage to a transformer that has a fault and may identify operating conditions that could cause a devastating transformer or grid failure. Relay protection features include thermal overload protection, differential protection, voltage protection, and automatic voltage regulation. Some relays also have configurable functionality for meeting specific requirements of various applications.

For instance, the transformer protection and control relay, RET615 from ABB, conforms to IEC standards and offers a compact and versatile solution for industrial and utility power distribution systems.

A dedicated protection and control relay, the RET615 offers supervision, protection, control, and measurement of power transformers. It offers several benefits such as a compact and versatile solution, while integrating supervision, monitoring, control, and protection is one single unit.

RET615 offers an extended range of control and protection functionality for power transformers with two windings. It provides the transformer high inrush stability, while offering fast and advanced differential protection.

Setting up and tailoring the RET615 protection and control relay is simple and easy because it has ready-made configurations that match the most commonly used vector groups. This includes swift installation and testing, thanks to its withdrawable plug-in unit.

The RET615 has a large graphical display that shows the customizable SLDs. Users have the choice of accessing the SLDs directly on the display or via a web browser human machine interface that is simple and easy to use.

Along with measurement facility, RET615 also offers voltage and differential protection. It supports several neutral earthing options, including the restricted earth-fault principles of numerical low-impedance or high-impedance. The relay offers high-speed outputs for optional arc protection.

RET615 conforms to IEC 61850 Editions 1 & 2 standards, which include PRP and HSR, and GOOSE messaging. It follows IEC 61850-9-2 LE standard for supervised communication and less wiring.

Time synchronization is highly accurate as the RET615 conforms to IEEE 1588 V2, offering maximum benefit of Ethernet communication at substation level. In addition, RET615 supports DNP3, Modbus, and IEC60870-5-103 protocols for communication.

The PiServer for the Raspberry Pi

If you were running an institution teaching computer programming to kids using Raspberry Pis (RBPis), then you would normally spend some time updating numerous RBPis with the latest Raspbian and copying over several files for the class. You can save a lot of time using the PiServer, and do away with the SD cards at the same time.

The PiServer is a new piece of software tool that can easily set up a network of client RBPis connected to a single x86-based computer acting as the server. The various RBPis connect over Ethernet, and do not need their SD cards to boot. The server can control all its clients, allowing addition and configuration of user accounts. This provides an ideal setting for the classroom, within the home, or even an industrial setting.

To recall the terminology, the server is the computer providing the boot files, the file system, and authenticates the password of the clients. The clients are several computers that communicate with the server to retrieve the boot files, and the file system from the server. Although several clients connect to one server, they share the same file system. A user, with a unique combination of a username and password, can log into a client system. Once logged in, the user can access the file system on the server. The user may log in from any client system using their credentials, but will always see the server and the same file system. As the system does not give sudo capability to any user on a client, users are unable to make significant changes to the software and its file system.

All client RBPis use the PXE or network booting, and therefore, do not require any SD card to boot. The advantages of this type of booting are a large number of clients can boot off a single server, which treats all clients as the same. Additionally, as the server runs on a regular x86 system, it offers higher performance, network speed, and disk speed.

Without the PiServer, creating such a network would involve a lot of work, setting up the required FTP and DHCP servers, and making them interact seamlessly with other components on the network. The entire network is prone to breakdown with a single error. The PiServer takes care of all the intricacies, and has automatic functionalities.

For instance, PiServer can automatically detect any RBPi trying to boot via the network, and locate its Ethernet address. PiServer also sets up a DHCP server, to act as a router to provide an IP address to each client, whether in proxy mode or in full IP mode. For the safety of the network, the DHCP server replies only to those RBPis you have specified.

The PiServer also has the task to create usernames and passwords on the server. Therefore, in the classroom, the teacher can set up all the users beforehand. This allows each user to log in individually and keep all their work separately in the central location. The PiServer uses a somewhat altered Raspbian build, which has the LDAP enabled.