Intelligent Electronic Devices in today’s FRTU

This article describes the various functions that IED (Intelligent Electronic Device) plays in today’s power networks.

Electric energy is a commodity, that like any other is bought under a contract between a customer and utility. The customer expects a high-quality power at a negotiable price. So growing customer expectations and new regulations, force the utilities to manage their grids in an intelligent way. To do that, the information has to be collected from the grid and processed in real time in order to enable automatic optimisation of the power delivery efficiency for demand and supply side management. The monitoring system consists of transducers, sensors and Intelligent Electronic Devices (IEDs), located throughout the distribution system, that gather the data, process it as necessary and transmit to control centres.

IEDs have been deployed extensively in power automation systems recently, and the shift from RTUs to IEDs is evident due to the integration and long-distance communication features of the IEDs. This technical article briefly explains the IED advanced functionality to present view of automation in power systems. In addition, the IED handles additional features like self and external circuit monitoring, real-time synchronisation of the event monitoring, local and substation data access, and an entire range of software tools for commissioning, testing, event reporting, and fault analysis.

Metering and Power Quality Analysis
Metering capabilities of the IEDs became acceptable to the power utilities quickly, and major cost saving was achieved by combining the non-revenue metering function into the IEDs. It may be noted that the primary CTs and VTs for protection purposes may not be accurate enough for normal current measurement for revenue metering. The normal metering functions include measuring the voltage and current root mean square (RMS) values and the real and reactive power. Hence for revenue metering we need a highly precise measured value with high 0.2s or 0.1 accuracy class current transformer.

Load profiling is another metering activity that can be achieved using Rishabh Load Manager LM1360 IEDs where the power factor profile, ampere demand, long-term RMS voltage value, harmonics, THD and phase sequence, can be monitored and can be used for load profiling along with data logging facility. Rishabh’s Power Quality Analyser eliminates the digital recorders because waveform recording during a voltage dip, voltage swells, inrush current measurement, distorted waveform can be performed by the IEDs, whereas the electro-mechanical relays as well as Normal Energy Meters did not have such capability. Event reporting can be easily done by RISH PQA IEDs eliminating sequence of events (SOE) recorders.

Ring Main Unit (RMU)
RMU is a compact, sealed for life metal enclosed switch gear widely used in Urban Power Distribution Network. A RMU includes a combination of one or more Load Break Switch (LBS) cum Earth Switch as incomer and outgoing feeder and Vacuum Circuit Breaker with associated Disconnector and Earth Switch for load feeders.

Depending on the requirement, RMU is available in different voltage ratings (mainly in medium voltage level) and are suitable for both indoor and outdoor installation. The RMU is an innovative solution that makes it easier to manage the numerous challenges of electrical distribution. RMU is an all-in-one solution and is safe, easy to install and maintenance free switch gear helping the utilities improve reliability and uptime of the network and reduce the operational costs.

Feature-Rich Compact Designs: RMU is an SF6 insulated compact Switch gear equipped with SF6 Switch disconnector and vacuum circuit breaker. Its compact design requires least possible space to install and operate.

Time and Cost Savings: RMU is a complete switchgear and is easy to install, saving installation and commissioning time. Independent of climate and resistant to environmental conditions, the operation and maintenance costs of these units are also low.

Smart Capabilities: When fitted with IEDs and Communication capabilities the RMU is easy to integrate with Distribution Management System and in building a Smart Grid solution.

In the modern urban power distribution system, RMU is widely used across India and around the world to efficiently meet the growing needs of reliable energy. It’s one solution with comprehensive capabilities.

Feeder Remote Terminal Unit (FRTU)
The FRTU system will monitor all the substation equipment and will check all the parameters of incoming and outgoing feeders in real-time. It also monitors the status of circuit breakers, relays, isolators etc. that are prevalent in any substation. The overall system needs planning, execution, forecasting, implementation and maintenance etc. Energy supply is to be balanced with the load demand continuously under normal and abnormal situations such as loss of generation, equipment failures, line faults, peak loads, load shedding etc. in order to maintain system stability, reliability and availability. The application of FRTU leads to automatic data logging, substation level control, energy management, load forecasting etc. Eventually it will lead to sub-station automation.

FRTU Functions
Acquiring analog values from multifunction transducers or alternatively through direct signal modules and the status inputs of devices from the substation, processing and transmitting to master stations. Two nos. PT100 sensor inputs shall be taken in FRTU directly or through transducers to measure temperatures for distribution transformers connected to the RMU.

Use of MODBUS over RS485 interface, protocol to communicate with the MFTs and FRTU shall be capable of acquiring analog values through transducers having output as 4-20 mA, 0-10 mA, 0-+10 mA etc. using analog input modules.

Minimum number of I/O tags handling capacity shall be 100 I/O tags and 20,000 data points. FRTU have inbuilt optical coupler to isolate field signals for digital inputs FRTU shall monitor suitable feature of external battery availability check through potential free contact as DI signal to FRTU and suitable provision in FRTU to supervise and prevent accidental serious discharge of battery.

Parts Of FRTU
Analog Inputs (6/8 Input AI Card): The real time values like active power, reactive power, apparent power, three-phase current and voltage, and frequency, and power factor and accumulated values of import/export energy values will be acquired FRTU from the following in the given manner:
1. MFTs installed in RMU
2. RTU shall also take 4-20 mA as analog inputs to acquire DC battery voltage etc.

Control Outputs: The FRTU shall provide the capability for a master station to select and change the state of digital output points. These control outputs shall be galvanically isolated and used to control power system devices such as circuit breakers, Isolators, relays and other two-state devices, which shall be supported by the RTU. Switching voltage is 24 V DC. Each control output shall consist of one set of potential free no contact. The output contacts shall be rated for atleast 3 Amp continuous at 24 V DC.

Power Supply: The FRTU will be powered from a 24 V DC power supply system. The RTU shall not place additional ground on the input power source. The battery and battery charger should be capable of performing all FRTU required operation and provide reliable aux. supply to Modem, MFT and DC transducers etc.

FRTU Requirement: Nominal voltage of 24 VDC with variation between 20.4and 28.8V DC (+20%/-15%).

AC input supply: The nominal input frequency is 50 Hz, which may vary from 47.5-52.5Hz. The input voltage shall be single phase (nominal 240V) varying from 190V to 265V for power supply for DCPS.

DC Output Required: The module shall be capable of operating in “auto float-cum-boost charge” mode depending on the condition of the battery sets being sensed by the control unit. The float voltage shall be continuously adjustable and presetable at any value in the range of 24 to 28V (for 24V) either at the module or may be set from the common controller.

Transducer: All transducers use a 24 V DC auxiliary power supply as provided for the FRTU. All transducers shall have a maximum power consumption of 10 watts. Transducer shall be din rail or wall/plate mounted.

Each FRTU shall be provided with two nos. DC voltage transducers for monitoring DC battery voltage for RMU and FRTU battery.

The input, output and auxiliary circuits shall be isolated from each other and earth ground. The transducer output shall be underground and shall have short circuit and open circuit protection. The transducers shall comply the IEC 60688, without damage to the transducer.

DC Transducer
The DC transducer (DCT) is of two types: Voltage, and Current.
The DC Transducer are required to measure battery charger current and voltage shall be suitable for 20 per cent continuous over load and able to withstanding 20 times the normal current rating for a period of one second. The DCT shall be able to accept the input up to 120 per cent of the nominal voltage. The DCT shall have low VA burden. Output of the device shall preferably be 4-20mA with an accuracy of 0.5.

Multifunction Transducers (MFT): It requires multifunction transducers for acquiring the real time analog inputs through 3-phase 3-wire CT/PT circuits/ 3 phases 4 wire CT/PT circuits (Based on the field requirement). Based on the CT/PT secondary rating, the multifunction transducer shall be designed for nominal 110 V (Ph-Ph voltage) and 1A/5A (per phase current). The MFT shall be suitable for 20 per cent continuous over load and shall be able to withstanding 20 times the normal current rating for a period of one second.

The MFT shall be able to accept the input voltages up to 120 per cent of the nominal voltage. The MFT shall have low VA burden. MFTs shall be mounted in the interface cabinet to be supplied by the contractor/RMU as feasible.

Multifunction transducers shall provide at least phase voltage, phase current active/reactive power, import and export energy (active and reactive), Pf, frequency with class 0.5 accuracy or better. The parameters to be acquired from multifunction transducers shall be selectable. Multifunction transducers shall accept nominal 24V DC as auxiliary power supply. Multifunction transducer shall be provided with RS485 interface to communicate with RTU over Modbus protocol.

Each FRTU shall have two MFTs for monitoring load current and other parameters in each of the breaker unit of the 4-way RMUs.

Ganesh P. Tupe
Executive Sales Engineer,
Rishabh Instruments Pvt. Ltd.