What is Fault Indicator

A fault indicator is a device which provides visual or remote indication of a fault on the electric power system. Also called a faulted circuit indicator (FCI), the device is used in electric power distribution networks as a means of automatically detecting and identifying faults to reduce outage time.

Overhead indicators are used to visualize the occurrence of an electrical fault on an overhead electrical system. Underground indicators, locate faults on an underground system. Often these devices are located in an underground vault. Some fault indicators communicate back to a central location using radio or cellular signals.

Basic principles

Typically fault indicators sense magnetic field caused by current flows through a conductor or cable. Some of them also use measurement of electric field caused by voltage in conductor.

During an electrical fault on a grounded system, additional current flows through a conductor, inducing a magnetic field, which is detected up by the fault indicator causing a state change on the mechanical target flag, LED, or remote indication device. Ground fault indicators for ungrounded systems sense the vector sum of the current and look for an imbalance indicating a fault on one or more of the three phases.

Systems with earthing through high resistance have low phase-to-ground fault currents so require high sensitivity of FI. In insulated neutral systems and systems with earthing through a Petersen Coil, ground faults cannot be located with classical FI's at all. Capacitive current appears in overall faulted system so directional fault location devices are required. Some modern network protection systems e.g. GFN have time to clear a fault as small as possible down to 60ms so fault indicators must not only be highly sensitive and directional but additionally very fast.

High-voltage fuses commonly drop down after operating, making it obvious where the fault is.

Products and reference designs of Fault Indicator

As shown in the figure below, the Fault indicator consists of 9 parts, each part is Energy harvesting、Energy storage、Non-isolated DC/DC power supply、Analog front end、Digital processing、Input user interface、Wireless interface、Self-diagnostics/monitoring、Output user interface

Energy harvesting

Overhead fault indicators are mounted on medium voltage transmission lines. Hence, self-powering and long battery operating life are the critical system requirements. To meet this requirements energy is harvested using solar cells or load current using CT. Energy harvesting includes Boost converters (integrated switch)、Shunt voltage references, the corresponding products are LM2735、TPS61021A、TL431、LM4040

Energy storage

Rechargeable batteries or supercapacitors act as storage elements while a primary battery is used as backup for short durations. Auto switching the load between energy storage elements is required to avoid unwanted system shutdown and deep discharge of batteries. Energy storage includes Battery charger ICs、Ideal diode/ORing controllers, the corresponding products are BQ25505、LM74610-Q1、LM74700D-Q1

Non-isolated DC/DC power supply

The Fault Indicator needs a constant voltage for measuring voltage and current signals accurately. The varying voltage of battery and energy storage elements are regulated using LDOs, PMICs or DC/DC converters. Depending on power levels TI has solutions that can help to reduce power dissipation and size as well. Non-isolated DC/DC power supply includes Buck converters (integrated switch)、Linear & low-dropout (LDO) regulators, the corresponding products are TPS628301、TPS62826、TPS715、TPS73801

Analog front end

Level Shifting, filtering and amplification of the voltage and current signals can be performed by opamps in the analog front end, before interacting to the MCU to convert the signals into the digital domain. A DAC can be used to for self-diagnosis of the current and voltage measurement channels. Analog front end includes Precision op amps (Vos<1mV)、General-purpose op amps、Instrumentation amplifiers、Analog current-sense amplifiers、Precision DACs (≤10 MSPS)、Series voltage references、Analog switches & muxes, the corresponding products are LT1013D、OPA320、LM358B、INA211、DAC101C081、REF5040、SN74LVC1G3157

Input user interface

Fault reset manually or self-test is performed by placing a strong magnet near to fault indicator using hot-stick in field. TI offers industry-leading ultra low power hall switch sensor with digital output for this application. Input user interface includes Hall-effect latches & switches, the corresponding products are DRV5033、DRV5032

Digital processing

Processing of analog input data can be configured using the MCU to sample at regular intervals or with a specific interrupt. Internal or external comparator is used to generate the interrupt to wake up the MCU when the analog input values exceeds pre-set limit(Fault mode). TI low power MCU MSP430 can be configured to run in Low Power Mode (LPM) under normal conditions to reduce the power consumption. Digital processing includes MSP430 microcontrollers、Low-power 2.4-GHz products, the corresponding products are MSP430FR2476、MSP430FR5729、CC2340R5

Wireless interface

Advanced fault indicators communicate with data concentrators or RTUs and are monitored at a central substation. License free Sub -1 GHz or 2.4GHz RF bands can be used for low power, short distance communication. It helps in reducing the fault location identification time and improves system restoration. Wireless interface includes Sub-1 GHz wireless MCUs, the corresponding product is CC1312R

Self-diagnostics/monitoring

Since lithium based batteries are used in fault indicators, ambient temperature is critical for safe operation of these batteries. As such, it is necessary to monitor the temperature for safe and reliable operation continuously using the MCU. LED current is controlled for lower current consumption and better visibility based on ambient light intensity. A TI ambient light sensor can be interfaced to MCU over the I2C bus. Self-diagnostics/monitoring includes Light sensors、Analog temperature sensors、Digital temperature sensors、Supervisor & reset ICs, the corresponding products are OPT3001、TMP235、TMP175、LMS33460

Output user interface

LED drivers are used for intensity control based on ambient light and uniform 360 degree visibility. LED drivers with integrated temperature compensation can be configured and controlled by the MCU over an I2C interface. Output user interface includes RGB LED drivers, the corresponding products are TLC6C5912、LP5813