**3.2 Application of a POF-based current sensor for measuring leakage current in 500 kV transmission line**

#### **3.2.1 Introduction**

The leakage current of insulators in a high voltage transmission line is due to the increasing conductive deposited yielded by environment pollution. The more common pollution is the salt-spray produced by winds in areas close to sea shore. The salt deposit on the insulator surface offers to the electrical current alternative paths to the ground, thus connecting the high voltage to ground potential. Although this current is only a few tens of milliamperes, when multiplied by the total number of insulators located in that particular transmission line, the total leakage current can reach so high values that can trigger over current protection devices leading to electrical power line interruption. When an electric arc occurs,

Optical Fiber Sensors 15

The method proposed to monitor the leakage current of high-voltage circuits is based on the fact that all leakage current flows through all insulator sectors (see Fig. 3.2.1). Therefore if we measure the current from the last section of the insulator to ground, we are actually measuring the leakage current and with the sensor connected at a low voltage level. Fig. 3.2.2 shows the location where the control and transmission box is installed and the location where the sensor

(a) (b)

Fig. 3.2.2. The arrows show the location where the control and transmission box is installed

The leakage current supplies energy enough to optically power a high efficient blue-green-LED (Marcou, 1997). The leakage current modulates the LED output power that is transmitted to the receptor by means of a specially protected POF. The emitted light from the LED is amplitude and wavelength modulated. In this work it was applied only

The optical signal is detected by the receptor circuit, inside the Remote Station that performs the half cycle demodulation. Fig. 3.2.3 shows pictures of the transducer which was designed to be connect to the test-insulator. Calibration and measurement procedures of leakage waveforms were made under controlled laboratory conditions by a salt-spray chamber

is installed at the insulator chain of a 500 kV transmission line tower.

and the location where the sensor is installed.

amplitude modulation/demodulation technique.

Fig. 3.2.3. Picture of the POF-based leakage current sensor

aiming to simulate the real conditions of the field.

**3.2.3 The transducer** 

it is called "flashover", leading occasionally to the destruction of the whole insulator [Kanashiro and Burani, 1996].

An optoelectronic sensor for real time leakage current monitoring of high-voltage (500kV) power lines insulators was developed as a continuation of a study presented on the 2003 POF Conference (Werneck et al, 2003). The leakage current drives an ultra-bright green LED. The optically intensity-encoded data measurement is coupled to a POF and transmitted from the high potential to ground potential. After demodulation, the RMS value of the leakage current and waveforms are sent to a remote station using a GPRS code in a cell-phone platform. This case study shows the sensing system, the telemetry technology and the results obtained.

The continuous monitoring of leakage current levels yields parameters to establish an operational preventive strategy of cleaning or substitution of insulators placed inside the area with favorable conditions to flashover occurrence.

Usually, in order to monitor leakage current, it is applied a resistor (Briggs, 1976; Amarh, 2001) or an induction coil (Maraio, 1992). However, these methods need the connection to potential, allowing electromagnetic interference and risks to personnel. In order to circumvent this problem it has been proposed the development of an optoelectronic system using plastic optical fiber (POF) technology. This method has some advantages, such as it does not suffer electromagnetic interference effect; it is of low cost, light weight and does not need power supply to work.

With this system in operational status it will bring benefits such as energy losses monitoring and increasing the grid reliability. It also promotes optimization of insulators washing causing therefore a reduction of maintenance cost.

#### **3.2.2 System description and methods**

The sensor previously developed and presented in the 2003 POF Conference was used for 13.8 kV whereas this same technology is now used for a 500 kV transmission line in the city of São Luis, State of Maranhão, Brazil. Fig. 3.2.1 shows the schematic diagram of the system.

Fig. 3.2.1. Schematic diagram of the system

The main problem is the insulation of the whole system, since at this so high voltage, nearly everything conducts, including POFs. Therefore, we should place the sensor at or near ground potential.

it is called "flashover", leading occasionally to the destruction of the whole insulator

An optoelectronic sensor for real time leakage current monitoring of high-voltage (500kV) power lines insulators was developed as a continuation of a study presented on the 2003 POF Conference (Werneck et al, 2003). The leakage current drives an ultra-bright green LED. The optically intensity-encoded data measurement is coupled to a POF and transmitted from the high potential to ground potential. After demodulation, the RMS value of the leakage current and waveforms are sent to a remote station using a GPRS code in a cell-phone platform. This case study shows the sensing system, the telemetry technology and the results obtained. The continuous monitoring of leakage current levels yields parameters to establish an operational preventive strategy of cleaning or substitution of insulators placed inside the

Usually, in order to monitor leakage current, it is applied a resistor (Briggs, 1976; Amarh, 2001) or an induction coil (Maraio, 1992). However, these methods need the connection to potential, allowing electromagnetic interference and risks to personnel. In order to circumvent this problem it has been proposed the development of an optoelectronic system using plastic optical fiber (POF) technology. This method has some advantages, such as it does not suffer electromagnetic interference effect; it is of low cost, light weight and does

With this system in operational status it will bring benefits such as energy losses monitoring and increasing the grid reliability. It also promotes optimization of insulators washing

The sensor previously developed and presented in the 2003 POF Conference was used for 13.8 kV whereas this same technology is now used for a 500 kV transmission line in the city of São Luis, State of Maranhão, Brazil. Fig. 3.2.1 shows the schematic diagram of the system.

The main problem is the insulation of the whole system, since at this so high voltage, nearly everything conducts, including POFs. Therefore, we should place the sensor at or near

[Kanashiro and Burani, 1996].

not need power supply to work.

area with favorable conditions to flashover occurrence.

causing therefore a reduction of maintenance cost.

**3.2.2 System description and methods** 

Fig. 3.2.1. Schematic diagram of the system

ground potential.

The method proposed to monitor the leakage current of high-voltage circuits is based on the fact that all leakage current flows through all insulator sectors (see Fig. 3.2.1). Therefore if we measure the current from the last section of the insulator to ground, we are actually measuring the leakage current and with the sensor connected at a low voltage level. Fig. 3.2.2 shows the location where the control and transmission box is installed and the location where the sensor is installed at the insulator chain of a 500 kV transmission line tower.

Fig. 3.2.2. The arrows show the location where the control and transmission box is installed and the location where the sensor is installed.
