Guidebook for Detecting and Mitigating Low-Level DC Leakage and Fault Currents in Transit Systems (2020)

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2 Nonintrusive Injection-Based Techniques The nonintrusive active clamp approach involves injecting a high frequency signal into the power line and determining the high frequency impedance of the selected line segment. Segmentation of the line is achieved by applying a virtual blocker at each end of the chosen segment, typically at the traction power substation. The high frequency imped- ance of that segment is thus linked to the condition of the cable and the level of the leakage current in the segment. A block diagram of the active clamp sensor concept is presented in Figure 1. The active clamp sensor has three main elements to measure the high frequency impedance of the selected segment in the transit system: • A high frequency signal injector (to inject the high frequency voltage into the system); • A current sensor that measures the current induced into the system to determine the high frequency impedance; and • A blocking mechanism to block the high frequency current from entering the train or other stations. Figure 2 presents the components of the active clamp sensor. The sensor can work as either the injector or the blocker. To control the injector, a microcontroller was programmed to process the measured injected voltage and current signal to determine the magnitude and the phase of the high frequency impedance of the line. The proposed method is designed to be nonintrusive and to work with live high voltage overhead lines. The output from this sensor can serve as a direct tripping signal for the circuit breaker or as a warning signal for the maintenance team. The complete system is shown in Figure 3. A unit similar to the injector is used as a blocker to block the injected high frequency signal far away from the injector. The blocker injects a signal with a 180-degree phase shift to bring the impedance level of the segment to infinity at high frequency. To operate the system wirelessly, the system was powered by a 12V rechargeable battery (TURNIGY 2.2), while two wireless ZigBee communication modules were used to control and receive data from the board, as shown in Figure 4. One ZigBee module is connected to the controller board in the injector side and configured to be a coordinator node that can send and receive data from all the modules using the same address. The other ZigBee module is connected to a laptop using a USB port and configured to be a router node to send data only to the coordinator. C H A P T E R 2

Nonintrusive Injection-Based Techniques 3 High voltage DC bus Blocking signalBlocking signal Injected signal L i-1 L i Rail InsulatorInsulator Si-1 Si+1Si Figure 1. High frequency impedance detection for DC railway system. Figure 2. Components of the active clamp sensor.

4 Guidebook for Detecting and Mitigating Low-Level DC Leakage and Fault Currents in Transit Systems 2.1 Segmentation of the DC Electric Network The maximum length of the network allowed to be monitored is as follows: • The nonintrusive injection-based sensor can measure impedances down to 2 Ω, which is equiv- alent to a line capacitance of 0.795 μF at 100 kHz. The recommended maximum line is 10 km, considering the transmission line modeling at high frequencies. • The maximum impedance measurable is 45 kΩ. The recommended minimum line is 10 m, considering the transmission line modeling at high frequencies. Beyond this limit, the picked up noise will dominate and affect the measurement accuracy significantly. Figure 3. Complete system with the printed circuit board (PCB) connected to the microcontroller and the clamp-on voltage and current transformer. Figure 4. The ZigBee wireless module and the battery.

Nonintrusive Injection-Based Techniques 5 Two factors must be considered in determining the maximum length of the network allowed to be monitored: 1. Electrical railway structure. To allow maintenance of the overhead line without having to power off the entire system, the overhead line system is divided into electrically isolated portions known as sections. The transition from section to section is known as a section break. It is set up to ensure the electric transit vehicles’ pantograph is in continuous contact with the wire to the extent possible and to keep costs down. 2. Leakage current detection. The length of the monitored section depends on the sensor range and sensitivity. The injected signal produced from the sensor should be strong enough to monitor the entire section with the required sensitivity to identify the location of the leakage current. The sensor monitors the high frequency impedance of the system to detect any leakage currents, and this high frequency impedance depends mainly on the capacitance between the power line and the ground. Therefore, the magnitude of the impedance decreases when the monitored area increases, and it becomes easier for the sensor to measure the impedance. In another words, the length of the monitored area should be long enough. The proposed sensor is nonintrusive and uses transformers to inject the high frequency monitoring signal into the system. The transformer must be designed with the constraints posed by the application in mind, namely, the potential for magnetic saturation as a result of the large DC current in the power lines. 2.2 Frequency Range Allowed to Be Injected into the Line Frequency limits. Depending on the frequencies used for the communication system, the frequency of the injected signal should not interfere with the electric transit signals or the communication system. The industrial electronics used in building the system have a bandwidth of 1 MHz minimum, and the filtering chain is designed with a bandwidth of up to 500 kHz, so the maximum frequency limit is 500 kHz, and the minimum frequency limit is 1.5 kHz. Modulation schemes for the injected signals. The signal injected into the line has an adaptive amplitude and a fixed frequency. The modulation scheme in the sensor has variable phase shift, DC bias, and amplitude depending on the conditions of the line. The signal processing chain is presented in Figure 5. The cut off frequencies of the high pass filter are chosen to eliminate the DC component from the measurement. If there are any AC components in the traction system, the filter cut off frequency should be adjusted to eliminate that frequency also. 2.3 Signal Level That Can Be Injected into the Line Voltage amplitude limits. The injector can inject a voltage of up to 30 VAC peak. The exist- ing design is set to inject a maximum of 1.25 VAC peak into the line. The applied voltage sets the current flowing through the line. The maximum value of the injected current will dictate the resolution of the measurement. Current amplitude limits. The injector can inject a current of up to 3 A continuous, 5 A peak. The maximum power that can be injected is 90 W. The existing sensor has a 40 W power supply, 37 W of which would be used in the injection process. The level of injection should not interfere with the electric system of the transit system. Thus, injection of voltage and current varies accordingly. Also, injected amplitudes should not be harmful to the outside environment.

6 Guidebook for Detecting and Mitigating Low-Level DC Leakage and Fault Currents in Transit Systems For the sensor design, the magnitude of the high frequency voltage needed to be induced on the power line is the determining factor of the design. This value, in conjunction with the desired maximum measurable line impedance, determines the minimum high frequency current level that should be detectable by the current-sensing transformer. Thus, the magnitude of the induced voltage has to be large enough to provide a measurable current. The magnitude of the high frequency voltage to provide the minimum measurable high frequency current on the power line will be as follows: 2V I Zmin max= where, Imin is the minimum current that can be sensed by the current sensor and Zmax is the maximum impedance of the power line section. Since the impedance value and the secondary winding turns are fixed values, the number of turns in the primary of the transformer has to be as small as possible to keep the induced voltage large. Then the number of turns in the primary (injecting) side is determined according to the following equation: 2 1 1 V V N = where, V1 is the primary voltage, V2 is the secondary voltage, and N1 is the number of turns in the primary. Choosing a low number of turns has disadvantages, like increased leakage currents and very low primary impedance. Due to the low primary impedance, the primary current will increase significantly to provide the same flux. This is limited by the current driver IC’s ability to produce high frequency signals because it has a limited current capacity. 2.4 Environmental Conditions for the Sensor Weight. Sensor weight (electronics, battery, and cores) < 8 lb in the first prototype version. Sensor weight (electronics, battery, and cores) < 4 lb in the finalized package. Temperature. The system operates on the industrial temperature range −40°C to 85°C. The operating conditions will be outside (open air); therefore, any sensor installation on the line would need to tolerate a maximum of 50°C. The sensor complies with the EN50155 standard with class TX (extended temperature range) category. Figure 5. Signal processing chain for the injector. High Pass Filter PGA Low Pass Filter Sampled intoMicrocontroller 50 MHz Crystal Digitally Controlled Osc. µ Controller Analog Multiplier µ Controller VT or CT Pre- amp

Nonintrusive Injection-Based Techniques 7 Humidity. The sensor needs to comply with the EN50155 standard with the requirement of 2 × 25 hr at 40°C. Tension. The tension on the railway lines typically varies between 9 kN and 20 kN (2,000 lbf and 4,500 lbf) per wire. The sensor’s weight of 4 lb produces no significant tension on the line. 2.5 Power Consumption for the Sensors and Communication Channels for the Sensors The following table details the power budget in the sensor: Device Power requirement Communication • Transmission 0.82 W • Reception 0.33 W • Idle 0.15 W Controller • Full capacity 3 W Injector and sensor • Running Up to 37 W • Idle 3 W Supply 12 VDC 2.6 Decisionmaking The sensor information can be used to initiate a direct tripping for the circuit breaker or just a warning signal for the supervisory control and data acquisition (SCADA) system for the operators. 2.7 Wireless Communications To make the system wireless, the system was powered by a 12V rechargeable battery (TURNIGY 2.2). Two wireless ZigBee modules were used for controlling and receiving data from the sensor. The Xbee modules were Xbee S1 802.15.4 with a range of 100 ft/300 ft for indoor/outdoor, respectively, with a maximum data rate of 250 kbit/s, at 2.4 GHz amateur band. For a higher range of communication, a Global System for Mobile Communications (GSM) or another Wi-Fi module like LoRaWan can be used.