Wireless Overcurrent Sensor RelayTeam 18 Zumbahlen, Rusev

Wireless Overcurrent Sensor Relay

Design ReviewSeptember 27, 2010ECE 445 Fall 2010

Team 18Ray ZumbahlenTsvetan Rusev

TA: Xiangyu Ding

Wireless Overcurrent Sensor RelayI. Objectives and GoalsThe title of our project is Wireless Overcurrent Sensory Relay. We will be working with S&C Electric Company (hereinafter S&C) in a collaborative effort to bring this system into industry to be used on their circuit switching line of products. This is part of the reason why we chose this project. It has a practical real-world application that has high chances of being implemented in an existing system to make the overall device more reliable and efficient, which is what engineering is about. The goals of our system are to accurately measuring current on a transmission line, and to relay this information wirelessly by sending a trip signal to a circuit breaker. Right now, S&Cs line of circuit switching devices does not monitor the current on a transmission line between a circuit switcher and a transformer. So the goal is to get this current measurement without making a physical connection to the transmission line and sending the current magnitude wirelessly to flag a trip signal at the circuit switcher depending on the value of the line current. Also, our system will be self-powered in the sense that it will use the current in the transmission line to power itself. It will need no source of external power. The device will have the ability to store power in order to successfully shut down after a trip signal is sent since then there would be no line current to harvest energy from. The high level goal is to have a functioning product at the end of the semester that S&C can use to implement this for a full three phase device.Device Features Wireless radio allows versatile placement of sensor Quick overcurrent signaling Stores energy to power down itself after a fault Real-time current monitoringDevice Benefits Overcurrent protection between circuit switcher and transformer Energy efficient via self-powering from the lines Low to no maintenance designII. DesignA. MSP430 Software Flow Chart Figure 1 Software Flow Chart

B. Schematics, Simulations, and Descriptions1. Crowbar CircuitThe purpose of the crowbar circuit is to protect the storage capacitor and the DC-Link from overvoltage conditions. During a fault, the voltage induced in the secondary of the CT may exceed the rated voltages of the capacitor and the DC-Link. Two reverse parallel SCRs are used to short the secondary of the CT when such a condition occurs. When the supply voltage exceeds the breakdown voltage of the Zener Diodes (D13 and D14) the SCRs are triggered reducing the voltage across load. Figure 2 - Crowbar Simulation Schematic

The circuit operation is evident from the simulation plots below in Figure 2. Figure 2 represents the voltage across the load and the current through the SCRs. In this simulation, the input voltage is below the Zener breakdown and the crowbar remains inactive; that is no current flows through the SCRs. The load sees input sinusoidal voltage equal to the input voltage. Under these conditions the storage capacitor will be charged to the input voltage (secondary of the CT).Figure 3 is a simulation of the same schematic. The input voltage however is now increased to above the Zener reverse breakdown. Every time the breakdown voltage is exceeded the SCRs are triggered and the CT is shorted. Thus the voltage across the load decreases to about 2-4V. Once triggered the SCRs remain active until a zero crossing of the current waveform. This allows the capacitor to charge even if the overvoltage condition continues indefinitely (which can only occur in case of communication malfunction and the circuit switcher remains closed).

Figure 3 - Crowbar Inactive Simulation Output

Figure 4 - Crowbar Active Simulation Output

2. Crowbar and StorageFigure 5 - Crowbar & Storage Simulation Schematic

Energy storage is needed to allow the device to remain powered during crowbar operation. A storage capacitor will provide smooth power to the electronics and allow the device to remain operational for a short amount of time after the circuit switcher has been triggered. Figure 4 describes how the storage capacitor is connected to the crowbar. A bridge rectifier is used to convert the AC input voltage. In this configuration the capacitor also provides filtering for the rectified AC signal.Below is a simulation plot describing how the overall power supply operates. It can be seen that the capacitor reaches rated voltage within 2 cycles. The red and purple traces represent the how the Zener breakdown operation and SCR triggering. Figure 6 - Crowbar Active & Storage

3. DC-Link The DC-Link draws power directly from the storage capacitor and converts it to adequate voltage to power the MSP430 microcontroller, XBee communications, and the Rogowski coil signal conditioning circuitry. It is based on the Supertex HV9961 LED driver and is a modified switching power supply. According to the manufacturer, the HV9961 is capable of operating in the range of 8-450V rectified AC and is capable of saucing 165mA. Given the voltage and current specifications, the DC-Link from Figure 5 should be able to handle power demands. Figure 6 - DC-Link Schematic

4. Op-Amps & MSP430The WOSR (Wireless Overcurrent Sensor Relay) shall have the capability to measure line current with 5% accuracy. In order to achieve that for the range currents specified (5-600A), two channel parallel signal amplification and filtering will be used. Channel one or the low current channel (as we will refer to it) will measure 5-60A and the channel 2 or the high channel will measure 60-600A. The gains for the two channels will be chosen according to the Rogowski coil specifications. The MSP430 is interfaced to the sensor circuit via two A/D channels. It will analyze the signals and make a decision to send or not to send a trip signal over the XBee radio. It will also measure and display in real time the current on the line.XBee transmitter receives serial data from the MSP430 and broadcasts it wirelessly. When a trip signal is sent, the XBee receiver will set a flag (light an LED for example) as an indication of the tripping. Figure 7 - Signal Conditioning & MSP430 Schematic

Below is a pSpice simulation schematic and output waveform of the op-amp circuit. The Rogowski coil is modeled as a transformer and its output is offset by 1.5V. This is necessary because it will allow the A/D converters to see the entire waveform (not just the positive half- cycle). The output waveform is what the MSP430 will see on its A/D channels. The gains and Rogowski coil output voltage for this simulation were chosen arbitrarily and will be adjusted to actual sensor output from testing.Figure 8 - Op-Amp Simulation Schematic

Figure 9 - Op-Amp Simulation Output

III. VerificationA. Performance Requirement The current sensing device shall be located on the Sensing Module and should be capable of sensing current from 5A through 600A symmetrical. The current sensing device should be able to withstand 10A of continuous load and a 600A fault current for 0.5 seconds. Magnitude accuracies of 5% or better will be implemented for the current sensing device. The maximum internal diameter of the current sensing device will be 2 and the total weight will be less than 10lbs. The power supply shall also be located on the Sensing Module. This device shall be able to harvest energy from currents greater than 5A in order to power up the processor. At 600A, the Sensing Module shall be able to measure current against a TCC curve and send a trip signal to the receiver module in less than 15 cycles. The self-powering supply shall provide power to all portions of the Sensing Module and keep supply voltage at 3.3V. The self-powering supply shall also provide enough energy to keep the Sensing Module fully functional after the fault occurs until proper shut down is completed. This however will not be utilized to power the Receiver Module. The Sensing Module shall utilize a low-power processor that is responsible for coordinating all of the signals and operations of the Sensing Module. For this, we have chosen to use the MSP430 FG4618. The Sensing Module shall include a means to communicate the trip command to the Receiver Module. Wireless communication shall be used, and the transmitter must be able to securely transmit the trip signal for a distance of 50 feet and operate at less than 150 mW. For this operation, we have chosen to implement the XBee radio since it gives us secure transmission that meets the above requirements seen in [1]. Once installed on the power line the device shall require no maintenance or calibrationB. Testing ProceduresMany of the requirements in our project were directed to us by S&C Electric. Although we are trying to implement the same general requirements, our system is much less involved than the overall system they intend to implement. Therefore, we modified some requirements stated in their internal company proposal [2] which we are not to disclose. However, we are referencing that document for many requirements listed below.1. SensingWe will first test the sensor (Rogowski Coil) by putting current through it and recording the output voltage. We will be using the Grainger Power Lab in the basement of Everitt Lab. Since we are not able to provide 600A in the lab, we plan to use 12 gauge wire and push 15 A through it. To emulate the 600A we need to test, we will feed the wire back through the coil 40 times. We will investigate the current vs. voltage relationship and calculate the voltage per ampere ratio of the sensor. Then we will have to test the sensor with the amplifiers as well. This is because we have a large range of current we need to sense and, the amplifiers will have different gains to put the sensor output signal in a preferred range. We also must carefully test the noise in the signal from the coming from the power supply. We do not have the Rogowski coil yet, so it is impossible to say what the gains of our amplifier need to be. However, according to the TAEHWATRANS INC Rogowski Coil data sheet, the output voltage will be between 16.72mV and 17.22uV as stated in [3].

2. Power SupplyTo begin this testing, we are running simulations to get an idea of what our power supply will be doing. Then, the next step will be similar to the Rogowski coil testing. We need to test our CT at various currents (0A to 15A) to ensure the CT core does not saturate. Again, using 15A and 40 turns on the primary, we can assume it will operate as if there were 600A on one primary turn. We will also test that the voltage supplied to the ICs remains 3.3V. Also, we will make sure that the power supply can charge up the energy storage capacitor to its max charge quickly.3. Wireless CapabilityFor the wireless capabilities, we will use the XBee devices. The requirement for this module is that it must transmit at least a distance of 50 ft. line of sight. However, we will ensure that the device can transmit at distances greater than this. How we will test this functionality is self-explanatory. We will also test that the noise coming from the transmission line will not prevent successful XBee transmission. For this, we will simply place the XBee transmitter as close to the power supply as possible with high current on the line and perform a transmission. As far as modular testing goes, we will use the processor to simulate sensor readings ranging from 5A to greater than 600A and ensuring that the XBee will transmit, receive, and relay the trip signal. The XBee should be a viable source for this module as the data sheet states it has the ability to transmit 100 ft. outdoors (line of sight) which is more than enough distance for our system. It also uses less than 150 mW of power also stated in [1].4. Measurement AccuracyWe require that the measurement accuracy be within plus or minus 5% at all times. For this, we will take the reading from the LCD display on the Experimenters Board and compare it to a measured value on the line using a multi-meter. 5. Successful FlaggingEach time we put more than 600A on our line, we will ensure that we successfully transmit and visually display (on an LED) a trip signal flag.6. System Upon completing testing on the modular components, we will test the system at a higher level as a whole. To do this, we will test the system at various levels of line current. We will check a variety of currents under 600 A (actually 15 A) and measure the accuracy of our sensing system. Also, we will put very close to 600 A through the line to ensure the system does not send the trip signal prematurely. Finally, we will test at greater than 600 A to be sure the trip signal will indeed be sent wirelessly to the receive unit and light up our LED.C. Tolerance AnalysisThe most important component that will determine the performance of our project is the Rogowski coil current sensor. Although it is wound in a way to minimize EM interference it will still be affected by noise. Identifying noises and providing adequate shielding and filtering of the signal is a key to the success of this project. Minimizing noise picked up by the current sensor will allow for higher measurement precession and faster operation. Precision of 5% or less of the actual current will suffice. Temperature will also be a factor as it might affect the sensors electrical properties.IV. Ethical ConsiderationsIEEE item number 7 in the IEEE Code of Ethics [4] says we shall seek, accept, and offer honest criticism of technical work, to acknowledge and correct errors, and to credit properly the contributions of others. Since we are developing this product in a collaborative effort with S&C Electric, it will be important for us to give appropriate credit for the aid and direction we receive from S&C Electric. Also, at the beginning of the project, we signed a non-disclosure agreement with the company. This is so that we do not use this idea and/or technology for our own personal financial gain. That is, we are not to sell this idea or any ideas presented by S&C Electric to any other organizations. The idea is property of S&C Electric so it is not ours to do whatever we please. So, the largest ethical consideration we have to make is when it is regarding disclosing too much or the wrong information to anyone that could use it for their personal gain. Other than this, there are not too many ethical issues regarding the actual use or functionality of the device.

V. Cost and ScheduleA. Cost Analysis Parts in PossessionPart NamePurposePriceQuantityTotal

Current TransformerHarvest energy from line$90.001$90.00

MSP 430 FG4618Take, process, and output data$9.671$9.67

XBee RadioWireless Communications$19.002$38.00

LEDTrip Signal Flag$0.151$0.15

Subtotal =$38.15

Parts on Hand in Everitt LabPart NamePurposePriceQuantityTotal

10 AWG WireSimulate transmission line$0.50/ft20$10.00

Op AmpsBoost sensor output voltage$0.614$2.44

.1 uF cap-$0.151$0.15

1 uF cap-$0.154$0.60

10 uF cap-$0.201$0.20

100 uF cap-$0.201$0.20

500 uF cap-$0.201$0.20

diode (1N4004)-$0.109$0.90

Zener diode (1N4730A)-$0.153$0.45

MUR160 diode-$0.201$0.20

1k resistor-$0.1012$1.20

3 M resistor-$0.201$0.20

IRFU320 regulatorVoltage Regulator$4.041$4.04

Subtotal =$22.58

Parts OrderedPart NamePurposePriceQuantityTotal

Rogowski CoilSensor current on line$150.001$150.00

SCR (30TPS08)Crowbar$3.012$12.04

HV9961 LED drivermay use ICM7215IPG$2.241$2.24

Subtotal =$164.28

Parts TotalDescriptionSub-Totals

Parts in Possession$38.15

Parts on hand in Everitt Lab$22.58

Parts Ordered$164.28

Grand Total =$225.01

B. Schedule Detailed Schedule for Remaining WeeksWeek #RayTsvetan

1 (Sept 6)- Meet with S&C to discuss project - Research parts- Meet with S&C to discuss project - Research parts

2 (Sept 13)- Develop project proposal- Choose processor, transceiver, sensor- Develop project proposal

3 (Sept20)- Order Rogowski Coil- Prep for Design Review- Determine MSP430/XBee connections- Design and simulate power circuitry- Prep for design review

4 (Sept 27)- Conduct personal and peer review- Test wireless functionality- Begin programming processor- Conduct personal and peer review- Continue circuit simulations- Begin CT testing

5 (Oct 4)- XBee limitation tests (distance/noise)- Help Steve characterize Rogowski Coil- Build and test power supply circuitry- Begin characterizing Rogowski Coil

6 (Oct 11)- Begin MSP430/XBee integration- Continue programming processor- Aid in sensor/op-amp tests- Continue power supply testing- Continue heavy sensor/op-amp testing

7 (Oct 18)- Begin sensor output/MSP430/XBee integration- Continue XBee radio limitation testing- Test IC supply voltages- Test over voltage protection (crowbar)- Begin op-amp optimization work

8 (Oct 25)- More sensor/MSP430/XBee integration- Begin current reading accuracy testing- Prepare individual progress reports- Continue same power supply tests- Test power supply cut-off and capacitor energy discharge on overcurrent

9 (Nov 1)- Request 1st version of PCB- Optimize processor programming- Request 1st version of PCB- Continue same tests as week 8

10 (Nov 8)- Test and revise 1st PCB- Request 2nd PCB at end of week- Test and revise 1st PCB- Prepare mock-up demo

11 (Nov 15)- Test and verify 2nd PCBs operation- Mock-up presentations- Test and verify 2nd PCBs operation- Mock-up presentations

12 (Nov 22)- Thanksgiving Break - Prepare presentations- Thanksgiving Break - Prepare presentations

13 (Nov 29)- Demonstrate project and present- Prepare final report- Demonstrate project and present- Prepare final report

14 (Dec 6)- Submit Final Report and checkout- Submit Final Report and checkout

VI. References[1]Digi International, IEEE 802.15.4 RF Modules, XBee/XBee-PRO RF Modules Datasheet, November 23, 2009

[2]A Montenegro, Requirements Specification for the Self-Powered Over-Current Relay Fault Detection & Signal System, S&C Electric, Inc, Oswego, IL, USA, Tech. Report. 17 Aug. 2010.

[3]TAEHWATRANS INC, Rogowski Coil Datasheet, Rogowski Coil, March 27, 2009

[4]IEEE, "IEEE - IEEE Code of Ethics." IEEE Board of Directors, Feb. 2006. [Online]. Available: http://www.ieee.org/portal/pages/iportals/aboutus/ethics/code.html [Accessed: 25 Sept. 2010].

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