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Current measurement fundamentals (draft 8) (1)
- 1. 1 Copyright © Yokogawa Meters & Instruments Corporation Practical Current Measurement with a Precision Power Analyzer
- 2. 2 Copyright © Yokogawa Meters & Instruments Corporation Host Fundamentals of Power Measurement Kristin Porche Marketing Specialist Yokogawa Corporation of America Newnan, GA webinars@us.yokogawa.com 1-800-888-6400 ext. 5656 tmi.yokogawa.com
- 3. 3 Copyright © Yokogawa Meters & Instruments Corporation Yokogawa Webinar – Housekeeping Issues Fundamentals of Power Measurement PC’s Speakers - Audio Broadcast To hear the audio through your PC, select the Communicate Tab and Join the Audio Broadcast. Recorded Presentation A recording of this presentation will be posted under our technical library of our web page. Chat: Select and send all questions to “Panelist” during the Webinar presentation.
- 4. 4 Copyright © Yokogawa Meters & Instruments Corporation Presenter Fundamentals of Power Measurement Sam Shearman Product Manager III Yokogawa Corporation of America Newnan, GA
- 5. 5 Copyright © Yokogawa Meters & Instruments Corporation Agenda ■Relevance of current to power measurement ■Current sensors •Selection criteria •Sensor alternatives ■Coupling with your measurement devices •Tips and techniques
- 6. 6 Copyright © Yokogawa Meters & Instruments Corporation A Fundamental Component of Power Measurement ■Measurement of DC power requires current measurement ■DC power: Current Measurement Background 𝑷 𝑫𝑪 = 𝑽 𝑫𝑪 ∙ 𝑰 𝑫𝑪 = 𝑰 𝑫𝑪 𝟐 ∙ 𝑹 𝑰 𝑫𝑪 𝑹
- 7. 7 Copyright © Yokogawa Meters & Instruments Corporation A Fundamental Component of Power Measurement ■Measurement of AC power requires current measurement ■AC power: Current Measurement Background 𝑷 𝑨𝑪 = 𝑽 𝑹𝑴𝑺 ∙ 𝑰 𝑹𝑴𝑺 = 𝑰 𝑹𝑴𝑺 𝟐 ∙ 𝑹 (For a Resistive Load) 𝑷 𝑨𝑪 = 𝑽 𝑹𝑴𝑺 ∙ 𝑰 𝑹𝑴𝑺 ∙ 𝑷𝑭 (For a Reactive Load) = 𝑰 𝑹𝑴𝑺 𝟐 ∙ 𝑹 ∙ 𝑷𝑭
- 8. 8 Copyright © Yokogawa Meters & Instruments Corporation Selection Criteria for Current Sensors ■Current range ■Amplitude response ■Phase response ■Propagation delay ■Voltage isolation ■Common mode rejection ■Absolute accuracy
- 9. 9 Copyright © Yokogawa Meters & Instruments Corporation Selection Criteria for Current Sensors (2) ■Zero offset ■Stability over temperature ■Stability over time ■Hysteresis ■Insertion impedance ■Magnetic sensitivity ■dV/dt sensitivity ■Sensor output matches up with power analyzer input ■Cost ■Size ■Ease of installation dV/dt sensitivity
- 10. 10 Copyright © Yokogawa Meters & Instruments Corporation Direct / Indirect Current Sensing ■Direct Sensing •Based on Ohm’s Law •Example - Current Shunt ■Indirect Sensing •Based on Ampere’s Law, Faraday’s Law, and/or Faraday Effect •Isolated from the Load •Examples - Current Transformer - Rogowski Coil - Hall Effect - Flux Gate 𝑰𝒊𝒏 𝑩
- 11. 11 Copyright © Yokogawa Meters & Instruments Corporation Current Shunts ■Low-resistance device usually placed in series with the circuit with current to measure. ■Measuring the voltage across the shunt and knowing the shunt resistance, you can calculate the current flowing through the shunt.
- 12. 12 Copyright © Yokogawa Meters & Instruments Corporation Current Shunt Pros and Cons ■Simple working principle ■High accuracy ■Passive—no power supply needed ■Measures AC & DC ■Convenient—Built-in to Power Analyzer ■Insertion Loss ■Not isolated ■Voltage output—Susceptible to noise ■Heat Dissipation
- 13. 13 Copyright © Yokogawa Meters & Instruments Corporation Yokogawa Shunts Yokogawa Part Number Ohms Accuracy 1323EZ 5 ohm +/- 0.05% 1324EZ 10 ohm +/- 0.02% 1325EZ 20 ohm +/- 0.02%
- 14. 14 Copyright © Yokogawa Meters & Instruments Corporation Current Transformer ■Produce an AC current in its secondary winding proportional to the current measured in its primary. They reduce a high current to a lower, more safely measured current. A 𝑰 𝒐𝒖𝒕 = 𝟏 𝑵 ∙ 𝑰𝒊𝒏 𝑰𝒊𝒏 𝑩 𝑵 turns
- 15. 15 Copyright © Yokogawa Meters & Instruments Corporation Current Transformer ■Simple working principal ■Passive—no power supply needed ■Low losses ■Provides isolation ■Current output—less susceptible to noise ■Current must be AC with low DC / offset components ■Magnetic core saturation due to DC current or high primary current ■Bandwidth limitations
- 16. 16 Copyright © Yokogawa Meters & Instruments Corporation Yokogawa Current Transformers Model 2241 Specs Current Input 0.5 ~ 1500 A Primary 10/15/30/50/100/ 250/300/500/750/1500A Secondary 5A Rated Load 15VA Relative Error 0.2% Phase Error ±0.166 degrees
- 17. 17 Copyright © Yokogawa Meters & Instruments Corporation A Word of Caution Fundamentals of Power Measurement NEVER Open Circuit the Secondary side of a Current Transformer while it is energized! • Doing so can cause serious damage to the CT and could possibly be harmful to equipment operators. • A CT is a Current Source. • By Ohm’s Law E = I x R • When R is very large, E becomes very high • The High Voltage generated inside the CT will cause a magnetic saturation of the core, winding damage, or other damage which could destroy the CT.
- 18. 18 Copyright © Yokogawa Meters & Instruments Corporation Rogowski Coil ■Helical coil of wire with the lead from one end returned through the coil to the other. ■Uses a non-magnetic core. ■Output is proportional to the rate of change of the current and must be integrated Integrator
- 19. 19 Copyright © Yokogawa Meters & Instruments Corporation Rogowski Coil ■Easy to install ■Low inductance ■No iron core to saturate ■Durable and lightweight ■AC Only ■Varying conductor location causes varying accuracy ■Requires power for sensing circuitry ■Voltage output
- 20. 20 Copyright © Yokogawa Meters & Instruments Corporation Hall Effect Current Sensors (Open Loop) ■Hall effect device generates voltage proportional to primary current + - A 𝑰𝒊𝒏 Rmeas
- 21. 21 Copyright © Yokogawa Meters & Instruments Corporation Hall Effect Current Sensors (Closed Loop) Rmeas + - A 𝑰𝒊𝒏 𝑰 𝒔
- 22. 22 Copyright © Yokogawa Meters & Instruments Corporation Hall Effect Sensors ■AC & DC measurements ■Capable of high current measurement ■Isolated from high current ■Split core ■Requires a small power source ■Temperature sensitivity ■External magnetic field sensitivity
- 23. 23 Copyright © Yokogawa Meters & Instruments Corporation Flux Gate Transducers
- 24. 24 Copyright © Yokogawa Meters & Instruments Corporation Flux Gate Transducers ■Higher Measurement Accuracy ■AC & DC measurements ■Less susceptible to stray magnetic fields ■Requires external power supply ■Complex operation ■Susceptible to shock and vibration
- 25. 25 Copyright © Yokogawa Meters & Instruments Corporation Coupling to your Measurement Devices ■Matching Wiring to Load ■Avoiding Stray Capacitance
- 26. 26 Copyright © Yokogawa Meters & Instruments Corporation Matching Wiring to Load (Larger Current Wiring) ■Connect Current Measurement Circuit to the Source Side
- 27. 27 Copyright © Yokogawa Meters & Instruments Corporation Matching Wiring to Load (Smaller Current Wiring) ■Connect Current Measurement Circuit to the Load Side
- 28. 28 Copyright © Yokogawa Meters & Instruments Corporation Minimizing Error due to Stray Capacitance ■Stray capacitance Cs exists between the internal shielding and the case.
- 29. 29 Copyright © Yokogawa Meters & Instruments Corporation Minimizing Error due to Stray Capacitance (2) ■Stray capacitance Cs exists between the internal shielding and the case.
- 30. 30 Copyright © Yokogawa Meters & Instruments Corporation Conclusion ■We’ve reviewed: •Relevance of current to power measurement •Common criteria for current sensor selection related to working with a power analyzer •Common current sensor types including current shunts, current transducers, Rogowski coils, Hall effect current sensors •Tips for connecting a current sensor to a power analyzer
- 31. 31 Copyright © Yokogawa Meters & Instruments Corporation Yokogawa’s Power Measuring Solutions Precision Power Analyzers
- 32. 32 Copyright © Yokogawa Meters & Instruments Corporation Yokogawa’s Power Measuring Solutions Digital Oscilloscopes with Power Analysis
- 33. 33 Copyright © Yokogawa Meters & Instruments Corporation Questions?
- 34. 34 Copyright © Yokogawa Meters & Instruments Corporation Future Webinars Join Us for Future Web Seminars Visit our Web Site tmi.yokogawa.com Go to > Technical Library > Webinars On-Demand
- 35. 35 Copyright © Yokogawa Meters & Instruments Corporation Educational Webinars ■Basic Power Measurements ■Harmonic Analysis ■Motor Analysis ■ScopeCorder Applications Archived at tmi.yokogawa.comTechnical LibraryWebinars On-Demand
- 36. 36 Copyright © Yokogawa Meters & Instruments Corporation Thank You For Attending
- 37. 37 Copyright © Yokogawa Meters & Instruments Corporation Contact Us Yokogawa Corp of America Test & Measurement Div. 2 Dart Rd. Newnan, GA 30265 tmi.yokogawa.com Tel: 1-800-888-6400 webinars@us.yokogawa.com Sam Shearman Product Manager sam.shearman@us.yokogawa.com
Editor's Notes
- [[ go through points ]]
- Current measurement is a fundamental component of both AC and DC power measurement. Measurement of DC power requires current measurement. DC Power is equal to the DC voltage multiplied by the DC current, which is equal to the DC current squared multiplied by the resistance as defined in the simple circuit on the right.
- Measurement of AC power also requires current measurement, but the formulas for power differ than those for DC Power because the voltage and current waveforms vary with time. AC Power for a purely resistive load is the RMS voltage multiplied by the RMS current. AC Power for a reactive load is the product of the RMS voltage, the RMS current, and the cosine of the phase angle between the voltage and the current. The cosine of the phase angle between the voltage and current is also known as the “Power Factor.”
- To start, let’s review some common parameters to consider when selecting a current sensor for use with a precision power analyzer. I’ll also relate some typical values that make sense in the context of using a current measurement device with a power analyzer. These are criteria that, depending on the sensor type, will appear on the datasheet of the sensor. Current range is the minimum and maximum current amplitudes for which the sensor is designed to respond with a valid measurement. Sensors are typically in the range 0 to +-5000A with up to 20kA available. Amplitude response is the gain applied to the output of the sensor as a function of frequency. It is usually specified in terms of the full-scale accuracy. Current sensors for a precision power analyzer may be better than 0.1% accuracy from DC to 10kHz. Phase response is the time delay of the phase of the signal being measured. It is a critical parameter when considered in the context of power analyzers because such devices simultaneously sample voltage and current to determine power. Propagation Delay is closely related to phase response and is the time it takes for the signal to travel from the CT to the power analyzer. It adds to the error introduced by the phase response. A typical propagation delay might be less than 1 degree phase error at 5kHz, which is less than 1usec. Voltage Isolation is the level of voltage that you can feed through the primary hole. For instance, a CT may not have appropriate insulation for a 5 KV signal. Common mode rejection refers to the ability of the sensor to reject the average voltage present at both terminals of the input. When two input terminals are connected to each other with the reference as device ground, ideally there should be no influence on the measurement result. Again with current sensors this may be specified with respect to the full scale voltage. A typical value might be better than 0.01% Vfs. Absolute accuracy is the maximum difference between the true value and the sensor measured value. It is typically expressed as a percent of full scale.
- Continuing with some selection criteria… The zero offset is the non-zero output of a current sensor when the expected output is zero. If it is constant, it can be compensated for by the power analyzer. Stability over temperature is the variation of the sensor gain as a function of temperature. Stability over time is the variation of the sensor gain as a function of time. A typical value might be 10ppm/month or less than 0.01%/year. Hysteresis is the output value that the current sensor returns to after experiencing full-scale voltage input. Insertion impedance depends on the type of CT. If it is an inductive, indirect there would be very little effect. Direct sensors have some impedance which must be accounted for. For instance, a current shunt, which we will talk about in a moment will exhibit some impedance that depends on frequency. Magnetic sensitivity is the sensitivity of the sensor to magnetic fields such as what is generated by the Earth. For instance Hall effect sensors are sensitive to magnetic fields so moving one around may cause the reading to shift. If the effect is stable it can be compensated for by the power analyzer. dV/dt sensitivity is sometimes specified. It is a measure of how rapidly the output voltage on the secondary can track changes on the input voltage. Matching the Sensor output to the power analyzer input involves multiple considerations including whether the voltage or current output of the current sensor can be coupled directly to the voltage or current input of the power analyzer. For instance, Yokogawa power analyzers can input voltage or current for the current input. With voltage, the device can input 0 – 10 V with 0 representing no input and 10V representing full scale input. They can also input current directly, with, for instance, a Yokogawa WT3000 offering two different options for intput meters: 2A and 30A. With both voltage and current options for current you should choose an output that uses the entire input scale, for instance, with a voltage input of 0-10V you should choose a current sensor that outputs 0-10V. Doing so ensures that you use the entire dynamic range of the power analyzer and get the best resolution. Cost, Size and Ease of Installation are self explanatory factors in current sensor choice.
- Let’s now move on to discuss current sensors. There are two general categories of current sensors: direct and indirect. The direct sensing approach is based on Ohm’s Law of resistance and requires a direct connection in the circuit being measured. This direct connection leaves measurement components potentially exposed to the line voltage. An example of a direct current sensor is a current shunt. The indirect sensing approach is based on Ampere’s Law, Faraday’s Law and/or the Faraday Effect—it relies on the magnetic field (B in the diagram) generated by current flowing through a conductor such as a wire or a cable. As we will see, indirect current sensors commonly work with a ring made of Iron or another ferromagnetic material encircling a current carrying wire. Indirect current sensors provide isolation which could be necessary for design safety. Examples of indirect current sensors include: current transformer, Rogowski coils, Hall effect current sensors and flux gate current sensors.
- The first current sensor we will talk about is the current shunt. Current Shunts are low-resistance devices placed in series with the circuit that has the current you are trying to measure. Current shunts have a simple working principle that’s based on Ohm’s law: voltage equals current times resistance. Current therefore equals voltage divided by resistance. With this in mind, you can calculate the current flowing through a shunt if you measure the voltage across the shunt and know the resistance of the shunt. Shunts typically have low resistance to minimize the amount of power they consume and heat they generate.
- So what are some of the pros and cons of current shunts? First, they have a simple working principle. Relying on ohm’s law operation is straightforward. Some of them can have high accuracy, we found models that hold 0.005% accuracy advertised on the Internet. Shunts are passive—no power supply is needed. Some shunts are designed to measure both AC and DC. The shunt in the upper right is a high-accuracy AC model available from Ohm Labs. Power analyzers typically have a built-in shunt or shunts to measure current directly. Insertion loss is a down-side of a current shunt. Not isolated – potential exposure to high voltages. Finally, heat dissipation is another potential problem with current shunts. As current flows through the shunt resistance, heat proportional to the square of the current occurs in the shunt. This heat will change the resistance value and therefore your measurement. You can use a low-resistance shunt to reduce this effect.
- Yokogawa offers a variety of shunts. Among them are “shunt boxes” that plug in to the external voltage inputs on power analyzers. The shunt boxes are used with external CTs that have current outputs when using the voltage input of the power analyzer. The bnc side of the shunt plugs directly in to the power analyzer.
- Current transformers produce an AC current in their secondary winding that is proportional to the current measured in their primary. They consist of a ring core and primary / secondary wire windings. A current carrying conductor is fed through the core ring. An AC current on the conductor creates a magnetic field that is concentrated by the core. This magnetic core induces an alternating current in the secondary. The secondary current is proportional to the primary current divided by the number of turns around the core in the secondary. CTs are specified by the primary to secondary current turn ratio. The CT may have taps that connect to different numbers of turns in the secondary. This enables a single CT to be “programmed” with different turns ratios.
- As discussed on the previous slide, a current transformer has a relatively simple working principle. This fact enables passive operation that doesn’t require a power supply. As an indirect measurement of current, the CT has low losses and provides isolation. Current output is another benefit, because the current signal is less susceptible to noise and interference than a voltage output signal than a low-level voltage output device. Current transformers only work with AC signals and any DC component or offset may saturate the core. They have a limited operational bandwidth over which they can provide high accuracy. For instance the Yokogawa 2241 is rated for 50-60 Hz operation over which it provides 0.1% error.
- Yokogawa offers 3 models of current transformers, and the table shows the specs for one of them. [[ Read specifications ]] Note that the phase error is at 60Hz and bandwidth is limited (narrow) around 60Hz.
- The Rogowski coil is a helical coil of wire with the lead from one end returned through the coil to the other end. Unlike a CT, the device uses a core of air or some other non-conducting material. The operating principal of a Rogowski coil is based on Faraday’s law of induction. The output is proportional to the derivative of the primary current, so an integrator circuit is used to enable the sensor to output a voltage that is proportional to current.
- Pros Rogowski coils are easy to install because they are open loop and can be wrapped around a conductor without breaking the conductor. They are a low-inductance device that can respond to a fast changing current. Frequency response can be to 30MHz (-3dB) They remain highly linear with higher current because there is no iron core to saturate. They are typically durable and lightweight. Cons AC measurement only, because the sensor circuit uses an integrator. Can be used to measure ac current superimposed on a DC current with high resolution. E.G. 20A Rogowski Coil to measure switching noise on DC current from an AC to DC converter. Accuracy is influenced by the conductor position and decreased when the coil isn’t centered because the winding density around the coil is never perfect. The sensing circuitry requires power. Outputs a voltage value that is susceptible to noise and voltage drop over longer connectors.
- Hall effect current sensors rely on a hall effect device, which is commonly integrated on a chip. Hall effect devices generate a signal based on a current passing through them. There are two general types of hall effect current sensors, open loop and closed loop. Both types require a Hall effect device to be positioned in a gap present in the toroidal core. The diagram on this slide shows a simplified open loop hall effect current sensor circuit. Open loop Hall effect current sensors use the Hall device in the gap to measure the magnetic flux generated by a current (Iin in the diagram) fed through the toroidal core. The output from the Hall effect device is amplified and conditioned by an electric circuit built in to the sensor. The conditioned output, either a current or voltage mirrors the input current signal. The output signal depends on the magnetic linearity and stability of the core.
- The diagram on this slide shows a simplified closed loop hall effect current sensor circuit. With a closed loop hall effect current sensor, a current through the primary (Iin in the diagram) creates a magnetic flux that is concentrated by the toroidal core. This magnetic flux is balanced by an opposing magnetic flux generated by a secondary winding. The secondary winding is excited by a feedback current (Is in the diagram) generated by a Hall effect device that senses overall magnetic flux present in the toroidal core. The compensating current generated by this feedback channel mirrors the current present in the primary. The core operates at zero magnetic field or “zeroflux”. Core losses are reduced enabling high frequency operation. The Closed Loop Hall effect sensor is typically higher cost than the open-loop variant. It also requires higher power since the current to drive the compensation winding is required. Unlike the open loop variant, the core linearity and stability are not important.
- Hall effect sensors are capable of high-current AC and DC measurement. As an indirect sensor, they are isolated from the current being measured. Depending on the model, hall effect sensors can be split core, which makes them easy to install or clamp on. Hall effect sensors require a small power source, which may be a battery depending on the model. They can exhibit sensitivity or gain error due to temperature changes that can cause the core to expand or contract.
- While other current sensors rely on the toroidal core not being saturated, Flux Gate transducers put the core into saturation to operate. What follows is a simplified description of the operation of the flux gate transducer. Flux gate transducers have two coils wrapped around a solid toroid. One coil supplies excitation to the core, saturating it with a positive and negative going waveform that is 50% positive and 50% negative. A current flowing through a conductor fed through the core will cause the saturated core to spend more time positive or negative depending on the direction of the current. The second coil senses the saturation direction and timing to determine the level and direction of the primary current.
- With it’s unique approach, the Flux Gate Transducer offer high accuracy and the ability to measure both AC and DC currents. With it’s solid toroid core, the flux gate transducer is much less susceptible to external magnetic fields than a Hall effect sensor. The core of the Flux Gate transducer is delicate and susceptible to vibration and shock. It may not be appropriate for applications where vibration is expected.
- Let’s now discuss some tips and techniques for connecting a current measurement device to your power analyzer.
- You can minimize power loss error by wiring your measurement circuit to match your load. With a relatively large current, wire the current measurement circuit to the Source side. When you do so the current measurement circuit sees the sum of Current flowing through the load of the circuit under measurement Current flowing through the voltage measurement circuit This can be seen on the diagram to the right, where Itotal = iL+iV, where iV is the current through the measurement circuit. iL is an error current due to the measurement circuit. As an example, the Yokogawa WT3000E voltage measurement circuit input resistance is 10 MOhm. This means that with an input voltage of 1000 V, the error voltage, iV, will be 0.1 mA. With a load current, iL, is 1 A or more, then the effect of iV on the measurement is 0.01% or less. With an input of 100 V and 1 A, iV = 100 V/ 10 MOhm = 0.01 mA then the effect of iV on the measurement accuracy is 0.01 mA/1 A = 0.001%.
- With a relatively small current, connect the current measurement circuit to the load side. When you do so, the voltage measurement circuit on your instrument will measure the sum of the load voltage and the voltage across the current measurement circuit. On the slide right, the load voltage is eL and the current measurement circuit voltage is eI., so eI is the source of error in this case. As an example, the WT3000E current measurement circuit input resistance is 500 mOhm for the 2A module and 5.5 mOhm for the 30A module. If the load resistance is 1 kOhm then the measurement accuracy effect is 500 mOhm/1 kOhm = 0.05% with the 2A module. For the 30A input module, the accuracy effect is 5.5 mOhm/1 kOhm = 0.00055%.
- Power analyzers can have stray capacitance which can effect measurement accuracy. With the Yokogawa WT3000E, a stray capacitance exists between the internal shielded cases found on the voltage and current measurement circuits and the grounded instrument casing. The plus/minus terminal of the current measurement input terminals is connected to the current measurement circuit’s shielded case. Also, the plus/minus terminal of the voltage input terminals is connected to the shielded case of the voltage measurement circuit.
- Consider the two current flows that result when one side of the power source and the outer case of the power analyzer are grounded. The first current flow is the current through the load, iL on the diagram. This is the current you want to measure. The second current flow is through the stray capacitance, iCs on the diagram and is the error. With a common-mode voltage of VCs, iCs can be found with iCs = VCs * 2pi * f * Cs, which tells us the error current increases as the frequency of interest increases. By connecting the current input terminal of the power analyzer to the side that is close to the power source earth potential, VCs becomes approximately zero and little iCs flows.