Introducción Cabina de compensación reactiva
- 1. Regional Product Specialist Regional Product Specialist Development Development RPC System Discussion RPC System Discussion
- 2. Objectives Explain what RPC is. Explain why RPC is required. Understand the function(s) that RPC serves on P&H mining shovels. Understand Inductive and Capacitive reactance. Understand power factor. Identify System components Understand how those components function
- 3. The acronym RPC stands for Reactive Power Compensation. Thyristor Switched Capacitor banks (TSC) Fixed Capacitor Banks Capacitors supply the REACTIVE power thus the utility doesn’t need to. Minimizes voltage drop on mine distribution system Power Factor Compensation Reduces Power Bills Reduces I2R losses in conductors Reduces loading on transformers Introduction: What Is RPC?
- 4. The capacitors form part of a series LC circuit that are connected and/or switched across the line(s) of the 600VAC converter supplies. Fixed bank, ½ Bank, and Full bank configuration. ½ Bank 675KVA (60HZ) & 750 KVA (50HZ) Full Bank 1350 KVA (60HZ) & 1500 KVA (50HZ) Introduction: What is RPC? – cont.
- 5. 1. Power Factor Correction 2. Support Mine Utility (Voltage Sag) Introduction – Why Do We Need RPC?
- 6. RPC For Power Factor Correction Any phase controlled AC-DC adjustable voltage conversion system has an out of phase component called volt-amp reactive (VAR) The SCR’s in P&H’s AC-DC static converters are phase controlled. Phase control means that the firing angle of the SCR’s can be adjusted to fire at any selected point on the AC voltage wave. The effect of phase controlled SCR firing is that conducted current lags the applied voltage and generates lagging volt-ampere reactive (VAR) power. This reactive power does no useful work and acts to lower the system power factor if compensation is not provided.
- 7. Most electric mining shovel loads are inductive and require a magnetic field to operate. Motors and transformers are both contributors. Their magnetic fields are necessary, but produce no useful work. The mine’s distribution system must supply the power to produce the magnetic field and the power to produce the useful work. These two types of current are the ACTIVE and REACTIVE components For P&H to maintain a power factor that averages near unity (1), it is necessary to provide compensating leading capacitive reactive power to cancel the effect of lagging inductive power. RPC For Power Factor Correction – cont.
- 8. Power In a Resistive Circuit Inductive Reactance and Impedance. AC resistive circuit If we were to plot the current and voltage for the circuit below consisting of an AC source and a resistor, it would look something like this:
- 9. Inductive Reactance and Impedance. Power in a resistive circuit We can calculate the power dissipated by this resistor, and plot those values. Power In a Resistive Circuit – cont.
- 10. Inductive Reactance and Impedance AC Inductive Circuit If we were to plot the current and voltage for this very simple circuit, it would look something like this: Power In An Inductive Circuit
- 11. Inductive Reactance and Impedance - cont. AC Inductive Circuit Expressed mathematically, the relationship between the voltage dropped across the inductor and rate of current change through the inductor is as such: e = L* di/dt The expression di/dt is one meaning the rate of change of instantaneous current (i) over time, in amps per second. The inductance (L) is in Henrys, and the instantaneous voltage (e) is in volts. To show what happens with alternating current, let's analyze a simple inductor circuit: Power In An Inductive Circuit
- 12. Inductive Reactance and Impedance - cont. Power in a inductive circuit We can also calculate the power dissipated by this inductor, and plot those values on the same graph: Power In An Inductive Circuit
- 13. Capacitive Reactance and Impedance - cont. AC Capacitive Circuit – cont. To show what happens with alternating current, let's analyze a simple capacitor circuit. If we were to plot the current and voltage for this very simple circuit, it would look something like this: Power In A Capacitive Circuit
- 14. Capacitive Reactance and Impedance - cont. AC Capacitive Circuit Expressed mathematically, the relationship between the current "through" the capacitor and rate of voltage change across the capacitor is as such: i = C*dv/dt The expression dv/dt is one meaning the rate of change of instantaneous voltage (v) over time, in volts per second. The capacitance (C) is in Farads, and the instantaneous current (i), is in amps. To show what happens with alternating current, let's analyze a simple capacitor circuit: Power In A Capacitive Circuit
- 15. Capacitive Reactance and Impedance - cont. Power In a Capacitive Circuit As with the simple inductor circuit, the 90 degree phase shift between voltage and current results in a power wave that alternates equally between positive and negative. This means that a capacitor does not dissipate power as it reacts against changes in voltage; it merely absorbs and releases power, alternately. Power In A Capacitive Circuit
- 16. Capacitive Reactance and Impedance - cont. Review In a purely resistive circuit, all circuit power is dissipated by the resistor(s). Voltage and current are in phase with each other. In a purely reactive circuit, no circuit power is dissipated by the load(s). Rather, power is alternately absorbed from and returned to the AC source. Voltage and current are 90o out of phase with each other. In a circuit consisting of resistance and reactance mixed, there will be more power dissipated by the load(s) than returned, but some power will definitely be dissipated and some will merely be absorbed and returned. Voltage and current in such a circuit will be out of phase by a value between 0o and 90o . Reactive & Capacitive Power - Summary
- 17. The Electrotorque RPC control system employs a reactive power compensation method that automatically accomplishes power factor correction onboard electric mining shovels. The RPC system combines fixed and switched tuned capacitor banks, the latter being switched online under essentially zero current conditions. How Does RPC Work?
- 18. The RPC components include a set of air core reactors. They are used in conjunction with the power factor correction capacitors to form a series LC tuned filter. These filters are tuned to absorb most of the harmonic currents generated by the static converters onboard, so that the harmonics present on the mine’s distribution system are minimized. The filter is a “bandpass” type tuned to a resonant frequency equal to the 5th order harmonic of the fundamental frequency. 300HZ @ 60Z 250HZ @ 50HZ How Does RPC Work? – cont.
- 19. Power Factor By Definition Power Factor = Real Power / Apparent Power Power Factor = Watts / VA Only watts or real power do any work. The reactive power merely increases the amount of apparent power that must be conducted to the end use. The effect is that the ratio of real power over apparent power is less than 1.
- 20. Power Factor Explained The Power Triangle
Editor's Notes
- #4: Why the difference between 50 and 60 HZ applications? A: The amount of capacitance required is inversely proportional to the frequency. C=1/2fXc
- #8: Because the resistor directly resists the flow of electrons at all periods of time, the waveform for the voltage drop across the resistor is exactly in phase with the waveform for the current through it. We can look at any point in time along the horizontal axis of the plot and compare those values of current and voltage with each other. When the instantaneous value for current is zero, the instantaneous voltage across the resistor is also zero. Likewise, at the moment in time where the current through the resistor is at its positive peak, the voltage across the resistor is also at its positive peak, and so on.
- #9: Note that the power is never a negative value. When the current is positive (above the line), the voltage is also positive, resulting in a power (IXE) of a positive value. Conversely, when the current is negative (below the line), the voltage is also negative, which results in a positive value for power (a negative number multiplied by a negative number equals a positive number). This consistent "polarity" of power tells us that the resistor is always dissipating power, taking it from the source and releasing it in the form of heat energy. Whether the current is positive or negative, a resistor still dissipates energy.
- #10: Remember, the voltage dropped across an inductor is a reaction against the change in current through it. Therefore, the instantaneous voltage is zero whenever the instantaneous current is at a peak (zero change, or level slope, on the current sine wave), and the instantaneous voltage is at a peak wherever the instantaneous current is at maximum change (the points of steepest slope on the current wave, where it crosses the zero line). This results in a voltage wave that is 90o out of phase with the current wave. Looking at the graph, the voltage wave seems to have a "head start" on the current wave; the voltage "leads" the current, and the current "lags" behind the voltage.
- #12: Because instantaneous power is the product of the instantaneous voltage and the instantaneous current (P=I x E), the power equals zero whenever the instantaneous current or voltage is zero. Whenever the instantaneous current and voltage are both positive (above the line), the power is positive. However, because the current and voltage waves are 90o out of phase, there are times when one is positive while the other is negative, resulting in equally frequent occurrences of negative instantaneous power. But what does negative power mean? It means that the inductor is releasing power back to the circuit, while a positive power means that it is absorbing power from the circuit. Since the positive and negative power cycles are equal in magnitude and duration over time, the inductor releases just as much power back to the circuit as it absorbs over the span of a complete cycle. What this means in a practical sense is that the reactance of an inductor dissipates a net energy of zero, quite unlike the resistance of a resistor, which dissipates energy in the form of heat. Mind you, this is for perfect inductors only, which have no wire resistance. An inductor's opposition to change in current translates to an opposition to alternating current in general, which is by definition always changing in instantaneous magnitude and direction. This opposition to alternating current is similar to resistance, but different in that it always results in a phase shift between current and voltage, and it dissipates zero power. Because of the differences, it has a different name: reactance. Reactance to AC is expressed in ohms, just like resistance is, except that its mathematical symbol is X instead of R. To be specific, reactance associate with an inductor is usually symbolized by the capital letter X with a letter L as a subscript, like this: XL. Since inductors drop voltage in proportion to the rate of current change, they will drop more voltage for faster-changing currents, and less voltage for slower-changing currents. What this means is that reactance in ohms for any inductor is directly proportional to the frequency of the alternating current. The exact formula for determining inductive reactance is as follows: XL = 2fL
- #13: The current through a capacitor is a reaction against the change in voltage across it. Therefore, the instantaneous current is zero whenever the instantaneous voltage is at a peak, and the instantaneous current is at a peak wherever the instantaneous voltage is at maximum change (the points of steepest slope on the voltage wave, where it crosses the zero line). This results in a voltage wave that is -90o out of phase with the current wave. Looking at the graph, the current wave seems to have a "head start" on the voltage wave; the current "leads" the voltage, and the voltage "lags" behind the current.
- #14: Capacitors oppose changes in voltage by drawing or supplying current as they charge or discharge to the new voltage level. The flow of electrons "through" a capacitor is directly proportional to the rate of change of voltage across the capacitor. This opposition to voltage change is another form of reactance, but one that is precisely opposite to the kind exhibited by inductors.
- #15: A capacitor's opposition to change in voltage translates to an opposition to alternating voltage in general, which is by definition always changing in instantaneous magnitude and direction. For any given magnitude of AC voltage at a given frequency, a capacitor of given size will "conduct" a certain magnitude of AC current. Just as the current through a resistor is a function of the voltage across the resistor and the resistance offered by the resistor, the AC current through a capacitor is a function of the AC voltage across it, and the reactance offered by the capacitor. As with inductors, the reactance of a capacitor is expressed in ohms and symbolized by the letter X (or XC to be more specific). Since capacitors "conduct" current in proportion to the rate of voltage change, they will pass more current for faster-changing voltages (as they charge and discharge to the same voltage peaks in less time), and less current for slower-changing voltages. What this means is that reactance in ohms for any capacitor is inversely proportional to the frequency of the alternating current: The exact formula for determining capacitive reactance is as follows: XC = 1/2fC