Reactive power battery charging apparatus and method of operating same

A battery charger may be capable of receiving power from a power distribution circuit. The charger may be configured to receive a request for reactive power and, in response, cause the requested reactive power to be present on the power distribution circuit.

BACKGROUND

Real power is the capacity of a circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. The apparent power may be greater than the real power due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source.

The power factor of an AC electric power system may be defined as the ratio of the real power flowing to the load to the apparent power (a number between 0 and 1).

In an electric power system, a load with a low power factor draws more current than a load with a high power factor, for the same amount of useful power transferred. The higher currents may increase the energy lost in the distribution system, and may require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities may charge a higher cost to customers with a low power factor.

In a purely resistive AC circuit, voltage and current waveforms are in phase, changing polarity at the same instant in each cycle. Where reactive loads are present, such as with capacitors or inductors, energy storage in the loads results in a time difference (phase) between the current and voltage waveforms. This stored energy returns to the source and is not available to do work at the load. Thus, a circuit with a low power factor will have higher currents to transfer a given quantity of real power compared to a circuit with a high power factor.

AC power flow has the three components: real power (P) measured in watts (W); apparent power (S) measured in volt-amperes (VA); and reactive power (Q) measured in reactive volt-amperes (VAr). Power factor may thus be defined as
P/S(1)

In the case of a perfectly sinusoidal waveform, P, Q and S can be expressed as vectors that form a vector triangle such that
S2=P2+Q2(2)

If θ is the phase angle between the current and voltage, then the power factor is equal to |cos θ|, and
P=S*|cos θ|  (3)

When power factor is equal to 0, the energy flow is entirely reactive, and stored energy in the load returns to the source on each cycle. When the power factor is equal to 1, all the energy supplied by the source is consumed by the load. Power factors may be stated as “leading” or “lagging” to indicate the sign of the phase angle.

If a purely resistive load is connected to a power supply, current and voltage will change polarity in phase, the power factor will be unity, and the electrical energy will flow in a single direction across the network in each cycle. Inductive loads such as transformers and motors consume power with the current waveform lagging the voltage. Capacitive loads such as capacitor banks or buried cables cause reactive power flow with the current waveform leading the voltage. Both types of loads will absorb energy during part of the AC cycle, which is stored in the device's magnetic or electric field, only to return this energy back to the source during the rest of the cycle. For example, to get 1 kW of real power, if the power factor is unity, 1 kVA of apparent power needs to be transferred (1 kW÷1=1 kVA). At low values of power factor, however, more apparent power needs to be transferred to get the same real power. To get 1 kW of real power at 0.2 power factor, 5 kVA of apparent power needs to be transferred (1 kW÷0.2=5 kVA).

SUMMARY

A battery charger may be capable of receiving power from a power distribution circuit. The charger may be configured to receive a request for reactive power and, in response, cause the requested reactive power to be present on the power distribution circuit.

An automotive vehicle may include a traction battery and a battery charger capable of receiving power from a power distribution circuit remote from the vehicle. The charger may be configured to (i) charge the battery, (ii) receive a request for reactive power, and (iii) cause the requested reactive power to be present on the power distribution circuit.

A method for operating a battery charger may include receiving a request for reactive power and causing, in response, the requested reactive power to be present on a power distribution circuit electrically connected with the battery charger.

While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention.

DETAILED DESCRIPTION

Referring now toFIG. 1, a power distribution circuit10may include power lines (lines)12,12′ return lines (neutrals)14,14′ and a ground line (ground)16and may be similar to, in some embodiments, power distribution circuits found in residential or commercial buildings. A fuse box18, battery charger20and other loads22are electrically connected with the distribution circuit10. (The battery charger20may, for example, be a stand alone unit or integrated within a vehicle.) The line12and neutral14are that portion of the circuit10electrically connected between the fuse box18and loads22. The line12′ and neutral14′ are that portion of the circuit10electrically connected between the charger20and loads22.

The fuse box18includes a fuse23electrically connected with the line12.

A power storage unit24, e.g., vehicle traction battery, may be electrically connected with (and charged by) the battery charger20.

As known to those of ordinary skill, power from a power source25, e.g., utility grid, etc., is delivered to the distribution circuit10(and thus the battery charger20and loads22) via the fuse box18.

In the embodiment ofFIG. 1, the loads22, (such as a refrigerator compressor, etc.) have both real and reactive power components (resulting in an AC current that lags the AC voltage.) This lagging current causes reactive power to flow between the loads22and power source25. (This reactive power flow will result in a current through the fuse23, for a given real power, that is greater than the current through the fuse23in the absence of this reactive power flow.) The loads22thus lower the power factor associated with the distribution circuit10and decrease the real power available for a given amount of apparent power.

As explained below, the battery charger20may determine the power factor of the distribution circuit10and operate so as to reduce and/or eliminate reactive power flow on the lines12,14caused by the loads22. (As apparent to those of ordinary skill, this reduction/elimination will be accompanied by an increase in reactive power flow on the lines12′,14′.)

Referring now toFIGS. 1 and 2, an embodiment of the battery charger20may include a bridge rectifier26, power factor (PF) controlled boost regulator28, buck regulator30and microprocessor32. Of course, the battery charger20may have any suitable configuration. The bridge rectifier26may be electrically connected with the line12′, neutral14′ and ground16of the distribution circuit10. The PF controlled boost regulator28is electrically connected with the bridge rectifier26and buck regulator30. The buck regulator30may be electrically connected with the power storage unit24. The PF controlled boost regulator28and buck regulator30are under the command/control of the microprocessor32.

The battery charger20may also include voltage sensors34,36and a current sensor38. The voltage sensor34measures the voltage between the line12′ and neutral14′. The sensor36measures the voltage between the neutral14′ and ground16. (As apparent to those of ordinary skill, this voltage is dependent on the current through the neutrals14,14′.) The sensor38measures the current through the neutral14′. The sensors34,36,38are in communication with the microprocessor32.

If the charger20is not operating, all load current due to the loads22passes through the neutral14. The neutral14, having an internal resistance R14, experiences a voltage drop between the loads22and fuse box18that is proportional to, and in phase with, the current through the loads22. This voltage drop can be measured at the charger20by the sensor36. If the loads22contain a reactive component, the voltage measured by the sensor36will be out of phase with the voltage measured by the sensor34. From (5) (discussed below), the power factor can thus be computed.

If the loads22were absent, the charger20could produce the same voltage drop by charging at a rate that causes a current through the neutrals14,14′ that is equal to:
((R14+R14)*Icharger)/R14(4)
where R14′is the internal resistance of the neutral14′ and Ichargeris the current through the charger20(the current through the sensor38).

If the charger20is operating and the loads22are present, the reactive component of power due to these combined loads will have an associated current that can be determined based on the measured voltage36. Due to this component of current, the measured voltage waveform at the sensor36(VNG) will be out of phase with the measured voltage waveform at the sensor34(VLN). If the charger20is commanded to operate as a load with a reactive power such that the measured voltage waveform at the sensor36is substantially aligned with the measured voltage waveform at the sensor34, the power at the fuse box18will have little or no reactive component.

From (4), if R14′is small relative to R14, the charger current necessary to correct and align the phase of VNGwith VLNwill be approximately equal to the current magnitude and phase of the example above where the charger20is not operating and thus all load current due to the loads22passes through the neutral14. If R14′is not small relative to R14, a portion of reactive power may still be observed at the fuse box18.

The microprocessor32may determine the power factor (and thus differences in phase between the voltage and current) of the distribution circuit10based on information from the sensors34,36. For example, the microprocessor32may determine the power factor based on the period, T, of the voltage waveform as measured by the sensor34and the phase between the voltage waveforms as measured by the sensors34,36. Other suitable techniques, however, may also be used.

To find T, for example, the microprocessor32may determine the time between two consecutive zero-crossings of the voltage waveform as measured by the sensor34, and multiply this time by a factor of 2. Alternatively, the microprocessor32may determine the time between alternate zero-crossings of the voltage waveform as measured by the sensor34. Other schemes are also possible.

To find the phase between the voltage waveforms as measured by the sensors34,36, the microprocessor32may determine the time, t, between a zero-crossing of the voltage waveform as measured by the sensor34and an immediately subsequent zero-crossing of the voltage waveform as measured by the sensor36.

The microprocessor32may then find the power factor of the distribution circuit10as
PF=cos((t/T)*360)  (5)

The microprocessor32may communicate this power factor to the PF controlled boost circuit28. The PF controlled boost circuit28(which may take the form of circuitry described in the UNITRODE Application Note “UC3854 Controlled Power Factor Correction Circuit Design” by Philip C. Todd, 1999, or any other known and/or suitable form) may control the power drawn in order to correct for reactive power caused by the loads22. This control may be accomplished, for example, with the addition of a digital or analog lead/lag of the current measured by the sensor38(or by a lag/lead of the voltage measured by the sensor34) prior to the signal being processed by the PF controlled boost circuit28. In this example, a lag in the current signal will produce a corresponding lead in the power factor at the input of the charger20, and the PF controlled boost circuit28will no longer be drawing unity PF at its input as originally intended. Conversely, a lead will produce a corresponding lag in the power factor at the input of the charger20, etc.

If the loads22are motors, for example, they will typically have an inductive reactance, Xl, that will cause a lagging power factor. A leading power factor equivalent to a capacitive reactance, Xc, may be provided such that Xc≈Xl. With this approximate match, little or no reactive power will flow on the line12and neutral14, and will instead flow on the line12′ and neutral14′.

If the reactive power needed to correct for reactive power caused by the loads22is known, the PF controlled boost regulator28may be directed to produce the needed (complementary) reactive power.

Alternatively, considering (4) and the prior discussion of current produced voltages at the sensor36, for small values of R14, relative to R14there will be little or no reactive power flow through the line12, neutral14and fuse23, and VNGwill be in phase with VLN. Even for larger values of R14when VNGis in phase with VLN, the reactive power flow through the line12, neutral14, and fuse23will be reduced. Of course, if the reactive power of the loads22is known, the reactive power produced current can be directly calculated and controlled.

Control signal inputs to the PF controlled boost circuit28may be based on the voltage (rectified) between the lines12′,14′, and the magnitude of the voltage between the lines14′,16(which, of course, is proportional to the current through the neutrals14,14′). As apparent to those of ordinary skill, the above control signal input scheme allows the PF controlled boost circuit28to substantially correct the power factor of the distribution circuit10(as opposed to just the battery charger20.)

The boost circuit28may measure, in a known fashion, the rectified AC voltage from the bridge rectifier26and control, in a known fashion, the current, i, through its inductor such that the instantaneous value of the magnitude of i is proportional to the instantaneous value of the magnitude of the voltage between lines14′,16.

If the battery charger20is the only load on the distribution circuit10, the line12will have a power factor of approximately unity. (Because the current, i, is proportional to the AC voltage on the line12(they are in phase), the power factor of the distribution circuit10is unity.) If, however, there are additional loads, such as loads22, with reactive components, the distribution circuit10will also have a power factor of approximately unity at the fuse box18because of the control input scheme discussed above.

Assuming the microprocessor32finds the power factor for the distribution circuit10as discussed above, it may control the PF controlled boost circuit28so as to produce reactive power sufficiently equal (and of opposite sign) to the reactive power caused by the loads22. The reactive power produced by the PF controlled boost circuit28will thus cancel with the reactive power of the distribution circuit10and increase the real power for a given amount of apparent power.

From (2) and (3), and assuming a lagging power factor of 0.8 and an apparent power of 375 VA for the distribution circuit10, the real power is approximately equal to 300 W and the reactive power is approximately equal to 225 VAr (current lagging voltage in this example). The PF controlled boost circuit28may thus operate to produce approximately 225 VAr (current leading voltage) and drive the apparent power to a value of 300 VA. Operation of the battery charger20may thus increase the efficiency at which power is delivered by the distribution circuit10under circumstances where non-power factor corrected loads (such as the loads22illustrated inFIG. 1) are electrically connected with the distribution circuit10. In this example, the distribution circuit10would need to provide 3.125 A at 120 V to provide the 375 VA of power. With the reactive power component substantially eliminated, the distribution circuit10would only need to provide 2.5 A at 120 V to provide the 300 W of power. (Thus, an additional 0.6 A of real current could now be drawn by the battery charger20without changing the amount of apparent current flowing through the fuse23.)

Referring now toFIG. 3(where like numerals have similar descriptions toFIG. 1), a power distribution system140includes a power source125and several power distribution circuits110n(110a,110b,110c, etc.). The power source125ofFIG. 3is configured to provide power to the distribution circuits110n.

Reactive loads electrically connected with the distribution system140via the distribution circuits110nmay cause a net leading or lagging reactive power. As discussed above, this net reactive power may cause inefficiencies in power delivery within the distribution system140.

In the embodiment ofFIG. 3, the power source125may request offsetting reactive power (leading or lagging) to be produced/generated by any battery chargers similar to those described with reference toFIG. 2and electrically connected with the distribution circuits110n. In other embodiments, the power source125may request offsetting reactive power to be produced/generated by other suitably controlled loads or added power sources capable of modifying, upon request, the power factor of the distribution circuits110nin a manner similar to the battery chargers described herein. Such loads or added power sources, for example, may have an architecture and input control scheme similar to the battery charger20ofFIG. 2.

The power source125may include, for example, a wireless transmitter/transceiver or modulator (for power line communication) to communicate such requests for reactive power (and receive information from battery chargers as explained below). Any suitable information transmission technique, however, may be used.

Referring now toFIGS. 3 and 4(where like numerals have similar descriptions toFIG. 2), an embodiment of a battery charger120may include a bridge rectifier126, PF controlled boost regulator128, buck regulator130, microprocessor132and transceiver133. The microprocessor132is in communication with the transceiver133. The battery charger120may also include voltage sensors134,136and a current sensor138.

The transceiver133is configured to transmit and/or receive wireless signals in a known fashion. The transceiver133may, for example, receive requests/commands for reactive power (of a particular sign) wirelessly transmitted by the power source125in a known fashion. These requests/commands may then be forwarded to the microprocessor132for processing. In other embodiments, the battery charger120may include HOMEPLUG-like (or similar) technology for receiving and/or transmitting over-the-wire communications from and/or to the power source125. As apparent to those of ordinary skill, such a HOMEPLUG module would be electrically connected with the power and return lines112′,114′. As known in the art, with HOMEPLUG, information is supper-imposed on AC lines at particular frequencies. With appropriate circuitry, this information can be read at the receiving end.

The microprocessor132may use the requested/commanded reactive power as a target by which to “tune” the reactive power of the distribution circuit110n. For example, if 5 VAr total of reactive power (current leading voltage) is needed to substantially correct the power factor of the distribution system140, and the microprocessor132has determined, using the techniques described herein, that 1 VAr (current leading voltage) is available to be produced by the charger120, the microprocessor132, in response to a request for reactive power (current leading voltage) from the power source125, may control the PF controlled boost regulator128to produce 1 VAr of reactive power (current leading voltage) by, for example, controlling the digital or analog lead/lag of the current measured by the sensor138(or the lag/lead of the voltage measured by the sensor134) as discussed above (thus driving the reactive power of the distribution circuit110nto 4 VAr (voltage leading current)).

The microprocessor132may also determine the capacity of the battery charger120to cause a specified reactive power to be present on the distribution circuit110and communicate this information to the power source125via, for example, the transceiver133. The power source125may aggregate this information from all such battery chargers electrically connected with the power distribution system140and issue requests for reactive power accordingly (e.g., based on the aggregate capacity).

Based on the apparent power and power factor of the distribution circuit110n(from (1) and (2)), the real and reactive powers may be found. The incremental reactive power available may then be found using the power/current ratings of the distribution circuit110n(which may be, for example, assumed, determined or input by a user). If, for example, the real and reactive powers are 10.6 W and 10.6 VAr (current leading voltage) respectively, and the power rating of the distribution circuit110nis 15 W, the battery charger120cannot produce additional leading reactive power (current leading voltage) because, from (2), the apparent power is equal to the power rating of the distribution circuit110n. (One of ordinary skill, however, will recognize that the battery charger120can still produce lagging reactive power if needed.) If, for example, the real and reactive powers are 0 W and 0 VAr respectively, and the available power rating of the distribution circuit110nis 15 W, the battery charger120has the capacity to produce 15 VAr of reactive power (of either sign).

In certain embodiments, the power source125may measure the PF (and determine whether voltage is leading or lagging current) using any suitable technique and broadcast a command for all battery chargers to produce, for example, 1 VAr of reactive power (having a sign opposite to the net reactive power). The power source125may then periodically measure the PF and broadcast commands for all battery chargers to increase the reactive power (of sign opposite to the net reactive power) produced until the net reactive power on the distribution system140has been sufficiently reduced and/or eliminated. In other embodiments, such as those having two-way communication between the power source125and any battery chargers120, the power source125may request, in a known fashion, that respective battery chargers120produce/generate different amounts of reactive power (based on their respective capacities) provided, of course, that each battery charger reporting its capacity also provides identifying information that may distinguish it from others. Other control scenarios are also possible.