WO2006037265A1

WO2006037265A1 - Dimmable lighting system

Info

Publication number: WO2006037265A1
Application number: PCT/CN2005/001584
Authority: WO
Inventors: Henry Shu-Hung Chung; Ngai-Man Ho
Original assignee: E. Energy Double Tree Limited
Priority date: 2004-10-01
Filing date: 2005-09-28
Publication date: 2006-04-13
Also published as: AU2005291756B2; CN101044800B; CN101044800A; AU2005291756A1; GB0421901D0; AU2005291756A8; GB2418786A; AU2005291756B8; HK1089614A1; GB2418786B

Abstract

There is disclosed a reactive power controller (RPC) for dimming AC mains lighting systems that use ballasts, for example magnetic ballasts. In one embodiment, a DC/AC converter is used to convert a DC voltage, from a DC voltage source, to an AC voltage that is out of phase with current flowing through the RPC. For example, if the AC voltage is 90° or 270° out of phase with the current then the RPC will appear, to an AC mains lighting system, to be effectively a capacitor or an inductor, thus increasing the effective impedance of the mains supply to the magnetic ballast. The RPC creates a controllable AC voltage at mains frequency, so that the net voltage to the load is adjustable to dim the load. The net voltage is the vectorial sum of the AC mains voltage and the AC voltage created by the RPC. A second RPC may be used to improve the power factor of the AC mains lighting system. The second RPC generates or absorbs the required reactive power of the whole system, so that the input power factor of the AC mains lighting system becomes substantially unity. In an advantageous embodiment, the first and second RPCs are connected to share a common DC voltage source.

Description

DIMMABLE LIGHTING SYSTEM

FIELD OF THE INVENTION This invention relates to apparatus and methods for providing (1) high power quality and (2) dimming control for individual electrical lamps or more generally electrical lighting systems including systems formed by a plurality of individual lamps. The invention relates in particular to a simple general purpose and non-intrusive dimming system that can be retro-fitted to existing lamps and which is non- intrusive in the sense that when not in use the dimming apparatus has no effect on the normal operation of the lamp. The apparatus and dimming methods maintain high power quality (with capability up to unity input power factor) from the power system at any dimmed power level .

BACKGROUND OF THE INVENTION

A wide range of different types of lamps and lighting systems are used in various applications. These include incandescent lamps, fluorescent lamps, high- and low- pressure discharge lamps. For both aesthetic and energy saving reasons, various attempts have been made in the prior art to provide such lamps with a dimming control so that the brightness of the lamps can be adjustable. Dimming function is particularly useful for high intensity discharge (HID) lamps, which are widely used in public lighting systems 'due to the HID lamps' manifold advantages such as longevity and high luminous efficacy. Unlike incandescent lamps, high intensity discharge lamps generally require a long warm up time to reach full brightness. After being shut off, they need a cooling down period before they can be restarted again. It is this "re-strike" characteristic that makes dimming a very- attractive alternative to simply turning the lights off, because dimming does not require the turning off the lamps and can therefore avoid considerable warm-up time of the lamps after re-ignition. Although numerous attempts have been made in the prior art to develop dimmable electronic ballasts for individual lamps, conventional magnetic ballasts are still the most reliable, robust, cost-effective, and dominant choice for high-wattage discharge lamps and large-scale lighting systems, such as street lighting systems.

Dimming also has other advantages such as reduction of peak power demand, increase of flexibility for multi-use spaces, safer driving in light traffic conditions and avoidance of light pollution. This invention relates to a dimming technology that can dim a lighting system formed by a plurality of lamps. Existing non-dimmable lighting infrastructure can be converted into a dimmable one with real energy saving. No major change of the lighting electrical network is needed. Of particular importance, the technology will maintain the principle of providing "environmentally clean" power conversion that will not introduce harmonic pollution problem into the power system.

PRIOR ART

Existing dimming methods for existing lighting systems include Triac-based dimmers for incandescent lamps and gaseous discharge lamps compatible with triac dimmers, dimmable electronic ballasts for gaseous lamps, and a range of disparate techniques for dimming lamps driven by magnetic ballasts. These prior art techniques will be discussed in turn. Triac-based dimmers have been popularly used as the dimming devices for Edison-type incandescent lamps and some triac-dimmable fluorescent lamps [1] . The circuit connection is illustrated in Fig. 1 (a) . A triac dimmer consists of a triac and also a triggering circuit which controls the phase angle of turning the triac on over a cycle of the mains voltage. As shown in Fig. 1 (b) , by controlling the delay angle (α) , the output root-mean-square voltage, and thus the power to the lamp, can be controlled. This control of ac voltage results in the ability to adjust the brightness of the lamp.

However, the waveshape of the mains input current through the triac dimmer is dependent on the delay angle. When the delay angle is nonzero, the input current will deviate from the sinusoidal shape of the mains voltage. When the delay angle is increased, the conduction time of the triac is diminished. The input current will then consist of high harmonic components and thus generates undesirable harmonics into the power system. In addition, as the input power factor is the product of the displacement factor and the distortion factor [2] , the input power factor becomes small when the delay angle is large. It is because the displacement factor is equal to the cosine of the delay angle (If the delay angle is large, the displacement factor will become small . ) and the distortion factor deteriorates as the current harmonic content increases . The ultimate effect of this low input power factor is the presence of reactive power flow between the ac mains and the lighting system. This reactive power could cause serious defects over the power system. The lower the power factor, the larger the rating of the transformers and the larger the size of the conductors of transmission must be. In other words, the greater the cost of generation and transmission will be. That is the reason why supply undertakings always stress upon the consumers to increase the power factor [3] . Recently, there has been an increasing trend of using dimmable electronic ballasts for discharge lamps such as fluorescent lamps and high intensity discharge (HID) lamps. A dimmable electronic ballast usually has a 4-wired connection arrangement on the input side. Two connections are for the "live" and "neutral" of the ac mains, the other two are for the dimming level control signal, which is typically a dc signal within 1V-10V. A general structure of the dimmable electronic ballast is illustrated in Pig. 2 (a) . It consists of an active or a passive power factor correction circuit, a high-frequency dc/ac converter, and a resonant tank circuit. The power factor correction circuit and the dc/ac converter are interconnected through a dc link of high voltage. The dc/ac converter is used to drive the lamp through the resonant tank circuit. It is usually switched at a frequency slightly higher than the resonant frequency of the resonant tank circuit. The resonant tank is used to provide a high voltage to ignite the lamp and ballast the lamp current . Dimming function is achieved by controlling the dc link voltage and/or the switching frequency of the dc/ac converter. The input power factor can be kept high at any power level. As illustrated in Fig. 2 (b) , the waveform of the input current i_ac is sinusoidal and in phase with the ac mains .

A typical circuit of electronic ballast for fluorescent lamps is illustrated in Fig. 2 (c) , in which the ac mains is rectified by a rectifier, the power factor correction circuit is realized by a boost dc/dc converter, the dc/ac converter is realized by a half-bridge inverter circuit, and the resonant tank circuit is formed by L_r and C_r. The dc link voltage is regulated at a level slightly higher than the peak value of the ac mains voltage. If the lamp is dimmed by increasing the switching frequency of Si and S₂, then the reactance of L_r is increased, so that the power delivered to the lamp is reduced.

Electronic ballasts for fluorescent lamps (low-pressure discharge lamps) have been widely used and have been shown that their use has an overall economic benefit [4] . Operating at high frequency (typically above 20-kHz) electronic ballasts can eliminate the flickering effects of the fluorescent lamps and achieve a higher efficacy than mains-frequency (50Hz or 60Hz) operated magnetic ballasts. Therefore fluorescent lamps driven by electronic ballasts consume less energy for the same light output when compared with lamps driven by magnetic ballasts. However, one major weakness of electronic ballasts is the relatively short lifetime (typically less than 3 to 5 years for high-quality products) . Magnetic ballasts can normally operate over 10 years without replacement and it is rare to have electronic ballasts with such long lifetime. Also, the inductor core materials and winding materials are recyclable, while electronic ballasts have more toxic and non-recyclable materials . In HID lamps market, the situation of using electronic ballasts is different. Electronic ballasts are only used in a minority of mainstream HID applications. They are limited to small-scale systems and consumer products, such as projectors, film lighting, and automobile headlamps [5] . One main reason is that HID lamps could suffer from acoustic resonance when they are operated at high frequency. Acoustic resonance is due to the power pressure variation in the lamp tube that could trigger various forms of resonance - as occurring in the woodwind musical instruments. In principle, it can be avoided by operating the lamps at a low frequency (< IkHz) or a very high frequency (> 35OkHz - 70OkHz) [5, 6] However, the lamp characteristics change with time and thus lamp stability is not guaranteed when the lamp's ageing effects become crucial. Unlike incandescent lamps, HID lamps require a long warm up time to reach full brightness. After being shut off, they need a cooling down period before they can be restarted again. It is this "re-strike" characteristic that makes dimming a very attractive alternative to simply turning the lights off - in order to avoid spending considerable time waiting for the lamp to warm up. Dimming also has other advantages such as energy saving, esthetic purpose, increase of flexibility for multi- use spaces, safer driving in light traffic conditions [2] and reduction of light pollution in outdoor lighting systems Hence, despite the increasing efforts on developing electronic ballasts, conventional magnetic ballasts are still the dominant choice, especially in outdoor applications. Unlike electronic ballasts, magnetic ballasts have the advantages of extremely high reliability and long lifetime, and robustness against transient voltage surge (e.g. due to lightning) and hostile working environment (e.g. high humidity and wide variation of temperature) .

Particularly, they offer superior lamp-arc stability performance in HID lamps. In the past, the major limitation of magnetic ballasts was their lack of flexibility in achieving dimming control . Apart from the technical issues, it is also not economical to use a dimming device for each individual lamp in a lighting system formed of a large group or network of lamps. The arrangement is particularly a concern for converting a non-dimmable lighting system into a dimmable one by replacing all dimmable control gears with dimmable devices. As illustrated in Fig. 3, the wiring and electrical installation will be complicated, since it is necessary to redesign the electrical networks for both power lines and control signals. The situation will be even more complicated in systems having multiple zones. Therefore, installation of individual dimmable electronic ballasts in all lamp posts in a road lighting system, for example, will involve high installation cost and will also be a maintenance nightmare for the road lighting management companies, in view of the relatively poor immunity of electronic ballasts against extreme weather conditions . Therefore, if magnetic ballasts can be made dimmable, the combined features of their long lifetime, high reliability and energy saving can make such "dimmable magnetic ballasts" an attractive solution for both indoor and outdoor applications. Moreover, it would be useful to have a technology that can dim a plurality of lamps with magnetic ballasts. Fig 4 (a) shows the general non-dimmable lamp system configuration with magnetic ballasts, in which the input of the ballasts is directly connected to the ac mains through a switch gear. The switch gear is used to turn on the lamps and is controlled by various means, for example, manual control, automatic timer control, and photo-sensor. To date several dimming methods for lamps with magnetic ballasts have been reported. The ultimate purpose is to control the lamp current, and hence the lamp power, so that the lamps' brightness can be varied. The strategies are mainly acted on the input side of the ballast or at the lamp side. As depicted in Fig. 4 (b) , they can be categorized into several methods .

Method I - Control of the supply voltage or current to the lamp

Reducing the voltage supplying to the ballast is a direct way of dimming. As illustrated in Fig. 4 (b) , when the supply voltage v_L is reduced, the supply current i_L, the lamp current i_hamp, and hence, the lamp power will be decreased. This method can be realized by various voltage transformation means, such as low-frequency transformers or high-frequency switching converters.

One of the most obvious methods of altering the voltage conditions on the ballast input is to provide means whereby the voltage ratio of the supply transformers in the system may be varied. As the voltage ratio is dependent on the turns ratio, it follows that if the turns ratio can be altered then the voltage ratio will be changed by the same amount [5] . Various methods have been adopted for effecting this desired change in the transformation ratio, the most simple one involving the use of a tapped winding on one side of the transformer, so that the effective turns-ratio can be altered. Another one is the use of autotransformer that the turn-ratio can be continuously varied. In [7] , a two-winding autotransformer is used to provide two voltage levels for implementing a two-level dimming system. In [8] , a multilevel dimming system that uses a more complicated transformer is proposed. All these methods involve the use of mechanical devices, such as contactors for changing the turns-ratio and motors for continuously adjusting the turns ratio. Another method is the use of high-frequency switching converters. The ac mains voltage is converted into a dc voltage by an ac-dc converter and the dc voltage is converted into an ac voltage by a dc-ac converter [2] . Thus, the overall system can flexibly provide a high-quality variable voltage and variable frequency output at v_L. However, as the input energy from the ac mains is processed twice, the overall efficiency is low. For example, if the efficiencies of the ac-dc and the dc-ac converters are 0.95, the overall efficiency is 0.95 * 0.95 = 0.90.

Apart from using ac-dc-ac conversion approach, an ac-ac converter such as cycloconverter can be used to provide a controllable voltage at mains frequency. In [9] , a power converter is used to chop the ac sinusoidal voltage into voltage pulses with the sinusoidal envelope. Similar approach is used in [10, 11] . However, considerable current harmonics will be generated in the process, leading to harmonic pollution problems in the power system. It is unsuitable for lamps, such as HID lamps, which are sensitive to the excitation voltage. It may cause undesirable acoustic resonance and flickering effect.

Another approach [12, 13] is the use of an external current-control power circuit to control the magnitude of the input current at mains frequency. Instead of transforming the voltage magnitude, this method adjusts the input current taken from the ac mains. As the active power taken from the ac mains is proportional to the product of the ac mains voltage and current, this can thus control the overall power delivered to the lamps.

Method II - Control of the ballast-lamp impedance path

Instead of transforming the ac mains voltage directly, Fig. 4 (c) illustrates another dimming method that the - apparatus.. ls._connected _.in series with the lamp system. The connected apparatus is a variable reactance that it does not dissipate any active power ideally. As the overall impedance of the lamp system is adjustable, the magnitude of v_L and input current becomes adjustable.

As discussed in [14] , a two-step inductor consisting of two series inductors is used for the choke in the ballast . With a switch that can bypass one of the two inductors, the overall inductance can be altered in a discrete manner. In [15] , a saturable reactor is used in the ballast that can dim the lamps continuously within a limited range. By adding an extra winding to the reactor and injecting a dc current into this extra winding, the reactor core can be saturated, so that the impedance of the inductor in the ballast can be changed. The resulting effect is to adjust the current flowing to the lamps. A variant in [16] uses a variable reactance with the current to the control winding being provided by a multi-tapped autotransformer, so that different combinations of equivalent series impedance can be realized.

Instead of using passive elements, another approach [17, 18] is based on creating a voltage source connecting in series with the lamp path. In [17] , a dc/ac converter is connected in series with the lamp system. The dc side of the converter is connected to another ac/dc converter, which is supplied from the ac mains. Both converters have to handle active and reactive power. In other words, there is a circulating energy between the two converters. Similar idea is used in [18] that the implementation is based on using transformer coupling. Nevertheless, this circulating energy will introduce energy loss in the system. Apart from lowering_._the__efficiency_,__it is also necess_ary_ to handle the thermal issue.

Method III - Control of the lamp terminal impedance As illustrated in Fig. 4 (d) , the third approach is to use an apparatus that can divert the current from the ballast. The overall effect is to reduce the lamp current i_hamp- In [19] , a switchable capacitor is connected across a lamp. If dimming is required, the capacitor is switched on, so that part of the current from the ballast will be diverted away from the lamp into the capacitor. In this way, the lamp current and hence the lamp power can be controlled in a discrete manner.

Comparing the above methods, Methods I and II are suitable for dimming a plurality of lamps, particularly for existing installation. Method III requires modification or installation of the dimming apparatus on each individual lamp. Although all of the above methods can dim the lamps with magnetic ballasts, they have their respective limitations of

1) requiring expensive and bulky mechanical construction

[7,8] ,

2) introducing undesirable harmonic pollution to the power system [9] - [13] , 3) inapplicable for dimming a plurality of lamps [14, 15, 19] ,

4) handling the total active and reactive power of the load

[7] -[13] ,

5) providing discrete dimming only [7, 8] , [14] - [16] , [19] 6) being practically difficult for central or automatic control [I₁ 8, 14, 15, 17] , 7) reducing the input power factor of the entire lighting system when the lamps are dimmed [14, 15, 16] , and

8) handling dissipative circulating energy [17, 18] .

SUMMARY OF INVENTION

According to the present invention there is provided apparatus for providing improved power quality and dimming control for an electrical lamp or generally electrical lighting systems including systems formed by a plurality of individual lamps. The apparatus achieves the dimming function with real energy saving by controlling the voltage available to the lighting system without handling the active power of the system. Instead, it only handles the reactive power of the entire system. The power rating of the dimmer will be much smaller than the full power rating of the lighting system. The structure of a preferred embodiment is shown in Fig. 5 (a) . Installation of the apparatus is similar to Method II in Fig 4 (c) that the apparatus is connected in series with the lighting system. Instead of a reactance, the apparatus is a reactive power controller (RPC) , which will insert an auxiliary voltage between the ac mains voltage v_ac and the ballast input v_L. Fig. 5 (a) shows the connection of a RPC, RPC_1, connecting between the ac mains and lighting system. The auxiliary voltage is maintained at 90 or 270 degrees out of phase with the current i_L flowing through said apparatus, and wherein the magnitude of the auxiliary voltage is used for varying the voltage applied the lamp.

In one preferred embodiment the RPC in the apparatus is a dc-ac converter, consisting of a half-bridge converter circuit in Fig. 6 (a) or a full-bridge converter circuit in Fig. 6 (b) . The dc side of either configuration is connected to a dc source. This dc source can be an externally controlled dc voltage source or a capacitor. The ac side of the bridge circuit is connected to the nodes ^VA' and ^λB' in Fig. 5 (a) through an inductor L.

The magnitude of the voltage, and hence the current, supplying to the lamp are varied by controlling the amount of reactive power generated from or absorbed by the reactive power controller RPC_1 in Fig. 5 (a) . However, with the use of RPC_1 to adjust the active power delivery to the lamps, the input power factor of the entire system in Fig. 5 (a) will be reduced. As illustrated in Fig. 5 (b) , a second reactive power controller RPC_2 is therefore connected across the ac mains. Its function acts as a power factor improvement circuit. RPC_2 absorbs reactive power from or generates reactive power to the entire lighting system with the RPC_1 in Fig. 5 (a) , so that the overall power factor of the system can be improved. Ideally, the overall reactive power absorbed or generated from the whole is zero, so that the input power factor of the whole system is maintained at unity. Because the two reactive power controllers RPC_1 and RPC_2 essentially handle reactive power only, theoretically no active power is involved in the circuit operation (except that there will be, in practice, some conduction and switching losses in the power electronic devices, and copper and magnetic core losses in the inductors) . Therefore, this proposed dimming method based on reactive power control has the ability of achieving high energy efficiency. It is important to note that the proposed RPC configuration operates in a different manner from conventional two-stage ac-dc converter and dc-ac inverter configuration [17] . While the former configuration handles reactive power only, the latter configuration handles both real and reactive power. There is no circulating energy between the two converters. The value of the dc link capacitor (i.e. the two capacitors (Fig. 6a) or single capacitor (Fig. 6b) that is used as a voltage source) can be made smaller than the one required in [17] , because the amount of energy handled by the two converters is less than that in [17] . Therefore, the proposed configuration has a much small Volt-Ampere rating and higher energy efficiency than those of conventional two- stage ac-dc-ac power converter systems.

Put another way, an advantage of the present invention over prior art devices such as those described in [17] and [18] is that in the present invention, the RPC develops a voltage that is substantially 90° or 270° out of phase with the current flowing through the RPC. Thus the RPC appears, to a lamp ballast or to the AC mains supply, to be either a substantially "pure" inductance or a substantially "pure" capacitance. By "pure", it is meant that the effective resistance of the RPC is negligible compared to its reactance impedance so that the RPC can be regarded as a pure capacitor or a pure inductor, without significant series resistance. As those skilled in the art will appreciate, capacitors and inductors do not dissipate energy (except that is, for secondary effects such as dielectric losses in capacitors or hysteresis losses in inductors) .

In contrast, prior art devices such as those described in [17] and [18] have a complex impedance that includes both a reactive impedance together with a significant real impedance. As those skilled in the art will appreciate, a real impedance is equivalent to a resistor. Thus significant electrical power will be dissipated across the resistive real impedance.

As the two RPCs are dc/ac power converters, the circuit complexity and control method can be simplified in an advantageous embodiment by connecting the dc sides of RPC_1 and RPC_2 together. However, there may seem to be some grounding problems for the two RPCs. The two RPCs may not be directly together and may require isolation transformers to isolate the RPC from either the series or parallel branch. In this patent, a half-bridge-derived converter circuit configuration will be discussed. Without using isolation transformer, the series and parallel RPCs can be developed with the same dc link (Fig.11) in this proposal.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1 (a) - (b) illustrate the operation of a prior art triac-based dimmer,

Fig. 2 (a) - (c) shows the general structure of electronic ballast for discharge lamps,

Fig. 3 illustrates the infrastructure of the lighting system using a dimmable electronic ballast for individual lamp, Fig. 4 (a) - (d) depict various prior art dimming methods for lamps with magnetic ballasts, Fig. 5 (a) - (b) show various configurations of the dimming apparatus,

Fig. 6 (a) - (c) illustrate the schematics of the reactive power controller, Fig. 7 (a) - (b) shows the phasor diagram and the power triangle of the system when the lighting system is inductive, Fig. 8 (a) - (b) shows the phasor diagram and the power triangle of the system when the lighting system is capacitive, Fig. 9 illustrates the structure of the RPC for improving the input power factor,

Figs. 10 (a) - (g) shows the possible implementation of the dimming control apparatus with high power factor, Fig. 11 show the preferred embodiment that RPC_1 and RPC_2 share the same dc source,

Fig. 12 (a) - (d) shows the equivalent circuit of the entire system at different combinations of v±_nv and v_P__±_nv, and Fig. 13 shows an embodiment which uses AC/AC converters.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A. Equivalent circuit of the RPC An equivalent circuit showing the operation of the RPC is illustrated in Fig. 6 (c) , in which the half-bridge [(Fig. 6 (a) ] or the full-bridge [Fig. 6 (b) ] converter is modeled by a sinusoidal voltage source Vi_nv with the same frequency as the ac mains . The dc side of the converter is connected to an externally controlled dc voltage source or a capacitor. The nodes ^VA' and ^λB' are connected to a fictitious ac source v_a through an inductor L. The fundamental frequency of v_a is the same as the line frequency. The switches in the two bridge circuits, [Si and S₂ in Fig. 6 (a) ] and [S₁ to S₄ in Fig. 6 (b) ] , are switched at a frequency much higher than the frequency of the ac mains. The gate signals driving the switches are modulated with the frequency of the ac mains. Many pulse width modulation (PWM) schemes for driving the switches can be used. Examples include sinusoidal PWM, PWM scheme with selective harmonic elimination, uniform sampled PWM, etc [2] . The frequency spectrum of the converter output across ^λC and ^λD' in Fig. 6 consist of mainly the line frequency, the switching frequency, and the inner and cross-modulation of the two frequencies. As the inductor L is used to filter out the undesirable high-frequency components appearing across the nodes 'A' and ^λB' , active and reactive energy flow between v^ and RPC can be represented by considering the line frequency only. Hence, using Vι_av in Fig. 6 (c) to model the converter is applicable.

B. Operating Principles of the RPC The line-frequency model shown in Fig. 6 (c) is used to explain the operating principles of the RPC. The power P and the reactive power Q flow through RPC can be expressed as [3]

P= ^{Vfl v}'"" sinδ (1)

2πfL _β=v.(v.-v»,∞sδ) ₍₂₎

2πfL where f is the mains frequency and δ is the phase difference between v_a and Vj_nv.

Based on Eq. (1) , the RPC has three possible operating modes :

Mode I - If δ > 0, P > 0, the inverter absorbs energy, and the capacitor voltage v_c increases.

Mode II - If 8 < 0, P < 0, the inverter releases energy and v_c decreases .

Mode III - If δ = 0, P = O and v_c remains unchanged. Q is determined by the magnitudes of \r_inv and v_a. If v_a > v±_nvι Q > 0. The RPC acts as a reactive power absorber. Conversely, if v_a < Vi_nv, Q < 0. The RPC acts as a reactive power generator.

Ideally, the RPC is operated in Mode III and acts as either a reactive power absorber or a reactive power generator. In practice, δ is slightly larger than zero, because the absorbed energy is used to compensate the practical energy loss in the RPC. As depicted in Fig. 6 (b) , the value of v±_nv determines the values of Q₁ v_a, and hence the power delivered to the lamps. Depending on the type of the dc source in the RPC, different control methodologies can be applied.

1. Externally controlled voltage source If the dc source is an externally controlled dc voltage source, the amount of Q can be controlled by adjusting the modulation index M of the bridge circuit and/or the magnitude of the dc source voltage v_dc. As the high- frequency ripples will be attenuated by the inductor, only the fundamental component (i.e., the line frequency) will be considered. As shown in [2] ,

where v_inv is the rms value.

Thus, both M and v_dc can be used to vary v_inv.

2. Dc capacitor

If the dc side is a capacitor, control of v_inv is based on varying M and/or δ. Changing the value of M is similar to eq. (3) that

Another way of varying the capacitor voltage v_c, and hence v_xnv, is to make RPC_1 enter into either Mode I or Mode II by adjusting δ. Afterwards, RPC_1 reverts back to Mode III. The ultimate function is to regulate v_c at a desired level so that there will be no net flow of power in the RPC. This function can be realized by using a phase-lock loop to synchronize v_inv with v_a and an error amplifier to adjust the value of δ and/or M.

C. Dimming of the lamps Dimming of the lamps is achieved by controlling the voltage across the lamp system. Fig. 7 (a) - (b) show the phasor diagram and the power diagram of the system, when the lamp system is inductive. Fig. 8 (a) - (b) show the respective diagrams of the system, when the lamp system is capacitive.

1. Inductive lamp system

Fig. 7 depicts the dimming operation at two power levels. When the input power of the lamp system is P₁, the supply voltage to the lamp system is v_L__x and the supply current is i_L_i• iχ,_i lags v_L_ι by a phase difference of φi. As the RPC is connected in series with the lamp system, the current flowing through it is also equal to i_L_i• As shown in Fig. 5 (a) , the required voltage vector of the RPC v_a__! is perpendicular to i_L__lr so that the RPC will not get any real power from the ac mains. i_L_χ lags v_a__x. It means that the RPC absorbs reactive power. As shown in Fig. 7 (b) , the reactive power consists of two components, including the reactive power absorbed by the lamp system Q₁ and the reactive power absorbed by the RPC Q_a_i•

If the lamp system is dimmed down to a power level of P₂, the supply voltage becomes v_L_₂ and the supply current is Ϊ_L_₂■ Ϊ_L_₂ lags v_L_₂ by a phase difference of φ₂. The magnitude of v_{L 2} is smaller than the magnitude of V₁^₁. The vector of the RPC voltage should be perpendicular to i_L_₂ and its magnitude should be higher than that of v_a_χ. Therefore, in order to achieve the dimming function, the magnitude of the dc-side voltage must be increased. In general, the reactive power absorbed by the lamp system decreases (i.e., Q₂ < Qi) . The reactive power absorbed by RPC is changed from Q_a_i to 2. Capacitive lamp system

Fig. 8 depicts the dimming operation at two power levels. When the input power of the lamp system is P₁, the supply voltage to the lamp system is Vχ,_i and the supply current is i_L_i. iz,_i leads v_Lj_ by a phase difference of (J)₁. As shown in Fig. 8 (a) , the required voltage vector of the RPC v_a_χ is still perpendicular to i_L_i- ±_L_I leads v_a__x. It means that the RPC generates reactive power. As shown in Fig, 8 (b) , the reactive power consists of two components, including the reactive power generated by the lamp system Qi and the reactive power generated by the RPC Q_a__±.

If the lamp system is dimmed down to a power level of P₂, the supply voltage becomes v_L_₂ and the supply current is 3-_L 2• Ϊ_L_₂ leads v_L_₂ by a phase difference of φ₂. The magnitude of v_L_₂ is smaller than the magnitude of v_{L x}. The vector of the RPC voltage should also be perpendicular to i_L_₂ and its magnitude should be higher than that of v_a_i. Therefore, in order to achieve the dimming function, the magnitude of the dc-side voltage must be increased. In general, the reactive power generated by the lamp system decreases (i.e., Q₂ < Qi) . The reactive power absorbed by RPC is changed from Q_a_i to Q_a_₂.

D. High Power Factor Configuration As depicted in Fig. 7 and Fig. 8, in addition to the reactive power absorbed or generated by the lamp system, the RPC will absorb or generate additional reactive power from the ac mains. This operation will cause a reduction of the input power factor of the whole lighting system. The power factor is reduced from cos Q₁ to cos Q₂, when the lamp power is adjusted from P₁ to P₂. Although connecting a capacitor across the ac mains input can improve the power factor, the required capacitor value is 1amp-power-dependent . The power factor can be kept unity at any lamp power by connecting another controller RPC_2 across the ac mains as in Fig. 5 (b) .

For inductive lamp systems, the required reactive power Qp [Fig. 7 (b) ] that needs to be generated by RPC_2 is equal to the sum of the reactive power absorbed by the lamp system and RPC_1 at any power level. If the input power of the lamp system is P₁,

If the input power of the lamp system is P₂,

Fig. 7 (b) illustrates the vectors of Q_P_ι and Q_P_₂. Thus, Q_P is load-dependent .

For capacitive lamp systems, the required reactive power Q_P [Fig. 8 (b) ] that needs to be absorbed by RPC_2 is equal to the sum of the reactive power generated by the lamp system and RPC_1 at any power level. Eqs. (5) and (6) are still valid. Fig. 8 (b) illustrates the vectors of Q_P__± and Q_P_₂. Again, Q_P is load-dependent .

In both inductive and capacitive lamp systems, Q_P is determined by eq. (2) . As illustrated in Fig. 9, the bridge converter voltage Vp_±_nv determines the magnitude of Q_P. Eqs. (1) and (2) show the power and reactive flow between the ac mains and RPC_2. That is,

_n _ ^Vac(^Vac-^VP »,v^{C0S δ}p) ,_.

where δ_P is the phase difference between v_ac and v_P_±_nv. Again, RPC_2 has three possible operating modes: Mode I - If δp > 0, Pp > 0, the inverter absorbs energy, and the capacitor voltage v_P__c increases.

Mode II - If δp < 0, P_P < 0, the inverter releases energy and v_P_c decreases. Mode III - If δ_P = 0, P_P = 0 and v_P__c remains unchanged. Q_P is determined by the magnitudes of vp__inv and v_ac. If v_ac > vp ±nv, Qp > 0. The RPC_2 acts as a reactive power absorber. Conversely, if v_ac < v_P_±_nv, Qp < 0. RPC_2 acts as a reactive power generator. Control of Qp is based on monitoring the phase shift between the ac mains voltage and that of the overall input current. Similar to RPC_1, the voltage Vp_i_nv determines Qp and is controlled by adjusting the modulation index of the bridge in RPC_2 and/or δp. For example, if the lamp system is inductive, RPC_1 will be operating as a reactive power absorber and RPC_2 will be operating as a reactive power generator. If the input current lags the supply voltage, the control circuit will increase the dc link voltage (i.e. the voltage of the capacitor, or capacitors, that are used as DC voltage source) of RPC_2, so that RPC_2 will generate more reactive power and hence the phase difference between the supply voltage and input current will be reduced. If the input current leads the supply voltage, the control circuit will decrease the dc link voltage of RPC_2, so that RPC_2 will generate less reactive power and hence the phase difference between the supply voltage and input current will be reduced. As shown in Fig. 7 and Fig. 8, the overall input currents i_L %' and ih_τ.' for power levels P₁ and P₂, respectively, will be in phase with the ac mains and thus the power factor is unity.

E. Sharing the same dc source for RPC_1 and RPC_2 Fig. 10 shows possible implementation of the dimming control apparatus with high power factor. Fig. 10 (a) shows one possibility that two half-bridge converters can be used for RPC_1 and RPC_2. In this method, four dc capacitors and four semiconductor switches are required. Fig. 10 (b) shows another possible method that two full-bridge converters can be used for RPC_1 and RPC_2. In this case, two capacitors and eight switches are required. Other possibilities are the combinations of using half-bridge and full-bridge converters for RPC_1 and RPC_2. Fig. 10 (c) shows the possibility that RPC_1 uses a half-bridge converter and RPC_2 uses a full-bridge converter. Fig. 10 (d) shows the possibility that RPC_1 uses a full-bridge converter and RPC_2 uses a half-bridge converter. If the dc link sources of RPC_1 and RPC_2 are combined, the number of capacitors can be reduced. However, if the two controllers shown in Fig. 8 are connected directly, a short circuit path between the live and neutral may be formed. In order to avoid the short circuit problem, RPC_1 and RPC_2 can be isolated by using transformers. Figs. 10 (e) - (g) show different combinations of using transformers to isolate the RPC_1 and/or RPC_2. This will introduce additional power loss on the transformers.

Fig. 11 shows an embodiment that RPC_1 and RPC_2 share the same dc source and no transformers are needed. As encircled in Fig. 11, RPC_1 is formed by Si, S₂ and the two capacitors, while RPC_2 is formed by S₃, S₄ and the two capacitors. Its operation can be explained by studying the overall operations of RPC_1 and RPC_2 in the dimming apparatus. Fig. 12 (a) - (d) shows the equivalent circuit of the entire system at different combinations of Vi_nv and v_P χ_nv. As Vi_nv and v_P ι_nv can be positive or negative, four possible combinations are resulted. Si, S₂, L, and the two capacitors formed the RPC-I, while S₃, S₄, L_P, and the two capacitors formed the RPC_2. When S₁ and S₃ are switched on, the circuit shown in Fig. 12 (a) is formed. When S₂ and S₃ are switched on, the circuit shown in Fig. 12 (b) is formed. When Si and S₄ are switched on, the circuit shown in Fig. 12 (c) is formed. Finally, when S₂ and S₄ are switched on, the circuit shown in Fig. 12 (d) is formed.

On the one hand, as explained above, v_L can be varied by controlling the modulating index of the converter formed by Si and S₂ and the phase shift between v_inv and v_a. On the other hand, the power factor can be improved by controlling the phase shift between v_ac and v_P_±_nv, and the modulation index of the RPC_2. The two converters do not handle any active power in the whole system.

Adjusting the phase shift between v_ac and v_P i_nv, and the modulation index of the inverter, formed by the switches S₃ and S₄, can control the amount of reactive power generated or absorbed by RPC_2. As the dc source is common to RPC_1 and RPC_2, dimming of the lamps can be performed by varying the modulation index of the inverter formed by the switches Si and S₂.

In alternative embodiments, an AC voltage source (instead of a DC voltage source) and an AC/AC voltage converter (instead of a DC/AC voltage converter) may be used to develop the auxiliary AC voltage across RPC_1 and/or RPC_2, as shown at Fig. 13.

The frequency of an AC main supply will typically vary. For example a nominally 50Hz AC mains supply may vary between 49Hz and 51Hz. Thus in many embodiments it will be desirable to include a frequency locking means such as a frequency locked loop, or a phase locked loop (PLL) . Ideally, the voltage and current flowing through RPC__1 and/or RPC_2 will be exactly 90° out of phase so that RPC_1 and/or RPC_2 behave as perfect inductors or perfect capacitors. In practice, there will be some losses in RPC_1 and/or RPC_2. Some losses will result from switching losses, e.g. from the half-bridge or full-bridge circuitry. Also, control circuitry will require electrical power to function and this electrical power will appear, to the magnetic ballast and to the AC mains supply, as a small real resistance. Thus although it is preferred that the voltage and current are exactly 90° out of phase as for a pure reactance, the invention may be put into effect when the voltage and current are within 1°, 5° or 10° of being 90° out of phase. As well as the full-bridge and half-bridge topologies described above, other types of power converter may be used to generate the auxiliary voltage. It is anticipated that in most embodiments, power semiconductor devices will be used as the switches for, say, the half-bridge or the full-bridge An example of an appropriate power semiconductor device is an insulated gate bipolar transistor (IGBT) although, as those skilled in the art will appreciate, other types of devices may be used instead.

The present invention may be applied to electronic lamp ballasts as well as to magnetic lamp ballasts. As those skilled in the art will appreciate, some electronic ballasts include power regulation, for example to allow operation of lamps from an AC mains supply in the range 110V to 240V. The present invention may not be so effective when used in conjunction with electronic ballasts that include power regulation as, if the RPC increases the effective impedance of the AC mains supply, such electronic ballasts will reduce their equivalent impedance which will to some extent counteract the impedance increase of the RPC.

The present invention may be used with gaseous discharge lamps such as high pressure sodium lamps, low pressure sodium lamps, fluorescent lamps and high intensity discharge lamps such as metal halide lamps.

The present invention relates in particular to a simple general purpose and non-intrusive dimming system that can be retro-fitted to existing lamps and which is non-intrusive in the sense that when not in use the dimming system has no effect on the normal operation of the lamp. As shown in Fig. 9, a by-pass switch may be connected across RPC_1 and is open if RPC_1 is in operation. If RPC_1 is not in use, the by-pass switch will be closed and the lighting system will be supplied from the ac mains with any interruption.

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[3] B. M. Weedy, Electric Power Systems, John Wiley and Sons, Inc., 1988.

[4] Global Electronic Ballast Markets Technologies, Applications, Trends and Competitive Environment, Darnell Group Inc., December 2000. [5] R. Simpson, Lighting Control - Technology and Applications, Focal Press, 2003.

[6] W. Yan, Y. Ho, and S. Hui, "Stability study and control methods for small wattage high-intensity-discharge (HID) lamps," IEEE Transactions on Industrial Applications, vol. 37, no. 5, pp. 1522-1530, Sept. 2001. [7] E. Daniel, "Dimming system and method for magnetically ballasted gaseous discharge lamps," US patent 6,271, 635, Aug. 7, 2001.

[8] E. Persson and D. Kuusito, "A performance comparison of electronic vs. magnetic ballast for power gas-discharge

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[10]L. Lindauer, et al, "Power regulator," CJS patent

5,714,847, Feb. 3, 1998. [11] J. Hesler, et al, "Power regulator employing a sinusoidal reference," US patent 6,407,515, Jun. 18, 2002.

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2000.

[13] B. Szabados, "Apparatus for dimming a fluorescent lamp with a magnetic ballast," US patent 6, 538, 395, Mar. 25,

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11, 1995. [16] R. Scoggins, et al, "Power regulation of electrical loads to provide reduction in power consumption, " US patent 6,486,641, Nov. 26, 2002. [17] E. Olcina, "Static energy regulator for lighting networks with control of the quantity of the intensity and/or voltage, harmonic content and reactive energy- supplied to the load," US patent 5,450,311, Sept. 1995. [18] Scoggin, et al, "Power regulation of electrical loads to provide reduction in power consumption, " US patent 6,486,641, Nov. 2002.

[19] R. Lesea, et al, "Method and system for switchable light levels in operating gas discharge lamps with an inexpensive single ballast," US patent 5, 949,196, JuI. 11, 1999.

Claims

CLAIMS :

1. A reactive power controller for dimming an AC mains lighting system, the reactive power controller comprising: first and second terminals for connection to an AC mains lighting system; voltage developing means for developing an AC voltage at mains frequency between the first and second terminals; signal receiving means for receiving a dimming level control signal; and control means for controlling the voltage developed by the voltage developing means on the basis of the dimming level control signal .

2. A reactive power controller according to claim 1, wherein the control means is operable to cause the voltage developing means to develop an AC voltage that is out of phase with current flowing from the first terminal to the second terminal .

3. A reactive power controller according to claim 2, wherein the control means is operable to cause the voltage developing means to develop an AC voltage that is either substantially 90° or substantially 270° out of phase with current flowing from the first terminal to the second terminal.

4. A reactive power controller according to claim 3, wherein the control means is operable to cause the voltage developing means to develop an AC voltage that is in the range 80° to 100° or in the range 260° to 280° out of phase with current flowing from the first terminal to the second terminal .

5. A reactive power controller according to claim 4, wherein the phase is in the range 85° to 95° or in the range 265° to 275° .

6. A reactive power controller according to claim 4, wherein the phase is in the range 89° to 91° or in the range 269° to 271° .

7. A reactive power controller according to claim 2 or 3, wherein the control means comprises a frequency locking means .

8. A reactive power controller according to any one of claims 2 to 7, wherein the control means comprises a phase locked loop (PLL) .

9. A reactive power controller according to any one of claims 1 to 8, comprising means for receiving a DC voltage from a DC voltage source, wherein the voltage developing means is operable to convert the DC voltage to the AC voltage.

10. A reactive power controller according to any one of claims 1 to 8, comprising a DC voltage source, wherein the voltage developing means is operable to convert a DC voltage from the DC voltage source to the AC voltage.

11. A reactive power controller according to claim 10, wherein the control means is operable to control the voltage of the DC voltage source on the basis of the dimming level control signal, and thereby control the voltage developed by the voltage developing means.

12. A reactive power controller according to claim 10, wherein the DC voltage source comprises a capacitor means .

13. A reactive power controller according to claim 10, wherein the DC voltage source comprises two series connected capacitor means.

14. A reactive power controller according to claim 12 or 13, when dependent on claim 2, wherein the control means is operable to control on the basis of the dimming level control signal the phase of the AC voltage relative to current flowing from the first terminal to the second terminal, to thereby store or release energy in the capacitor means, to thereby change the voltage of the capacitor means, and thereby control the voltage developed by the voltage developing means.

15. A reactive power controller according to any one of claims 9 to 14, wherein the voltage developing means comprises switching means to convert the DC voltage to the AC voltage by alternately connecting the DC voltage source to the first and second terminals with a forward polarity and then a reverse polarity.

16. A reactive power controller according to claim 15, wherein the switching means comprises a half-bridge converter.

17. A reactive power controller according to claim 15, wherein the switching means comprises a full bridge converter.

18. A reactive power controller according to any one of claims 15 to 17, wherein the switching means comprises a power semiconductor switching device.

19. A reactive controller according to claim 18, wherein the switching means comprises an insulated gate bipolar transistor (IGBT) .

20. A reactive power controller according to any one of claims 15 to 19, wherein the voltage developing means comprises inductor means for attenuating high frequency current ripples resulting from the switching means.

21. A reactive power controller according to any one of claims 15 to 20, wherein the control means is operable to control on the basis of the dimming level control signal a modulation index of the switching means, and thereby control the voltage developed by the voltage developing means .

22. A reactive power controller according to any preceding claim, wherein the signal receiving means is operable to receive a DC voltage as the dimming level control signal.

23. A series-shunt reactive power controller for dimming an AC mains lighting system, the series-shunt reactive power controller comprising: a first reactive power controller according to any^¬ one of claims 1 to 22, for series connection to the AC mains lighting system to reactively control the power supplied to the AC mains lighting system; a second reactive power controller according to any one of claims 1 to 22, for shunt connection across the first reactive power controller and the AC mains lighting system to improve the power factor of the first reactive power controller and the AC mains lighting system, wherein the first and second reactive power controllers are electrically connected to share a common DC voltage source.

24. A series-shunt reactive power controller according to claim 23, wherein the first and second reactive power controllers are directly connected to share a common DC voltage source.

25. A series-shunt reactive power controller according to claim 23, comprising a transformer means to connect the first and second reactive power controllers to share a common DC voltage source.

26. A dimmable AC mains lighting system comprising: means for receiving an incoming AC mains supply; one or more lamp ballasts; and a first reactive power controller according to any one of claims 1 to 22, wherein the first reactive power controller is series connected via its first and second terminals between the incoming mains supply and the one or more magnetic lamp ballasts.

27. A dimmable AC mains lighting system according to claim 26, wherein the one or more lamp ballasts comprise one or more magnetic lamp ballasts.

28. A dimmable AC mains lighting system according to claim 26, wherein the one or more lamp ballasts comprise one or more electronic lamp ballasts.

29. A dimmable AC mains lighting system according to any one of claims 26 to 28, comprising a second reactive power controller according to any one of claims 1 to 22, wherein the second reactive power controller is shunt connected via its first and second terminals across the incoming mains supply.

30. A dimmable AC mains lighting system according to claim 29, wherein the first reactive power controller and the second reactive power controller are electrically connected to share a common DC voltage source.

31. A dimmable AC mains lighting system according to any one of claims 26 to 30, wherein the means for receiving an incoming AC mains supply comprises switchgear.

32. A dimmable AC mains lighting system according to any one of claims 26 to 31, comprising one or more lamps respectively connected to the one or more lamp ballasts.

33. A dimmable lighting system according to claim 32, wherein the one or more lamps comprise gaseous discharge lamps.

34. A dimmable lighting system according to any one of claims 26 to 33, comprising by-pass switch means across the first reactive power controller.

35. A reactive power controller as hereinbefore described and/or with reference to any one of Figures 5 to 12.

36. A dimmable AC mains lighting system as hereinbefore described and/or with reference to any one of Figures 5 to 12.

37. There is disclosed a reactive power controller (RPC) for dimming AC mains lighting systems that use ballasts, for example magnetic ballasts. In one embodiment, a DC/AC converter is used to convert a DC voltage, from a DC voltage source, to an AC voltage that is out of phase with current flowing through the RPC. For example, if the AC voltage is 90° or 270° out of phase with the current then the RPC will appear, to an AC mains lighting system, to be effectively a capacitor or an inductor, thus increasing the effective impedance of the mains supply to the magnetic ballast . The RPC creates a controllable AC voltage at mains frequency, so that the net voltage to the load is adjustable to dim t"he load. The net voltage is the vectorial sum of the AC mains voltage and the AC voltage created by the RPC. A second RPC may be used to improve the power factor of the AC mains lighting system. The second RPC generates or absorbs the required reactive power of the whole system, so that the input power factor of the AC mains lighting system becomes substantially unity. In an advantageous embodiment, the first and second RPCs are connected to share a common DC voltage source.