The present invention relates to AC voltage supply systems, particularly uninterruptible power supply (UPS) systems that can be connected in parallel to share power to one or more loads.
The following publications and patents describe AC supply systems, uninterruptible power supplies, parallel operation of AC generators, and other aspects related to the present invention:
The present invention provides a modular AC supply that can be connected in parallel to drive loads. For example, a 1 MW load could be driven by ten 100 kW modular systems. A potential problem of parallel operation is instability and coordination of operation and the outputs of the modules. An advantage of the present invention is that communication lines between the supply modules is not required. Rather, each supply incorporates a discrete controller functioning based on parameters sensed locally and without reference to other supplies.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
The primary purpose of an uninterruptible power supply (UPS) is to provide conditioned, continuous power to one or more loads. With the proliferation of electronic loads like computers, the incidence of power quality-related problems is growing. Similarly, sensitive communication equipment, such as cellular telephone cell sites, may typically be powered by a utility AC power grid, but it is necessary or desirable that uniformly conditioned power continue to be supplied if grid power is interrupted or degrades. Preferably, the UPS system will provide backup power by way of battery strings or another DC generator in the case of utility power outages, and will also correct for high and low power events (surges and sags).
Two general types of UPS systems are illustrated in
In the “on-line” system represented in
Similarly, in the line-interactive UPS system of
In either the line-interactive or on-line system, when multiple UPS modules 18 are provided, it is desirable that such modules share the load equally or in accordance with their capacities, and their outputs should be coordinated both in voltage and frequency. In some systems, this is achieved by a “central controller” and data communication wiring between the controller of each AC supply. The central controller dictates the current sharing of the different modules based on signals received from and communicated to them. In many cases, a more desirable system is a “distributed” system in which no central controller is used and no data communication line is required between the different AC supplies. Rather, each supply has a discrete controller which adapts the unit for current sharing based on locally sensed parameters. The distributed system is particularly preferred when the supplies are located remotely, and when it is desired to add supplies such as for expansion of a power system.
In the present invention, as represented in
With reference to
The current regulator 26 of
The relationships of the actual voltage and current values and the reference values are given in these figures. In addition, the relationship between the actual voltages and the α, β, P, Q, R, S values are provided in these figures. See also
In the case of a single module, rather than paralleled modules, it is not necessary to provide current feedback to the voltage reference generator, and the system of
In practice, the system is implemented with a digital controller as represented in
While described with reference to UPS systems, the AC modules of the invention can be used as primary AC supplies with or without an associated utilities grid. The DC supplies for the inverters are not limited to batteries, but can include any other source of power. Without limiting the generality of the foregoing, such source can include photovoltaic arrays, batteries, wind power generators, fuel cells, flywheels, microturbines, and so on.
Consider the relationship among the ac power supply's output voltage, load voltage and the isolation inductor between the ac power supply and the load. Suppose that the load voltage is Vload=Vsin(2πft), the output voltage of the ac power supply is Vsupply=(V+ΔV)sin(2πft+Δθ), and Lf may be the leakage inductance of an isolation transformer. Here the V is the amplitude of the voltage, the f is the voltage frequency, ΔV is the voltage amplitude difference between load side and the ac power supply side. 2πft is the phase angle of the load voltage and Δθ is the angle difference between load side voltage and the power supply side voltage. Then the current through the inductor is
i(Lf)=iP+iQ,
iP=V/(2πf*Lf)*[(1+ΔV/V)*sin(Δθ)]*sin(2πft),
it is the real power current because it is in the phase with the voltage, and
iQ=V/(2πf*Lf)*[(1−cos(Δθ)−ΔV/V*cos(Δθ)]*cos(2πft),
it is the reactive current because it has 90 degrees angle shift respect to the voltage. The above relationship reveals that the current iP and iQ can be controlled by controlling the ΔV and Δθ. In contrary, ΔV and Δθ can also be calculated from measured Ip and IQ.
Normally, ΔV/V is less than 0.05, and Δθ is less than 3 degree or 0.05 radians because Lf is small. In this case, real power iP is mainly determined by Δθ, and the reactive power iQ is mainly decided by ΔV.
Because θ is the integral of frequency with respect to the time, Δθ can be reached by change of the frequency f of the ac power supply. The droop characteristics of iP-f and iQ-V are the known fundamental current sharing control principle.
A 3-phase voltage system can be represented by a voltage vector Vα-β. (see
Vα-β=⅔*{Van(t)+Vbn(t)*A+Vcn(t)*A2}=Vα+jV⊖,
Where A=e−j2π/3
The trace of the tip of voltage vector Vα-β of a balanced 3-phase voltage should be a circle with amplitude of V and rotates at the angle speed of 2πf, and the phase angle of the Van is θ=2πft=atan−1(Vβ/Vα).
The voltage control of a 3-phase system is normally not in the physical a-b-c frame but in a rotating P-Q frame. The axis P overlaps the vector Vα-β. The 3-phase voltage system can be presented also by a vector VP-Q in P-Q frame.
VP-Q=VP+jVQ
VP=Vα*cos(θ)+Vβ*sin(θ)
VQ=−Vα*sin(θ)+Vβ*cos(θ)
For an ideal 3-phase voltage system, VQ should be zero, VP-Q=VP, and it is a constant.
In the same way, a 3-phase current can be presented by vector Iα-β=Iα+jIβin the stationary α-β frame and IP-Q=IP+jIQ in the rotating P-Q frame. IP represents the real power current and IQ represents the reactive current. The power factor is cos(Φ), Φ=atan−1(IQ/IP). However, for an unbalanced 3-phase load, except the forward sequence current component, the current also has a backward sequence current. The backward sequence current component can be represented in a backward rotating R-S frame as IR-S=IR+jIS. R-S frame rotates at a same angle speed as the P_Q frame but at inverse direction, and the angle between axis R and axis P is always equal to 2θ. This means that a 3-phase voltage supply with an inner current control loop must control all four current components IP, IQ, IR and IS so that a balanced 3-phase voltage system can be realized.
For a single operating ac power supply, the reference IPref, IQref, IRref and ISref are created by voltage regulator VP and VQ in P-Q frame and VR and VS in R-S frame respectively. The reference VPref=V, VQref=0, VRref=Vcos(2θ) and VSref=−Vsin(2θ), where V is the nominal voltage amplitude, and θ=2πft.
When ac power supply operates at parallel mode, the angle θ has to be modified by Δθ determined by the droop of id-f curve, and the voltage amplitude V has to be modified by an adjustment ΔV determined by the droop of iq-V curve. Here, iq normal is the actual sensed value. In the present invention the predicted value iQ
The 3-phase ac power supply has a DSP based controller with an inner current-loop and an outer voltage-loop control. It works for either on-line UPS system or line-interactive UPS system. For line-interactive UPS system, the seemly transition from UPS mode to by-pass mode is an additional requirement. This requirement is also the responsibility of the controller of each ac power supply.
The seemly transition from UPS mode to by-pass mode is realized by take of the advantage of the zero-inertia of the ac power supply. The controller monitors the grid voltage continuously, and compares the grid voltage vector with the ac power supply's voltage vector in the ad frame, when the amplitude and the angle of the two voltage vectors are close enough, say voltage amplitude within 5% difference and angle within 5 degrees, the ac power supply will switch to current mode and takes grid's phase angle as its angle reference while keeping Ipref, IQref, IRref and ISref unchanged for tens milliseconds waiting the close of the main switch. After the main switch closed, the ac power supply will switch to charge mode.
the voltage vector's peak and phase angle*/
s.inverter_v_alpha = (1.0/3.0) * (s.inverter_vab −
s.inverter_v_beta = (SQRT3/3.0) * s.inverter_vbc;
s.inverter_v_peak = sqrt(s.inverter_v_alpha * s.inverter_v_alpha +
go to voltage mode directly.
waiting!
take the measured voltage vector's angle as inverter's phase
The application claims the benefit of U.S. Provisional Application No. 60/359,431, filed Feb. 22, 2002, the benefit of which is hereby claimed under 35 U.S.C. §119.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US02/08976 | 2/25/2002 | WO | 00 | 1/12/2005 |
Publishing Document | Publishing Date | Country | Kind |
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WO03/073190 | 9/4/2003 | WO | A |
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Number | Date | Country | |
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20050117375 A1 | Jun 2005 | US |
Number | Date | Country | |
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60359431 | Feb 2002 | US |