Electrical transmission or interconnection systems – Plural supply circuits or sources – Substitute or emergency source
Reexamination Certificate
2003-10-02
2004-10-12
Toatley, Jr., Gregory J. (Department: 2836)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Substitute or emergency source
C363S065000
Reexamination Certificate
active
06803679
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a method for controlling a parallel redundant power system composed of UPS modules coupled in parallel, and more particularly to a method that utilizes the internal impedance of the UPS modules in accompaniment with a current shift method to simulate a virtual inductor so that the drawbacks caused from the actual inductor are eliminated.
2. Description of Related Arts
With the almost total-dependence of modern societies on electricity for employment, living, communication and entertainment, any disruption to an electricity supply brings about a temporary halt to civilization as we know it. Especially in the present age where a breathtaking variety of electronic products are widely used, serious economic loss may possibly occur if a company loses critical data when the electricity supply is suddenly interrupted.
To solve the above problem, the uninterruptible power supply (UPS) is developed. However, because of the quick evolvement of the load specifications, a single UPS device may be unable to satisfy the electricity requirement, especially when the amount of the loads is increased. Therefore, it is a natural trend to design the UPSs as modules to perform a parallel redundant power supply system.
The control techniques for the parallel redundant power supply system are mainly categorized into two types, the wired-connected mode and connectionless mode. For example; U.S. Pat. No. 5,257,180 “Controlling system for parallel operation of AC output inverters with restrained crossed current” adopts the wire-connected mode. In another aspect, the connectionless mode is applied in U.S. Pat. No. 5,745,356 “Independent load sharing of AC power systems connected in parallel”, U.S. Pat. No. 6,118,680 “Methods and apparatus for load sharing between parallel inverters in an AC power supply” and U.S. Pat. No. 6,356,471 “Dynamic feedback adaptive control system and method for paralleling electric power source and an uninterruptible power supply including the same”.
At present, many kinds of loads need the steady electricity supply for operating normally whereby strict control of the power quality in the parallel configuration is essential. One example of the connectionless mode being superior to the wire-connected mode is that the connectionless mode does not have the problem of system-level failure of single point failure, whereby the entire UPS system can achieve the highest reliability. In U.S. Pat. No. 5,745,356, only the DC energy is calculated and only the differentiation of the active power is concerned; however the reactive power is difficult to control. The differentiation manner in the prior art has some further drawbacks, such as the low anti-interference ability and being unable to process harmonic waves. In U.S. Pat. No. 6,118,680, the phase locking is determined by whether the voltage area is zero. Not only should the calculation accuracy be considered, but also whether the output voltage contains the harmonic wave must be considered. Further, with regard to the active power and the reactive power, U.S. Pat. No. 6,118,680 can not provide an efficient control manner. In U.S. Pat. No. 6,356,471, an inductor must be additionally coupled to the output of the power system in parallel.
The basic concept of the parallel connectionless operation is from the parallel connection of the power generators in the power system. However, there are still some different physical characteristics between the UPS and the power generator. The internal impedance of the generators represents as a large reactance, whereas on the contrary, the UPS has a small internal resistance.
With reference to
FIG. 1
, by simulating the power generator models to perform the parallel configuration of the UPS modules, the output terminal of each UPS module is coupled with a large inductor (Z
1
, Z
2
) in series.
The UPS module is simulated by an ideal voltage source {right arrow over (V
oi
)}=|{right arrow over (V
oi
)}|<&dgr;
i
and an equivalent impedance Z
oi
, wherein the impedance represents the resistance character. Further, Z
si
=jX
si
represents the output inductance of the “i
th
” UPS module (i is an ordinal), and Z
si
is much larger than Z
oi
, (Z
si
>>Z
oi
). If the internal impedance Z
oi
of the UPS module is ignored, the output power of the UPS module is calculated by the following equation:
P
oi
=
&LeftBracketingBar;
V
oi
→
&RightBracketingBar;
·
&LeftBracketingBar;
V
o
→
&RightBracketingBar;
X
si
sin
δ
i
(
1
)
Q
oi
=
&LeftBracketingBar;
V
oi
→
&RightBracketingBar;
·
&LeftBracketingBar;
V
o
→
&RightBracketingBar;
cos
δ
i
-
&LeftBracketingBar;
V
o
→
&RightBracketingBar;
2
X
si
(
2
)
According to the equation (1), the active power P
oi
is directly proportional to the phase angle &dgr;
i
that is defined between the {right arrow over (V
oi
)} and {right arrow over (V
o
)}. The reactive power is approximately directly proportional to the |{right arrow over (V
o
)}| to represent the output voltage amplitude.
FIGS. 2A and 2B
show a first line indicating the relationship between the active power and frequency and a second line indicating the relationship between the reactive power and voltage.
&ohgr;=&ohgr;
o
−k
&ohgr;
*P
(3)
V=V
o−k
V
*Q
(4)
where &ohgr;
o
can be set to 50 Hz or 60 Hz, and V
o
can be set to 120Vac or 230Vac depended on the power output requirement.
Based on the equations (1) to (4), the parallel connectionless configuration is able to be established by the droop method.
With reference to
FIG. 3
, the relationship between the output voltage of each UPS module and the total output voltage of all UPS modules is shown by two vectors.
In
FIG. 3
, vector {right arrow over (V
oi
)} means the equivalent output voltage of the inverter of the i
th
UPS module (i is an ordinal), and {right arrow over (V
o
)} represents the output voltage that is composed by all parallel connected UPS modules. In the condition that {right arrow over (V
o
)} remains at a constant, when the frequency of {right arrow over (V
oi
)} is increasing, the phase angle &egr;
i
will accordingly increase. Further, the reactive power output from the ith UPS module also gets larger based on equation (1). According to equation (3) and the relationship of P−&ohgr;, the increase of the active power will cause the decrease in the frequency of {right arrow over (V
oi
)}, thus the phase angle &dgr;
i
will be accordingly decreased. If the frequency of {right arrow over (V
oi
)} is decreased, the same result still will occur. Finally, the {right arrow over (V
oi
)} and {right arrow over (V
o
)} will finally have the same frequency and the phase angle &dgr;
i
between the {right arrow over (V
oi
)} and {right arrow over (V
o
)} is keep at a constant.
With reference to
FIG. 2B
, the relationship between the reactive power and the voltage amplitude is shown. In the event of {right arrow over (V
o
)} remains at a constant, when the output voltage of {right arrow over (V
oi
)} is increasing, the reactive power output from the UPS i
th
UPS module will accordingly increase based on equation (2). Further referring to the Q−V relationship in equation (4), the output voltage amplitude will then decrease. The balance relationship between {right arrow over (V
oi
)} and {right arrow over (V
o
)} will ensure that both the amplitude of {right arrow over (V
oi
)} and {right arrow over (V
o
)} can remain at a static status.
From the foregoing description, the droop method can be applied to accomplish the parallel connectionless operation on the premise that the output of each UPS module is coupled with a large inductor. Since the inductor is composed of windings, the entire size and weight of the UPS system will become extremely large and heavy if the inductor is coupled to the output of the UPS module. Moreover, wh
Luo Han-Sheng
Tian Shou-Long
Tsai Chia-Ming
Pelton, Esq. William E.
Phoenixtec Power Co. Ltd.
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