Wireless controller of parallel inverter in distributed online UPS system
Josep Guerrero', luis garcia de Vicufia', José Martas' *, Homme Miret "and Miguel Castilla"
. Industrial automation and information system engineering department. Catalonia University of Political Science
C. Count de Houguere, 187.08036- Barcelona. Spain. E-mail: josep.m.guerrero@upc.es Electrical Engineering Department. Catalonia University of Technology
AV。 victor bal guer s/n . 08800 I-Vilanova I la Geltrh。 Spain
In this paper, a new parallel controller is proposed.
On-line UPS inverter without control line
The interconnection is introduced. Wireless control technology is
Based on the well-known drooping method, the method comprises the following steps
P-oan and Q-V schemes are introduced into the inverter to
Distribute the power of load correctly. Sagging method
It has been widely used between load sharing applications.
Different parallel inverters. However, this method
There are several disadvantages that limit its application, such as
Trade-off between output voltage regulation and power sharing
Accuracy, slow transient response, and frequency and phase.
Deviation. This last disadvantage makes
Method, because in this case, each module
Must be in phase with the public AC power supply. In order to overcome these
Limitations, we put forward a new control scheme, giving
Strictly speaking, parallel UPS system has appropriate transient response.
Synchronize with the frequency and phase of the AC power supply, and
Excellent power sharing. Simulation and experimental results
The validity of the proposed method is verified.
1. Introduction
Parallel operation of distributed uninterruptible power supply
Power supply (UPS) is a suitable power supply solution.
Critical and sensitive loads at high reliability and high power.
Availability is necessary. In the past few years, many controls
The scheme of parallel inverter is proposed,
It comes from the parallel scheme of dc-dc converter.
[I], such as master-slave control [2], or democracy
Control [3]. In contrast, the new control scheme
Recently, such as chain structure control [4], or
Distributed control [5]. However, all these schemes require
Control the interconnection between modules, so
The reliability of systems decreases because they may be a source.
Noise and malfunction. In addition, these communication lines
The physical location of the module is limited [6].
In this sense, several control technologies have been proposed.
There is no control interconnection, such as droop method.
In this method, the control loop realizes good power sharing.
Strictly adjust the output voltage frequency.
And the purpose is to
Compensate the imbalance between active and reactive power [7].
This concept comes from the power system theory.
The frequency of the generator drops.
Attracted to the utility line increased [8].
0-7803-7906-3/03/$ 17.00 02003 IEEE。 1637
However, this control method has an inherent trade-off.
Between voltage regulation and power sharing. Besides,
This method shows a slow dynamic response because it requires
Low-pass filter for calculating active average value
And reactive power. Therefore, stability and dynamics
The whole system is almost unaffected by these characteristics.
With these filters and the values of attenuation coefficients,
It is limited by the maximum allowable deviation.
Output voltage amplitude and frequency.
In addition, when the active power increases, the voltage drop
Characteristics cause the frequency to deviate from the nominal value.
Value, and thus lead to variable phase.
Difference between output voltage of power supply and inverter.
When the bypass switch must
Connect the public line directly to the critical bus instead of
Its phase difference. In [9], two possibilities are put forward.
In order to realize the phase synchronization of parallel lines
UPS system. The first one is location-specific
The modules near the bypass switch must be synchronized.
Support the output voltage of power supply when overload occurs.
The state before opening. The second possibility is to wait.
At the moment when the phase matching produces the connection.
Bypass
However, the two folds mentioned cannot be applied to
Parallel online UPS system, because the maximum transmission time
Should be less than 1% of the line period, and all modules
Must always be synchronized with the power supply.
At present. Therefore, the module should be ready for transfer.
Supply power directly from the main power supply to the critical bus under the following conditions
Refers to overload or failure.
In our previous work [1 1][ 12], we proposed different.
Control scheme to overcome
Conventional drooping method. However, these controllers are controlled by
They are not suitable for applying for parallel online-
UPS system. This paper presents a novel wireless control scheme.
It is suggested to connect different online UPS modules in parallel.
Performance and limitation requirements. controller
Provide: 1) appropriate transient response; 2) Power sharing
Accuracy; 3) stable frequency operation; And 4) good phase.
Matching between output voltage and public line.
Therefore, this new method is especially suitable for parallel.
With real redundancy, high reliability and
Power availability. The simulation and experimental results are as follows
The report confirms the effectiveness of this control scheme.
Figure 1. Equivalent circuit of 10 a bus
t "
Figure 2. P-odraop function.
1 1. Rotation speed of traditional drooping method
Figure 1 shows the equivalent circuit of inverter connection.
Connected to a common bus through a coupling impedance. When this
Impedance is inductive and absorbs active and reactive power.
The directional load can be expressed as
EVcosQ - V2 Q=
Where Xis is the output reactance of the inverter; Q is the phase.
The angle between the inverter output voltage and.
Voltage of common bus; E and v are amplitudes.
The output voltage and bus voltage of the inverter,
They are.
It can be deduced from the above equation
Power p mainly depends on power angle q,
While the reactive power q mainly depends on the output voltage.
Amplitude. Therefore, most wireless controls
The parallel inverter uses the traditional droop method,
This introduces the following drop in the amplitude e.
And the frequency u of the inverter output voltage.
u = w -mP (3)
E = E ' - n Q，(4)
W* and E' are that frequency and amplitude of the output voltage.
When they are empty; M and n are decline factors.
Frequency and amplitude respectively.
In addition, coupling inductance is needed between the.
Key bus of inverter output and fixed output
Impedance to ensure proper power flow. however
It is bulky and increases: Size and cost of UPS
Module. In addition, the output voltage is severely distorted.
When supplying power to a nonlinear load, due to the output impedance
It's pure inductance.
As we all know, if the sag coefficient increases,
Then good power sharing is at the expense.
The voltage regulation is reduced (see Figure 2).
The inherent trade-offs of this scheme limit
Referring to the coefficient, this may be a serious limitation.
Transient response, power sharing accuracy and
System stability.
On the other hand, in order to perform the descending function,
Represented by (3) and (4), it is necessary to calculate
Output the average value of a valid line period.
Reactive instantaneous power. This can be achieved in the following ways.
Means that the bandwidth of the low-pass filter is smaller than it.
Closed loop inverter. Therefore, power
The calculation filter and droop coefficient largely determine.
Range, Dynamics and Stability of Parallel Inverter
System [13 1.
In short, there are several inherent drooping methods.
Application of 1.0 wireless parallel system
Online UPS can be summarized as follows:
Static balance between output voltage regulation
(frequency and amplitude) and power sharing
Accuracy (active and passive).
2) Limited transient response. system dynamics
Depend on that characteristics of the pow calculation filter,
Drop coefficient and output impedance.
The AC power supply is out of synchronization. Frequency sum
Due to frequency drop, phase deviation
This method is not suitable for parallel connection.
On-line UPS system, in which each UPS shall
Continuously synchronize with public AC power supply.
1)
3)
1 1 1. Suggested control of parallel online UPS.
inverter
In this work, we will strive to overcome the above limitations.
And synthesize a new control strategy.
Suitable for high-performance communication lines
Parallel industrial UPS. The goal is
When not in use, connect online UPS inverter in parallel.
Control interconnection. This system, also known as
Inverter is preferred and should be synchronized to continuously.
Utility line. In case of overload or inverter failure
When occurs, the static bypass switch can connect the input line to
Load, bypassing inverter: rter [14][ 15].
Fig. 3 shows a distributed online
UPS system. The system consists of two buses: public facilities
A bus connected to a common AC power supply; and
Safety bus, connected to distributed critical load. this
The interface between these buses is based on some online.
UPS modules are connected in parallel to provide
Continuously supply power to the load [16]. UPS module
Comprises a rectifier, a group of batteries, an inverter and static electricity.
Bypass switch.
1
1638
alternating-current main
bus
I I I
J distribute the load!
Figure 3. Online distributed UPS system.
syposr /
I 4
(4
Figure 4. Operation mode of online UPS.
(1) Normal operation. (b) bypass operation. (c) power failure
Main operation modes of distributed online UPS
1) Normal operation: power flows to the load, from
Power is supplied through distributed UPS devices.
2) Power failure: When the public AC power fails.
UPS inverter supplies power to load.
Battery, without interruption.
Bypass operation: When overload occurs,
The bypass switch must be connected to the critical bus.
Directly connected to the AC power supply to ensure that
Continuously supply load to avoid damage.
UPS module.
Therefore, the output voltage waveform should be
When the last one appears, synchronize with the power supply.
The system is listed as follows (see Figure 5):
3)
However, as we said before, the traditional droop
Method cannot meet the requirements of
Utility, due to the frequency change of inverter
Cause phase deviation.
In order to obtain the required performance, we give a transient.
Power drop without frequency deviation at steady state,
Previously proposed by OUT in [1 1 1
w=o -mP (5)
Where there is an active power signal with no DC component,
This is made up of
. -
Identification number t- 1
P= p，
(s + t - ' ) ( s + o，)
Z is the time constant of the instantaneous descending action.
The transient drop function ensures a stable frequency.
Regulation under steady-state conditions, at the same time
Time, through adjustment to achieve active power balance.
Frequency of the module during load transients. In addition, in order to
Adjust the phase of the module, and we suggest adding one.
Synchronous cycle, concession
o=w'-m%k，A$，(7)
Where A$ is the phase difference between the inverter and.
Power supply; K is the proportional constant of frequency.
Adjust. The steady-state frequency reference w* may be
Obtained by measuring the mains frequency.
The second term of the former equation tends to zero in 2005.
Steady state, lead to
w = w' - k4($ -@ ')，(8)
Is the phase angle of the output voltage inverter
And public power mains.
Considering w = d $/d t, we can get
Next differential equation, stable bifurcation, positive
d$ *
dt dt
- + km$ = - + k，$ '。 (9)
Therefore, when the phase difference increases, the frequency will
Slightly reduced, so all UPS modules will
Synchronize with public utilities and share the consumed power at the same time.
Go and load the goods.
Four. Control implementation
Fig. 5 depicts the proposed
Controller. Average active power p, no DC
Component, can be obtained by the
Output voltage through output current, and filter the product.
........................................................................................
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Sj 'nchronirorion loop
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Figure 5. Block diagram of the proposed controller.
Use a bandpass filter. Similarly, the average level
Reactive power is obtained, but in this case it is the output voltage.
It must be delayed by 90 degrees and a low-pass filter is used.
To adjust the output voltage frequency, the equation
(7) Whether it is realized, which corresponds to the frequency.
The power supply drops due to two transient terms: transient active power
Power signal term; Phase difference term
In order to synchronize the output voltage with
Alternating current power supply adopts phase locked loop (PLL) mode. Output voltage
Amplitude is obtained by using conventional
Sagging method (4).
Finally, physical coupling inductance can be avoided in the following ways.
Use virtual inductance [17]. This concept includes
Simulate inductance behavior by reducing output.
The voltage is proportional to the time derivative of the output.
Currently. However, when a nonlinear load is provided, the higher order
Current harmonics will increase the output voltage too much.
THD。 This can be easily solved by using Qualcomm.
The filter replaces the pure derivative term of the output current,
This is very useful for sharing linear and nonlinear loads [I 1][ 12].
Furthermore, an appropriate design of the output inductor may
Reduce unbalanced line impedance to a great extent.
Influence on power distribution accuracy.
Verb (abbreviation for verb) simulation and experiment ULTS
The proposed control schemes (4) and (7) are simulated.
Use the parameters listed in table 1 and the scheme shown.
In fig. 6, for two parallel inverter systems. this
The coefficients m, n, t and kv are selected to ensure stability.
Appropriate transient response and good phase matching. Figure 7
Shows the frequency, circulating current,
The phase difference between the module and the public line,
And the evolution of active and reactive power. attention please
Excellent synchronization between modules and
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Figure 6. Parallel operation of twa online UPS module,
Power supply, at the same time, good power sharing.
Get. This feature enables us to apply the controller to
On-line UPS parallel system.
Two I-kVA UPS modules were manufactured and tested to
The validity of the proposed method is proved. Each UPS
The inverter consists of a single-phase IGBT full bridge and a
20 kHz switching frequency and an LC output filter.
The following parameters: 1. = 1 mH，C = 20 WF，Vi" = 400V，
V，= 220 V，I50 Hz。 The controllers of these inverters are
Based on three loops: internal current loop and external PI.
Controller to ensure voltage regulation and load distribution
A controller based on (4) and (7). The last controller
It is realized by TMS320LF2407A fixed point.
Texas 40 MHz digital signal processor (DSP)
Instrument (see Figure 8), using the parameters listed in the table.
I. the DSP controller further includes a PLL block to
Synchronize the frequency converter with the common bus. When this
When occurs, the static bypass switch is turned on and is based on droop.
Start control.
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Big7wa \ CFC) rms of twu.invencr; Mnectcd parallel. rpchrontred io Ihc ac mdnl。
(a) Frequency of uninterruptible power supply; (b) Current circulation between modules. (CJ Phmc d！ Nercn:betucen ihc UPS a #> Mom
(d) active participation and active participation
Please note that iimc-acs is directly caused by the decision of \ inrblrr.
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Table 1.
Parameters of the PARALLELESDYS project.
Filter order I I
Filter cutoff frequency I 0, I 1i rags
Figure 8 shows the output current transient response of the.
UPS inverter. First, two UPS run in parallel.
No load. Please note that there is a small reactive current circulating.
Between modules, due to measurement mismatch.
Then, connect a nonlinear load with crest factor of 3.
Suddenly. The results show good dynamics and load sharing.
When sharing a nonlinear system
Load.
Figure 8. Output current of two parallel UPS when connecting B.
Common nonlinear loads with crest factor of 3. (x axis: 20 mddiv. Y axis:
5 Mdiv。 ).
Conclusion of intransitive verbs
This paper presents a new parallel load sharing controller.
Online UPS system. controller
Based on the droop method, it avoids the use of
Control interconnection. In sharp contrast to ...
With the traditional droop method, the proposed controller can
In order to strictly maintain the frequency and phase of the output voltage
Synchronize with the mains AC power supply, while maintaining
Good load distribution of linear and nonlinear loads. This fact makes
The United States extended the droop method to parallel online UPS.
On the other hand, the proposed controller simulates
Special type of impedance, avoid using physical.
Coupling inductance. The results reported here show that
Effectiveness of the proposed methodology.