Decentralized control of a different rated parallel UPS systems Ryszard Strzelecki, Daniel Wojciechowski Department of Ship Automation Gdynia Maritime University Gdynia, Poland
[email protected],
[email protected], Abstract— The paper presents the single phase uninterruptible power supply (UPS) system with galvanic separated DC-AC-DC-AC converters operating in parallel. The CAN physical layer based system of communication between converters has been developed and applied, which allow to utilize a decentralized master-slave control providing high availability factor of the whole UPS system. The control system of particular converters has been developed to ensure a high quality of the output voltage for both linear and nonlinear load. The selected simulations and experimental results obtained for 5kVA prototype modules are presented. Keywords- uninterruptible power supply, decentralized control, parallel operation of the voltage source inverters
I.
INTRODUCTION
Uninterruptible power supplies are the key element of the supply systems that provide electrical energy with very high availability and of very high quality. In the special cases, i.e. for selected loads in a data centers, a requirement for the availability is equal to 99,9999999%, what means that statistically the system doesn’t operate only for 30 ms per a year. Such a high requirement can be fulfilled by using on-line VFI power electronics systems with redundancy, without the single points of failure (SPOF) and with the special maintenance procedures. The redundancy and reduction of SPOFs is possible to obtain only by using a modules which operate in parallel supplying a common loads [1], [2], [3], [4]. Such a configuration makes it necessary to implement an appropriate hardware and software solutions, in particular:
•
control of parallel operating voltage source inverters (VSI) with the LC output filter, which constitute an output converter of each UPS module,
•
reliable, disturbances immune communication between particular modules.
In the proposed configuration an effective master-slave control method (Fig. 1) of VSIs operating in parallel for the autonomous system has been applied. The master unit provides the reference current for itself and for all the slave units, and slave units only realize this reference current control [1], [3]. In consequence, a master unit is controlled in the system with both the outer voltage control loop and inner current control loop, whereas slave units are controlled by using only the inner current control loop. It is important for master unit to fulfill the following requirements: •
precise control of the output AC voltage in case of
iLset
Master Slave 1
Slave n
iL ,M L iL ,S1
io,M
C iC,M L io,S1
uo io
Loads
C iC,S1
iL ,Sn L
io,Sn
C iC,Sn
Figure 1. UPS system with parallel operating master-slave units (without bypass)
P
A B
Battery IW
IW K1
IW
A1K 2
A1K 2
A2
A2
+15V 15V
IW
-15V
12V 0V
-15V
A B
+15V
P
K1
N
N
IW
N
+15V
12V 0V
L
IW
Output
+15V 15V
Figure 2. The main circuit of the single UPS module of rated power 5kVA (control circuits and EMI filters are not depicted)
supplying both linear and nonlinear loads (up to required crest factor (CF) of the loads, •
controlled power sharing between VSIs of all the connected modules in steady as well as transient states.
Therefore, it is necessary to provide the reference current samples from master unit to every slave unit on each pulse width modulation (PWM) period, so as to provide fastest possible control response for changes of load. To ensure elimination of SPOFs it is necessary to provide a full functionality of each module. It means, that each of the module has to be equally capable to operate as a master unit and a slave unit. It is the only method to provide a functionality of the whole UPS system in case of malfunction of the master unit. The proposed system utilizes a specially developed communication, that realizes all the tasks which are the result of requirements listed above. Both the physical and logical layers of this communication are described in the paper. The paper presents results of simulations and experiments realized for the 5 kVA UPS modules.
proportionally to its rated power. The bypass is switched on synchronously on each parallel operating module in case of system overload. The control system was designed based on the following equations which describe the output AC circuit (Fig. 1):
L
du diL = U DC ⋅ d − uo , C o = iC = iL − io , dt dt
(1)
where UDC denotes the voltage on the DC side of VSI of each UPS module, and d denotes a state of VSI related with particular combination of transistors states. The state d can be equal to –1, 0, or 1. For the purpose of control system formulation it is sufficient to use the simplified averaged model without modulation. For this case equation (1) has the following form:
L
du diL = uinv − uo , C o = iC = iL − io , dt dt
(2)
where uinv denotes the VSI output voltage. II.
THE CONTROL SYSTEM OF MASTER AND SLAVE UNITS
The electrical circuit of single UPS module is presented in Fig. 2. The main components of each module are: •
DC-DC converter transformer,
•
voltage source inverter with the output LC filter,
•
thyristor AC switch for UPS bypass circuit.
with
the
separating
pulse
The control system of each UPS module includes control of all the components listed above with taken into account a parallel operation of VSI as well as AC switch. The instantaneous power is shared between particular modules
The control system of UPS module VSI is presented in Fig. 3. The main parts of the system are the outer voltage control loop and the inner current control loop. From the current controller viewpoint the UPS module output voltage uo constitutes a disturbance. The controller employs both feedback and feedforward control. As a feedforward the measured UPS module output voltage is used, which decouples controller from disturbance. Similarly, the output current constitutes a disturbance for the voltage controller, and it has been decoupled in the same manner. The purpose of the switch which is depicted in Fig. 3 is the selection of master or slave mode of control. The VSI output voltage is generated by using an unipolar PWM with the switching times updated (and also control system computed) in its every half period.
iLset
Model of the controlled system (equation 2)
slave
uoset +
−
KU io,M
+ or io ,S
master
+
+
−
KI
+
K=1 +
uo
+
uo
−
1 sL
iL
+
− io,M
1 sC
uo
or io ,S
Figure 3. The block diagram of the UPS module VSI
III.
COMMUNICATION BETWEEN UPS MODULES
A. The Physical Layer The proposed solution of communication between UPS modules is based on reliable ring topology with the physical layer consistent with the CAN standard, and specially developed logical layer with master/slave units selection procedures and system diagnostics.
The general diagram of the physical layer of CAN communication is shown in Fig. 4. The system utilizes two independent CAN communication channels. Each channel is built-up form the bus controller, galvanic isolation and integrated CAN controller. The Microchip MCP2515 CAN controllers with serial user interface has been utilized. The maximum transfer rate of this controller is equal to 1 Mb/s. Data is transferred to/from the CAN bus from/to the floating point digital signal processor ADSP21065L via FPGA chip FLEX6016. In FPGA data is bidirectional converted from series to parallel form, depending on the direction of data transfer. The data is transferred between FPGA and CAN controller using SPI interface. The serial user interface is timed ADSP21065L
with 30MHz clock signal which is generated inside FPGA. The CAN controller is separated from the bus controller with insulation strength equal to 5 kVrms, and propagation time equal to 32 ns. For this purpose the separation chip ADUM2402 from Analog Devices has been used. It is based on the MOS structure and coreless pulse transformers. The PCA82C250 CAN bus controller has been utilized. The whole physical communication layer provides the data transfer rate equal to 1 Mb/s. The system is very immune to EMI disturbances from power electronics converters and its environment. B. Tasks of the Logical Layer To fulfill the requirements described in chapter II of the paper regarding the control of UPS system, and in order to provide its high availability factor, the logical layer of communication provides:
•
transmission of reference current samples from master unit to all slave units in every PWM impulse period,
•
procedure of determining the master unit during a system startup,
•
fast procedure of determining a new master unit after malfunction of present master unit,
•
system checkup and master/slave determining after hot (realized during UPS system operation) connection of a new module,
•
correct operation of UPS system in case of momentary or permanent communication malfunction,
•
correct transmission in case of single brake of the communication bus (ring topology).
FLEX6016
MCP2515
MCP2515
ADUM2402
ADUM2402
PCA82C250
PCA82C250
Channel 1 connector
Channel 2 connector
Figure 4. Basic diagram of the CAN communication channels
In order to provide maximum robustness of the system all the communication procedures are synchronized with the main software interrupt of the each UPS system module, what has been shown on Fig. 5. C. The First Communication Channel The first channel is assigned to transmit the control signal (reference current) from master unit to all the slave units only. The lack of competitive access to this channel ensures transmission of the control signal with the predictable, constant delay time. The data frame for the single transmitted sample is longer that the sampling frequency of the DSP controller, and
Master CAN
Data 1
Slave 1 Slave 2 Slave 3
Data 2
Receive of data 1 Receive of data 1 Receive of data 1
Data 3
Receive of data 2 Receive of data 2 Receive of data 2
Figure 5. Exemplary transmission in channel 1
from that reason data is transmitted in every second sampling period (which corresponds to the UPS system VSI PWM period). The lack of control signal transmission for the predetermined period of time leads to initialization of the new master unit determination procedure in channel 2. Fig. 5 presents the typical transmission in channel 1. The impulses which are visible in transients 1, 3, 4, and 5 shows the instants of interrupts in master unit and three slave units. The second transient shows the bus activity. It is clear from that transients, that the control signal data transferred by the master is received by the slave units in the next software interrupt following the end of transmission on the CAN bus. Because the software control interrupts in the particular modules of the UPS system are not synchronized together, the control signal is received by slave modules at different instants of time, but always within the time of one sampling period. D. The Second Communication Channel The second communication channel is used to determine the hierarchy of parallel operating modules based on predefined parameter which determines the priority of each module. After connection to the system the module tries to set itself to the master mode by transmitting several times the data with its priority parameter. The remaining modules of the system receives that message, compare the received priority parameter with its own parameter and in case when the own priority is higher it begins to transmit it to other modules. If the received priority parameter is not higher than the own one, the module sets its control mode to slave. In case when several modules have the same priority, the last connected module sets its mode to master.
IV.
RC load. The simulations were realized in POWERSYS PSIM 7.0 environment. It is clear from the results, that the instantaneous current of the particular modules is determined and controlled at the level which is proportional to its rated power. The transient states related with turning on and turning off the load do not substantially disturb the output voltage of the UPS system. TABLE I.
THE UPS MODULE CIRCUIT PARAMETERS
Quantity
Value
Battery voltage
48 V
VSI input DC voltage
370 V
VSI output voltage
230 V RMS
Output voltage frequency
50 Hz
VSI AC filter inductance
150 uH
VSI AC filter capacitance
100 uF
PWM carrier frequency
10 kHz
Sampling frequency
20 kHz
V.
CONCLUSION
In the paper the complex hardware, software, and control solutions for the decentralized UPS system with parallel operating modules has been presented. The system was designed to provide the supply of the critical loads with very high availability factor. Presented results confirm high dynamics of the output voltage control and precise power sharing between the UPS modules controlled according to the proposed method. The development work was realized within the Development Project of Polish Ministry of Science and Higher Education R01 002 01. ACKNOWLEDGMENT Authors wish to thank Mr. Piotr Reiter and Mr. Mariusz Rutkowski for their important contribution to this project.
THE RESULTS OF UPS SYSTEM INVESTIGATION
Fig. 6 shows the prototype of UPS module of rated power 5 kVA. Basic parameters of the circuit are listed in Tab. 1. The module has been constructed by the PEDC group in Gdynia Maritime University [5]. The selected properties of the module during autonomous operation (current, output voltage, voltage harmonics, load parameters) are presented in the experimental results in Fig. 7 and Fig. 8. The measurements were realized using power quality analyzer Fluke 434. For the linear load the total harmonic distortion of the output voltage is equal to THDu=1,1%, and for the nonlinear load with crest factor CF=2,99 – THDu=2,3%. In Fig. 9 are depicted the simulation results with transients of currents and voltage obtained for the UPS system of three parallel operating modules of rated power 5 kVA, 3kVA, and 2kVA supplying the bridge rectifier with
Figure 6. Prototype of the 5 kVA module of UPS system
Figure 7. The autonomous operation of an UPS module supplying a linear load of power 5,2 kW
Figure 8. The autonomous operation of an UPS module supplying the a nonlinear load of power 3,18 kW and crest factor CF=2,99
uo
REFERENCES [1]
io [2]
io ,M io ,S1
[3]
io,S2 [4]
Figure 9. Parallel operation of the UPS system with three modules of rated power 5 kVA, 3 kVA oraz 2 kVA. Transient state related with turning on and turning off the loads. Simulation
[5]
Woo-Cheol Lee, Taeck-Ki Lee, Sang-Hoon Lee, Kyung-Hwan Kim, Dong-Seok Hyun, In-Young Suh, “A master and slave control strategy for parallel operation of three-phase UPS systems with different ratings,” Proc. of APEC 2004, Vol. 1, pp. 456–462, 2004. Josep M. Guerrero, Luis García de Vicuña, Jose Matas, Jaume Miret, Miguel Castilla, “A high-performance DSP-controller for parallel operation of online UPS systems,” Proc. of APEC 2004, Vol. 1, pp.:463–469, 2004. Hongtao Shan, Yong Kang, Xikun Chen, Mi Yu, “Novel & practical digital parallel UPS system based on CAN BUS,” Proc. of INTELEC 2006, pp. 1–5, 2006. S.J. Chiang, C.H. Lin and C.Y. Yen, “Current limitation control for multi-module parallel operation of UPS inverters,” IEE Proc.-Electr. Power Appl., Vol. 151, No. 6, pp. 752–757, Nov. 2004. www.pedc.am.gdynia.pl
