Analysis of synchronous and induction generators in parallel operation mode in an isolated electric system

This paper presents first an analysis of a parallel connection of one synchronous generator and one self-excited induction generator, each coupled to its dc machine as well as a capacitor bank with a resistive load connected to the system. In this stage, the voltage and frequency adjustments were done manually. In the second portion, a similar circuit was implemented considering a voltage loop control and a speed loop control to the synchronous generator. These controls kept on the system voltage and frequency stables. The induction generator supplies the part of active power required by load. The synchronous generator controls the frequency and supplies the reactive power and the additional active power. The experiment demonstrated the feasibility and stable operational performance for this alternative and creative parallel arrangement of generators.


Abbreviations and Acronyms
In recent years, self-excited induction generators have been employed as suitable isolated power sources in small hydroelectric and wind energy applications [2][3][4]. In wind power generating systems, physical size of the individual machines operating at maximum efficiency and dealing with regular routine maintenance related to necessary interruptions, future growth and reliability are the reasons to be operated in parallel.
This paper presents results that show remarkable characteristics by operation in parallel mode among one SG and one IG. Some of characteristics as weight, size, easier maintenance, shorter lead time are more associated to induction generator and also be relevant to oil platforms or offshore installations concerns.
One of the potential motivation for this study target consists in being a potential alternative capable of optimizing the main electric system currently adopted in oil Platforms and FPSO ships to become cheaper, simpler, lighter and efficient.
The potential application of this study is basically the replacement of one or two Synchronous Generators by one or two Induction Generators in oil offshore platforms or FPSO. A offshore typical electric system uses currently 3 or 4 turbo-generators in 13800 V, 60 Hz, driven by dual fuel (fuel gas or diesel oil) turbines. In normal operation, 3 three Main turbo-generators operating and are able to supply all unit consumers with the fourth in standby. During the peaks of load, the fourth Main turbo-generator may be required to meet the demand. Therefore, the whole electric system shall be suitable for this operational condition.
The main advantages of induction generators over the synchronous generators are: 1-brushless construction with squirrel-cage rotor, 2-reduced size and weight, 3-absence of DC supply for excitation, 4-reduced maintenance cost, and 5-better transient performances. [7] Therefore, the study target is related to take advantages or fit alternatives from induction generator used as substitute of Synchronous Generator for application in cited electric system, delimiting the operational conditions and its advantages. Establishing operational scenarios and the steps required to start up, voltage and frequency regulation and shutdown based on the schematic using a synchronous generator in parallel to induction generator.
The equipment weight is one of the relevant induction generator advantage over synchronous generator for that specific application, how much less weight in a vessel such as a FPSO, the gains are so much including load capacity, space, energetic efficiency and so on. Another advantage is the lower price, almost 40% cheaper than the Synchronous Generator as presented at [7].
This paper presents conclusions about the both start up methods and its dynamics interactions considering the transitions among scenarios including load in and no load system. The laboratorial experiments and its results were presented along with this paper throughout graphs, curves and experimental observations for each scenarios transition. Firstly, the circuits were mounted without automatic control, open loops. The sets were done manually in order to observe dynamic responses for each generator as the load was in or out In sequence, the automatic voltage and frequency control for synchronous generator were implemented. The results shows that voltage and frequency remains steady as load is put or taken out. Besides, the induction generator has the control of active power required by load and synchronous generator complements the rest of active power and reactive power necessary. The capacitor bank is capable of supplying all induction machine demand and to relieve the reactive power from synchronous generator when either the load is on and induction generator has only a little of generation or induction generator is off.

II.
OPEN LOOP SCHEME

A Synchronism Switch vs. Interconnection Switch
For open loop control were mounted two electric schemes which consist of a circuit with synchronism switch and a circuit with just a interconnection switch, both are showed in fig. 1. The fig. 2 shows the electric schemes mounted at laboratory.
After startup and operation steps in the interconnection switch circuit and synchronous switch circuit was checked that interconnection switch circuit is simpler than synchronous switch for startup and operations as long as it is possible use less components and requirements for putting the two generators in operation. The induction machine can be started as motor and in monitored mode it can be turned into an induction generator as in [8].

• Capacitors Bank Sizing
As informed at IM dataplate cosΦ = 0,8, then senΦ = 0,6 Reactive power and capacitors calculus to attend the reactive demand of Induction Machine = √3 * * * (7) = √3 * 220 * 7,5 * 0,6 => 1.714,7 VAr Q = Fc * Q (8) Fc =1,2 [6] For the machine coupled at a resistive load which requires 7,5A, it is necessary the reactive power generation about 2.057,6 VAr as demonstrated bellow. The capacitor bank used in the circuit has 40 µF per phase and each 10 µF capacitor can be shutdown in order to set the number of capacitors connected to circuit depending on the scenarios under focus.

D Scenarios -Interconnection Switch Scheme
The transitions among the eight scenarios from interconnection switch circuit which are described in table 4 were analyzed in order to focus on the voltage and frequency automatic control in an isolated electric system supplied for one synchronous generator in parallel to one induction generator.  In this case, the IG speed was led on to close and below the fluctuation point, it means some value close to point where the torque or conjugate is null as it can be seen in fig. 3 bellow. The fluctuation point is at the intersection with rpm axis. The interconnection switch was closed when the Induction motor speed is in the fluctuation area. From this IG speed, the DCMIG speed should be raised up to overcome the fluctuation point to become the induction motor into induction generator. It was checked the master and slave responses from generators, it means that all adjusts done by Synchronous Generator were followed by Induction Generator keeping on the same Voltage and Frequency determined by Synchronous Generator.

• Observation from scenario 3 to scenario 4
After the load in, the generators voltage dropped to 192 V and frequency decreased to 47,5 Hz. The Induction Generator assumed almost full of resistive load reducing the contribution from SG. Note that the frequency even been reduced to 47,5 Hz, the shaft speed point ( 1485 rpm) is upper than synchronous rotation (1425 rpm) what denote the operation as induction generator (6).
• Observation from scenario 4 to scenario 5 It was inserted more 10 µF/ phase, totalizing 40 µF/ phase at capacitors bank, what led to the Generators voltages gone back level before closing of load.
• Observation from scenario 5 to scenario 6 In order to set the generator voltage to 220 V and its frequency to 50,0 Hz, it was increased the IfieldSG regarding to elevation of Voltage and the same for IfDCMSG regarding to elevation of frequency. In both sets, it were checked the master and slave responses from generators, it means that all these adjusts done by Synchronous Generator, master, were followed by Induction Generator, slave, keeping the same Voltage and Frequency determined by Synchronous Generator.
• Observation from scenario 6 to scenario 7 In order to prepare the system for turning off and avoid over voltage upon Synchronous Generator, it was necessary removal of 10 µF/ phase from capacitors bank and reduce the generators terminal voltage to some value close 200 V actuating at IfieldSG.
• Observation from scenario 7 to scenario 8 Set the Induction Generator speed to some value close and below to fluctuation point in order to become the induction motor almost imperceptible for Synchronous Generator.
After the step above, it was opened the interconnection switch uncoupling the generators and turning off the capacitors bank.

III. CLOSED CONTROL LOOP SYSTEM
After checking the open loop system responses from parallel operation to synchronous and induction generator, the closed loop circuit was implemented as shown in figs 6, 7 and 8. Regarding to prior circuit, this scheme had a load and capacitor bank positioned in another side of interconnection switch in order to keep on the load supplied even when the interconnection switch and I.G. have been turned off.
It was inserted a rotor resistance in order to permit the induction motor to start directly by interconnection switch.

A. Equations -Portion II
As ISG, IIG, IC and Iload are measured values, table 6, it is a  system with 4 variables and 4 equations. Then, for each  scenario, the four variables  ,  ,  and  are  calculated by Matlab software and shown in table 6. After then, PSG, Power from SG, is calculated as well as PIG, Active Power from IG. The sum of PSG and PIG is demanded by load.
Besides, the whole group efficiency and its each subgroup were calculated based on PIG, PSG, PDCMSG and PDCMIG. as follow:

B. Close Control Loop Scheme
Through controlled loops it is established two different controls over SG, one is field control and another is the SG speed control as highlighted by blue and green lines in fig.6. Firstly, it was started up the SG and set the working point for each loop considering the rated or desired voltage and frequency. After this, close the interconnection switch starting up the induction machine as motor. At this stage, it was necessary connecting the rotor resistance at the rotor to available initial increased torque and motor starts.
While the induction machine operating as motor, its speed increases until it overcomes the synchronous speed, it means, for 50 Hz, synchronous speed is 1500 rpm as (6). The speed measurement is done by manual taco over the machine shaft. As long as the active power from induction generator is increased through by DCMIG speed (2), the synchronous generator is less required supplying less power. When the load in and DCMIG is set to 1720 rpm, it was realized the most load active power was supplied by induction generator, table 6. During this stage, it was realized that the electric system promptly recover the rated voltage and frequency. It occurred immediately after a voltage and frequency have dropped, demonstrating good control and automation.
The induction machine starts directly as motor and after becomes a generator what is a advantage because it is only necessary close the interconnection switch after SG startup. Induction Machine becomes motor first and after its speed is raised until overcomes the synchronous speed when it becomes IG and from this point, it operates supplying the part of active power required by load.
The configuration of electronic control board, table 5, as to SG field as to SG speed loop were done empirically by setting the time constant and gains throughout the system responses analysis over the load removal and load insertion. Other configuration factors were set considering the convertors type(1B6C).
• The resistors chose based on the sized resistor were: • Speed Control Loop Resistive Divisor as fig. 6.
• Resistor Power sizing: = 36 (9 * 10 ) = 1,14   fig. 6, was turned on through by speed electronic control board MP410T and then, the speed reference (working point) was defined and adjusted based on the specified speed. The speed reference was adjusted in 60,2.
After this, the field voltage loop, highlighted by blue color in fig. 6, was turned on through by field voltage electronic control board MP410T and then, the field voltage reference (working point) was defined based on the specified generator terminals voltage. The voltage reference was set in 69,4.

E. Voltage and Speed Control Loops
The fig.7 and 8 show the closed control loop used to control the SG voltage and frequency via electronic boards presented in figs. 10 and 11. The fig. 9 shows the configuration 1B6C used for the two thyristors convertors bridges which were used in these control loops.

Fig. 9 . Conversors Configuration used for controlling Operational Scenarios
The fig. 10 shows the electronic boards used to control the SG field and SG speed, both are also presented in fig. 6  and fig.11.

F. Scenarios
Follow the operational scenarios which were taken in account for getting the data and the qualitative analysis presented forward

G. Graphs
The graphs bellow express some relevant qualitative analysis from table 6 data  fig. 12, as long as the supplied IG power decreases by actuating on the DCMIG speed by machine flux as (2), the SG power increases automatically by SG speed and SG field loops running. Note that the sum of each generator power result to load required power. In the same way of fig. 12 content, in the fig. 13, the DCMSG compensates automatically the IG power decrease, that is resulted from the manual actuating on DCMIG speed by strengthening of the DCMIG field as (2).
At the transition between 5 and 6 scenarios, there is a short elevation of total power supplied due to the SG efficiency loss when it is operating over its nominal conditions. It requires more power from DCMSG.

H. Wave Forms:
The follow ISG, IIG and VpnSG wave forms were gotten for scenarios 2 and 6 as well as the Iload and VpnSG wave forms representing the scenarios 2 to 7.

III-CONCLUSION
The experiment presented in this paper verified that the startup method with interconnection switch is simpler than synchronism switch, what became the operations simpler and showed the synchronism switch is not necessary. In closed loop circuit was implemented a rotor resistence in order to permit direct startup instead of initial procedure used in open loop circuit which consist of approaching the induction motor synchronism speed prior to close the interconnection switch. It became the startup quite simpler.
In this arrangement of generators is necessary a capacitors bank to supply reactive power to auto-excited IG and contribute to SG as well as it can be seen in table 6, currents Ic1 and Ic2. It is relevant that this bank is connected to SG side in order to ensure the load feed continuity in face of the IG has been turn off, as demonstrated during these experiences.
In all scenarios in both circuits were verified the master and slave behavior by the generators considering that SG is the master and IG is the slave. The system frequency and voltage are determined by SG while active power consumed at load is controlled by induction generator as long as its generated power minus loss goes to load and the load power is complemented by SG automatically as presented in figs 12 and 13.
It was verified that IG is capable to supply active power freely to load and the SG regulates the frequency, voltage and complement the load power through its voltage and speed control loops as shown in table 6 and figs. 12, 13 and 14.
The experiment implemented in laboratory had responses that indicates good voltage and frequency stability as presented in fig. 14 as well as a steady power supply, a machine helping another mutually. And in this case, it was added with the IG advantages such as lower complexity, lower price, shorter lead time and lesser weight which is so relevant in offshore environment.
In the next stage of this work, the current and voltage transients will be measured and analyzed when contingencies to take a place as each machine be abruptly disconnected.

IV. ACKNOWLEDGMENT
The authors would like to thank our dear colleague and friend Dr. Paulo Fernando Ribeiro, for the help in the revision of the English language of this paper and also the PETROBRAS for the logistical and financial support along the study and trip to this conference.