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Electricalsystemsofagridconnected2MWminihydropowerprojectatSiripagama

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International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

1

Electrical Systems of a Grid Connected 2 MW Mini Hydro Power Project at Siripagama G.R.C.B.Gamlath1), A. Arulampalam2) and I.H.D.Sumanaratne3)

1) Ceylon Electric ity Board, New town, Ratnapura, Sri Lanka 2) Department of Electrical and Electronic Engineering, Faculty of Engineering, University of Peradeniya, Sri Lanka 3) No.49, School Lane, Nawala, Rajagiriya, Colombo, Sri Lanka Email: spesbp@ceb.lk and atpu@ee.pdn.ac.lk

ABSTRACT This paper presents the electrical system design of a 2 MW Mini Hydropower Power Plant (MHPP) project at Siripagama. Catchments area of 19.1 km2 and annual average rainfall of 3935 mm yield to 3.5 m3/s designed flow. Net head was found as 68 m. Plant was sized to 2 MW and accordingly structural components and electro-mechanical system were designed. Two numbers of electro-mechanical units in which each unit consists of Francis turbine of 1054 kW, synchronous generator of 1250 kVA, 400 V and two number of 33 kV 1800 kVA step up transformers. The generator systems were modeled using PSCAD simulation package to check the performance of generator. Also the system together with the transformer was studied during the normal loading and load rejection operations. Grid interconnection lines were modeled using SYNERGEE software. This was used to study the line flows and voltages in the feeder from Ratnapura grid substation to the Siripagama MHPP. The simulation results confirm the proper operation of the MHPP and the grid without violating the limits of voltage and conductor ratings. Finally the total system was commissioned on 19th June 2006. Initially the plant factor was about 36% and in October it rose to 56%. This is one of the projects, which confirmed the better performance of the micro hydro plant in Sri Lanka. 1 INTRODUCTION One of main sources of power generation in Sri Lanka is hydroelectric. According to the statistical data published by Ceylon Electricity Board (CEB) for the year 2006, 49.4% of total generation is met by hydropower. In 1996, Sri Lanka Government encouraged private sector participation in power generation. As a result at the end of year 2006, 60 numbers of mini hydropower plants were connected to the national grid adding total of 109 MW with an annual generation of 346 GWh (4% of gross production) [1]. In the mini hydro power projects initially site selection is done by analyzing mean stream flow and available head. This helps to take the decision on the plant capacity and to determine the anticipated annual power generation. Then electro-mechanical equipments such as turbine and generator and accessories are selected to suit with design parameters. Generators of directly coupled to turbine shafts are commonly used [2]. Generator control unit was used to adjust the power generation according to the availability of water. Protection system was used to isolate the MHPP when a grid fault occurs. In addition monitoring system was also included. Embedded generators are classified into five and required protection requirements are stipulated in the CEB guide [3]. In addition to that a generator protection was made coupled to generator breaker.

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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The power house site is remote from the national grid. Therefore a transmission line was designed. Constrains used in this design are allowable voltage variations and reduced power losses. Availability of materials is also taken into account when designing the power line. 2 SELECTION OF ELCTRO-MECHANICAL EQUIPMENT Selection of Turbine A mini hydropower scheme converts hydro potential energy into mechanical energy and then to electrical energy. Energy is power delivered in unit time. Turbine output power = ηΤ × Turbine Input Power Turbine output power (kW) = ⋅Tη netw HgQ ⋅⋅⋅ ρ / 1000

Where: ηΤ - Efficiency of turbine (90 %) g - Gravity (9.81 m2/s)

Hnet - Net head available at the turbine (68 m) Q - Water volume enters into turbine per second (3.5 m3/s)

ρw - Density of water (1000 kg/ m3) Turbine output power (kW) = 0.9 × 3.5 × 1000 × 9.81× 68 / 1000 = 2101 kW Therefore, out power of one unit = 2101 / 2 = 1050 kW Net head, flow rate and turbine output power were furnished to the turbine manufacturers in order to select a turbine from available standardized sizes. This head fall into the medium group and therefore it is supposed to use impulse type turbines like Crossflow, multi-jet Pelton, Turgo or reaction type turbines like Francis [4]. According to the manufacturers data and the site design parameters annual average power production was calculated. It was calculated for the Francis turbine 12,560,000 kWhr and for the Crossflow turbine 11,696,000 kWhr. The power difference of 864 000 kWhr is equivalent to Rs.4.6 million in terms of income. Selection criterion was also based on the cost of the unit. Unit price of a Francis and Cross flow turbine inclusive of all controls and accessories were Rs.9.6 million and Rs.10.8 million respectively. Cost of Cross flow turbines was high due to the additional drive system. Considering all those facts, Francis type turbine was selected in this project. Two numbers of Francis turbines each 1050 kW, 96% of efficiency were used in order to increase the part flow efficiency, increase reliability and easiness in overhauling the turbines without shutdown the plant. Effective head was same for both units. But flow was shared equally and subsequently output power was divided equally. Selection of Generator Considering the facts that the synchronous generators are of high efficiency even in partial loading, independent control of real and reactive power and commonness, it is decided to use a Synchronous generator as the alternator of this design. Synchronizing and operation was bit complex in the past and the recent development has simplified it [5, 6 and 7]. The generator input power is same as the turbine output power 1050 kW. Taking the efficiency of a generator as 95%, the output power of the generator is calculated as 998 kW and taken as 1000 kW. As the plant is design to operate at 0.8 power factor, the apparent power rating of the generator becomes 1250 kVA. Two generators were selected to produce 2 MW output power. Generator performance tests Some of the performance tests were carried out to and obtained the test certificates. On top of the test results the start-up transient and running conditions were also checked in simulations. Both generators were modelled in EMTDC/PSCAD to examine the start-up transient and running operations of the system. Figures 1 and 2 show simulation model and results of the two generators connected system.

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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Excitation voltage (E1 and E2) and the torque (Tm) were the inputs to the generator. Excitation voltages are adjusted to maintain the 0.8 power factor according to the CEB requirement. This was done by multiplying the generator active power output by tan(cos -1(0.8)) to calculate the reactive power corresponds to 0.8 power factor. This was compared with the measured reactive power output and the error was regulated to find the required excitation voltage. The torque of the 1st generator was changed from 0.1 p.u to 1.0 p.u at 0.5 seconds and the 2nd generator was changed at 1.5 seconds. The simulation result shown in Figure 2 illustrates that the transient are within the acceptable limits when the generator output power is changed drastically. 3 ELECTRICAL SYSTEMS Generating voltage of the plant was selected as 415 V in order to avoid auxiliary transformer and related controls. This voltage was step up to 33 kV to interconnect with national grid. Therefore, electrical system of the power plant consisted of Low Voltage (415 V) switch boards and the Medium Voltage (33 kV) Switchyard [8].

Siripagama Mini Hydro Plant Generator

1.25 MVA, 415 V

W1

H G

w

Te

A

B

C

IfEfEf If

Tm

TmTM1

IF

A

B

C

A

B

C

A

B

C33.0

# 2# 1

0.415

1.6 [MVA] A

B

C

3 PhaseRMS

Pow

erAB

PQ

0.001

0.001

0.001

A

B

C

Qgen Pgen

Siripagama Mini Hydro Plant Generator

1.25 MVA, 415 V

H G

w

Te

A

B

C

IfEfEf If

Tm0Tm

Tm

A

B

C

A

B

C33.0

# 2# 1

0.415

1.6 [MVA]

0.1

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Ctrl

Ctrl= 11.0

0.1

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B

Ctrl

Ctrl= 11.0

TIME

TIME

Ib_grid

Ic_grid

Ia_grid

TM2

W2

T

*

0.75Tan(cos-1(0.8))

D +

F

-I

P

E1

E1

1.0

A

B

Ctrl

Ctrl = 1

TIME

Pout_gen1*

1.25MVA rating

Qout_gen1 *

1.25MVA rating

*

0.75Tan(cos-1(0.8))

D +

F

- I

P

E2Pout_gen2

*

1.25MVA rating

Qout_gen2 *

1.25MVA rating

E2

1.0

A

B

Ctrl

Ctrl = 1

TIME

Figure 1 Simulation Model of Siripagama MHPP

Simulation result from 2.5 seconds to 5.5 seconds

0.00 0.50 1.00 1.50 2.00 2.50 3.00 ... ... ...

32.00

32.50

33.00

33.50

34.00

kV

V_grid_rms

-0.080

0.080

kA

Ia_Grid Ib_Grid Ic_Grid

0.00 0.50 1.00 1.50 2.00 2.50 3.00

MW

& M

VA

r

P_Grid Q_Grid

0.00 0.20 0.40 0.60 0.80 1.00

pu

Tmechanical_Gen1 Tmechanical_Gen2

314.0

322.0

rad

/ s

Gen1_speed Gen2_speed

0.0

1.80

MW

& M

VA

r

Pout_gen1 Qout_gen1 Pout_gen2 Qout_gen2

Figure 2 Simulation result of the generator operation when generated power changed from 10% to 100%

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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a. Low voltage system and controls

The panel was made with synchronoscope, volt meter, generator voltage controls, manual synchronizing controls, emergency stop push button, generator run hour meter, surge arrestor for switch yard side of breaker, meters for active power, reactive power, apparent power, power factor, kWh, kVArh, line to line voltages, line to ground voltage, current in each phase, indicators for mains and generator supply, indicators for breaker position etc. [9 and 10]. Protection for each Generator and Turbine unit were provided. Synchronous generators was operated with 0.8 power factor lagging to make sure that it supplies reactive power to the network as per utility requirement. Reverse power protection was made with minimum power setting. Minimum flow requirement to rotate the turbine is 0.05m3/s, which is about 3% of the rated turbine flow, was made as threshold to trip. Therefore reverse power relay was set to operate trip circuit at 30 kW while making 5 seconds alarm at 40 kW output. Negative sequence current protection prevents unbalanced current flow in the generator, which causes overheating. Generally, generators are able to withstand for operation with short time negative sequence current. Typical set value of 4% of generator full load current for 10 seconds was used. Therefore, negative sequence current of 65 A was set to trip while 10 seconds alarm set at 80 A. Loss of excitation operates the generator with drawing the reactive power from the network. Field failure protection relay operates and gives an alarm in order to restore excitation or initiate shutdown. The relay operation was based on the impedance detection. Operating impedance value was set to 0.7 times of synchronous reactance plus transient reactance of generator. Surge protection was made by installing 33 kV, 10 kA lightning arresters at the end of the Circuit Breaker. This diverts surge voltages entered into the LV side. The electrical over-current protection is generally set to higher values and cannot sense the continuous overloads of less value. Continued over-loading may increase the winding temperature to such an extent that the insulation will be damaged and lifetime will be reduced. Resistance Temperature Detectors (RTD) was embedded in the stator slots to sense the winding temperature for thermal protection. Thermal protection was set to activate an alarm at 1200C according to the manufacturer’s recommendation. Temperature detectors were also inserted into bearings to prevent damages due to temperature rise caused by failure of cooling system. This temperature indicator was set to give an alarm at 950C. 3.2 Medium voltage (33 kV) system Generated voltage of 415 V was stepped up to 33 kV as the plant supply to 33 kV distribution system of national grid. Two numbers of transformers were used to step up the generated voltage of each generator.

Metering

LV Switch Gear

Synchronous cope

Protection Module

Mains Monitoring

From Generator

Generator Monitoring

Panel 1 Panel 2

To Grid connection

Figure 3 Single line diagram of LV Electrical system

Generator capacity = 1250 kVA

Maximum current =415310001250

××

= 1739 A Therefore, rated current of each breaker connected to a generator was selected as 2000A. Rated voltage was selected as 440V. Air Circuit Breaker was considered due to comparatively low cost. A motorized switch gear was used since the system was automated. Auxiliary contacts were used for indications and control circuit. This breaker was used in synchronizing the generator with the grid.

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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(a) Over and under voltage: Embedded generator is maintained the system voltage by means of AVR. Tolerance level of the 33kV system of CEB is ± 6%. Therefore, over voltage protection was set to alarm at +6 % and trip at +10 % and the under voltage protection was set to alarm at –6% and trip at –10 %. (b) Over and under frequency: Partial or total loss of the grid supply to the local network may overload the embedded generator. Under frequency relays send tripping signal, if the power frequency drops from set value for a defined time. However, the under-frequency settings of embedded generators should be below the same setting of feeders of the grid as it helps to recover overloading of the grid with load shedding of some feeders. The under -frequency setting of CEB in stage I is 48.75 Hz. The set value of the plant was 47.8 Hz with 30s delay for alarming and 47.7 with 2s delay for trip signal. Over-frequency relay give signals to trip off the breaker when over speeding of the generator taken place due to loss of load. This provides back-up protection for speed control governor. The over-frequency setting was 50.5Hz. (c) Neutral Voltage Displacement (NVD): In a balanced three phase system, under normal conditions the sum of the phase to neutral voltages will be nearly zero for an unearthed system. Under earth fault condition, a voltage difference is produced between the system neutral and earth, which is called as Neutral Voltage Displacement and detects by the NVD relay. NVD relay was an over voltage relay connected to trip Circuit Breaker installed in the switch yard. The NVD relay detects three times of neutral voltage.

Transformer 2

Transformer 1

DDLO DDLO To NVD protection and Metering

Energy Metering

Circuit Breaker

To National Grid

Metering

Lightning Arrestor

Isolator

G 59 protection

Lightning Arrestor

Lightning Arrestor

From Generator Breaker

Figure 4 Single line diagram of MV Switchyard

Figure 4 shows the single line diagram of the medium voltage system. The outdoor switch yard was mainly equipped with two transformers of 33kV/415V 1600kVA, SF6 Circuit Breaker of 33kV 25kA/1sec 630A, horizontal manual operated isolator with earth switch of 33kV 25kA/1sec 630A, NVD transformer of 33kV /110/110V, three current transformers of 33kV 100/1A, six DDLOs of 33kV 25kA/1sec 50A, three lightning arrestors with surge counter of 33kV 40kA, and earthing system. Rating of the generator is 1250 kVA. Considering the reliability of the transformer, partial loading of 80% of a transformer was taken. Therefore 1600 kVA rated transformer was selected. The Circuit Breaker was installed before connecting to the CEB grid to isolate the system during maintenance and any abnormal conditions such as faults. Based on 1250 kVA load the maximum current flow in the breaker is 44 A. Short time withstand current is taken as 525 A (12x44). Available 33 kV breakers are 630 A, and it was selected. Network Protection System was made according to the CEB requirements guide [3]. In this project ratio between maximum installed capacity (2.0 MW) and maximum captive load (1.5 MW) was greater than 0.8. Therefore, the MHPP project felt under case 3 of the guide and following protection requirements were installed.

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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(d) Loss of mains protection: Islanding of an embedded generator result an unearthed a section of the CEB network. With the removal of grid supply, embedded generators should be automatically disconnected and remain disconnected until the supply restored. Since this islanded section is unearthed, fault level is inadequate to operate protection relays. The other reason is restoration of power may do without check synchronizing which could result severe damages to the network. Disturbances due to islanding are used to detect loss of mains. 4 SYSTEM STUDIES

a. Power flow studies

Power flow studies were carried out to check the normal operating status of the distribution network after connecting an embedded generator. Voltage at every point in the distribution network should be within the acceptable limits for all expected loading conditions and with and without the embedded generator. Thereby the interconnection line of the embedded generator to the distribution network was determined. Computer package called SYNERGEE-version 2.1 developed by Stoners Electric, USA was used in this simulation [11]. Siripagama MHP was connected to the feeder no.2 of the Ratnapura Grid Substation. This feeder was modeled in SYNERGEE giving all data of feeders, conductors, embedded generators and load. Feeder was divided into several line sections for ease entering of load data. Figure 5 shows the modeled feeder.

Figure 5 Feeder No.2 of Ratnapura Grid Substation

The feeder 2 of Ratnapura Grid Substation run towards Balangoda and it was tapped at Malwala to feed Siripagama area. There were three other embedded generators connected to this feeder and two were in this section. Loads were modeled as PQ buses and the net active and reactive power was entered. Generators were modeled as PV busses where it was taken that the AVR of the generator keeps the voltage at set value. Ratnapura Grid Substation was taken as the reference bus. Feeder, conductor and existing embedded generator data were taken from CEB. There loads were entered in two sets. Six months average values of maximum demands of each heavy consumer were calculated and entered as spot loads. Distribution loads

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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were the ordinary consumers of single phase and three phase supplies connected to low tension distribution system at each transformer. Generally, loading pattern of National grid shows two peaks during 6.00pm to 9.00pm and 9.00am to 11.00am. Therefore, the network was analyzed for both of peak and off peak loading conditions. The night load peak gives the highest value and distributed load data was measured during this period manually. Nineteen distribution transformers were connected to the modeled Feeder 2 of Ratnapura grid substation and load data were measured for all low tension feeders. The off load was collected with measurements and calculations and found that the off load is around one fourth of peak load. Therefore, the model was run for two loading conditions 100% and 25% of peak time load. Results showed that the both loading conditions were acceptable with conductors’ ratings. The demand of the feeder (30A) during peak load is less than the demand (51A) at off peak load. It was also notice that during the peak load, feeder fed power to the distribution system and during off peak load, power was fed back from distribution network due to the embedded generators in this feeder. In this study it was observed that voltage variation was significant. The base voltage was taken as 33 kV. Feeder 2 of Ratnapura grid Substation was divided into several sections. Voltages of two sections were higher than the acceptable limit (106%) of CEB. Conductor type and length are given in the Table 1.

Section Conductor type Line length / km % Voltage BA07-034 7/0.102 1.70 109.9 BA07-035,36 7/0.102 7.20 109.9 BA07-029 LAB 7/0.161 3.10 102.5 Total length/km 12.0

Table 1 Section data The over voltage can be solved either by changing the conductors with low impedance or reducing the line length. Usually Aluminum Conductor Steel Reinforce (ACSR) conductors are available in Sri Lanka for long span distribution lines. Changing of line length was also not possible as it was through a fairly difficult terrain and many land constrain exists. Therefore, only option was to have a higher diameter conductors to reduce line reactance and hence losses. Conductors were changed to 7/0.102 to 7/0.161 and run the model again. Then the all parameters were within the acceptable limits.

Figure 6 Grid interconnection line route of Siripagama MHP

Accordingly the grid interconnection line was made as follows; 1. Existing 7/0.102 conductors to be replaced by 7/0.161 conductors and necessary rehabilitation of the

line section, 8.90 km from Malwala to Guruluwana (tapping point of Siripagama MHP) 2. New 1/0.161 line 3.10 km from tapping point to Switchyard. 3. Installed a switch break disconnector at tapping point.

Iluktenna

H

Malwala

To Balangoda

G

Ratnapura GSS

B

K Guruluwana

A

To Siripagama

F

To Guruluwana

MHP

Siripagama MHP

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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b. Fault Studies

There can be faulty conditions such as failure of insulators, broken or touching bare conductors, which creates short circuit between conductors or between conductors and earth. The distribution system was designed in such away that fault currents are large enough to be detected under all operating conditions. Before accommodating an embedded generator to the distribution network, Fault Level (FL) calculations are also made to check whether the existing switchgear capacities in the grid substation can withstand the FL with the embedded generator. The configuration of the system has a significant effect on the FL. Synchronizing additional generators or connecting parallel lines, reduces the impedance of the network and hence increase its FL. This affects the ratings of the grid substation components. Therefore, FL calculation was done in order to check whether the present switchgear ratings are sufficient to connect the embedded generator. Figure 7 shows the generator at Siripagama MHPP connected to the grid through the distribution network. Fault level calculation at Ratnapura Grid Substation due to interconnection of this generator is given below.

Figure 7 System configuration for the fault studies

It was taken that the common base parameters as 100 MVA and 33 kV. All resistive parts of impedances & link charging capacitances were neglected. Fault level at the Grid Substation was taken from the forecasted CEB reports. Fault level at Ratnapura Grid Substation without Sripagama MHP is 7.2 kA. Existing switch gear capacity is 25 kA. Reactance of 7/0.161 conductor line is 0.375 Ω/km. Therefore with the 12 km line and the MHP generator and transformer transient impedances, the fault level will not be increased by more than 4.23 kA. It shows that the fault level at the Ratnapura Grid Substation with Sripagama MHP will not exceed the switchgear rating. 5 CONCLUSION

Sri Lanka Government changed its policy of power generation change to overcome power crisis in the country and encourage private sector in this field. Sri Lanka has many renewable energy sources all over the country. Since the south-west monsoon region gets high rainfall, Ratnapura area has a high potential for hydro power generation. Country benefits in many ways with an implementation of a mini hydropower project especially in means of improving infrastructure and the standard of living in these remote villages.

Plant sizing, selection of electro-mechanical equipments were done in proper manner. Adequate protections for the network as well as for the generator and the personnel were accommodated. Control system for power flow and synchronizing were also accommodated as per the practical applications. Monitoring system give a better transparency on operation. Generator performances were tested even by modeling the system. Power line was design in such a way that to satisfy utility constrains and to minimize line losses.

In overall this design comprised of a complete electrical system and it confirmed the performances of the project. In October 2006, plant factor was 56% which is fairly good even with the power interruptions.

S G

3 phase fault

Vg=1<0

Zs Zf

Grid Substation bus

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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6 AKNOWLEDGEMENT

This design is submitted in partial fulfillment of the requirements for the award of degree of Master of the Science of Engineering in Electrical and Electronics Engineering conducted by University of Peradeniya. The Authors would like to thank project developer for the cooperation and the panel of examiners of thesis evaluation for their valuable comments and corrections.

7 REFERENCES Statistical digest 2006, published by Statistical unit of Ceylon Electricity Board, Sri Lanka. Harvey A., Brown A., Hettiarachchi P., Inversin A., Micro-Hydro Design Manual – A guide to small-scale

water power schemes), Intermediate Technology Publications 1993, 153-304, 321-348. Ceylon Electricity Board, Guide for grid interconnection of embedded generators-Part 2, December 2000. Sayann K. S., Hydro turbines for SHP, International course on Small Hydropower development, Indian

Institute of Technology, Roorkee February 2004, 186-216. Singla A.K. , Management of operation and maintenance in SHP- a case study, International course on

Small Hydropower development, Indian Institute of Technology, Roorkee February 2004, 115-124. Bijelwan R.C., Design and Selection of SHP hydro generators, International course on Small Hydropower

development, Indian Institute of Technology, Roorkee February 2004, 217-226. Verma H.K. , Auto and remote control of SHP stations, International course on Small Hydropower

development, Indian Institute of Technology, Roorkee February 2004, 243-248. Say M.G., Alternating Current Machines, book, Halsted publisher, February 1984. Jenkins N., Allan R., Crossley P., Kirschen D. and Strbac G., Embedded Generation, book, IEE power and

energy series 31, 2000. Fernando M.A.R.V., Investigation of synchronizing parameters in paralleling a remotely located mini hydro

unit to an unbalanced loaded bus bar, IESL, Transactions 2002, Vol. I – Part B. Integrated software manual, Synergee electric computer software, Stoners electric, Sc ott and Scott Systems

Inc, USA.