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Index Terms--AC motor protection, Induction motor protection Overcurrent protection.

I. NOMENCLATURE

I Fault current in Amps A Conductor Transversal section in Circular

Mils T2 Cable Short-circuit Temperature T1 Cable Service Temperature In Nominal Current in amperes Ist Starting Current in amperes ST Starting time assumed 3s (can be changed) MINscale Is the minimum value of decade (in amperes

or seconds on the equation) Med Length from the minimum value of scale to

the unknown point (*) Dim Length of a decade on hard copy catalog. (*)

(*) med and dim shall be measured in the same unit (centimeters, inch, etc)

II. INTRODUCTION

ow voltage coordination is a very important and fundamental tool for the analysis of an electric system in

industrial plants. The safety of the electric system’s operation depends on this analysis as well as the security of personnel who work at or operate an industrial plant.

Protection Coordination software is very popular these days, but sometimes these kinds of software are very expensive, or have limitations. For example, they only work with a group of trademarks or sometimes the report summary cannot be customized.

The electric department of Inelectra makes Coordination protection analysis for many clients using Microsoft Excel worksheets in a manual way. This paper realizes an automation of the process, using the programming module of Microsoft Excel called Visual Basic Application (VBA). The use of this tool is very helpful to customize any study or

analysis. In this case, it is used to develop an application to execute Protection Coordination for Low voltage systems. The main reason to use Microsoft Excel is because it is the most popular electronic worksheet.

III. THE APPLICATION

Basically it consists of a Microsoft Excel Chart with data series plotted. Each series represents curves and points related to the study. Figure 1 shows the application screen.

Fig. 1 – Application screen

IV. CURVE PARAMETERIZATION

The user can obtain curves from a hard copy catalog or electronic files. Sometimes it is difficult or imprecise to obtain data point.

Figure 2 shows two typical cases that can be presented in a hard copy catalog. In case “A”, the current is known and time is unknown. Time can be calculated applying the next equation.

Current: ×= dim10med

ScaleMINt (1)

In case “B”, the time known and the current is unknown, the current can be calculated applying the next equation.

Time: ×= dim10med

ScaleMINI (2)

Software Design for Low Voltage Protection Coordination based on Visual Basic

Applications (VBA) for Microsoft Excel A. Borrero, and M. Azarie

L

1-4244-0288-3/06/$20.00 ©2006 IEEE

2006 IEEE PES Transmission and Distribution Conference and Exposition Latin America, Venezuela

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Fig. 2 – Parameterization Chart

V. SOFTWARE OPTIONS

The user can get trip curves from any manufacturer. These curves can be:

A. Cables Damage Curves

The application can be represented on the drawing as the cable damage curve for copper and aluminum cables, and a different insulation.

Fig. 3 – Conductor properties dialog box

Copper conductor

++

⋅=5.2345.234

log0297.01

210

2

TT

AI (3)

Aluminum conductor

++⋅=

228228log0125.0

1

210

2

TT

AI (4)

B. Molded Case Circuit Breaker

The application has a data base loaded, with typical trip curves for a magnetic-only circuit breaker and thermo-magnetic circuit breaker. The user can load another trip curve from any other manufacturer by just obtaining points from the minimum curve and the maximum curve.

Fig. 4 – Fixed Breaker properties dialog box

Also the user can add trip curves from an adjustable circuit breaker modifying values such as: long time pickup, long time delay, short time pickup, short time delay and instantaneous pickup.

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Fig. 5 – Adjustable breaker properties dialog box

C. Motor Starting Curve

A low voltage motor starting curve is very difficult to obtain from the manufacturer and a design engineer needs to obtain this information to start the protection coordination. The application considers seven points to construct the motor starting curve.

TABLE I POINTS TO CONSTRUCT MOTOR STARTING CURVE

Point Current Time 1 In 1000s 2 In ST x 1.2 3 In x 1.2 ST 4 Ist x 0.8 ST 5 Ist ST x 0.8 6 Ist 0.03s 7 Ist x 1.6 x 1.1 0.01s

Fig. 7 – Motor properties dialog box

D. Transformer Damage Curves

The application can represent the transformer damage curve based on [4]. The through-fault protection curves presented in IEEE Std C57.12.00-2000 takes into consideration the fact that transformer damage is cumulative, and the number of

through faults to which a transformer can be exposed is inherently different for different applications.

TABLE II TRANSFORMERS CATEGORY

Category kVA One Phase

kVA Three phase

I 5-500 15-500 II 501-1667 501-5000 III 1668-10000 5001-30000 IV > 10000 > 30000

For unbalanced faults, consider the ANSI factors and the connection type as well, this through-fault protection curve can be represented for ANSI factors..

Fig. 6 – Transformer properties dialog box

E. Thermal Relay Operation Curve

On industrial plants, it is very common to use thermal relay based on a bimetallic device. It’s operation curve is an inverse time type. Manufacturers often represents this curve in multiples of a current called “Ir”. This current is set by the designer and it depends on nominal load current and restrictions made by National Electric Code.

Also plotted is the in-rush current for transformers and nominal current.

TABLE III TYPICAL INRUSH CURRENTS

Power (kVA)

Time (s)

Current x In

500-2500 0.1 8 In > 2500 0.1 10 – 12 In

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F. Fuse melting curves

Low voltage fuse curves can be plotted from an existing data base. Also, new manufacturer’s curves can be added.

Fig. 8 – Fuse properties dialog box

G. Special curves

Sometimes it is required to show special data on the protection coordination drawing. This information can be obtained from a hard copy catalog or any computer report. The application has an option called “Dispositivo de Usuario”, which allows the user to represent any curve or any points not contained into the group of curves mentioned from A to F.

VI. EXAMPLE

The example shows a typical one-line section. The intention of this example is to show the low voltage coordination of a 20Hp induction motor.

Industrial plants use several numbers of induction motors, in this example, a 20Hp motor will be connected to an existing motor control center. The application will be used to adjust the protective devices located into a typical full voltage non-reverse starter. The starter is equipped with a thermal relay to protect the motor against overloads and a molded case circuit breaker to protect the feeder against short-circuits currents.

Fig. 8 – One-line Example

First it is necessary to obtain all the required data.

- Motor: Induction type, nominal power 20Hp, 3 phase, nominal voltage 460V, nominal current 27A (taken from table 430.150, NEC), starting current 145A (taken from table 430.151(B), NEC). Starting time assumed 3 sec.

- Feeder: Assumed length 200m, #2 AWG, insulation XLPE, 600V

- Thermal Relay: Telemecanique, Class 10, current range 23A – 32A. Model: LRD-32

- Molded Case Circuit Breaker: Square-D, Only magnetic circuit breaker, Mag-Gard Model: FAL3605016M

- Motor Control Center: Square-D, 480VAC, 3 phase + Ground, Short-circuit withstand 65kA at 1 second.

- Main Breaker: Micro Versa trip, Circuit Breaker, 150A, 65kA, Current Sensor: 150A, Rating Plug: 60A, Long Time Pickup: 1 x (Rating Plug), Long Time Delay Band: 4, I^2t: In, Sort Time Pickup: 9 x (Long Time Pickup), Sort Time Delay Band: 1, INST: None

- Transformer: 500 kVA, 4.16 kV / 0.48 kV, Δ-Y, solid grounded, Z = 7%

- Short circuit currents: Typical values for short circuit current at 480V motor control centers are between 20kA and 45 kA. The assumed value is 38kA.

After completing all the required data, the designer engineer is ready to star the study. Start by filling in the general information: project name, elements to coordinate, date, project number, revision, document number, one-line drawing

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number, one-line image file, client image file, time and currents limits to plot.

Fig. 9 – General properties dialog box

VII FINAL REPORT

Figure 10 shows the final report for the current study. From the report it is possible to verify that the 20 Hp new motor is successfully added to a system. The settings for the thermal relay are appropriate to allow the motor start. And the setting for the circuit breaker provides an integral protection against high currents until the real short circuit level (38kA). The motor’s feeder is totally protected and the existing settings for the motor control center main breaker keeps selectivity with the downstream circuit breaker

Thermal Relay: Ir = 33.75A (Class 10) Breaker: In = 20A, A, Trip 1300%

VIII. CONCLUSIONS AND FUTURE WORK

This paper has discussed one of many protection coordination studies. Protective and damage curves are shown in different colors and the user can manage the labels, colors, line width, one-line figure and add any other note or drawing.

This application can be performed to be used in medium voltage systems. It is a starting point of motivation for many design engineers to maximize the potential of the information tools.

As a future work, the incorporation of a new module that will verify the coordination and selectivity between protective devices or curves has been contemplated.

The migration of this application to be used online into the INELECTRA’s Intranet and will be accessible for all engineers who work in Inelectra, S.A.C.A.

IX. REFERENCES

[1] IEEE, (2001) IEEE Recommended Practice for Protection and Coordination of industrial and Commercial Power Systems 242, ANSI/IEEE

[2] IEEE, (1990) IEEE Recommended Practice for Power System Analysis 399, ANSI/IEEE

[3] NFPA, (2005) National Electric Code, NFPA-70.

X. BIOGRAPHIES

Alfredo Borrero was born in Caracas - Venezuela, on December 21, 1976. He graduated from the Simon Bolivar University.

His employment experience includes Inelectra, S.A.C.A., working in hydrocarbon industry.

Michel Azarie was born in Anzoátegui - Venezuela, on July 16, 1983. He graduated from the “Universidad de Oriente”.

His employment experience included Inelectra, S.A.C.A., as engineer assistance

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0.001

0.010

0.100

1.000

10.000

100.000

1000.000

10 100 1000 10000 100000 1000000Current (Amp.)

Tim

e (s

ec.)

20HP/480V/3F MOTOR 1

RELÉ TÉRMICO Clase 10

RELÉ MAGNÉTICO AJUSTE - 1300

CONDUCTOR CALIBRE: 2 - ICC: 38KA

M-VERSATRIPS:150- RT:60LTC: 1- LTD:4I^2t In- STC:9- STD:1INST: None

ICC TRIFÁSICA Valor: 38 KAmp.

TX 1-TR3F-500KVACURVA DE DAÑOS

TX 1-TR3F-500KVA1er Pico

TX 1-TR3F-500KVA2do Pico

TX 1-TR3F-500KVACURVA DE DAÑOS - ANSI

TX 1-TR3F-500KVACORRIENTE NOMINAL

Fig.10 – Final report for current study