This paper discusses common methods of power cable design for electrical submersible pumps and presents a new method which can result in a more optimal design. A procedure is developed for estimating the maximum expected operating temperature of the conductor for each cable size. An accurate selection of power cable insulation type can be made for each cable size based upon the maximum operating conductor temperature. Total cable investment cost can then be determined for each cable size. Power loss calculations are also provided to predict the power cost of the cable. The final cable selection can be determined by economic analysis. Other factors that should be considered when selecting the cable for a specific ESP installation are also discussed.
Electrical submersible pumps are a major form of artificial lift in the industry. The cable string is often the most expensive component of the ESP system. The choice of cable size and insulation is based on operating conditions. Proper cable selection can significantly improve the economics of an ESP installation.
The cable insulation and cable size are two major determining factors of the investment cost of the cable. The major variable to consider in proper selection of ESP cable insulation is the maximum expected operating cable temperature. This paper presents a method of estimating the maximum expected operating conductor temperature due to electrical resistance in a given ESP cable based on motor amperage and the ampacity/temperature curves for the cable. Maximum cable operating temperature can then be compared to a maximum recommended temperature for a given insulation material or jacket material to determine which temperature dependant materials are acceptable for a given cable size in a given well. The final cable selection is based on the economics of investment cost versus operating costs.
The major factors to consider in selecting cable size and insulation are maximum expected operating temperature, downhole clearances, and operating versus capital investment costs. The industry has tried to incorporate many of these considerations into conservative, easy to use cable selection guidelines. Four common guidelines used are:
Maximum Voltage Drop per 1000 ft (305 m) of Length
A voltage drop of 30V/1000 ft is a common figure used as a maximum limit for sizing a given cable. Voltage drop in itself is not detrimental to cable. The increased conductor temperature due to resistive heating caused by high amperage flow is the most critical factor in cable operation.
Percentage Rule versus Nameplate Voltage Percentage Rule versus Nameplate Voltage This rule sets a maximum total cable voltage drop limit of 5% of the motor nameplate voltage. Cable selections are limited to those that will not exceed a total voltage drop of 5% of the motor nameplate voltage. The rule has previously been shown to result in a less than optimum design in some situations.
Percentage Rule versus Total Surface Voltage
This rule allows consideration of only those cables which limit the total cable voltage drop to 5% of the total surface voltage.