When we talk about commissioning an ESP (Electric Submersible Pump) system using a direct-on-line (DOL) contactor or a soft starter, selecting the right transformer tap is a "make or break" step. Unlike systems with a VFD (Variable Frequency Drive), where you can adjust the output frequency and voltage on the fly, here you are locked into the physical connection on the step-up transformer (SUT).
Your goal is to provide the motor with its rated voltage while accounting for the voltage drop across the power cable. In the oilpatch, we call the documentation for such equipment the Data Sheet or Equipment Records, and getting the numbers right here prevents a premature motor failure or a "nuisance trip" during startup.
Selecting the right tap for these types of Switchboards (Control Panels) is a fairly straightforward task. To do it correctly, you just need to calculate the voltage drop across the downhole cable and add this value to the motor’s nominal voltage. Then, find the nearest higher tap on the transformer and set the step-up transformer (SUT) switches accordingly.
I’ve attached a standalone calculation utility (an executable file ) to this post so you don’t have to crunch the numbers manually. This tool handles the cable loss math for you. Just a heads-up: the calculation should be based on two conductors.
If you're out at the wellhead without the tool, you can use these "back-of-the-envelope" estimates:Note: Please be aware that this calculation utility is in Russian. Unfortunately, I don't have the source code to provide an English version. However, the interface is intuitive for any field engineer - you just need to input your current and cable length to get the voltage drop. If you run into any trouble with the translation, feel free to drop a comment in the thread.
- For motor currents around 20A, figure a drop of about 24.4V per 1,000 ft (80V per 1000 m).
- For the 40-50A range, you're looking at roughly 30.5V per 1,000 ft (100V per 1000m).
Make sure to measure the voltage while the ESP is running to account for the grid load. Then, at the first opportunity (the next time the unit trips or is shut down), adjust the tap to the calculated value.
Calculation Example:
Let’s look at a typical scenario. We have a motor (ESP motor/PED) with a rated voltage of 2,000V and a rated current of 45A. The cable string is 8,202 ft (2500m) long with a #5 AWG (16mm2) conductor cross-section.
Before the initial startup, we run the numbers using the calculation tool. For this setup, we get a voltage drop of 246V. We then set the transformer tap to the value closest to 2,246V (2,000V motor + 246V drop).
Once the unit is up and running, measure the operating line voltage. After about an hour of stable operation, record the motor currents, update the settings in the Switchboard, and perform a "final" tap calculation.
Let’s say the measured voltage under load is 400V and the operating current is 32A. First, recalculate the voltage drop for the new current: it’s now 175V. This gives us a target secondary voltage of 2,175V. Now, we account for the actual line voltage:
(2,175V / 400V) * 380V = 2,066V.
As shown by the recalculation, the optimal tap is 180V lower than our initial estimate.
Note: Please keep in mind that this specific calculation is based on the GOST standard (380V). However, the logic remains exactly the same for the American system (480V). To adapt this for US operations, simply substitute the 380V reference with 480V in your formula to match the local grid standards.