Soft Start of Submersible Pumped Oil Wells
Authors: Neely A.B. , Patterson M.M. 
Published: December 2007

Cyclic operation of submersible pumping is considered undesirable because of the start-stop characteristics of the installation. "Across-the-line" starting results in large current surges, typically five to eight times running current. Such surges can damage the motor and its cable and also cause line voltage irregularities. Also, stopping can result in significant amperage and voltage spikes. Because of this, most operators are very reluctant to start and stop a submersible pumping installation any more often than absolutely necessary. This creates design problems in sizing the pump. Reduced-voltage starting should minimize the severe strain imposed on the electrical system by an instantaneous start. A reduced-voltage solid-state starter using six silicon-controlled rectifiers was tested between Nov. 1981 and March 1982. The current surge on start was limited to about 2.5 times full-load amperage. The time required to bring the motor to full speed was increased from about 0.25 seconds to 1.3 seconds. The well was cycling on-off about six times per day. The unit was start-stopped about 900 times during the test. A prototype production model is available.

Introduction

The undesirable characteristics of cyclic operation of submersible pumps can create several design problems. If the pumping unit is undersized, it must either be pulled and a larger pump run or production loss will occur. When the pump is oversized, the well will tend to pump off. Cyclic operation will occur unless the backpressure at the wellhead is increased to reduce the capacity of the centrifugal pump. This is undesirable because of the energy waste caused by imposing the backpressure, as well as the possibility that thrust problems will occur in the pump, leading to early failure. If it can be shown that reduced-voltage or "soft" starting of a submersible pump will eliminate the detrimental effects of cycling, then installation design becomes much easier. A pump capacity 25 to 30% above well capacity can be selected and the well operated sufficient hours each day to meet well capacity. This is a commonly used and successful technique with sucker rod pumping.

Reduced-Voltage Starting

The standard electrical unit used today to start high-voltage, high-horsepower induction motors is an electro-mechanical or across-the-line starter. It can handle voltages up to 2,500 and horsepowers to 1,000 [746 kW] without problems. Starting is done by pushing a button that energizes a 100-VAC relay, which in turn causes the main power relay to pull in. Starting is almost instantaneous. Field tests and data from starter manufacturers show that large current surges can occur during this start period. Such surges can damage the motor and its cable and also cause line voltage irregularities. In submersible pumped wells, since the motor controls must be on the surface, there can be up to 10,000 ft [3048 m] of electrical cable between the starter and the motor. The problem is more severe with higher voltages and long cable runs like those used in submersible pumped oil wells, or in cases where the load is separated from its transformer by great distances. Whenever controlled starting is required, the usual methods are wound rotor motors, special connections using a wye to delta conversion or in-line transformers. Some of the disadvantages of these methods are the added cost of the motor, the expense of the switch gear and voltage-dropping components, the maintenance of necessary mechanical devices, and step changes in torque and acceleration during startup. Improvement in silicon-controlled rectifiers (SCR's) has led to the development of a reduced-voltage starter that is well suited for controlling large power levels. The basic circuit uses six SCR's connected as three full-controlled AC switches. One switch is interposed between each power phase and the motor. Fig. 1 shows how the SCR's are connected in a full bridge to control or limit the current. When hooked up as shown, the circuit can control the AC power to a load by varying the turn-on point of each SCR during each half cycle (see Fig. 2). The switching is done by applying the properly timed signal to the gate of the SCR. By gradually changing the back phase angle in each half cycle, it is possible to increase the power flow to the motor smoothly. Typical operation of such a system is shown in Fig. 3. When the start button is pushed, the current rapidly ramps up to a preset value of the full-load amperage (FLA), usually 40%. This level is set just below the breakaway torque of the motor-load unit. From this point the current will slowly begin to ramp up to some preset level of the FLA that will ensure that the motor runs at the correct voltage. This time period is adjustable and can range from one to 30 seconds. During this soft start the voltage and the motor speed are also slowly changing as shown in Fig. 4.

JPT

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