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Electrical distribution systems
The electrical-distribution system furnishes electrical power and partial protection of the electrified oil field and consists of a primary system and a secondary system. It is important to the economics and longevity of the overall system that distribution be designed adequately before installation.
Primary distribution system and voltages
To reduce power losses, electricity distributed to an oil field is brought to the field at higher voltages of between 4,000 and 15,000 V. This higher-voltage distribution system is called a primary system. Higher voltages allow the use of smaller conductors, but require more expensive transformers. Even so, when the primary system must deliver electrical power over a long distance, a higher voltage generally is favored because the lower cost of the smaller cable over the longer distances offsets the higher costs of the transformers and protective equipment.
An electrified oil field has a high degree of exposure to electrical storms. Electrical storms cause high static voltages and, sometimes, high transient voltages, the latter being caused by lightning. Static lines and lightning arresters are used to reduce the damage to electrical equipment by the static voltages and lightning strikes. During electrical storms, the formation of rain clouds creates a difference in potential between the cloud and the earth. Above-ground primary electrical systems in a storm’s vicinity might inherit a high static-voltage level that, if not reduced by properly sized and grounded lightning arresters, can cause motor-winding-insulation damage. When the potential difference between the cloud and the earth becomes large enough, there will be an electrical discharge, or lightning strike. If lightning strikes the primary system, it will create high transient voltages. These voltages must be arrested by lightning arresters; otherwise, the insulation of the motors will fail, as will other electrical equipment in the system, including transformers and reclosures.
Secondary electrical system
The secondary electrical system includes devices that operate at the same voltage as the motors, including the transformer at the end of the primary system, the cables, the disconnect switches, and the controls. In general, the voltage of all the devices within the secondary system should not be greater than 600 V.
A special case of the secondary system is the installation of a 796-V system. This voltage is obtained by Y-connecting three transformers, each with a secondary voltage of 460 V, yielding a line-to-line voltage of 796 V at the motor. (The Y connection is discussed below.) The 796-V system is used to reduce line drop to the motor; however, many operators who installed 796-V systems years ago have since converted to 460-V operation because, although operating at 796 V requires less current than operating at 460 V, this benefit is more than offset by the 796-V system overstressing the insulation of motors and control components.
The secondary system consists of one or more transformers that convert the primary-system voltage to the motor-operating voltage. Voltage from the transformer is provided to the motor starters through a fused disconnect switch or a circuit breaker. The motor starter provides for control and protection of the motor itself.
Wherever the secondary system uses overhead cables, it is exposed to electrical storms. An electrified oil field has a high degree of exposure to electrical storms. Electrical storms cause high static voltages and, sometimes, high transient voltages, the latter being caused by lightning. Static lines and lightning arresters are used to reduce the damage to electrical equipment by the static voltages and lightning strikes. During electrical storms, the formation of rain clouds creates a difference in potential between the cloud and the earth. Above-ground primary electrical systems in a storm’s vicinity might inherit a high static-voltage level that, if not reduced by properly sized and grounded lightning arresters, can cause motor-winding-insulation damage. When the potential difference between the cloud and the earth becomes large enough, there will be an electrical discharge, or lightning strike. If lightning strikes the primary system, it will create high transient voltages. These voltages must be arrested by lightning arresters; otherwise, the insulation of the motors will fail, as will other electrical equipment in the system, including transformers and reclosures.
All the devices in the secondary part of the system should be sized to allow full loading of the motors without thermal damage to the equipment. Sizing of this equipment also should consider the protection of the electrical devices. Select fuses, circuit breakers, transformers, and wire sizes on the basis of the full-load rating of the motors.
Distribution transformers reduce the primary high voltage to a lower voltage used by the motors. The distribution transformers are rated from 3 to 500 kVA. Transformers larger than 500 kVA are classified as power transformers.
To obtain full-load capability of the transformers and the motors, it is desirable to use three single-phase transformers in place of a single three-phase transformer. One advantage of using three single-phase transformers is the convenience of replacing one, should it fail.
Distribution transformers can be connected in several different configurations to deliver three-phase power: wye-delta, delta-delta, delta-wye, wye-wye, and open-delta (Figs. 1 - Figs. 2 - Figs. 3). All these configurations are used in the oil field, although some have distinct advantages.
The most desirable transformer connection is the wye-delta. The transformer primary is wye-connected, and the secondary is delta-connected. Though it is done in many cases, the primary winding of the transformer, known as the Y point, should not be grounded at the wellsite because if one of the primary wires were to go to ground at some point in the primary system, the ground wire at the wellsite might be at primary voltage to ground potential, creating a danger of electrical shock for personnel. The transformer "ground" should not be connected to the grounding system at the pumping unit because the latter includes the enclosures for the electrical equipment.
The wye-delta connection has the advantage of allowing harmonic voltages in the system to have a self-canceling effect in the delta-connected secondary. It is not necessary for three single-phase units connected wye-delta in a three-phase bank to have equal impedances; however, it is important for the primary to have balanced voltage because unbalanced primary voltages can cause circulating currents in the delta secondary.
In a delta-delta connection, the primary and secondary windings both are delta-connected. Delta-delta is an acceptable transformer connection, although not as desirable as wye-delta. The delta-delta connection requires all units in a three-phase bank to have impedances with less than a 10% differential. If a delta-delta connection is used, none of the endpoints or midpoints of the primary or secondary winding should be tied to the ground system at the wellsite. If any is, and if the ground is not satisfactory, the ground wire could be at a potential anywhere from zero to the line-to-ground voltage available at the transformer.
The delta-wye connection is undesirable because it allows a harmonic voltage in the distribution system to be applied to the motor and its control system. Harmonic voltages can cause erratic behavior of control components, as well as excess motor heat. If a delta-wye system is used, neither the primary nor the secondary windings of the transformer should be connected to the ground system at the motor. If grounds are attached to any part of this winding, they might be subject to the same voltage discussed under delta-delta. It is not necessary for the impedance of each unit in the three-phase transformer bank to be the same.
The wye-wye connection is the least desirable because harmonic voltages in the system are unable to circulate in the transformer winding. If harmonic voltages exist, they will be transmitted to the motor and its control system. If a wye-wye connection is used, no part of the transformer winding should be connected to the ground system at the wellsite. If a primary circuit has a phase-to-ground fault, a grounded wye will carry ground-fault current. This connection does not require transformers to have equal impedance. Using a delta secondary will eliminate harmonic voltage in the motor and its control system. It is not necessary for transformers to have equal impedances.
The open-delta is an incomplete delta-delta and is possible when three single-phase transformers are connected in delta-delta fashion to provide the three-phase power. If one of the three transformers on the delta-delta connection is removed, the connection is an open-delta circuit. This type of connection provides unsatisfactory performance of induction motors. The open-delta connection will have unbalanced voltages, which prevents utilization of full-load rating of the transformer and motor.
At no-load and with balanced voltages supplied to this transformer, the output will be a balanced three-phase voltage. While this two-transformer system is loaded, the impedance changes, providing an unbalanced voltage to the motor. Using the two-transformer open-delta connection does not allow full use of transformer kVA and the output rating of the motor.
Figs. 2 and 3 compare a three-transformer delta connection and an open-delta transformer connection.
In the open-delta connection, the total kVA is only 57% of the original 100 kVA. The two 33.3-kVA transformers remaining in the circuit would have a total of 66.6 kVA. With one unit removed, the remaining units with 66.6 kVA provide only 57.7 kVA, or only 86.6% of the rating. This example shows that the transformers used in open-delta connections must be derated to obtain the desired kVA rating of an open-delta connection system.
While the open-delta-connected transformers are loaded, the voltage shifts from a balanced voltage at low load to a seriously unbalanced voltage at rated load. Unbalanced voltages will contain a negative sequence component of voltage. When applied to a three-phase induction motor, this causes excessive heating in the rotor, as well as some lost torque in the motor. Unbalanced voltages cause unbalanced currents.
Unbalanced voltage causes a 3 to 5 times greater current imbalance. This means, for example, that for a 3% voltage imbalance, a current imbalance of 9 to 15% can be expected. Such unbalanced conditions require that the motor be derated. Fig 4 can be used to determine the derating factor for percent voltage imbalance at the motor terminals. It shows, for example, that for a 5% voltage imbalance, the motor will operate at 75% of capacity.
Fig. 4—Effects of unbalanced voltages on performance of three-phase induction motors. (Reprinted from Motors and Generators, MG-1-1978, by permission of the Natl. Electrical Manufacturers Assn., © 1978; all rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Library and Artistic Works, and the International and Pan American Copyright Conventions.)
Many older oilfield installations use open-delta transformer connections. The only way the open-delta transformer will operate successfully is if both the transformers and the motors are oversized to handle the load. An open-delta transformer connection should be used only briefly and in an emergency situation in which one transformer has failed. For this emergency condition, the transformer and motor must be derated.
Sizing of the distribution transformer is a very important part of satisfactory operation of rod-pumping motors. The industry rule of thumb for sizing transformers is 1 kVA per connected hp. Because of the cyclic nature of oilwell pumping loads, some operators use 0.9 kVA/hp. Ultrahigh-slip motors do not have horsepower ratings; therefore, to determine the required kVA, use a factor of 0.75 times the full-load current of the motor in the high-torque mode.
- Bradley, H.B. 1987. Petroleum Engineering Handbook. Richardson, Texas: SPE.
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