Basic terminology and how the electric-power system works from generation to the end consumer was covered in Part 1 of this series (TCI Magazine, October 2023). This article will focus on the distribution system. Trees are often near these lines, so arborists need to be familiar with the associated electrical-system equipment.
Arborists need to be able to recognize these devices and have a fundamental understanding of their functions. The electrical-system equipment can be divided into three categories:
1) Voltage-management devices.
2) Protective devices.
3) Support devices.
This article will cover voltage-management devices and many protective devices. Some protective devices, switching devices and support devices, such as insulators and poles, will be discussed in Part 3 of this series.
There is a critical need to manage the voltage in the distribution system. Small appliances in a house operate efficiently within a narrow range of voltage, so service voltage is managed between 114 and 126 volts for 120 volts, and between 228 to 252 volts for the 240 volts needed by larger appliances. Voltages that fluctuate beyond these limits can damage appliances.
There are three devices on the pole that manage voltage. These are transformers, regulators and capacitors. Transformers are designed to transform voltage higher or lower. Capacitors adjust the power factor, and voltage regulators correct voltage irregularities.
The function of transformers was briefly discussed in Part 1 of this series. The transformers at the power plant step up the voltage at generation to that needed to flow through the transmission lines. The transformers at the transmission and distribution substations and on the poles along the distribution system step down the voltage. Transformers on the distribution system poles are the ones discussed in this article.
Transformers convert electric power from one voltage to another. The conversion is through two sets of coils – one winding for higher voltage and the other for lower voltage – through a magnetic field. The current also is transformed by the winding, but opposite the voltage, i.e., decrease the voltage and the current increases. A single-phase 50-kVA (kVA = 1,000 volts x amps) transformer can be 12.4 kV (7.2 kV phase-to-ground) with a full-load current of about 7 amps (A) on the primary coil, while on the 120/240-V secondary coils, the 240-V is about 200 A.
This is a good reminder that it is the current that kills. Secondary lines and service drops, despite their lower voltage, still can cause electric shock. The voltage is low enough that serious injuries are rare on service drops, but they do happen. In one recent instance, a climber received a shock while cutting a branch when it deflected into the secondary lines. Groundworkers have suffered fatal injuries from holding a rope in contact with a service drop.
The typical pole-mounted transformer tank is a short cylinder attached to a single hanger bracket beneath the primary line. This single-phase transformer is about 30 inches in diameter and 40 inches long. There are porcelain, high-voltage bushings on the cover connected to the primary line. A high-voltage surge (lightning) arrester is attached to the tank to protect it from faults. There are low-voltage bushings on the side that connect to the secondary lines.
While the transformers on the pole step down the voltage from primary to secondary, they also can step up the voltage. This may occur during an outage if a residence or business has a generator that is back-feeding up the service drop to the transformer. Building codes require there be a physical disconnect when the building switches over to a generator to prevent back-feeding. Buildings with solar or wind turbines can back-feed excess power to the grid. These must automatically disconnect during outages to protect utility workers repairing lines.
Residential distribution lines may be buried rather than overhead. The transformers for underground systems are the large rectangular boxes set away from the houses in a neighborhood, usually one behind or between the homes. These have a warning to remain away from the boxes, a warning that is frequently ignored. While fatal and non-fatal injuries are rare from contact with these pad-mounted transformers – usually through broken access panels – they do occur. Do not use them as a platform to set gear on or as a seat. Establish drop zones so cut branches will not fall on these pad-mounted transformers.
Voltage regulators are found along feeders to adjust the primary voltage. Think of a voltage regulator as a mini transformer, reducing minor variations in voltage, usually less than 10% above or below the normal line voltage. Greater variation can occur along long feeders as people come home and turn on appliances.
The regulators are taller cylinders than transformers. A voltage-regulator tank is about 35 inches in diameter and 65 inches long. They are usually set in threes on a platform between two poles, rather than attached directly to a pole as transformers are. Some other distinguishing features between regulators and transformers are that regulators will have wires coming down from a primary line to two or three large bushings on top of the tank cover. There are no wires running to secondaries as seen on transformers.
Also, there is usually a large dial on the side of the voltage-regulator cylinder near the top. This is the position indicator, which shows the voltage being added or reduced. There also may be a control box on the pole near the ground. A control box or dial will not appear on transformers.
Capacitors are found on poles along the distribution system. They will appear as narrow rectangular units, each with two bushings on top, set below the crossarm supporting the three phase lines but above the common neutral. There will be one unit per phase line. There will also be small cylinders with bushings attached that are attached to the same pole mount. These function as power transformers or switches to turn the capacitor on or off. Capacitors will have wires connected to the primary lines. They are not connected to secondary lines.
Capacitors function to adjust the power factor, the ratio of actual power and apparent power. kVA is a measure of apparent power, while kW (1,000 watts) is actual or working power. Apparent power is the total power in the system, while actual power is what runs all the appliances we plug into outlets. Beer, rather than water, is a good analogy for this concept. A beer poured into a glass has liquid and foam. When buying a beer, you want liquid – the beer – all the way to the top of the glass. Beer is the actual power. The apparent power is the beer plus the foam. No one wants to buy a glass of beer that is more foam than beer. Same with power. Capacitors increase the amount of actual power (the beer).
Another way to look at a capacitor is as a battery, but one that stores a charge in an electrical field rather than a chemical one. Capacitors can charge and discharge very quickly – half a cycle. Total discharge time, however, can be as long as five to 10 minutes. Utility employees must wait this long before a disconnected capacitor is grounded. Never directly or indirectly contact a capacitor – even during an outage. Always maintain the minimum approach distance. Electrical flash burns have occurred from working too closely to a capacitor.
The distribution system and its hardware need protection from faults. These are unintended paths to current flow. Another name is a short circuit, as they bypass the intended pathway of the circuit. The protective devices include cutout fuses, automatic line reclosers and sectionalizers. Insulators – to be discussed in Part 3 of the series – also protect against faults.
Lightning and trees are the cause of most faults. Most tree-related faults are caused by falling trees or their limbs. These may break the lines, push lines together or serve as a bridge between two primary lines. A tree limb that brushes against one phase line will not fault; the overcurrent is too low to cause a fault.
Distribution cutouts are found on crossarms or poles where a single-phase feeder is tapped off on a primary line in a three-phase main. This limits the number of customers affected by a fault. Cutouts also are associated with electrical-system equipment – capacitors, reclosers, transformers and voltage regulators – on the poles.
A common cutout is the fuse cutout. Most are the open type with an insulator attached to a fuse tube. There are also enclosed types with fuses within porcelain housings. Both these devices have an expulsion fuse that blows or melts when an overcurrent passes through it. The melting causes the tube to drop, which interrupts the circuit.
Distribution fuse cutouts are rated for different overcurrents, from 8 to 200 amps. A 25-amp fuse will not blow in response to a 10-amp current, but will open at 30 amps. The greater the current, the faster the fuse blows. Another value to cutouts is that the open fuse tube serves as a visual indicator that the circuit is open, though arborists should not depend on this to determine if the line is still energized.
A particularly important note to arborists: The fuse tube has attachments to open or close the fuse. While employees of the utility can open and close cutouts, arborists may not. This operation requires special training and tools. It cannot be performed by an arborist, only utility personnel.
Automatic line reclosers
If the only protection from faults on our distribution lines were cutouts, that would mean that every time there was a fault, power would be off until someone from the utility came out to close the tube. Automatic line reclosers are self-contained devices that detect an overload (overcurrents) due to a fault, opening to interrupt the fault and then automatically reclosing the line. If the fault has cleared, the recloser will remain closed. The recloser will open after a preset number of operations that isolate the faulted section of the line from the rest of the distribution system.
Automatic line-recloser controls are either electronic (including electromechanical) or hydraulic. Hydraulic controls, which use a piston in oil to trip, are common on single-phase reclosers. These have two bushings on top of a cylindrical tank. The tank dimensions are about 20 to 25 inches long and 10 to 12 inches in diameter. There is a sheet hood on the upper side of the tank that covers a handle that can be used for manual operations. Three-phase hydraulic reclosers will have three pairs of bushings on top of a rectangular tank. These tanks may be 25 or 30 inches long, 12 inches wide and 20 inches tall.
Electronic controls are used for most three-phase reclosers and some single-phase. These use vacuum bottles to interrupt a fault current. Electronic reclosers will have a control box at the bottom of the pole.
If the actions of an arborist caused the fault, for example, a tree felled into the lines, it is important to remember the reclosers will open three or four times before being locked out. The first two will be short, less than half a second, but enough to cause the clocks and other electronic devices to reset in your house. If the fault has still not cleared, the next two times will have a longer interval. There have been instances when arborists have felt four separate shocks before the recloser locked out – not a pleasant experience!
Utilities want to limit outages to the fewest number of customers. Sectionalizers are designed to isolate faults on distribution circuits. They are not reclosers, as they do not interrupt a fault. But sectionalizers work in conjunction with reclosers.
After the upstream recloser interrupts a fault for a pre-determined number of operations, while the recloser is open, the sectionalizer will open to isolate the fault portion of the distribution line. This action limits the number of end consumers from being affected by a fault. Sectionalizers may look like reclosers and have similar dimensions.
This article covered some of the electrical equipment found on distribution systems. Support devices – poles, cross-arms and guys – and a few more protective devices – insulators and lightning/surge arresters – will be covered in the next article in this series.
John Ball, Ph.D., BCMA, CTSP, A-NREMT (Advanced-National Registry of Emergency Medical Technicians), is a professor of forestry at South Dakota State University.