What are the types of power disturbances and where do they come from?
Sags and Surges
Sags/ Surges are short duration changes in voltage level. Sags (low voltage) are much more common than surges (high voltage). Voltage sags are caused by starting electric motors and other equipment, ground faults, undersized power systems, and lightning. Surges may be generated when large electrical loads are shut off.
Impulses, Spikes and Transients
Impulses, spikes, and transients are all names used to describe very short duration, high amplitude voltage pulses on power lines. These voltage pulses often reach 6,000 volts. They are caused by lightning that strikes on or near the power lines, utility switching, static electricity, and switching electrical devices on or off.
Impulses damage all types of electronic and electrical equipment. The high voltage levels puncture or weaken insulation. The fast rate of voltage change stresses the turn-to-turn insulation of windings in motors, transformers, solenoids, etc. The damage may not cause immediate failure. Often the equipment is weakened and may fail days or weeks after the event. Besides equipment damage, impulses cause machine resets, data processing errors, and other apparently random malfunctions.
High or Low Voltages
Sustained high and low voltages are usually caused by the distribution system. This system of wires and transformers connects all electrical loads to the utility generators has impedance. This impedance causes the voltage to drop when current flows through the system. The further you are from the power source and the more current drawn, the more the voltage will drop.
To maintain the voltage as best they can, the utility will increase the voltage so customers close to the power source will have higher than nominal voltage while the furthest customers will have lower than nominal voltage (maybe much lower). Because the utility will only increase the voltage by a certain amount, and because additional voltage drop is caused in the user’s building wiring, voltages are usually low.
Brownouts are intentional undervoltage instituted by the utility. When power demand exceeds the capacity of the utility generators, the utility lowers the voltage to all or some customers. This reduces the load on the generators, so they will not burn out, but causes even more acute equipment malfunctions and damage.
Harmonic Distortion is a distortion in the shape of the normal voltage sine wave. It is generated by devices with non-sinusoidal load currents interacting with the impedance of the distribution system. This combination produces non-sinusoidal voltage drops and thus, non-sinusoidal voltages. Electronic power supplies, solid state motor drives, and transformer magnetizing current all produce harmonic distortion.
The distorted voltage may disrupt the operation of many devices connected to the AC line. Harmonic distortion causes overheating and burnout of three phase transformers, and adversely affects motor operation.
Learn about how variable transformers can be used to reduce harmonic distortion here
Three Phase Voltage Unbalance
Three Phase Voltage Unbalance simply means the voltages on a three-phase system are not equal. Utilities generate three-phase AC power because it is produced and distributed at lower cost than single-phase AC or DC power, and because three phases are needed to produce steady torque in AC generators and motors. To power single phase loads, any two of the three power wires are connected. Voltage unbalance is usually caused by connecting more single-phase loads to one of the three phases. This situation produces unbalanced load currents, uneven voltage drops, and thus, unbalanced voltages.
For three-phase loads, a voltage unbalance of one or two percent is usually not a problem. However, larger voltage unbalances can cause many problems. For example, three-phase motors with 5% voltage unbalance exhibit 25% decrease in torque, 50% increase in losses, 40% increase in temperature, and a whopping 80% decrease in life. In transmitter applications, voltage unbalance causes severe ripple in high voltage power supplies, straining the power supply filtering and increasing AM noise.
Impact of Power Problems on Machinery
How do overvoltage and undervoltage conditions impact machinery performace? Let’s look at common by industry: