A power system is a network of electrical components that generate, transmit, and distribute electricity to end users. The power system operates at a certain frequency, which is the number of cycles per second of the alternating current (AC) voltage and current. The most common frequencies used for power systems are 50 Hz and 60 Hz, depending on the region of the world. But why do we use these frequencies and not others? What are the advantages and disadvantages of different frequencies? And how did these frequencies become standardized? This article will answer these questions and explain the history and technical aspects of power system frequency.
What is Power System Frequency?
Power system frequency is defined as the rate of change of the phase angle of the AC voltage or current. It is measured in hertz (Hz), which is equal to one cycle per second. The frequency of a power system depends on the speed of rotation of the generators that produce the AC voltage. The faster the generators rotate, the higher the frequency. The frequency also affects the performance and design of various electrical devices and equipment that use or produce electricity.
How Did 50 Hz and 60 Hz Frequencies Emerge?
The choice of 50 Hz or 60 Hz frequency for power systems is not based on any strong technical reason but rather on historical and economic factors. In the late 19th and early 20th centuries, when commercial electric power systems were being developed, there was no standardization of frequency or voltage. Different regions and countries used different frequencies ranging from 16.75 Hz to 133.33 Hz, depending on their local preferences and needs. Some of the factors that influenced the choice of frequency were:
- Lighting: Lower frequencies caused noticeable flickering in incandescent lamps and arc lamps, which were widely used for lighting at that time. Higher frequencies reduced flickering and improved lighting quality.
- Rotating machines: Higher frequencies allowed smaller and lighter motors and generators, which reduced material and transportation costs. However, higher frequencies also increased losses and heating in rotating machines, which reduced efficiency and reliability.
- Transmission and transformers: Higher frequencies increased the impedance of transmission lines and transformers, which reduced power transfer capability and increased voltage drop. Lower frequencies allowed longer transmission distances and lower losses.
- System interconnection: Interconnecting power systems with different frequencies requires complex and costly converters or synchronizers. Having a common frequency facilitated system integration and coordination.
As power systems expanded and interconnected, there was a need for standardization of frequency to reduce complexity and increase compatibility. However, there was also a rivalry between different manufacturers and regions that wanted to maintain their own standards and monopolies. This led to a split between two major groups: one that adopted 50 Hz as the standard frequency, mainly in Europe and Asia, and another that adopted 60 Hz as the standard frequency, mainly in North America and parts of Latin America. Japan was an exception that used both frequencies: 50 Hz in eastern Japan (including Tokyo) and 60 Hz in western Japan (including Osaka).
What are the Advantages and Disadvantages of Different Frequencies?
There is no clear advantage or disadvantage of using 50 Hz or 60 Hz frequency for power systems, as both frequencies have their pros and cons depending on various factors. Some of the advantages and disadvantages are:
- Power: A 60 Hz system has 20% more power than a 50 Hz system for the same voltage and current. This means that machines and motors running on 60 Hz can run faster or produce more output than those running on 50 Hz. However, this also means that machines and motors running on 60 Hz may need more cooling or protection than those running on 50 Hz.
- Size: A higher frequency allows smaller and lighter electrical devices and equipment, as it reduces the size of magnetic cores in transformers and motors. This can save space, material, and transportation costs. However, this also means that higher-frequency devices may have lower insulation strength or higher losses than lower-frequency devices.
- Losses: A higher frequency increases losses in electrical devices and equipment due to skin effects, eddy currents, hysteresis, dielectric heating, etc. These losses reduce efficiency and increase heating in electrical devices and equipment. However, these losses can be minimized by using proper design techniques such as lamination, shielding, cooling, etc.
- Harmonics: A higher frequency produces more harmonics than a lower frequency. Harmonics are multiples of the fundamental frequency that can cause distortion, interference, resonance, etc., in electrical devices and equipment. Harmonics can reduce power quality and reliability in power systems. However, harmonics can be mitigated by using filters, compensators, converters, etc.
How is Power System Frequency Controlled?
Power system frequency is controlled by balancing the supply (generation) and demand (load) of electricity in real-time. If supply exceeds demand, frequency increases; if demand exceeds supply, frequency decreases. Frequency deviations can affect the stability and security of power systems, as well as the performance and operation of electrical devices and equipment.
To maintain frequency within acceptable limits (usually ±0.5% around the nominal value), power systems use various methods such as:
- Time error correction (TEC): This is a method to adjust the speed of generators periodically to correct for any accumulated time error due to frequency deviations over a long period. For example, if the frequency is below nominal for a long time (e.g., due to high load), generators will speed up slightly to make up for the lost time.
- Load-frequency control (LFC): This is a method to adjust the output of generators automatically to match any changes in load within a certain area or zone (e.g., a state or a country). For example, if the load increases suddenly (e.g., due to switching on appliances), generators will increase their output accordingly to maintain frequency.
- Rate of change of frequency (ROCOF): This is a method to detect any sudden or large changes in frequency due to disturbances such as faults or outages in power systems. For example, if a large generator trips offline unexpectedly (e.g., due to a fault), ROCOF will indicate how fast frequency is changing due to this event.
- Audible noise: This is an audible indication of any changes in frequency due to mechanical vibrations in electrical devices and equipment such as transformers or motors. For example, if frequency increases slightly (e.g., due to low load), some devices may produce a higher-pitched sound than normal.
Power system frequency is an important parameter that affects the generation, transmission, distribution, and consumption of electricity. The choice of 50 Hz or 60 Hz frequency for power systems is based on historical and economic reasons rather than technical ones. Both frequencies have their advantages and disadvantages depending on various factors such as power, size, losses, harmonics, etc. Power system frequency is controlled by various methods such as TEC, LFC, ROCOF, and audible noise to ensure the stability and reliability of power systems and the performance and operation of electrical devices and equipment.