Why Do We Use 50 Hz or 60 Hz Frequency for Power Systems?

 

A power system comprises interconnected electrical components responsible for generating, transmitting, and distributing electricity. This system functions at a designated frequency, typically measured in hertz (Hz), indicating the number of cycles per second of the alternating current (AC) voltage and current. Frequencies commonly used are 50 Hz and 60 Hz, varying by region. But why are these frequencies chosen? What are their advantages and disadvantages? And how did they become standard? This article addresses these queries, delving into the historical background and technical considerations of power system frequency.

What is Power System Frequency?

Power system frequency refers to the speed at which the phase angle of AC voltage or current changes, measured in hertz (Hz), with one hertz representing one cycle per second. This frequency is determined by the rotational speed of the generators generating the AC voltage; a faster rotation results in a higher frequency. Additionally, frequency plays a crucial role in influencing the performance and design of electrical devices and equipment.

How Did 50 Hz and 60 Hz Frequencies Emerge?

The selection of either 50 Hz or 60 Hz frequency for power systems is primarily influenced by historical and economic factors rather than strong technical considerations. During the late 19th and early 20th centuries, there was no uniform standard for frequency or voltage in power systems. Various regions adopted frequencies ranging from 16.75 Hz to 133.33 Hz based on local requirements and preferences. Factors influencing the choice of frequency included:

1. Lighting: Lower frequencies resulted in noticeable flickering in incandescent lamps and arc lamps, prevalent lighting sources at the time. Conversely, higher frequencies minimized flickering and enhanced lighting quality.

2. Rotating machines: Higher frequencies enabled the development of smaller and lighter motors and generators, thereby reducing material and transportation expenses. However, these higher frequencies also led to increased losses and heating in rotating machines, consequently lowering efficiency and reliability.

3. Transmission and transformers: Elevated frequencies raised the impedance of transmission lines and transformers, limiting power transfer capability and increasing voltage drop. Conversely, lower frequencies permitted longer transmission distances and decreased losses.

4. System interconnection: Integrating power systems with varying frequencies necessitated complex and expensive converters or synchronizers. Utilizing a common frequency simplified system integration and coordination efforts.

As power systems expanded and became interconnected, there arose a necessity for standardizing frequency to streamline operations and enhance compatibility. However, this initiative encountered resistance from various manufacturers and regions keen on preserving their individual standards and market dominance. Consequently, two major factions emerged: one advocating for 50 Hz as the standard frequency, predominantly in Europe and Asia, and another endorsing 60 Hz as the standard frequency, primarily in North America and certain parts of Latin America. Japan notably deviated from this dichotomy, employing 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 delivers 20% more power compared to a 50 Hz system with the same voltage and current. This translates to machines and motors running on 60 Hz being able to operate faster or yield higher output than those on 50 Hz. However, it also implies that equipment running on 60 Hz may require additional cooling or protection measures compared to their 50 Hz counterparts.
• Size: Higher frequency enables the design of smaller and lighter electrical devices and equipment by reducing the size of magnetic cores in transformers and motors. This can result in savings in space, materials, and transportation costs. However, it also means that higher-frequency devices may have lower insulation strength or higher losses compared to lower-frequency ones.

• Losses: Elevating the frequency heightens losses in electrical devices and equipment attributable to phenomena like skin effects, eddy currents, hysteresis, and dielectric heating. These losses diminish efficiency and escalate heating in electrical apparatus. Nonetheless, employing appropriate design techniques such as lamination, shielding, and cooling can mitigate these losses.
• Harmonics: A higher frequency generates a greater quantity of harmonics compared to a lower frequency. Harmonics, which are multiples of the fundamental frequency, have the potential to induce distortion, interference, and resonance in electrical devices and equipment. They can compromise power quality and reliability in power systems. Nevertheless, the adverse effects of harmonics can be alleviated through the application of filters, compensators, converters, and similar measures.

How is Power System Frequency Controlled?

Power system frequency is regulated by maintaining a balance between the supply (generation) and demand (load) of electricity in real-time. When the supply surpasses demand, the frequency increases, while if demand exceeds supply, the frequency decreases. These deviations can influence the stability and security of power systems, impacting the performance of electrical devices.

To ensure that frequency remains within acceptable bounds, typically around ±0.5% of the nominal value, power systems employ various methods such as:

1. Time Error Correction (TEC): This method periodically adjusts the speed of generators to rectify any accumulated time error caused by prolonged frequency deviations. For instance, if the frequency remains below nominal for an extended period (e.g., during high load conditions), generators will slightly increase their speed to compensate for the lost time.

2. Load-Frequency Control (LFC): LFC automatically regulates the output of generators to match changes in load within a specific area or zone (e.g., a state or country). If the load suddenly increases (e.g., due to the activation of appliances), generators will boost their output accordingly to maintain frequency.

3. Rate of Change of Frequency (ROCOF): ROCOF detects abrupt or significant frequency changes resulting from disturbances like faults or power system outages. For instance, if a large generator unexpectedly goes offline (e.g., due to a fault), ROCOF measures the rate at which frequency is changing due to this event.

4. Audible Noise: Audible noise serves as an indication of frequency variations caused by mechanical vibrations in electrical devices and equipment such as transformers or motors. For example, if the frequency experiences a slight increase (e.g., during low load conditions), some devices may produce a higher-pitched sound than usual.

Summary

Power system frequency is a critical parameter influencing electricity generation, transmission, distribution, and consumption. The selection of 50 Hz or 60 Hz frequencies in power systems stems from historical and economic factors rather than purely technical considerations. Each frequency offers distinct advantages and drawbacks, influenced by factors like power, size, losses, and harmonics. Frequency control in power systems employs methods such as TEC, LFC, ROCOF, and audible noise monitoring to maintain stability, reliability, and optimal performance of electrical devices and equipment.