The primary reason for accurate frequency
control is to allow the flow of alternating current power from multiple
generators through the network to be controlled. The trend in system frequency
is a measure of mismatch between demand and generation, and so is a necessary
parameter for load control in interconnected systems.
Frequency of the system will vary as load and
generation change. Increasing the mechanical input power to a synchronous
generator will not greatly affect the system frequency but will produce more
electric power from that unit. During a severe overload caused by tripping or
failure of generators or transmission lines the power system frequency will
decline, due to an imbalance of load versus generation. Loss of an
interconnection, while exporting power (relative to system total generation)
will cause system frequency to rise. Automatic generation control (AGC) is used to maintain scheduled
frequency and interchange power flows. Control systems in power plants detect
changes in the network-wide frequency and adjust mechanical power input to
generators back to their target frequency. This counteracting usually takes a
few tens of seconds due to the large rotating masses involved. Temporary
frequency changes are an unavoidable consequence of changing demand.
Exceptional or rapidly changing mains frequency is often a sign that an
electricity distribution network is operating near its capacity limits,
dramatic examples of which can sometimes be observed shortly before major
outages.
Frequency protective
relays on the power system
network sense the decline of frequency and automatically initiate load shedding or tripping of interconnection lines,
to preserve the operation of at least part of the network. Small frequency
deviations (i.e.- 0.5 Hz on a 50 Hz or 60 Hz network) will
result in automatic load shedding or other control actions to restore system
frequency.
Smaller power systems, not extensively
interconnected with many generators and loads, will not maintain frequency with
the same degree of accuracy. Where system frequency is not tightly regulated
during heavy load periods, the system operators may allow system frequency to
rise during periods of light load, to maintain a daily average frequency of
acceptable accuracy.
Frequency affects the power system in following ways;
Lighting
The first applications of commercial electric power were incandescent lighting (normal bulb) and commutator-type electric motors. Both devices operate well on DC, but DC could not be easily changed in voltage, and was generally only produced at the required utilization voltage.
If an incandescent lamp is operated on a
low-frequency current, the filament cools on each half-cycle of the alternating
current, leading to perceptible change in brightness and flicker of the lamps.
Rotating machines
Commutator-type motors do not operate well on high-frequency AC because the rapid changes of current are opposed by the inductance of the motor field; even today, although commutator-type universal motors are common in 50 Hz and 60 Hz household appliances, they are small motors, less than 1 kW. The induction motor was found to work well on frequencies around 50 to 60 Hz but with the materials available in the 1890s would not work well at a frequency of, say, 133 Hz. There is a fixed relationship between the number of magnetic poles in the induction motor field, the frequency of the alternating current, and the rotation speed; so, a given standard speed limits the choice of frequency (and the reverse). Once AC electric motors became common, it was important to standardize frequency for compatibility with the customer's equipment.
Generators operated by slow-speed reciprocating
engines will produce lower frequencies, for a given number of poles, than those
operated by, for example, a high-speed steam turbine.
For very slow prime mover speeds, it would be costly to build a generator with
enough poles to provide a high AC frequency. As well, synchronizing two
generators to the same speed was found to be easier at lower speeds. While belt
drives were common as a way to increase speed of slow engines, in very large
ratings (thousands of kilowatts) these were expensive, inefficient and
unreliable.
Transmission and transformers
With AC, transformers can be used to step down high transmission voltages to lower customer utilization voltage. The transformer is effectively a voltage conversion device with no moving parts and requiring little maintenance. The use of AC eliminated the need for spinning DC voltage conversion motor-generators that require regular maintenance and monitoring.
Since, for a given power level, the dimensions
of a transformer are roughly inversely proportional to frequency, a system with
many transformers would be more economical at a higher frequency.
Electric power transmission over long lines favors lower
frequencies. The effects of the distributed capacitance and inductance of the
line are less at low frequency.
System interconnection
Generators can only be interconnected to operate in parallel if they are of the same frequency and wave-shape. By standardizing the frequency used, generators in a geographic area can be interconnected in a grid, providing reliability and cost savings.