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1. Power
System Frequency

2. 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.

3. 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.

4. 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.

5. 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

6. 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.

7. 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.

8. 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.

9. 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.

10. 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

11. An electric power system is characterized by two main important
parameters: Voltage and Frequency.
In order to keep the expected operating conditions and supply
energy to all the users (loads) connected, it is important to control
these two parameters within predefined limits, to avoid unexpected
disturbances that can create problems to the connected loads or
even cause the system to fail.
The most commonly used nominal frequency (Fn) in power
systems is 50 Hz (Europe and most of Asia) and 60 Hz (North
America). The reasons for this choice are based on technical
compromises and historical situations.

12. Generally, when the system operates in a range of frequency Fn±0.1
Hz, it is in the standard conditions, while when the frequency ranges
from 47.5 to 51.5 Hz (in 50 Hz network for example), it is called
emergency condition or restoration condition. These values can
change from country to country.
Frequency variations in a power system occur because of an
imbalance between generation and load. When the frequency value of
a power system reaches the emergency condition, the control strategy
is initiated. The frequency control is divided in three levels: primary,
secondary and tertiary controls. Each frequency control has specific
features and purposes.

13. Primary Control
The primary control (or frequency response control) is an automatic
function and it is the fastest among the three levels, as its response
period is a few seconds. When an imbalance between generation and
load occurs, the frequency of the power system changes.
For example, with a load increase, the generated power doesn’t
immediately change, so the energy to compensate for this load
increase arrives from the kinetic energy of the rotating generators
that start decreasing the velocity (this is called the inertial response).
After this moment, the speed controller (called the “governor”) of each generator
acts to increase the generation power in order to recover this speed decreasing
and try to clear the imbalance..

14. Generally, in about 30 seconds, each generation unit shall be able to
generate the required additional power and then keep it for at least 15
minutes (this timing depends on the requirements of the transmission
system operator, or TSO). All the generation plants connected in the
HV power system are called to supply this service, except the
renewable energy source (RES) not schedulable (ie. wind, solar,
biogas, hydraulic flow water), so, for this reason each generation unit
shall have a dedicated and proper “reserve” power in order to
accomplish this regulation when active.
The purpose of the primary regulation is to clear the unbalance between generation and
loads, in order to take the system to a stable condition. This service is mandatory for all
the generators entitled to provide it and not remunerated.

15. Regarding the not schedulable RES, these generators must be able to work
with a defined P(f) function, in order to modulate their power according to
the frequency value. This is easier in case of over-frequency, which requires
power decrease. However, it could be really complex (almost impossible) in
case of under-frequency, which would require a power increase, not always
possible (even with a reserve power) due to the volatility of the primary
resource itself.
The continuous growth of RES implies the reduction of thermoelectric plants in
operation, with consequent difficulties to perform this frequency regulation, for
the reasons explained above. There are already different solutions under analysis
and some of them already in place in several power systems (battery energy
storage systems are one of the most promising). This is one of the main
challenges to the massive deployment of RES in the power systems.

16. Secondary Control
Once the primary regulation accomplished its target, the frequency value
it’s different from the nominal one, the reserve margins of each generator
have been used (or partially used) and also the power exchange between
the interconnected power systems is different from the predefined one.
So, it’s necessary to restore the nominal value of the frequency, the
reserve of each generator previously used, and the power exchange
among the power systems. This is the purpose of the secondary control.
In order to perform this task, there are some generators entitled to perform the
secondary control, through a dedicated reserve power. This reserve depends on the
requirement of each TSO and usually, it’s a percentage of the maximum power
available, with a predefined minimum value to guarantee independently from the
maximum power of each generator.

17. If the frequency value is less than the nominal one, additional
generation capacity needs to be started, while if the frequency value
is higher than the nominal one, some generation capacity must be
stopped, or the load has to increase. The secondary control is usually
performed in an automatic way, by all the generators that participate
to this regulation, through specific “set-point” sent by a central
controller.
Figure 1 shows an example of the first two levels of control after a
frequency event in the system. The green line and the red-
dashed line show two different responses according to the inertia
level of the system (power systems with low generation produced by
rotating machines will have low inertia level).

18. Figure 1 - Example of frequency response after a frequency event. Source Scientific paper Impact of Distributed Energy
Resources on Frequency Regulation of the Bulk Power System

19. Tertiary Control
After secondary control is completed, the reserve margin used for this control shall be restored too and this is the
purpose of the tertiary control (or replacement reserve) the last level of frequency control. In order to perform this
restoring, the TSO calls send single producers (even the ones not involved in the secondary control) the operating
prescriptions related to power variation for the generators already in operation and if needed asking start-up
generators not operating at that moment. This control level is not automatic but it’s executed upon request from
the grid operator, and its remuneration follows the same rules of the secondary control.