Page 67 - Energize June 2022

TECHNICAL

Frequency protection explained:

Variants and rationale

As the world transitions to distributed energy resources and renewable generation, frequency
protection has become a commonly specified technique for reliable asset integration.

INFORMATION FROM NOJA POWER

requency protection monitors the power system frequency and signals when the and the frequency would increase if
frequency departs from normal. the power generation remained the
FIn this article, we explore what normal frequency is, what scenarios cause power same.
system frequency to vary, and some of the common protection elements which act on This is why frequency is a useful
these fault scenarios. protection metric. It allows us to see if
the generation and load is balanced and
Power system frequency allows protection engineers to intervene if
Following the famous disagreements between Edison and Westinghouse in the early days of a mismatch becomes an issue.
electrical engineering, the victor of the debate between Alternating Current (AC) and Direct
Current (DC) was the former. Why is frequency protection being
Alternating current is a constantly varying magnitude of current over time, while DC is more commonly used?
constant. The main reason is the adoption of
While beyond the scope of this article, AC has advantages in electromagnetic induction, distributed generation. Generators need
which allowed technologies such as transformers to scale and transport bulk energy. While protection against frequency events,
HVDC is a viable alternative today, the vast majority of power transmission and distribution and these connections need frequency
is still AC. protection.
When “frequency” is discussed in a power system context, it is the cycles per second Furthermore, these distributed
of the electrical current. For most transmission and distribution, it is either 50 Hz or 60 Hz generators often use “asynchronous
(although some rail applications use lower frequencies). generation”, such as power inverters in
solar power plants. With no mechanical
What scenarios cause power system frequency to vary? inertia behind them, such generation sites
Frequency is a fundamental quantity in the design of an AC electrical network. All would have less capacity to respond to
equipment selected must be designed to operate at this frequency, and in a healthy system, fluctuations in power system frequency
there should be very little movement. without disconnection.
The frequency of an electricity network is generally governed by the “synchronous
generators” in conventional power plants. These massive machines generate power at a With this in mind, let’s explore some
specified frequency. typical frequency protection elements:
At a steady state, at rated power, the frequency should not vary. • Over-frequency
When the power demanded of generator changes, we can start to see variations in • Under-frequency
frequency. The power system “inertia”, or stored energy, will attempt to maintain the • Rate of change of frequency
frequency output under higher load.
A simple analogy is the behaviour of a car. When driving at a constant speed, say 50 Over-frequency
km/h, the car’s engine rotates at a specific RPM. RPM is a form of frequency. As discussed in our car analogy, a network
When that car reaches a hill, more power output is needed to achieve the same speed. node will experience an over-frequency
When this happens, the RPM will start to fall, as the engine is placed under more load. event when either the generation is too
This is frequency falling while the power output remains unchanged. If you can high, or the load is too low.
increase the power output, the car avoids stalling. If you can’t, the car stalls. In our power Over-frequency protection is
system example, if our generator can’t match the load, the generator stalls, and the power configured by applying a set point above
disappears. normal operating frequency. When this
In the same way, when the car reaches a downhill, less power output is needed to threshold is reached, the protection
maintain the same speed. If you don’t reduce your accelerator application, the RPM relay operates, triggering an alarm or the
(and car speed) would increase. In this case, the generator is exceeding the demand, operation of a circuit breaker.

energize | June 2022 | 65