IEEE Power & Energy Magazine - November/December 2017 - 66

Measuring Frequency: Not As Easy
As You Might Think
For more than a hundred years of utility practice, the speed
of synchronous generators has been the proxy for grid frequency. Turbine-generator governors have used deviations
in rotor speed as the measure of departure of the grid from
50 or 60 Hz. Today, nonsynchronous resources must measure
frequency directly from the grid. This is conceptually simple,
but there are challenges. For example, when is a disturbance
a change in frequency, and when is it something else? Figure S1 shows a single phase of a 50-Hz voltage sine wave. In
the middle of the signal, a 20o phase jump occurs, as might be
associated with a transmission-line switching event. The elongation of the half-wave means that the "frequency" (in this
case, defined by time between zero crossing) has dropped
dramatically to 45 Hz for the next half-cycle. Of course, a synchronous machine would filter this out to a negligible deviation. Suitable filtration of the signal here would do likewise.
But be careful what you ask for: rapidly measuring frequency

2
1.5
1
0.5
0
0
-0.5
-1
-1.5
-2
-2.5
-3
0

Phase Jump: 20° Normal
Voltage-One Phase, No Distortion

90
85
80
75
70
0.01 0.02 0.03 0.04 0.05 0.06 65
60
55
50
45
ROCOF = -500 Hz/s
40
0.01 0.02 0.03 0.04 0.05 0.06
Time (s)

Frequency (Hz)

Voltage (%)

can have some strange outcomes.

figure S1. A single phase of a 50-Hz voltage sine wave.

only when the frequency deviation exceeds a "normal" deviation (200-500 mHz). This usually has to
be provided without keeping any reserve. (For units
operated at maximum power, they will not provide
any control in cases of decreasing frequency.)
✔ Fast frequency control: so-called synthetic or emulated
inertial response. To limit frequency deviation, converters
can provide short-term frequency regulation. The energy
provided to the grid can come from different sources: the
kinetic energy of a wind turbine, a dedicated battery, and
other such reserves. The main difference between emulated inertia and actual inertia is that the emulation requires
the frequency to be measured (see "Measuring Frequency:
Not As Easy As You Might Think"); therefore, its effect
on the grid is delayed, and, for very-fast-frequency transients, it may not be enough.
✔ Ancillary services for active power/frequency control:
defined based on specific system needs and procured
through market mechanisms. This would allow suitable
technologies to provide the services in the most efficient
way. For example, large industrial loads with under-frequency relays are capable of providing fast, effective, and
sustained response during under-frequency events.
✔ Specific fault ride-through behavior: converters formerly used to disconnect when the voltage was too low
now providing current even during faults. Importantly,
the current is fully controllable, and it can be tuned to
provide active only, reactive only, or any configuration
of the two. This can be tuned to limit the voltage drop
on the grid during a fault transient. Today, converters
are even able to provide unbalanced current during
faults for specific needs such as protection.
Other mitigation measures, such as power-production limits due to weak grid concerns (e.g., in the Electric Reliability
Council of Texas), nonsynchronous generation penetration limits and/or inertia limits (e.g., in Ireland), use of synchronous
condensers throughout the network, and other methods can
be applied in the interim to maintain system reliability.

ROCOF: rate of change of frequency.

STATCOM capability. It is a very important ability because it ensures a stable voltage at any time of the day
(with or without sun/wind).
✔ Active power control:
* Frequency regulation: very similar to what is done
presently for synchronous machines. There is a droop
control between grid frequency and the active power
delivered by the converter. The converter measures
the electrical frequency and adjusts its active power
accordingly. (This requires some reserve of active
power to be able to deliver more power when frequency decreases.)
* Limited frequency control: similar to frequency
control but with a large deadband that makes it act
66

ieee power & energy magazine

Long-Term Solution: To Push the Limit
of Converter Penetration Up to 100%,
New Controls Will Be Necessary
The first capability is for converters to be grid forming. One
of the consequences of being grid forming is that the converter's output power is now driven by the ac grid and not
directly by the primary source. From a consumer's point of
view, a basic requirement is to be certain that when a new
load is connected, it will be fed by the grid. From a producer's
point of view, this is more challenging.
The converter is basically a link between an uncontrollable load and varying but predictable primary energy. Controls can change the operating point of the converters to
adjust the output to the input, but, during transients, some
additional energy will be needed, leading to the requirement for some type of storage or headroom. (The amount of
november/december 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2017