IEEE Power & Energy Magazine - July/August 2019 - 40

For the short circuit current to be interrupted
and the breaker contacts opened, the short circuit
current must experience a zero-crossing.
some margin to allow for unstudied conditions. The identified 230-kV operating region reflects the result of numerous power flow simulations. The figure shows that the grid
requirements can be met while respecting the Mvar capability of the synchronous condenser and the acceptable voltage
range at the machine's terminals in relation to the voltage at
the transmission system. The machine terminal voltage limit
reflects physical constraints on the synchronous condenser,
step-up transformer, and the auxiliary system.
As indicated by the letters in Figure 3, a list of operating
points is specified to determine the synchronous condenser
and step-up transformer design. For each operating point, the
following results were reported for a complete summary of
the installation operation under different conditions:
✔ transmission-system-side voltage
✔ transmission-system-side reactive power
✔ transmission-system-side current
✔ synchronous-machine-side voltage
✔ synchronous-machine-side reactive power
✔ synchronous-machine-side current
✔ auxiliary system busbar(s) voltages.

Transient Study
The installation of a synchronous condenser introduces a
big change in operating conditions and equipment stresses.
Higher fault levels and the corresponding interrupt capability of high-voltage circuit breakers need to be investigated.
Studies need to fully consider transient conditions to verify
that the circuit breakers will be capable of performing their
intended roles without surpassing acceptable operating limits.
For a projected synchronous condenser installation, two
transient phenomena are investigated:
✔ machine-fed short circuit current dc component
✔ transient recovery voltage (TRV).
In most cases, this circuit breaker would be on the highvoltage side of the synchronous-machine step-up transformer.
For both transient phenomena, the investigations were conducted in the electromagnetic transient domain. The electrical
network in the vicinity of the synchronous condenser installation was modeled in detail to capture the system's physical
behavior and assess stresses on substation components, especially the interrupting capabilities of circuit breakers.
Synchronous-Machine-Fed Short Circuit
Current dc Component

Although any short circuit current might have a high dc offset, synchronous-machine-fed faults typically have a low
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ieee power & energy magazine

damping ratio that could lead to delayed zero-crossings of
the short circuit current. The electrical network between
the short circuit source (the synchronous machine, which is
characterized by a low subtransient time constant) and the
interrupting device is usually composed of a step-up transformer and short-length cables. This arrangement results in
a very high reactance-to-resistance (X-to-R) ratio, leading to
delayed zero-crossing.
For the short circuit current to be interrupted and the
breaker contacts opened, the short circuit current must
experience a zero-crossing. If the first zero-crossing takes
place beyond the breaker's interrupting window, it will fail
to interrupt the short circuit current and might be physically
damaged due to energy buildup while it is arcing. The arcing characteristics of a circuit breaker, which are manufacturers' proprietary information, also need to be considered
in transient studies because the arc resistance could have a
significant impact on the damping of the short circuit current
dc component.
Transient Recovery Voltage

Recovery voltage appears across the terminals of a pole of a
circuit breaker after interruption. This voltage may be considered in two successive time intervals: one during which
a transient voltage exists (i.e., TRV), followed by a second
interval during which only a power frequency voltage exists.
The TRV wave shape is determined by the operating
point of the electrical network surrounding the circuit
breaker prior to interruption and the electrical characteristics of that network. Because TRV is a determining
parameter for successful current interruption, breakers are
normally type-tested in a laboratory to withstand a standardized TRV.
Standardized TRVs are fixed in international standards
such as IEC Standard 62271-100, IEEE Standard C37.04,
ANSI Standard C37.06, or IEEE Standard C37.09. This
standardized TRV is usually determined by the maximum
allowed rate of rise of recovery voltage and a maximum
crest voltage.
Circuit breaker ratings are defined within TRV envelopes
that specify a maximum allowable TRV. Parameters of the
envelope are adjusted as a function of the interrupted short
circuit current. If a breaker's TRV rating is exceeded, reignition in the breaker chamber can lead to permanent damage
to the breaker and nearby equipment.
For TRV investigations, all network details surrounding
the breaker being studied should be modeled. In particular,
july/august 2019

IEEE Power & Energy Magazine - July/August 2019

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