There are number of standards where
NERC (North American Electricity Reliability Corporation) dictates specific
requirements for the equipment, protection, control and operation of the
transmission facilities. The below NERC standards can be referred to for this
specific requirement:
·
NERC PRC 023-2 (Transmission Relay
Loadability)
·
NERC FAC-008-3 (Facility Ratings)
·
NERC PRC-025-1 (Generator Relay
Loadability)
Although the current industry
accepted design based on transmission code -referred to as "Good Utility
Practice"- would inherently meet normal NERC requirements, the
Engineer shall ensure all the requirements are met since the Client (Generator)
would be subject to a NERC audit for compliance after the installation and
commissioning of the generation station.
The Engineer, needs to focus on the
design requirements. However, there are more operational requirements that the
Generator shall be responsible for and shall be taken into
account. In other words, once the Engineer's design meets NERC requirements
for reliability, that is where their obligations end.
The existing facilities that do not
comply with the latest NERC requirements are allowed to continue to operate.
But any major retrofit project or expansion to the existing facilities shall
include additional equipment / systems to comply with the new requirements.
Our engineering and design of
stations in Ontario -which is based on Ontario Energy Board's Transmission
System Code- generally complies with NERC requirements. Two important aspects
of generation stations that are mandatory and need to be taken into account
during the initial estimate and subsequent design are as follows:
1- There shall be two battery banks
for protection and control equipment. Unlike load stations one common battery
bank with two chargers would not be acceptable for generation stations.
2- There shall be a circuit breaker
for switching at the switching station. Unlike load stations motorized
disconnect switch would not suffice.
Those are the major two features
that affect the generation stations. Other NERC requirements shall be similar
to those of load stations and would not have a significant impact on the
project estimate and the design.
The concern here for generation
stations in wind power projects is that while in some cases the generators are
derated to suit project requirements, NERC PRC 023-2 and NERC PRC-025-1 require
150% setting on transmission, transformer protection, 130% of rated nameplate
of the generator (not de-rated).
This shall be dealt with closely as
it could mean that the station and collector system shall be so designed to
carry nameplate rated load. This will have a huge impact on the equipment while
it can never happen in practice.
In wind generation facilities,
loadability is limited by inherent current-limit in WTG's and the cables / transformers
will not be overloaded however the relays are generally set to 10%-15% above
maximum load per worst case scenario identified in the power flow study and fault
overcurrent protections would be based on the fault fed from the grid. If WTG's
are derated and the derated MVA has been the base for the design of stepup
transformers and the collector cables, then the settings have to be selected
for derated equipment loadability again by the power flow study.
The NERC standard has observation to
synchronizing generator plants and transmission grid loadability which have
130% generation capability. In wind power projects with Type-4 Generator /
Inverters, each generator is able to run up to 105% of its nominal rating then a
130% setting is not effective. If the design is based on /contracted for the
derated WTG then the system is registered / recognized to the utility for the
derated MVA not nominal generator MVA. Regardless, the generation is limited by
generator manufacturer’s setting to the derated MVA.
If the generation is comparable to
the grid MVA at the POI (point of interconnection), then the system stability
is critical and loadability is important. In most of windfarm projects the
source is considered weak-infeed and has no impact on stability then the
loadability is more important to the client as profitability!
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