IEEE PES T&D Conference & Exposition 2022 - 110

of testing and calibration procedures of electromechanical
relays. It is still applicable and valuable during manufacturer
development processes and acceptance tests performed by
utilities to check element operation against the manufacturer
specification. However, this method is no longer enough for
commissioning and maintenance to guarantee proper reliability
of the protection system. As the technology evolved,
data show that relay failure is no longer the main cause of
protection system misoperations. Overall misoperations
have decreased, but the reasons why they still occur have
changed. Incorrect settings, logics, and design errors along
with communication failures account for a big portion of the
cause codes as indicated by the North American Electric
Reliability Corporation reports.
System-based testing is a method based on the simulation
of real fault scenarios and verification of the protection
system response. Simulations are based on a power system
model representing the application being tested. Ideally, and
when feasible, all protection devices used to protect the system
are part of the test and simultaneously exposed to the
simulated test scenarios. Not only individual elements, but all
interactions between them, communication interfaces, and
logics are checked. For testing protection of an entire substation
simultaneously, the test setup may get cumbersome
and demanding. Since interactions are not present between
all IEDs in the substation, it is wise to first define which scenarios
should be covered within the test procedure, and based
on these test cases, decide on a subset of IEDs that should be
part of each test case. Each subset will contain a limited number
of IEDs making the test practical and efficient.
System-based testing is not a new concept. This method
has the potential to unveil mistakes made by different stakeholders
throughout the whole process of setting a protection
system. It helps increase system reliability by finding
system design flaws, setting calculation mistakes, IED
Inject V/I
IED_T1
PTOC
PDIF
Monitor Element
PTRC
configuration errors, communication failures, and wrong
logics. In both " Moving the Focus from Relay Element Testing
to Protection System Testing " and " Why System-Based
Testing Should be Part of Every Commissioning, " the importance
of system-based testing is highlighted with examples
of errors found by using this method during field and factory
acceptance tests (FATs).
In digital substations, system-based testing principles
have even higher importance due to the high number of
communication interfaces and distributed user-configurable
logics present in such projects. Another benefit is that test
procedures can be mostly defined during the system-specification
phase, regardless of which exact physical architecture
and IEDs are chosen during design. The test cases are
based on the primary power system model, specified functions,
and which test scenarios should be covered. A more
standardized test procedure can be achieved with less effort
and improved quality. In reference to the examples of Figure
3 and Figure 4, testing procedures for both distributed
(Fig.3) and centralized (Fig.4) design options are almost the
same. Only little details need to be adapted, like the import
of SCL configuration files of involved IEDs, so the test sets
can simulate and/or subscribe to the desired messages.
Protection Subsystems and Test Coverage
When defining test procedures and which equipment to
involve, there are several ways this can be done. If the entire
system is to be tested together, this could be accomplished
by doing primary injection to CTs and VTs to simulate fault
scenarios and observe breaker operations to assess the system
response. For testing protection schemes, this approach
is not entirely feasible, as it would require simultaneous primary
injection in multiple CTs/VTs. Even a secondary injection
to merging units is challenging if multiple units must
be involved. On-site during commissioning, since different
groups are working to install and wire the PIU cubicles in
the substation yard, it is challenging to get all equipment
together. An overall test would be possible only at the very
end of the installation, and, therefore, planning it this way
may not be the most efficient choice. For maintenance, it is
also common that only the IED is taken out of service while
the PIU is kept in service.
One option is to separate the entire system into subsystems
as shown in Figure 8. Test coverage should overlap
each other, providing maximum reliability.
figure 6. Settings-based testing of an individual element.
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System-Based Test of P&C IED Subsets
This system-based test can verify the correct operation of
the protection and control (P&C) IEDs when exposed to
close-to-real conditions as, for example, faults in the system.
The devices under test are a subset of P&C IEDs, which are
relevant for the scheme being tested, and the communication
network infrastructure, i.e., network switches. The test
is performed as illustrated in Figure 7 by simulating SVs and
monitoring GOOSE messages published by IEDs. The MUs
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