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Protective relays are crucial to the reliability of electrical
power systems. Serious failures in the electrical power system can
lead to blackouts. Protective relays need to be tested for the following
reasons:
 Degradation
and drift: Traditionally, electromechanical relays degraded
in characteristic with age due to such effects as capacitor aging.
Leakage of volatile materials in coils and insulating materials,
contact oxidation, component failure due to age or transient overloads,
and effects of dust and humidity could also prevent proper operation,
therefore relays needed periodic testing and calibration. Newer
digital and microprocessor based relays although free from some
of these problems have other areas of concern such as susceptibility
to ESD, harmonics, radiation (on solid state memories), RF interference
and transients, or power supply failure due to electrolytic capacitors
drying out.- Regulations & Insurance: In
order to ensure the reliability of the electrical power system,
regulations from certain government bodies or regional power co-ordinating
councils may dictate that testing be done at specified intervals.
Insurance companies may also demand testing of protection systems
as a condition to insuring critical equipment.
- Increased system demands and loading:
With increasingly tighter technical and environmental regulations,
our electrical power systems are being loaded more and more to
their maximum capacity, and often are being upgraded to utilization
beyond their original design capacity. This puts increasing demands
on the accuracy, reliability and performance of our protection
systems. Utilities are re-evaluating the reliability and performance
of their protection systems to meet current demands. Customers
with critical processes, such as automotive manuafacturers have
a Special Manufacturing Contract (SMC) with their power suppliers.
The power supplier is bound to pay penalties for each disturbance
in the power supply. Quick isolation and fault location by protective
relays is required to prevent and minimize these disturbances
if they do occur.
- Settings and logic verification:
Newer electronic and microprocessor-based devices are fully programmable,
and the settings and logic must be carefully checked and verified
by testing to ensure that their operation is as desired and intended.
This is typically done when the device is installed or commissioned.
- Commissioning: Before a protective
relay or system is put into service, a complete functional test
is performed to verify the relay settings and the operation of
the entire protection system. This may serve as the benchmark
for future periodic tests of the system.
- Evaluation: Customers need to evaluate
the suitability of devices before installing them on their system.
Typically conditions specific to their power system are simulated
using test equipment to determine if the device will meet their
requirements. Digital fault recorder records may be played back
into the device to determine its response. For power systems which
have not been yet, or for hypothetical fault conditions, waveforms
obtained from an EMTP simulation may also be played back into
the device to characterize its perfomance.
- Troubleshooting/repair: Testing
is required in the process of troubleshooting devices which failed
to operate properly, and after they are repaired to ensure that
all of the device functions are as designed.
- Design verification: Manufacturers
need to verify the design of new devices. The hardware and software
of new designs must be proven and tested under a wide range of
power system conditions.
- Manufacturing quality control:
Manufacturers need to test devices being to ensure the product
is correctly manufactured, correct software has been loaded and
no defects exist prior to delivery to the customer.
- Receiving quality control: Users
of devices test the product when they receive it, in order to
check that no damage occured to the device in shipping, and to
ensure that it meets the required specifications. This type of
acceptance testing was more common for electromechanical relays.
See the following articles for more information:
Protective
Relay Maintenance: Need For Periodic Maintenance Testing
Periodic
Relay Maintenance Tests Extend Relay Functioning Life and Save Money
Potential
Electrical Protective Relay Problems
Learn more about power system protection.
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