Press release Distance relays – how to get accurate settings?
Distance relays – how to get accurate settings?
OMICRON’s CPC 100 + CP CU1 helps to determine correct impedance values
The results of a false operation of a distance relay can be extremely severe. Non-operation may
lead to damage or instability of the system and incorrect tripping leads to unnecessary outages with
all the problems that result. A common cause of such problems is the incorrect setting of impedance values of the relay. The positive sequence impedance is relatively easy to calculate because
the parameters of the transmission line are known. By contrast, the impedance of the return path
through the ground, or zero sequence, is much more difficult to estimate. This is because a typical
impedance value is used but the ground is not homogenous and the influence of objects such as
pipes, underground cables and rocks can significantly change its value.
CP CU1 makes the difference
As the calculated values for zero sequence impedance are unreliable, a far more effective approach is to measure the impedance of the fault loop directly. Traditionally this would require big
mobile generators in order to deliver the power necessary to allow a measurement to be made
against interference from the power system. This made such measurements costly and they were
A more elegant approach to this problem is to use the OMICRON CPC 100 with the CP CU1 accessory. This combination allows the injection and measurement of values at variable frequencies
so that the effects of power system frequency can be eliminated. As a result this compact, lightweight combination can perform extremely accurate measurements of the line and ground impedances. This provides the data to allow the distance relays protecting the line to be set very precisely with impedance values and the calculated k-factor.
To ensure operational safety, the CP CU1 comes with the CP GB1 grounding unit. This unit can
divert currents of up to 30 kA if its voltage limit value is exceeded due to unexpected induced potentials on adjacent lines.
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In order to guarantee highest safety levels within and around substations CPC 100 and CP CU1
can also be used to measure the ground impedances inside a substation as well as step and touch
voltages on its periphery. This can confirm the effectiveness of the substation’s grounding system.
CP CU1 with grounding unit
Line impedance measurement with CP CU1
OMICRON is an international company providing the electrical power industry with innovative testing and
diagnostic solutions. The application of OMICRON products gives users the highest level of confidence when
making condition assessments of primary and secondary equipment on their systems. Services offered in the
area of consulting, commissioning, testing, diagnosis, and training make the product range complete.
Customers in more than 140 countries rely on the company's ability to supply leading edge technology of
excellent quality. Service centers on all continents provide a broad base of knowledge and extraordinary
customer support. All of this together with our strong network of sales partners is what has made our company a market leader in the electrical power industry.
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OMICRON electronics GmbH
OMICRON electronics Corp. USA
3550 Willowbend Blvd
Houston, TX 77054, USA
Phone: +1 713 830-4660
+1 713 830-4661
OMICRON electronics Asia Ltd.
Suite 2006, 20/F, Tower 2
The Gateway, Harbour City
Kowloon, Hong Kong S.A.R.
Phone: +852 3767 5500
+852 3767 5400
Europe, Middle East, Africa
OMICRON electronics GmbH
Oberes Ried 1
6833 Klaus, Austria
Phone: +43 5523 507-0
+43 5523 507-999
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