# Transient relay for high resistance, intermittent and recurrent earth faults (Part 1)

Info Letter No. 17

The statistics show that earth faults constitute a large proportion of network faults. Conventional relays are designed only for low resistance faults under steady state conditions. They are not suitable for high resistance faults that occur especially in rural networks with transmission lines, nor for recurrent faults that occur mainly in compensated cable networks. As a result the earth fault is very often not recognized as the cause of the fault or an incorrect cause is identified. This increases the time required to localise the earth fault. On the other hand, effective protection of the network in a deregulated market is becoming increasingly important.

This Info Letter introduces a new algorithm for detecting high-resistance earth faults in the range of several Kilohms and describes its benefits.

#### Basics of Earth Faults

The behaviour of a single-pole earth fault can be described by superimposing three different operations. While all three operations start at the same time, each of them has a different duration.

The three operations are:

• Discharge of the faulty conductor across earth
• Charging the two healthy conductors across earth
• Stationary state of the earth fault

The explanation of the three operations is done using the network shown in Figure 1, with three outputs (A, B and C) and an earth fault in the conductor 1 of output A.

#### Discharge of the faulty conductor across earth

The lines can be considered as a distributed network with a complex series impedance ZLXX and a phase-earth capacitance CXX . Most highest probability for the first ignition of the arc is near the maximum of the phase-earth voltage U1E. At this time the load on the conductor is also the largest. The discharge of the conductor 1 begins at the point of failure and spreads like a wave in both directions to the ends of conductor 1. The propagation of the wave to the two healthy conductors is consequently blocked. The effect is also suppressed by a Petersen coil. More often there is a reflection of the wave at the ends of the conductor or at any point with a change of the characteristic impedance, such as in a substation or at a distribution point with a transition from one line to two or more. This reflection can be seen as a high-frequency oscillation in both the zero-voltage and zero-current.

Important parameters for the behaviour of the discharge are:

• Phase-earth capacitance of conductor 1
• Charge state of the phase-earth capacitance before the first ignition of the arc
• ZL series impedance of conductor 1 in the faulty output and in healthy outputs
• Impedance ZF at the fault point, which also includes the earth impedance.

The discharge frequency substantially depends on the series impedance and the phase-earth capacitance, which in a first approximation is proportional to the conductor length. The discharge frequency is higher for small networks and lower for large networks. Usually, the discharge frequency is in the range above 10 kHz.

Contact

## News from the product group

### News

General Knowledge

#### The Future of the Distribution Grid: Focus on Digital Secondary Substations

In this technical report, Gerald Jacob (Product Manager »EORSys«) uses a specific example to illustrate how distribution system operators can meet the challenges of energy transition in the medium and low-voltage grid with future-proof secondary substations.

General Knowledge

#### Earth fault explained – basics and functions of an electrical phenomenon

Everything you need to know about earth faults - from the basics, causes and examples to the consequences. Find out more in this article about earth fault clearing, compensation, location and the benefits of constant monitoring to ensure a safe power grid.

General Knowledge

#### Ready for the smart grid of the future?

All information about smart grids/intelligent power grids, the technology behind them, advantages, disadvantages, challenges and current distribution.

Info letter

#### Info Letter No. 21: The influence of the contact resistance at the earth-fault location on the zero sequence voltage UEN in compensated networks

The earth fault in compensated networks is usually presented without a contact resistance at the fault...

Info letter

#### Info Letter No. 20: Capacitance of conductors

In the conductors of electrical power supplies, a distinction is made between the operating capacitance...

Info letter

#### Info Letter No. 18: Transient relay for high resistance, intermittent and recurrent earth faults (Part 2)

Figure 3 can also be used for the determination of static earth faults. In an isolated network the entire...

Info letter

#### Info Letter No. 15: Earth fault location using the harmonic procedure (Part 2)

For the study of the behaviour of the harmonic currents and voltages in the compensated network, at...

Info letter

#### Info Letter No. 14: Earth fault location using the harmonic procedure (Part 1)

For the following considerations a network with three outputs and an earth fault in line 1 in output...

Info letter

#### Info Letter No. 12: E-coil control: Resonance curve procedure – Location curve procedure

In "compensated networks" the current through the point of failure is minimised with the help of an...

Info letter

#### Info Letter No. 6: Detection of transient earth faults

When there is an earth fault in a conductor, at the start of the earth fault three events occur at the...

Info letter

#### Info Letter No. 4: Summation of Alternating Currents

For the parallel connection of power sources, the principle of superposition applies, when all resources...

Info letter

#### Info Letter No. 3: The E-coil in general

The earth fault compensation coil, e-coil or also called the Petersen coil after its inventor, is connected...

Webinar

#### »EORSys« Webinar: Short-Circuit & Earth Fault Detection in Digital & Analog Substations

Webinar recording from 07.05.2024: »EORSys - Short-Circuit & Earth Fault Detection in Digital & Analog Substations«.

Special Publication

#### Cost Efficient Management of Digital Secondary Substations: Earth Fault Indicator EOR-3DS as Digitisation Unit for Secondary Substations

Digital secondary substations are becoming a key element for distribution system operators in terms of automation, monitoring, and efficient operations of their networks. Read this technical report about the EOR-3DS in its role as digitisation unit for secondary substations.