DNV GL China

Smart Cable Guard system

Compared to offline protection systems, DNV GL's Smart Cable Guard uses online monitoring to ensure even irregular partial discharge activity is detected in time to act proactively

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DNV GL's Smart Cable Guard online monitoring system safeguards your medium-voltage underground cable network by pinpointing weak spots and faults.

Each Smart Cable Guard system comprises two sensor  injector units (SIU) and two control units (CU), one of each placed at each cable end. A cable end is a cable termination or sealing end, as can be found in a ring main unit, a substation or outdoor, for instance connecting the cable to an overhead line. As can be seen in Figure 1, there can be other ring main units or substations placed between the cable ends. The SIU measures the pulses coming from partial discharges (PDs) or faults. It also injects small pulses for time synchronisation. The CU handles the measured data and sends it via the internet to the SCG server. When a fault occurs, the grid operator receives an email notification within 5 minutes. In addition, status updates are made available to the operator every hour showing any PD activity.
SCG system - image
Figure 1: Measurement setup of Smart Cable Guard. At each cable end there is an SIU and a CU for signal processing and internet-based communication with the SCG server.

Time synchronisation Using injected pulses, Smart Cable Guard's time synchronisation is accurate to 100 ns (0.0000001 second) or better. Based on our patented pulse injection technology, this time synchronisation approach is: 

  • Economical - uses SIU for PD / fault pulse recording and regular pulse injection 
  • Practical - doesn't use GPS, therefore does not need a GPS module or antenna on the roof of a building 
  • Reliable - the correct measurement of the regularly injected time sync pulses acts as a built-in reliability check for the detection of PD / fault pulses
Maximum monitoring distance between two SIUs The maximum distance that can be diagnosed effectively depends on:
  • What the grid operator wants to monitor (i.e. small pulses from PDs, large pulses from faults or both)
  • Whether the injected time sync pulses can successfully be detected by SCG at the other cable end
  • Noise levels
As a result, the following rules of thumb in Table 1 below show the maximum monitoring distances that can be bridged by Smart Cable Guard.
Paper cablesXLPE cables
PD measurement3.5 km8.0 km
Fault locator6.5  km15.0 km
Sensor injection unit The SIU contains both a sensor to measure pulses from the cable, and an injection device to inject small pulses into the cable. A  sensor is placed around the cable or earth lead. For safe installation, the inductive sensors have a split core that allow to be clamped around the earth lead(s) without them needing to be disconnected. Depending on the safety rules, it can even be mounted while the cable is in service.
Control unit The control unit (CU) is a small dedicated cpmputer that is connected with the SIU by an optical fibre, eliminating possible EMC interference which could easily occur in the environments where these units are used. The CU controls the measurement sequence, data collection, signal processing and data communication with the  Smart Cable Guard server. The CU also has communication facilities on board (LAN or mobile phone card), so can upload the resulting data to the Control Centre at DNV GL for further interpretation.  Centralised control and surveillance Measured data is uploaded via internet tothe SCG server for centralised surveillance. Network owners can access information about their network condition via a password-protected portal - information on their cables can only be seen by themselves and DNV GL. You can check your cable condition status with a single click. You are also automatically informed in high-risk situations and faults. After the data from both sensors  is uploaded and time combined, the SCG Knowledge rules are applied. The knowledge rules are  various statistical parameters  calculating and analysing  the behaviour of groups of PD's e.g. from a specific time period or location. Suspected weak spots found by the knowledge rules are presented to a DNV GL  experts in this subject which assignes the warning level and informs the network owner. From the server, control data can also be sent, via internet, to adjust parameters such as the measurement repetition rate or to update software in the CUs and SIUs.

Overview

performance
Smart Cable Guard
performance
Percentage detected of cable faults
Smart Cable Guard
95%
performance
Percentage detected cable weaknesses
Smart Cable Guard
65%
performance
Fault detection in isolated neutral networks
Smart Cable Guard
Yes
performance
Fault detection producing high currents
Smart Cable Guard
Yes
performance
Intermittent faults detection
Smart Cable Guard
Yes
performance
Location of faults/weaknesses
Smart Cable Guard
Accuracy of 1% of cable length
performance
Maximum measurable cable length, depending on local conditions
Smart Cable Guard
XLPE 20 km PILC 6 km