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 with the location of the fault. In addition, status updates are made available to the operator every hour showing any PD activity.
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.
By using a different technology for fault location Smart Cable Guard detects:
- More than 95 % of all faults in the MV network which is being monitored by Smart Cable Guard
- Faults producing reduced short circuit currents (Peterson coil / isolated neutral networks)
- Intermittent Faults
- The location of the fault with an accuracy of 1% of the cable length.
>> Smart Cable Guard is not looking at the short circuit current, but at the high frequency wave, which is always there when a fault happens and that makes the Smart Cable Guard system such an effective tool. More information on fault location.
DNV GL’s patented pulse injection technology is the reason why Smart Cable Guard is such an effective PD online measurement system, because:
- By using two time synchronised sensors Smart Cable Guard immediately pinpoints with an accuracy of 1% of the cable length the location of the PD activity
- By using two sensors Smart Cable Guard has a very strong noise reduction tool as the systems only check the information coming from the cable circuit in between the two sensors. All noise coming from outside the monitored circuits is disregarded.
>> Smart Cable Guard realizes an accuracy of correctly locating 65% of the weaknesses in the monitored MV cable circuit. More information on partial discharge.
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
Centralised control and surveillance
Figure 2: Customer web interface, showing the location of increasing partial discharge activity over time
Measured data is uploaded via internet to the 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 period or location. Suspected weak spots found by the knowledge rules are presented to a DNV GL experts in this subject which assigns the warning level and informs the network owner. More information on control centre.
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.
The control unit (CU) is a small dedicated computer 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.
Overview performance Smart Cable Guard