Power Line Communication - Protection Systems (Part I)

MV and HV Power Lines incorporates protection systems which takes measurements of information from Power Line. This information is regarding; current, voltage, impedance, frequency etc, with one objective, just for avoiding possible damages in lines (fires, etc) and having in consideration, if the Power System is having more voltage in the Power line, the protection must act quicker.

Normally, for determining, if the line must be disconnected (protection activation), the decision is able to  be taken by one of the points which is monitoring the information (current, voltage, etc), or also, it used to be by "mutual agreement", between the understanding of the two "end points" of the line, where is located the protections systems. This last solution is slower but gives more reliability, understanding that one outage, is able to give serious consequence in the stability of the frequency of the Electric System, with possible cascade reaction for unbalancing in the system by suddenly change in the loads (production/demand).

In case of activation, not necessary being one outage, because the activating of the protection systems is not necessary having one outage, simply by mesh architecture of Electric Networks.

In case of outage managing, the Electric Operators (Distribution or Transmission) uses EMS or DMS systems (Energy or Distribution Management Systems) by WASA (Wide Area Situational Awareness) or ADA (Advanced Distributed Automation; in case of Smart Grid Electric Network).

Figure 1_ Example of possible risk (fire) in whatever situation of non properly functionality of protection systems in the Over Head Power Lines.

The propose of Smart Grid Network is achieved information from sensors installed along the Grid for monitoring faults and predict further outages. PLC (Power Line Communication ) great implemented; AMI (Advance Metering Infrastructure), is able to report information to the Utilities about local outages, when there is no electric consumption in one determined time. All this kind of operations are enclosed in PSSE (Power Systems Simulators for Engineering), examples of those software are: "BigSilent", "PS CAD" or "PSS E"  which give the possibility to simulate protection systems, harmonics, production planning, voltage stability, earth system, contingency analysis etc. 

For having the control between the two points in the power line, this communication is able to be by fiber optic, radio frequency (RF) or PLC (Power line Communication), also called Broadband Power Line.

PLC has been used long time ago (decades), but it has one disadvantage, when there is one fault, the communication channel (Power Line) gets with high noise, and in that case, the communication is critical to assure the protection system. For solving this issues, the PLC needs to work with signals with high robustness and low latency, following the standard IEC 60834-1 (Tele-protection equipment of power systems).

Inside this application explained, the most safe option with great implementation is; "Line differential Protection". This system is working measuring the current waveform in the input of the line and this measurement is compared with the current waveform of the output of the line. For that, both waves must to be equal sample by sample (Nyquist Theorem: sample limited in band range and sampling process higher than double of Bandwidth). If there is no this condition, the system understands there is a short in this delimited line.

One issue presented in the waveform sample by sample compared is the propagation delay, being this waveform as sinusoidal. For solving this propagation delay, the system uses time slogans coming from GPS (Global Positioning System).

The maximum allowable time error depends of voltage level in the Power Line. The solution implemented nowadays is one system which monitor the phasors (PMU: Phasor Monitoring Unit), using synchrophasors. Being this application optimal for controlling the Protection of Distribution and Transmission Lines in a wide area of Smart Grid infrastructure: WASA (Wide Area Situational Awareness).

In HV Power Lines, this time of reaction is related 150 µs. In concrete, for PLC communications, when there is one fault and/or short, the noise and/or distortion are getting higher.

In this situation, PLC application implemented for a big range of protection (distance) is able to lose the synchronization and to spend more time than 150  µs in recovering the state.

In this situation, when the system needs the most proper reliability in the complete system of: differential protection + PLC, this systems is not working properly.

For that reason, PLC for differential protection is not used, and is used by Fiber Optic systems integrated in the cable lines (OPGW: Optical Power Ground Wire).

In MV, the time for disconnecting are lower, being 200 or 300 ms, and having the same issue than in HV Power lines, not being properly solution (PLC) for big range of protection area (distance).

When the protection systems are monitoring currents and voltages (over current and over voltage protections) used to work such as isolated system, with no issues regarding noise or other disturbance. In this case, the system is able to work jointly by systems designed properly for that application, specially for non loosing the synchronization, even in outages. This systems are called Tele-protection.

Figure 2_ Examples of Power Line: from subterranean to Overhead Power Line for crossing a river. Where there is the possibility for PLC is able to establish the communication in this subterranean line and after change to the wireless system or Fiber Optic in the OH Power Line.