The expansion of electric power transmission over greater distances has necessitated the reliable control of interconnected systems. The utilisation of existing HV transmission lines with a voltage of 110 or 400 kilovolts (kV), as is the case in Czech Republic, was chosen as the most suitable alternative.
The so-called “electricity telephony” – the principal of adding amplitude or frequency modulated signal (in frequency band 30 kHz – 750 kHz) to the fundamental harmonic voltage of 50 Hz. This system was able to carry not only speech, but also data. It is very robust – almost independent of weather conditions and greatly resistant to line failures. From the 1990s, traditional electricity telephony began to be replaced by optical links (a telecommunications link that consists of a single end-to-end optical circuit). The optical fibre is built into protective earth rope.
In the case of a medium voltage (MV) distribution network (working with levels 22 kV or 35 kV in Czech Republic), the situation is fundamentally different.
These lines do not use protective earth rope when the installation of a new parallel optical path is not economically viable. Moreover, the total length of MV lines is much greater compared with HV and EHV lines.
Utilisation of a conventional cellular network is a method that is leaned upon the most. Operators boast in advertisements of the continuous increase in coverage. Financial profit is the main goal and therefore they focus only on lucrative regions – with high population density. These, however, do not necessarily correlate with the location of all MV/Low-Voltage (LV) substations.
Other factors not always included when HV and EHV lines are considered are undefined latency data transfers, the need to ensure appropriate conditions for GSM signal propagation (e.g. in reinforced concrete buildings) or the fact that the fate of the entire network is in hands of another commercial entity.
There is a different idea to reintroduce electricity telephony in a new form – directly on MV lines. Communication over MV lines A key part of the whole MV communication channel is the coupler. It realises bidirectional signal interconnection between a modem’s output stage and the MV line.
Communication over MV lines
General requirements for the coupler can be the following:
- Voltage breakdown immunity – according to the EN 60060-1, test voltage 50 kV for 1 minute and impulse surge 125 kV during 50 μs.
- Maximal 50 Hz attenuation (including harmonics)
- Minimal attenuation in communication band
- Linearity at maximal output power
- Dimensions, technical design and price
Commonly used couplers can be split into two groups based on connection principal – serial (Figure 1a) and parallel (Figure 1b).
The main disadvantage of serial coupling (often referred to as current or inductive) is the necessity to use a transformer designed for handling maximal (short) MV line current. This causes the oversaturation of its core, which is then no longer able to transfer the modulated data signal. With suitable materials and processing technology, this effect can be partly eliminated, but only for frequencies very distant from the mains frequency of 50 Hz and its harmonics.
In the band of tens or hundreds of kHz, the parallel coupling principal is the only way (Fig. 1b), where the transformer does not have to fulfill such high demands.
Present and future
The most commonly used LV PLC options:
- Narrowband – long range, lower communication speed (typically 102 to 104 bit/sec)
- OFDM (Prime, G3-PLC) – medium range, higher speed (typically 104 – 105 bit/sec)
- Broadband BPL – Low reach, very high speeds (typically 106 to 108 bit/sec)
There is always a compromise between bandwidth and range, but also the price of the PLC modem.
The direction in which the field of PLC communications is going is the pursuit of greater range and higher data transfer speeds. If we start from the well-known theorem of channel capacity, known as socalled, “Shannon limit”.
It contains three variables that affect the bit rate (and other properties) of the data channel crucially.
Using a modem able to provide very high output power (S) is obviously beneficial, but only to a certain extent. The maximum transmission power is limited by legislation. Moreover, dimensioning the output stage and the coupler has a major impact on the overall cost and size of the modem.
Achieving greater throughput by increasing the bandwidth (B) is an alternative, upon which BPL PLC modems are built. Due to the fact that the supply system (both HV and LV) have a Low Pass characteristic, connection cannot be ensured for more than several hundred meters. This implies the need to use repeaters. In the field of high voltage distribution, connection of barrier filter (band-stop) can partially help, but costs of this improvement are very high.
S / N, expressing signal to noise ratio, determines the upper bound of the amount of data transmitted, while maintaining a certain error rate. In this context, an amount of data is presented commonly as a number of states we are able to encode into one symbol period. For example, standard Prime allows you to select 2, 4 or 8 states per symbol (corresponding to 1, 2 or 3 bits). In practice however, due to low quality of most lines, data almost always uses the mode of the two states per symbol.
What to do?
The above brief analysis shows that achieving better PLC performance by only increasing the transmit power or frequency range is not feasible. It is necessary therefore to choose an alternative method, whose common feature is the optimisation of existing channel at the current bandwidth and at the same transmit power.
One such method is the effort to maximise utilisation of the entire reserved spectrum and thus eliminate the unused frequencies (see Figure 2). This brings an elegant solution to a recently neglected modulation scheme called MCM (multi-carrier modulation). This system fills the whole target spectrum with a set of subchannels, which are much wider and capable to work independently in comparison with OFDM.
Another way to make the most of the PLC channel is using appropriate coding techniques. Let’s consider a typical canal full of interference and noise. This may be due to natural and physical phenomena, but in most cases is generated by human activity.
Most noise signals coming from surrounding devices and machinery interfere with other wired and radio communication channels or possibly caused by a deliberate jamming.
To reduce the channel sensitivity to external influences, we can use several approaches (usually in combination):
- ARQ (Automatic Repeat Query) – Repetition of the data block which has been evaluated as damaged by the receiver, based on a detection algorithm (e.g. checksum).
- FEC (Forward Error Correction) – code which is capable of using the redundancy bits in the packet together with a correction algorithm to correct certain amount of bit errors (e.g. Reed Solomon).
- Interleaving – process responsible for the data arrangement when the main communication channel is divided into more subchannels (e.g. MCM or OFDM). Its setting is crucial for proper functioning of the modem and is based on the coding used. The aim is to transform burst errors into isolated errors that are usually better repairable by forward error correction (FEC).
Long-term successful use of PLC transmission at low voltage lines and their continuous development (see Prime 1.4) confirm that Powerline Communication for grid applications is still far from having its last word. Accessible and cheaper computing power, modern components and new applications – such as just MV transmissions – are giving PLC new life. MI