Power utility companies constantly monitor high voltage power lines in order to ensure a secure and uninterrupted supply of electricity. Protection relays are used at substations and other critical locations controlled through communications links in order to isolate faults, protecting equipment and plant.
The communications channels used for protection applications demand the highest level of availability with very low delays. Legacy Telecom networks historically have been interconnected using metallic circuits the problem being that the substation environment is characterised by high levels of electromagnetic fields, which can disturb transmissions on copper wires.
To avoid the problems electromagnetic fields can have on transmission circuits it is recommended to use optical links using the IEEE C37.94 standard. The IEEE C37.94 standard defines the rules to interconnect teleprotection and multiplexer devices from different manufacturers using optical fibres.
Protection of High Voltage Power Lines
Authorities use a teleprotection schema to enable substations to communicate with one another to selectively isolate faults on high voltage lines, transformers, reactors and other important elements of electrical plant. This requires the continuous exchange of data over the utilities telecommunication network in order to assure correct operation. It is imperative that the telecom network therefore should always be in perfect condition in terms of availability, performance, quality and delay.
Telecom networks in the past have used transmission circuits that were vulnerable to electromagnetic and radio interference (EMI/RFI), signal ground loops, and ground potential rise. Communications circuits vulnerable to interference and disturbance are not acceptable where the reliable transport of teleprotection protocols is required.
The electrical substation environment is usually characterised by a high level of electromagnetic fields caused by the high voltages and currents in power lines. Moreover, during times when high voltage fault conditions occur electromagnetic disturbances can rise significantly causing communication errors on copper based communication circuits. The reliability of the communications link interconnecting the protection relays is critical and must be resilient to the effects encountered in these high voltage areas such as high frequency induction and ground potential rise.
To prevent these issues the electrical distribution industry moved to optical fibres to connect protection relays installed in substations. Fibre optic connections do not have a ground path and are therefore immune to the interferences caused by electrical noise. The use of fully optical links from power relays to multiplexers using the IEEE C37.94 standard has become widespread in the industry.
Protection schemes using IEEE C37.94 interfaces can transport signals across fault tolerant networks of different kinds including PDH/SDH and Packet networks. These fault tolerant networks help increase the reliability and availability of communications circuits helping ensure critical teleprotection data interchanges.
Before C37.94 interfaces and circuits are handed over for protection purposes they should be tested and verified that they meet the required standards and requirements. There is a major difference between point-to-point C37.94 circuits and multiplexed channels running over SDH or Packet based transmission networks although both require testing to ensure correct operation.
Direct point-to-point C37.94 circuits perform very well and are easy to test because they are symmetrical, have a fixed latency and do not share bandwidth with other applications. On the other hand C37.94 interfaces that are multiplexed and transported over synchronous digital hierarchy (SONET/SDH) or packet networks are not symmetrical, have variable delays and also carry other data that can impact on the transmission of critical protection data.
Bit Error Rate (BER) Testing
A BER test verifies the ability to deliver error free data between two network ports. This test is widely deployed in communications networks as it can detect a number of issues occurring anywhere between two point on a network including:
- Noise interfering with the transmission medium and network devices.
- Failing or marginal components in active network devices.
- Marginal electrical signals or power in lasers on fibre connections.
- Framing errors due to issues with communications clocks causing framing errors and lost data.
- Interruptions due to network switching and other events.
- Delays and loss of data due to network overloads or buffer overflow in network and multiplexer devices.
A BER test can be run for a short period of time or over a number of hours in order to expose cyclical network events, such as changing bandwidth usage patterns in the utility’s network or random events like the switching of high-voltage lines. Depending on the transmission technology deployed longer periods of testing may be required if for example microwave or wireless links are used or when a packet based (Ethernet) transport network is being used.
Multiplexed and Packet networks normally suffer from some form of network delay and latency. Network delays can have serious implications on the performance of protection systems and it is vital to be able to test that any network delays are within the required specification.
Through the use of a loopback device it is possible to measure the round-trip delay between two ports on a network and by halving the delay estimate the one-way delay. This assumes however that the transmission path between the two points is symmetrical, in reality when there are multiplexers and other network equipment in the path this is unlikely to be the case. The probability of asymmetrical network delays increases further when packet based (Ethernet) networks are used or when wireless, microwave or transport circuits are deployed.
With the use of the Global Positoning System (GPS) now being a viable option for portable and hand-held test equipment it is possible to measure the one-way-delay (OWD) between two network points. Performing a OWD test means that the delay/latency can be measured independently in each direction allowing for checks of network asymmetry.