Time synchronization source

A review of types of time synchronization sources is done. A total of 29 conference proceedings and journal articles were reviewed, Figure 2.2. Only one was published before 2010 and most were published in the last decade. Precision Time Synchronization Protocol (PTP) and Simple Network Time Protocol (SNTP) are reviewed for use in process bus networks.

Figure 2.2 Time synchronization documents reviewed per year

The equipment status collected at the process level by protection and control devices need to be time stamped and published in a frame format on the substation communication network. All the devices therefore need internal clocks that are synchronized with a substation GPS clock. Bhardwaj et al.,(, 2014: 4) showed that these synchronised device clocks can in addition to time stamping, also be used to calculate delays in the communication network. The synchronization is performed through IRIG- B, or indirectly over a network using one of several standards.

The IEC 61850 standard recommends the Network Time Protocol (NTP) as a synchronization method (Rinaldi, Della Giustina, et al., 2016). The NTP time accuracy

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DOCUMENTS REVIEWED VS YEAR

(0.1 to 1 ms) can be considered enough for data acquisition and control applications. The IEEE came up with IEEE 1588 standard (De Dominicis et al., 2011; Guo & Crossley, 2017) to synchronize multiple devices over a network where their clock is in master/slave mode (Bhardwaj et al., 2014: 4). A single network implementation can be accomplished using IEEE 1588 Precision Time Synchronization Protocol (PTP) (Skendzic et al., 2007: 5). Part 9-3 of IEC 61850 specifies a PTP profile of IEC 61588:2009. The IEEE Std 1588- 2008 applicable to power utility automation allows compliance with the highest synchronization classes of IEC 61850-5 and IEC 61869-9 (IEC, 2016a). IRIG and PTP can be considered as alternatives to NTP.

The timing class is defined in IEC 61850-5 standard (IEC, 2013b).

The substation under discussion for the research project uses a GPS receiver for time synchronization by distributing IRIG-B encoded time signals to protection and control IEDs. IRIG-B is a time code format, which provides date and time in a coded form. The accuracy of 1 µs can be achieved by 1 pulse per second (PPS) input (Bhardwaj et al., 2014). The SEL-2440 Discrete Programmable Automation Controller (DPAC) is considered as an input/output (I/O) unit for control and collecting equipment status in the yard (Schweitzer Engineering Laboratories, 2016a). The DPAC can use IRIG-B or SNTP for time synchronization. The 1 µs IRIG-B accuracy offered is better than ±5ms SNTP accuracy for this device. The IRIG-B network requires additional cabling compared to SNTP that can use the communication network to all the devices to be synchronised. A time synchronization source is required for the MUs to time stamp the sampled analogue signals and convert these into digital information. Skendzic et al. (, 2007: 5) explain that the SV streams must be synchronized to a common time reference. This is of importance when this information is used for power system protection and especially differential protection where data are sampled at different locations, sent over a communication network, and compared in a protection IED.

Network Time Protocol (NTP) as synchronization method is previously discussed. The NTP time accuracy (0.1 to 1 ms) can be considered insufficient for SV applications which require <1 µs accuracy.

Time synchronization by distributing IRIG-B encoded time signals to the MUs in the high voltage yard can provide better accuracy but additional cabling is required to the MUs. A single network implementation can be accomplished using PTP on the communications network (De Dominicis et al., 2011). PTP is recommended and used for the synchronization of SV messages in process bus implementations (Ingram et al., 2013a: 5935). The data traffic can affect the synchronisation accuracy (Liu et al., 2016). The coexistence of SNTP and PTP by using Time Gateways to translate the time from a time domain to the other are analysed (Ferrari, Flammini, Rinaldi & Prytz, 2011; Ferrari, Flammini, Rinaldi, Prytz, et al., 2011).

Attacks on time synchronisation and other vulnerabilities can affect the functionality and security of the power network (IEEE, 2019; Fodero et al., 2017).

The paper (Ingram, Schaub, et al., 2012: 1179) demonstrates that PTPV2 is a viable method of providing time synchronization for an SV process. The paper also notes “transparent clocks” and how this impacts the PTPV2 timing system where “sampling errors increase as transparent clocks are added to the system”.

2.2.5 Equipment

PRP uses IEDs with two Ethernet ports or Doubly Attached Nodes (DAN) to provide redundancy. The IED with two Ethernet ports, DAN running PRP (DANP), makes it possible to connect the IEDs to two separated and independent networks. Duplicated Ethernet packets are sent simultaneously through these two networks. The destination IED can still receive the data from one network if one data frame fails to reach the IED from the other network. A Redundancy Box (Redbox) is needed for IEDs with Singly Attached Nodes (SAN) to connect to PRP networks (Liu et al., 2014: 2).

HSR also uses IEDs with DAN but running HSR (DANH) it is not connected to separate networks as with PRP. A frame is duplicated when a multicast frame is sent from the DAN on the same network. Each duplicate frame is tagged with the destination MAC address and a different sequence number. The first of the two duplicated frames received is passed on at the destination, and the other frame is discarded. A Redbox is needed for SAN to connect to HSR networks in the same way as with PRP networks (Alstom, 2002: 440).

The HSR nodes require more processing power because every node will need to process a frame twice. This also has an impact on the bandwidth used due to every frame sent twice over the same network. This is important when a network is used for multicast SV sent from MUs (Goraj & Harada, 2012: 4).

Managed network switches allow configuration on how it functions for different IEC 61850 messages e.g. GOOSE and SV. VLANs and Quality of Service (QoS), which is defined in IEEE 802.1Q. can be configured (Oliveira et al., 2016: 3).