Terrestrial-based time dissemination

The above mentioned reliability issues of GNSS led to the IEEE recommendation to pursue an alternative method of synchronization using terrestrial system as stated in [4, 45]. Few of the available terrestrial-based time dissemination technologies gua- rantee the accuracy level required by synchrophasor networks. Recently, Packet-Based Time-Synchronization Protocols (PBTSPs) like the precision time protocol version 2 (PTPv2) [60] raised the interest of some PMU developers due to the increased accuracy with respect to its predecessors. Still, the higher cost of such a time dissemination infrastructure (the network needs to be deployed if not in place) does not yet attract many PMU developers. The PTPv3 (also known as White rabbit [61]) is expected to largely increase the synchronization accuracy (i.e., in the order of sub-nanoseconds) and thus make terrestrial-based time dissemination appealing also for synchrophasor networks.

2.2. Time synchronization

PPS

A simple way to synchronize two clocks is to use a train of positive pulses at a rate of one pulse per second (1 PPS). The rising edge of the pulses coincides with the seconds change in the clock and provides a very precise time reference (better than 100 ns). Because the PPS signal does not provide any indication of the date or time of day, it has been largely replaced by the IRIG-B.

IRIG-B

According to a 2014 survey on synchrophasor system networks [3], the IRIG-B [62] is the second most spread technology to synchronize synchrophasor networks after GPS. The unmodulated IRIG-B code can deliver accuracy limited only by the slew rate of the digital signal, usually better than 1 μs and, with care, even 100 ns. The IRIG-B includes control bits that enable its support for real-time applications thanks to the inclusion of leap second, daylight savings or summer time status, year of the century and time quality. These assignments extend IRIG-B to a complete time message as needed by the PMUs. The IRIG-B suffers from a limitation: the time dissemination is performed either with twisted pair wires or coaxial cables. This means that data exchange and time synchronization are over two different infrastructures. This limitation is overcome by the precision time protocol.

Precision time protocol

IEEE Std 1588-2008 [60] allows sub-microsecond time accuracy for devices connected via a network such as Ethernet. IEEE Std C37.238-2011 [63] specifies a subset of IEEE 1588 functionality to be supported for power system protection, control, automation and data communication applications using an Ethernet communication architecture. The IEEE Std 1588 specifies a way to evaluate the link delay between two nodes (master and slave) through the exchange of time-tagged messages, see Fig. 2.11. The master node sends a Sync message to the slave and stamps it with a time-stampt₁as it leaves its networking interface. The message is received at timet₂ in the slave’s time base. The process is then reversed, with a message Delay_Req sent from the slave at timet₃ and received in the master at timet₄. The Follow_Up and Delay_Response messages are used to transport the timestamps recorded at the master clock to the slave clock, so that the slave has the information needed to adjust its time. Indeed, assuming the one-way delay through the network is exactly half of the two-way delay, the offset of the slave’s clock with respect to the master is:

δ = (t2+t3−t1−t4)

Chapter 2. Modern synchrophasor networks and their time synchronization

Figure 2.11 – Simplified PTP message exchange diagram.

Following the same principles, the grandmaster clock at the top of the time distribution chain, synchronizes the clocks in the entire system to UTC. Each device in the time distribution chain (including Ethernet switches) is required to support IEEE C37.238- 2011 to achieve 1 μs time accuracy. Ethernet switches supporting IEEE C37.238-2011 should perform measurements and corrections for cable delay and queuing time. IEEE C37.238 offers the use of the same communication infrastructure (Ethernet) for PMU/PDC data and time distribution, and reduced use of GPS connectivity whenever possible.

Advantages:The adoption of the IEEE Std 1588-2008 in power system enables:

• The automatic time compensation for the dynamic changes of path (dynamic delays);

• The automatic choice of the grandmaster clock thanks to the Best Master clock algorithm. Additionally, the protocol selects the backup clocks in case of failure of the grandmaster;

• The usage of existing network cabling;

• Additionally, all protocol adaptations relevant for the electric power industry are taken care of in the so-called Power Profile IEEE C37.238-2011 [63].

Disadvantages:

• Special network switches (i.e., transparent clocks) are needed to achieve the sub-microsecond accuracy;

• Need to compensate for the time delay between the GPS antenna and the master clock (if grandmaster is not integrated in the antenna).

2.2. Time synchronization

• Time-stamping by means of dedicated hardware. A software-only time-stamping allows an overall accuracy in the range of 20 to 100 μs.

White Rabbit

The White Rabbit (WR) project was initiated at CERN in 2008. The idea is to deliver the following functionality while using (or improving) existing standards. As reported in [64], the functionality are:

• Sub-nanosecond accuracy in synchronization and stability better than 50 ps; • Coverage over distances of 10 km (proven nowadays to be able to cover more

than 100 km);

• Ability to serve more than 1000 nodes;

• Guaranteed upper bound in the overall latency; • Open hardware, firmware and software [61].

To achieve sub-ns accuracy, in addition to an improved version of the PTPv2 protocol, the WR operates the so-called Layer-1 syntonization. Indeed, typical PTP implementa- tions use free-running oscillators in each node. This means that the time base of each node drifts during the time interval between two calculations ofδin Equation (2.4). The Layer-1 syntonization ensures equal clock frequencies in all nodes, therefore elimi- nating this drift. Because all the system clocks oscillate at the same rate, it is possible to compensate for the phase shift between two clock signals. This is achieved by means of a phase-shifting circuit in the slave that creates a phase-compensated clock signal despite the delay introduced by the fiber link.

Disadvantages:

• Requires optic links and dedicated switches;

• Not standardized (although in the process of standardization).

In terms of performance, WR has been proven to be able to reach synchronization accuracy in the order of 200 ps with standard deviation of approximately 6 ps for 15 km links (e.g., [64]). A large number of applications is currently taking advantage of the WR performance. Recent literature has considered WR as the future synchronization method for PMU-based power systems (e.g., [65, 66]).

Chapter 2. Modern synchrophasor networks and their time synchronization

Table 2.5 – Summary of the accuracy of time dissemination technologies for synchrop- hasor networks.

Technology Typical accuracy

GNSS 100 ns

1PPS 100 ns

IRIG-B (unmodulated) ∼1 μs

PTPv2 (HW) < 1 μs

3

Time critical functions and appli-

cations

This chapter presents functions and applications that we envisioned as time critical for a successful operation of a synchrophasor network. The chapter starts by describing two functions that enable the network operator to (i) collect meaningful data from its geographically distributed measurement devices and (ii) obtain a reliable estimate of the state of its network. Since several applications can exploit the availability of PMU data, the second half of the chapter presents two state-estimation-based applications with different time constraints. First, ahard real-time application is described. It leverages on the state estimation results to provide protection functions to the mo- nitored network, in time-scales of hundreds of milliseconds. As a second example, a soft real-timeapplication is described. It is conceived to be able to control the state of the system in time-scales of seconds in order to maintain its voltage level within predefined limits.

Original contributions of this chapter:

• Definition of a data-pushing logic for synchrophasor data concentration; • Definition of a protection mechanism relying on PMU-based state estimation

processes.

3.1

Situation awareness functions