IP-based Communications for Line Current Differential Protection

Authors: A. Aichhorn, H. Krammer, T. Kern, Sprecher Automation GmbH, Austria, and R. Mayrhofer, Johannes Kepler University, Austria

Introduction

Line Current Differential Protection
The functional principle of 87L protection works according to Kirchhoff’s Current Law, the algebraic sum of all currents entering and exiting a node have to be equal zero.
The single line diagram of the arrangement of the protection relays is shown in Figure 2.

                          IA + IB = IF                                                                      (1)

Equation 1 shows the simplified calculation of the fault current by summing up the measured current values. In the case of a healthy state these currents cancel each other. If the fault current IF is not zero, a faulty state occurs.
There are in principle two different approaches to transfer and evaluate the measurement data. First, to calculate vectors for each period or a predefined number of periods and only transmit this vector data. In the second approach, all sampled values are transferred.

This article assumes the second approach, because transferring sampled values offers more options for the subsequent fault detection algorithms. Therefore, higher bandwidth on the communication channel and a more accurate time synchronization are required to transfer all samples with sufficient time accuracy.

Protection Interface
Essential for the line current differential protection is the communication between the relays. The relay is only able to calculate the differential and the bias current if the data of the remote relay is available and includes a precise timestamp.
In general, there are two characteristics of the communication decisive: the path delay (latency) and the path delay variation (PDV), also known as the jitter. Path delay causes a delay in receiving the measurement data at the remote relay, which has to be considered in the algorithm and may affect the tripping time depending on the occurring delay.
A typical requirement for the maximum latency is between 5 and 10 ms, depending on the applied voltage level. These recommended latencies are mentioned in the design considerations of the IEC 61850-90-1.

The jitter has an influence on the synchronization accuracy, depending on the used implementation. The implementation presented in this paper synchronizes sampling of data acquisition with the remote relay by using IEEE 1588 PTP based synchronization approach with a hardware timestamping unit over the communication path.
The higher the synchronization accuracy, the more accurate and faster the detection of a fault can be realized.
The IEC 61850-90-1 recommends a synchronization accuracy of lower than 10 µs if high fault current sensitivity is required.

This article focuses on the sampling synchronization of 87L protection devices, because synchronization is the precondition for the realization of a proper and fast line current differential protection. The following section describes the currently used communication technology as well as laboratory investigations on possible technologies and the proposed communication technology for the communication between substations.

Communication Technology for the Protection Interface
The communication technology of line current differential protection is a major part of this protection type. It influences the behavior (e.g. accuracy and speed of detecting a fault) and cost-efficiency of the overall solution.

Currently Used Communication Technologies in the Field
A communication technology with traditionally widespread use in the field of teleprotection is SDH. This kind of communication network is basing on TDM, which reserves the dedicated bandwidth for the applied service by using fixed timeslots. SDH is still used for protection applications, whereas the network providers discontinue native SDH services successively and change over to the PSN technology.  Packet Switched Networks offer the major advantage (compared to TDM technologies) that the available bandwidth can be more effectively utilized, allowing to use fewer network devices and connections for the same average load. This has the further advantage that less floor space is needed for the realization of a network as well as a reduction of the demanded electrical energy.

A whitepaper by the company Ciena Corporation describes a case study comparing legacy SDH equipment to equipment of packet switched technology.  The result of this case study reveals in a floor saving of 95% and energy savings of up to 90%. This economic factor and the advanced development of PSN networks is a convincing reason to discontinue TDM and change over to PSN technology.

As there are still devices which use SDH networks for their protection interface, research results have been published towards using Circuit Emulation Service over Packet Switched Networks (CESoPSN).
These investigations show that it is possible to emulate TDM communication by using a PSN network as the backbone system.
Due to the fact that TDM needs a fixed bandwidth, which is reserved for the needed timeslots, it does not provide effective bandwidth utilization anymore.
Therefore, both the advantages of packet switched networks (higher utilization) as well as TDM networks (hard real time capability) are sacrificed for emulating legacy technology over PSN backbones. Unsurprisingly, we conclude that it is advantageous to communicate natively over PSN technology.

Investigations on PSN Communication Technologies
For a realization of a 87L protection, different approaches are already discussed in the literature. Laboratory tests have been made by using Wi-Fi communication, as well as WiMAX. Considering these technologies for a 87L protection communication from the availability aspect, centrally controlled radio communication should be avoided as it may result in a single point of a global failure. Additionally, the maximum possible distance is an important point, which may be limited by using Wi-Fi.
S. Fukushima et al. recently published a 87L protection scheme with data communication basing on Ethernet frame messages (ISO/OSI layer 2) and sampling synchronisation based on Ethernet round-trip measurements. This approach already uses a native PSN network and is the approach most closely related to the one presented in this article.
However, all of these approaches neither use a higher level (layer 3 and upwards) protocol to be routable across multiple sites over WAN networks nor apply Quality of Service to prioritize the messages. The following section describes the proposed communication protocol and the related time synchronization.

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