Analysis of Protection Scheme Dependencies on Communications

Galina S. Antonova ABB Inc., Canada

Eduardo Colmenares ABB Inc., USA

Ilija Jankovic ABB Inc., USA

Abstract: Many protection and control schemes highly depend on the quality of the underlying communications. Communications availability and reliability have a direct impact on security and dependability of the protection systems. Classical use cases include but are not limited to line differential protection, teleprotection schemes, etc.

This paper analyzes how the quality of the underlying communications affects various protection schemes security and dependability. A number of protection and control applications will be discussed with a focus on the requirements that they impose on communications. An overview of current communications technologies and their characteristics will be given. Practical uses cases will be discussed and suggestions will be made on appropriated use of certain communication technologies for particular protection and control schemes.

Key words: protection, communication, latency, bit error rate, time synchronization, protocol, data loss.

I. Introduction Most commonly, protection, control and communication fall into responsibilities of different departments within a power utility company, with limited or no communication, and understanding between different groups. This has been appropriate and sufficient for manually operated protection and control in radial power utility systems.

Historically, developments in communications led to developments in protection. As a good example one could consider a line differential, now a common protection in transmission level relays, which was not possible prior to advancements in communications.

On-going evolution of power utility systems leads to grid transformations from simple unidirectional radial into more diverse bi-directional grids with sources and loads distributed throughout the system, such as systems with distributed generation (DG). Such system operation would highly depend on use and quality of communications, assisting in performing critical protection and control functions. Thus, lack of interactions and understanding between protection, control and communication engineers is no longer appropriate nor sufficient. Previous studies on this subject are available in [31],[32].

This paper aims to connect communications with protection and control functionality, to make protection and control engineers more aware of how quality of communication affects operation of protection and control schemes. Five examples of communication-assisted protection schemes are given. Effect of specific communication parameters on security and dependability of these schemes is discussed. Some considerations on communication technologies that are appropriate and not appropriate for a given scheme are given. Implemented schemes that are currently in operation are used as examples.

II. Dependencies of protection schemes on communications

Five types of protection schemes and analysis of their dependencies on communications is discussed in this section.

A. Line Current Differential protection

Line differential protection is a classical example of a protection scheme highly dependent on and in fact not possible without communications. This scheme typically uses current samples from 2 or more ends of the power line. Some more advanced schemes suggest using negative and zero sequence components for improved security and dependability. The main principle of current differential schemes, as well as any differential scheme, is based on Kirchhoff’s law for currents. Clearly, only current samples taken at the same time shall be compared, thus precise time synchronization is required. It should be noted that some current differential schemes allow for phase delay also. Although current differential is a common and widely available scheme, inter-vendor interoperability has not been achieved. While standardized protocols, such as IEEE C37.94 and transport systems, such as Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) are commonly used for these applications, actual algorithms are proprietary and vary from vendor to vendor, such that a relay from vendor A on one end of the line will not work with a relay from vendor B at the other end of the line. A basic diagram for a single line protection using current differential element (87L) is shown on Figure 1. It assumes the use of multiplexed communication channels, e.g. using IEEE C37.94 standard [1]. It also notes the time synchronization path.

Figure 1. Basic line current differential protection diagram

A more complex 5 terminal system is shown on Figure 2. For this case 10 dedicated communication channels are required, as shown by dotted arrows, to enable each 87L relay to communicate with 4 others to protect the entire zone. Each relay is responsible for making independent trip decisions (master mode).

Figure 2. Line current differential protection for 5 terminal system Samples can be lost or corrupted due to errors in communication channel (e.g. a value could change from 0 to 1 due to noise, attenuation, etc). Such errors are characterized by the Bit Error Rate (BER) of a channel. Some are detectable and possibly correctable, others are not, and result in dropping the data, i.e. losing the sample. Current differential schemes impose rigorous requirements on BER of 10-12 - 10-9 during normal operation, 10-6 during disturbance and 10-4 when channel is blocked [2]. As comparisons are made on per-current sample basis, reliable and timely delivery of these samples is the key requirement for the communication system. For proper operation samples shall be delivered in less than the data reporting interval, for example 5 ms. Loss of a data sample delays decision making by a data reporting interval, e.g. 5 ms, while loss of 4 samples (20 ms) can lead to blocking protection operation if no communication received in 200 ms. Obviously, quality of communication channel has a direct effect on system security and dependability. Dependency of current differential schemes on time synchronization deserves special attention. Diagrams explaining samples alignment / misalignment are shown on Figure 3. If sampling times are not synchronized, values with different sampling times will be compared and wrong decisions will be made, hugely compromising the dependability and security of the whole system. As well, the larger the time synchronization error the less sensitive differential measurements will be

Thus, sampling times have to be synchronized, not necessarily to a global time, but at least to a common reference for a given transmission line. Providing sample sequencenumbers (sequential numbers that increments each sampling interval) or timestamps time with current measurement are common ways of communicating time at which measurement was taken.

a) Misalignment of samples

b) Misalignment of waveforms

Figure 3. Alignment of currents between local and remote ends of the line

One way to synchronize samples is to provide Global Positioning System (GPS) synchronization at each 87L terminal. GPS time source typically provides 100-200ns time accuracy to Universal Time Coordinated (UTC). This method is independent of communication channel, communication technology and communication system architecture, as communication system is not used for synchronization. If GPS synchronization is not viable or possible, synchronization can be performed over the available communication system, imposing even stricter requirements on it. For achieving high synchronization accuracy the communication system shall have low and symmetrical delays (as synchronization mechanisms typically measure two-way delay, and use calculated one-way delay for clock adjustments, assuming delay symmetry). Different mechanism can be used for measuring delays, e.g. echo mode, path delay, etc. For real life systems achieving delay symmetry between transmit and receive delays is only possible within a given practical margin, e.g. 200 microseconds. Knowing the delay allows accounting for it, which is a definite asset. The requirement for delay symmetry, in turn, imposes a requirement for fixed routing, i.e. data should always use the same exact path. This is normally not possible for Internet Protocol (IP), and Multiprotocol Label Switching (MPLS)-based networks. Furthermore, if the selected fixed path fails, a new route has to be established quickly, e.g. in 2 seconds, because local clocks will be drifting without receiving proper synchronization, and will exceed an acceptable time accuracy limit if they are disconnected from a time source for longer than certain time, in our example 2 seconds. This time depends on type of local oscillators used, their frequency stability and holdover characteristics. Better (more expensive) clocks can withstand longer times without receiving a reference. It is interesting to note that there is a direct connection between time synchronization error / accuracy and sensitivity of line differential scheme. Figure 4 shows the dependency of error for different fault currents on delay asymmetry, leading reduced synchronization accuracy and unwanted trips [3]. In particular, it shows that virtual error increases with the increase of time synchronization error. Thus, the larger synchronization error, the large measurement error and the smaller sensitivity of the scheme. This further explains the connection between non-synchronized samples and poorly synchronized samples and false tripping (dependability) and not tripping when required (security).

Figure 4. Error for different fault currents and delay asymmetries.

As discussed above, communications is an essential part of current differential scheme operation. Communications for line differential schemes must have low BER, low (lower than sampling rate) transmission delay. Current differential schemes are also highly dependent on quality of sample synchronization. If communication system is used for synchronization, it shall provide low and symmetrical delays, fixed path routing, and low switching times on failures. As shown above these communication parameters differently affect dependability, security, availability, sensitivity and operation time of current differential schemes.

2.-Distance Protection

Distance protection relays are the most common relays used for transmission line protection. The reason for this is the simple measuring principle, the built-in back-up and the low bandwidth requirement on communication with the remote end [4]. While distance protection can be performed without communications, communication-assisted schemes operate faster (especially for faults near the end of the line), provide 100% protection of the protected line, cover more protection zones. They are also more selective.

In most distance protection scheme applications, communication channels between the two ends are utilized to improve the protection system behavior, to ensure high voltage system faults are typically detected within 1-2 cycles, and are cleared within 5 cycles or 83 ms for 60Hz system. The typical protection-system architecture on a high-voltage transmission line consists of three main components:

Figure 5. Typical protection system architecture

The transmission of commands/trip signals are typically send over an analog channel or digital channel, e.g. 64 kilobit per second. They can use electrical, optical and microwave communication media.

Figure 6. Available communication channels and communication media The most important characteristics of teleprotection equipment are security and dependability.

Typically, communication requirements needed for protection are more demanding than the ones needed for telecommunications. When a message is sent over the Internet and the message is not received it does not have any consequences; the message is just retransmitted. This is not acceptable for teleprotection commands. Teleprotection is not only about sending commands but also guaranteeing that the commands are received within the expected transmission time.

Disturbances in the telecommunication channel must neither simulate a command at the receiving end when no corresponding command signal was transmitted (security), nor suppress a command that was actually transmitted (dependability). In this regard, all teleprotection equipment must comply with relevant security, dependability and transmission time requirements [5].

The transmission of tripping signals via pilot wires or voice channels of PLC equipment is widely used. However, voice channels have limited bandwidth which limits the transmission time and the number of commands per channel. When using PLC where the channel is the protected transmission line, the relay system must be designed to mitigate loss of channel issues due to the possibility of the fault causing the loss of channel. There are two basic systems used for transmission line protection. The simplest is the directional comparison blocking (DCB) system, as shown on Figure 7. This system uses a blocking signal that is transmitted when the fault is outside of the transmission line protection zone, to prevent the remote end tripping for fault beyond the local terminal. This signal is not transmitted (off) during normal conditions and is turned on for a fault in the reverse direction. This is the “blocking” characteristic of the system. It utilizes the on-off type or the amplitude modulation channel [6].

Figure 7. Directional-Comparison Blocking (DCB) Scheme Another system is the permissive system, shown on Figure 8, which requires a signal from the remote end to give permission to trip for a fault. This system provides a continuous block or guard signal and shifts to a trip frequency to provide the permission. This is the frequency shift keyed (FSK) channel.

Figure 8. Permissive Overreaching Transfer Trip (POTT) Scheme There are two types of permissive systems: POTT and PUTT. POTT is more commonly used. A variation of the POTT scheme is the unblocking scheme. The POTT scheme must transmit a signal through the fault and the unblocking scheme considers the possibility that both frequencies might not make it through the fault so it includes a short time window where permission is assumed if neither frequency is received. Alternatively, an independent communication channel such as microwave can be used for the POTT signals to avoid transmitting through the fault. A system cannot be 100% dependable and 100% secure. Increasing dependability will decrease security. DCB schemes are dependable since they do not need a signal to perform a trip. They will trip for internal faults even if the communication is lost but they are less secure because they will trip for external faults under loss-of-channel channel conditions. Permissive systems are secure since they will not trip for external faults in case communication is lost but they are less dependable because they will not trip for internal faults either. When using digital channels, the speed of transmitting tripping signals is limited by the security requirements. The maximum permissible transmission time depends on the application. Direct transfer tripping schemes, however, must have the highest possible security and dependability, resulting in reduced demands on channel speed [7] . The reliability of a protection command sent over a digital channel is related to bit errors. If the digital teleprotection equipment is connected to a communication system free from bit errors, (bits in = bits out) security and dependability are the highest that can be achieved. Bit errors occur when sent data bits are not equal to received bits (bits in bits out).

IEC 60834-1 standard specifies the worst-case requirements for security and dependability for systems with tripping commands, transferred over digital communications [8]:

- probability of an unwanted command, PUC, a measure of security, shall be less than 10-8

- probability of missing command, PMC, a measure of dependability, shall be less than 10-2

Bit errors will result in the protection equipment detecting incorrect information. The way that this affects protection depends greatly on the equipment design, communication media and communication protocol used, but in most cases, the equipment will ignore erroneous bits. If the bit errors become excessive the likelihood of false operation increases. In this case the communication channel and all functions that use this channel should be blocked. As mentioned above protection and communication are measured using the same parameters. These are dependability (probability of missing command), security (probability of unwanted command), and speed (latency or transmission time). As in the case of current differential protection, the BER of communication will affect the dependability and security of the overall protection scheme. The higher the BER, the lower the transmission time and the higher latency will be. Therefore, proper knowledge of the expected behavior of the communication channel, specially under disturbance conditions will be needed in order to properly set the parameters of the selected teleprotection scheme. 3.-Bus Blocking Scheme Dedicated bus differential protection is normally not considered in distribution substations because it requires a dedicated set CT’s on each feeder connected to the bus. One of the approaches used to achieve bus protection in distribution is the blocking-based bus protection scheme driven by binary I/Os. This is shown on Figure 9 and described in more details in [9]. All bus blocking schemes require communication even if the communication is over copper wires. Traditionally, bus blocking schemes are widely used with copper wires on low voltage systems. In scheme shown on Figure 9, the 50 element of the relay A must be blocked when any Feeder relay senses a fault. This mean that the pickup signal of the feeder relays must be “communicated” to the Incomer relay to block 50 element. This scheme is implemented with hardwire connections between relays. However taking advantage of communications, it can be converted to GOOSE-based bus blocking schemes.

Figure 9. Zone- interlocking scheme The principle of operation in this scheme is the same of the binary I/O driven system but introduces some advantages. In the hardware-based bus blocking scheme each wire connection is a potential failure point that is not supervised. Using GOOSE, the communication path is constantly monitored with a heartbeat. This has a direct impact on the reliability of scheme. Operation time of the GOOSE-based blocking scheme is expected to be faster than that with a hardwire-based blocking scheme. This is achieved because the delay related to the binary outputs in the feeder relays and the filtering time in the relay A can be omitted. Binary GOOSE message transfer times can be as little as 4 ms or about one quarter of a cycle. Per IEC 61850-5 standard [10], Class P1 (distribution) point-to-point communication time for high-speed applications is 10 ms. Class P2/3 (transmission) requirement is <4 ms point-to-point communication time (GOOSE Trip 1A within a substation delay is 3ms, between substations is 8-10 ms). Commissioning time is also reduced since it is no longer necessary to test each wire one by one. Testing can be done using software tools that allow quick diagnosis of the complete application. Although the GOOSE-based zone interlocking scheme offers significant benefits, protection engineers are concerned because GOOSE messages use Ethernet technology that is non-deterministic and “best effort” in nature. Although GOOSE messages are mapped directly into Ethernet frames, Ethernet enhancements and GOOSE transfer mechanisms support data delivery in a timely manner. . For example, introduction of full-duplex Ethernet mode illuminated Ethernet collision issues. Special Ethernet buffering for time-critical applications, data prioritization and Virtual Local Area Networks (VLANs) enhanced data delivery capabilities over Ethernet. Priorities, in

particular, can be used to guarantee that high priority data (e.g. GOOSE messages) get delivered under strenuous conditions, at the cost of dropping low priority data. Note that GOOSE messages use high priority (default priority is 4) and can use VLANs also (by default VLANs are not used). The use of VLAN and Priorities is specified in [30]. Although the standard allows 8 priority levels, i.e. 8 priority queues, most Ethernet switches have only 2 priority queues, some have 4 queues, and very few devices have 8 queues. The number of priority queues corresponds to number of priorities supported, the higher the priority the faster the processing and the more reliable data delivery. For 2-queue devices, frames with priority levels 0-3 will use low priority queue, and frames with priority levels 4-7 will use high priority queue. While frames with priorities 0-3 can be dropped, frames with priorities 4-7 will still be delivered, even with heavy traffic. This is why the default priority for GOOSE messages is 4 to allow devices with 2 priority queues to process these frames faster and more reliably. Direct mapping into Layer 2 Ethernet is also a more efficient way of carrying data, as it does not require processing and carrying in the data stream higher layer protocol headers (overheads), e.g. for Internet Protocol (IP), Transport Control Protocol (TCP) or User Datagram Protocol (UDP). To deal with “best effort” nature of Ethernet and improve reliability of data delivery GOOSE messages are send periodically (heartbeat) and upon a failure or abnormal system condition. GOOSE messages are sent to all configured peer devices (multicasted), with randomized retransmission rates. This reduces probability of not receiving GOOSE data, while increasing network traffic and placing a burden on Ethernet network devices. GOOSE messages are normally small in size, and it is even harder for Ethernet switches to process a lot of short messages at the same time (GOOSE storm). GOOSE repetition rates, and message size should be carefully selected to avoid network Denial-of-Service (DoS) condition, that could block all other (including critical) communications and applications [11], [12]. The reliability of the bus blocking scheme can be greatly improved by the use communication-assisted schemes due to the continuous supervision of the communication path and data integrity of the messages.

Reduction in operation time is expected. The reliability of the bus blocking scheme can be greatly improved by the use communication-assisted schemes due to the continuous supervision of the communication path and data integrity of the messages. Reduction in operation time is expected. Flexibility of the scheme is also improved since communication-assisted schemes are easily extendable. Network load is an aspect to be considered, but the concerns can be minimized by using traffic separation, dedicated bandwidth, VLANs, priorities and message filtering techniques. The probability of not receiving GOOSE data is minimized by the use of priorities, multicasting and data retransmissions.

4.-Synchrophasor-based protection and control

Synchrophasors are phasors (magnitude and angle) that are synchronized to Universal Time Coordinated (UTC). Definition and notation of synchrophasors are defined in IEEE C37.118.1-2011 [13] and illustrated on Figure 10.

Figure 10. Definition of synchrophasor Synchrophasor data is typically collected over wider area, thus communications is required for applications using synchrophasor measurements. IEEE C37.118.2-2011 standard defines synchrophasor data transfer for power systems [14] while IEC 61850-90-5 Technical Report defines how to use IEC 61850 to transmit synchrophasor information [15].

Although most common applications of synchrophasor measurements today remain to be monitoring and data collection for post event analysis, synchrophasor measurements are also being used for protective relaying. IEEE PSRC Working Group C14 report summarizes the current status of these developments [16]. An example such scheme is shown Figure 11 and described in [17].

Figure 11. Angle differential protections scheme using

The rational for using synchrophasor measurements for protecting relaying lies in relating accurate frequency measurements with the load and angle measurements with reactive power. Synchrophasor data could also be integrated with SCADA systems, for example in case of phasor-enchanced State Estimator (SE) [18]. It was shown that with PMU coverage of 30% of buses and lines, the error of state estimation can be reduced by 90%. Other applications include Remedial Action Schemes (RAS) and System Integrity Protection Schemes (SIPS). Further developments of synchrophasor-based applications have been encouraged and supported in USA by the Department of Energy. Synchrophasor data collection network is shown on Figure 12. Details of this and other possible configurations are discussed in details in [19]. If synchrophasor measurements are to be used for control, and also protection functions, data quality and data latency become critical factors. Communications used for these schemes must meet reliability requirements for protection and control applications.

Figure 12. Example of synchrophasor data collection network Latency effect has been covered in [16] as well as IEEE Draft Guides [20], [21]. Determinism of one-way latency is of particular importance. Note that typical telecommunication systems normally specify statistical latency values, while for control and protection, guaranteed worst-case values are required. Common telecommunication technologies, such as SONET, Wave Division Multiplexing (WDM), etc with dedicated bandwidth can be used to transfer synchrophasor data, as long as they are fine-tuned for utility-grade operation, in particular low and deterministic delay, fast recovery upon path switch over, etc shall be provided. Communication latency also effects data quality. An interesting analysis on how latency affects SCADA data is given in [22]. Studies on effects of communication protocols on data quality and availability have been recently conducted. While TCP has been commonly used to transfer synchrophasor data, the use of UDP was also suggested, it is also the only protocol used in IEC 61850-90-5 [15]. UDP benefits include multicast data transitions (when the same data can be sent to multiple recipients), the main disadvantage is that UDP is an unreliable protocol, i.e. there is no confirmation on data reception, as in TCP. Thus, data sent over UDP can easily be lost. The use of UDP triggered numerous industry discussions and presentations questioning if UDP in fact can be used for reliable data transfers. Tests were conducted to verify that UDP can be used without data losses. During testing it was found that configuration of buffers in end machines (e.g. Windows workstations) affects data loss greatly, and in

certain configurations, data rates and packet sizes, data loss can be prevented. These results were presented during NASPI meeting [23], and are described in more details in [24]. Even with the use of UDP for data, it could still be wise to use TCP for commands (IEEE C37.118 Configuration Frames that control start and stop of data transmissions, etc), to make sure that at least start / stop data transmissions commands are delivered reliably. UDP can still be used for synchrophasor data transmissions. As discussed above, protection and control schemes with use of synchrophasors are highly affected by determinism of communication latency. Communication protocol (UDP vs. TCP) also greatly affects data quality and availability, and should be considered.

5. – Distributed Generation related protection

Distributed generation (DG) interconnection brings significant technical challenges as existing power systems were designed as radial, for unidirectional power flow, without flexible and reliable communications in place. Many of DG interconnection schemes could be improved and made faster using communications. Extensive power simulation studies have shown that introducing distributed generation (DG) can cause various protection issues like false tripping of feeders, protection blind spots, changed fault levels, undesired islanding, automatic reclosing blocking or unsynchronized reclosing, obviously compromising security and dependability of electrical system. Consider a few examples of DG interconnection protection and feeder protection schemes, and in particular, how characteristics of communications affect these protection schemes. The presence and quality of communications is of particular importance for these schemes, as timely information of DG connection / disconnection, provided by communications, is the key parameter that defines expected system behavior, settings, switching times, etc. Interconnection protection Figure 13 shows an example of a typical system with a co-generator with its own local loads, connected to a utility system.

Figure 13. Common setup for a system with DG

Common protection elements used for DG interconnection protection include [25]: • Directional power (32) for anomalous power flow • Synchro-check (25) for restoration and reclosing • Over/Under voltage (59/27) Over/Under frequency (81O/U) for detection of loss

of parallel operation with the grid • Directional elements (67/51P, 67/50P, 67/51N, 67/50N, 59N for zero sequence

voltage) for fault detection and backfeed protection. IEEE 1547 standard defines the technical requirements for interconnecting distributed resources with electric power systems, but utilities develop and mandate their own criteria and procedures [26]. An on-going work on interconnecting generation with transmission system, undertaken by IEEE PSRC Working Group C18 should be noted as well [27]. As examples of interconnection protection schemes, reliance on communications of load shedding and automatic transfer switch could be considered. Load shedding strives to maintain generation – load balance under all conditions, and require information about current system conditions to determine whether load should be shed. Frequency and rate-of-change-of-frequency could be used for this. This scheme also requires communication to perform control actions – shed or restore the load. Obviously, when the required load is not shed quickly, serious damage to key power system elements (generators, transformers, transmission / generation lines) can occur. Failure to timely shed required load led to huge currents of 8KA imposed on transmission lines during the southwestern U.S. event on September 8, 2011 [28].

An Automatic Transfer Switch (ATS) scheme is appropriate when the DG site connects to the utility over multiple (usually 2) feeders. Upon a failure of one path, ATS establishes a connection to the alternative feeder. However, without communications, the DG gets re-connected manually, during the next personnel visit to the site, which can take hours, days of even month(s), depending on site location, work load, priorities, etc. Use of communications allows fast supervised re-connection, while saving time and money on site visits.

Feeder Protection

Another level of protection to be considered with the introduction of DG is feeder protection. A system example is shown on Figure 14.

Figure 14. Feeder protection diagram, with DG connected The following protection schemes for feeder protection for systems with DG and co-generations could be considered: - Dynamic settings - Directional protection - In-rush detection - Auto-reclosing Because system parameters, such as fault current, system and feeder impedances differ depending on whether the DG is connected or not, different settings should be used for these cases. Furthermore, dynamic information on DG connection status is required. This information is available by means of communications. DG addition affects coordination as well, and can lead to false tripping, if not configured properly. For example for a fault on an adjacent feeder, the healthy feeder with a DG

connection will deliver a current in the direction toward the fault. Unless a directional element is used, the healthy feeder would trip, compromising system’s security. During energization of the DG interconnection transformer, a large inrush current will be present. This current can lead to false tripping, unless instantaneous blocking is activated using communications. A second harmonic detection function could be used to block tripping as well. Auto-reclosing sequence timing differs significantly depending on whether the DG is connected or not. Communication-based supervision can be used to control the reclosing function, as described in [25]. Communication is considered to be one of the main challenges that utilities face when introducing the DG. Existing networks and infrastructures were not designed to support bi-directional power flow, non-radial feeds, etc. Network management has not been provided for coordinating actions in the various parts of the grid. The most difficult part of this could be maintaining a reliable and secure communications between all devices. For the cases described above, IEC 61850 GOOSE messages can be used. Communication media and technology to carry GOOSE messages should be carefully selected and analyzed. Field experiences with WiMax wireless communication media to carry GOOSE messages for DG control shown challenges in achieving required performance (high message drop rate) and led to realizations that buffer size for some WiMax devices may not be sufficient for supporting defined GOOSE message size and repetition rates. A particular profile B of the WiMax standard [29] was selected, and the required performance was achieved. This is an example of how communication media profile and buffer capabilities of communication devices directly affect security and dependability of protection system.

IV Conclusions

This paper presents the analysis of dependencies of protection schemes on the quality of the underlying communications. Line current differential, distance protection, bus locking, synchrophasor-based protection and DG-related protection use cases were discussed. Effect of various communications parameters on protection system security and dependability was explained. These parameters include bit error rate, latency (determinism of one-way latency), accuracy of time synchronization (time synchronization error), probability of wanted and unwanted commands, communication protocols (TCP vs UDP), communication media profile, etc. Each of these use cases deserves a separate more detailed discussion and will be subject to future work. The goal of this paper is to introduce audience to this broad and complex subject and prepare them for more detailed discussions on the topic.

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