Deploying Multiple DA Applications

Author: Tony Burge, RuggedCom, USA

Multi-Application Requirements

Networks are often driven by one application, but once in place these networks are often required to support many others. In the case of utilities, distribution automation applications provide the most immediate efficiencies; however, other applications, such as video surveillance/monitoring, voice services, and field force automation are desired to provide additional operational efficiencies and grid robustness (Figure 1.)

Throughput: This article is not written with the intention of providing full capacity modelling, rather, it provides general guidelines of the most critical requirements, of which throughput is one. Surveillance-quality video throughput requires approximately 2 mb/s using MPEG-4 compression. Lower resolution, lower frame rate video monitoring can be accomplished using as little as 256 kb/s, which still exceeds many wireless communications capacities. Most of this communication is uplink data.
SCADA polling, Volt/VAR control, Fault Detection, Isolation, and Restoration, and Capacitor Bank monitoring applications do not require substantial data - perhaps 10-15 kb/s each. However, when aggregated, the throughput for these mission critical applications can exceed 100 kb/s. Most of the communication for these applications is uplink data.

In certain substation environments, cell phone coverage is poor and a dedicated phone line is impractical or too expensive. Therefore, a Voice over IP (VoIP) line is often required to provide field engineers the ability to communicate with advisers at the back office. A good, toll-quality VoIP connection requires 64 kb/s. This communication equally splits between uplink and downlink. Finally, operational efficiencies can be extended by providing field personnel with wireless connectivity while performing maintenance - for work orders, access to agency Intranet for schematics and manuals, and more. This communication is mostly downlink data traffic and can easily require 500 kb/s. With this analysis, it is easy to infer the following throughput requirements per remote location (i.e., substation) and based on two video surveillance requirements: See Table 1.

Range: The range of the radio frequency (RF) technology and interference mitigation features implemented contribute directly to coverage. The more coverage provided by a solution, the less amount of infrastructure and capital expenditure is required. Range is determined by occupied channel bandwidth, power allocation, deployed frequency, and features such as Orthogonal Frequency Division Multiplexing (OFDM). There is often a trade off between throughput and coverage, so it is important to find the balance that provides the throughput required for multiple applications while maximizing the range (Figure 2.)

 

Latency: Latency, or response time, for power utility applications is measured in milliseconds for point-to-multipoint wireless communications. Mission-critical distribution automation applications often operate effectively with wireless technologies that support less than 100 millisecond roundtrip response time. Even the most efficient, transparent transceivers support at best 8 to 10 milliseconds, which is why point-to-multipoint RF communication is not used as a primary communication technology for generation and transmission protection and control applications that require a quarter of a cycle, or 4 millisecond response time to protect high valued equipment. (However, point-to-point RF communications may be used as a redundant communications technology for fiber in these protection and control applications.) A good target for round trip, point-to-multipoint latencies for distribution automation is under 40 milliseconds. This latency metric provides room for retries without adversely affecting distribution automation applications.
Security: Power utility infrastructure is arguably the most important asset to industrialized states, provinces, and nations. Protecting data that monitors and controls this infrastructure is a primary requirement for wireless networks deployed for such purposes. At a high level, data security can be discussed in three categories: Transmission Security, Network Authentication, and Data Segregation.

Transmission Security involves encrypting data at a transceiver prior to sending the message. The receiving transceiver then decrypts the message. This process protects the contents of the message as it is transmitted over the airwaves
Network Authentication - While transmission security facilitates the integrity of data as it is transmitted over the airwaves, network authentication is intended to prevent unauthorized access to the network itself, which is a vital component of network security. The unauthorized network access can result in denial- of-service, access to sensitive, utility-wide operational data, and manipulation of the data to interfere with the power grid itself
 Data Segregation - Within a physical network, administrators may require virtual networks to segregate data such that access to data can be limited to certain functional groups
Other Security Considerations - Other measures to be considered are intrusion detection features, tamper- evident/tamper-proof fabrication, and facility security

Prioritization: With multiple applications running over a single network, it is important that mission-critical data receives priority. Without such a mechanism, VoIP calls or field force network access could hinder fault detection notifications, out-of-tolerance voltage variation corrections, SCADA polling responses, and more.  IEEE 802.1P, Quality of Service (QoS), was defined to provide multiple levels of prioritized service. QoS provides bandwidth prioritization from the least-restrictive Best Effort to the most restrictive Unsolicited Grant Service. Additionally, QoS assists in defining service flows within each prioritization level to further improve the latency and jitter required for specific applications.
The appropriate point-to-multipoint broadband technology (advocated in this article), which balances throughput and range and provides prioritized service flows, has come under questioning regarding the lowest possible round-trip latency. A properly engineered broadband solution is able to demonstrate round-trip latencies less than 20 ms for applications requiring low response times. Table 2 is representative of round-trip latencies demonstrated in the field based on the latency requirements of the application.

Proprietary and Standards-based Protocol Support: The utility grid requires the support of legacy protocols that may have been deployed (and still in valid operation) decades ago. Key requirements for protocols are physical interfaces (e.g., DB connectors and RJ connectors) and protocol support for active and passive serial support (Modbus, Modbus TCP, DF1, and DNP-3), standards-based Layer 2 messaging (e.g., GOOSE messaging), and full TCP/IP Ethernet communications.

Uplink Biasing: Commercially-focused wireless communications solutions provide more downlink throughput than uplink. The concept of uplink biasing for power utilities is to dedicate more throughput on the uplink (from the substation to the back office.) Without a mechanism to support this uplink biasing, an organization may have 2 mb/s to a substation; however, less than 1 mb/s would be available for uplink communications.
At a minimum, a wireless broadband network for power utilities should support an uplink/downlink duty cycle of 70% uplink and 30% downlink, configurable.

Redundancy: Equipment redundancy with hot- or cold-standby assists in restoring communications when a transceiver fails. This becomes especially important when the locations of these devices are not easily accessible. Equipment provided for a wireless communications network should provide levels of redundancy at the master station/base station/access point and at the remote/subscriber unit to improve network reliability and availability.

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