Wind Park requirements for Detnet
draft-vilajosana-detnet-windfarm-usecase-00

Abstract

This document analyses the use case requirements for deterministic flows in a wind park network. It inlcudes the intra-domain and inter-domain requirements.

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. Use Case description
• 4. Traffic Demand
• 5. Intra-Domain network considerations
• 6. Inter-Domain network considerations
• 7. Security Considerations
• 8. Wind Park Networks Future
• 9. Wind Park Network Wish List
• 10. Acknowledgements
• 11. References
• 11.1. Normative References
• 11.2. External Informative References
• Authors' Addresses

1. Introduction

The wind power industry has been selected as a representative example of industrial networks with strict performance, security, and reliability requirements. A Wind Park network is part of a Supervisory Control and Data Acquisition (SCADA) system that regulates power production from each wind turbine and from the entire park. The SCADA system extends beyond the Wind Park, over the Internet, to a remote control centre. Moreover, this network interconnects sensors and actuators and a hierarchy of purpose-built controllers and repositories via domain-specific protocols (e.g., IEC 1041, MODBUS2) in a static and secure topology. In this document the Intra domain requirements, referring to the network consiserations in terms of latency, jitter, delay tolerance, within the same administrative domain will be presented. Analogously, and as Wind Parks are connected to remote Control Centers the requirements and considerations for the Inter domain reliability, jitter, latency and delay tolerance will be outlined.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

3. Use Case description

In a Wind Park, the wind turbines represent a complex system of sensors, actuators and an internal controller, located offshore or in remote locations that communicate with a central control or master SCADA station over reliable communication network. Wind turbines are grouped in radials to maximize the energy production. The size of the wind park varies significantly; having the biggest offshore wind parks up to 200 wind turbines. Depending on the size of the park, there might be an additional substation located close to turbines to facilitate power transportation to the utility grid with minimum losses [Sie13], [Kri03]. On another side, local control center combines several functionalities necessary for control and management of the wind park. SCADA comprise power plant control function to synchronize and coordinate operation of the wind turbines in the park, network management system (NMS) for network configuration, performance and fault monitoring and different servers to collect and store the metering data and status information from sensors, as well as gateway to the other control centers and Internet [Spe09], [Pet11]. The communication system between field devices and SCADA has to be reliable to facilitate control and management of the wind park. Wind power plant control and monitoring system have stringent latency requirements, and reconfiguration of the network in the case of a failure has to be done quickly. The most common way to improve reliability is to connect wind turbines in a ring in order to provide resistance to single link or node failure. Additionally, backup microwave links can be built to improve the overall system availability.

Figure 1: Wind turbine control network

     |
     |
     |  +-----------------+
     |  |   +----+        | 
     |  |   |WTRM| WGEN   |
WROT x==|===|    |        |
     |  |   +----+    WCNV|
     |  |WNAC             |
     |  +---+---WYAW---+--+
     |      |          | 
     |      |          |        +----+
            |WTRF      |        |WMET|
            |          |        |    |
     Wind Turbine      |        +--+-+
     Controller        |           |
       WTUR |          |           |          
       WREP |          |           |
       WSLG |          |           |
       WALG |     WTOW |           |

4. Traffic Demand

Figure 1 presents the subsystems that operate a wind turbine. This subsystems include the rotor (WROT) control, the nacelle control (WNAC), the transmission control (WTRM), the generator (WGEN), the yaw controller of the tower head (WYAW), the in-turbine power converter (WCNV), the in-tower power transformer (WTRF) and an external meteorological station providing real time information to the controllers of the tower (WMET). Traffic characteristics relevant for the network planning and dimensioning process in a wind turbine scenario are listed below. The values in this section are based mainly on the relevant references [Ahm14] [Spe09]. Each logical node (Figure 1) is a part of the metering network and produces analogue measurements and status information that must comply with different specifications in terms of data rate.

Table 2: Wind turbine data rate constraints

+----------+----------+----------+------------+----------+-------------+
|Subsystem |# Sensors | # Analog | Data  rate | # Status | Data  rate  |
|          |          |  Samples |(bytes/sec) | Samples  | (bytes/sec) |
+----------+----------+----------+------------+----------+-------------+
|   WROT   |    14    |     9    |     642    |     5    |     10      |
|   WTRM   |    18    |    10    |    2828    |     8    |     16      |
|   WGEN   |    14    |    12    |   73764    |     2    |      4      |
|   WCNV   |    14    |    12    |   74060    |     2    |      4      |
|   WTRF   |    12    |     5    |   73740    |     2    |      4      |
|   WNAC   |    12    |     9    |     112    |     3    |      6      |
|   WYAW   |     7    |     8    |     220    |     4    |      8      |
|   WTOW   |     4    |     1    |       8    |     3    |      6      |
|   WMET   |     7    |     7    |     228    |     -    |      -      |
+----------+----------+----------+------------+----------+-------------+
|   Total  |   102    |    73    |  225544    |    29    |     58      |
+----------+----------+----------+------------+----------+-------------+

Quality of Service (QoS) requirements of different services are presented in the Table 2. The requirements are defined by IEEE 1646 standard [IEEE1646] and IEC 61400 standard [IEC61400].

Table 3: Wind turbine Reliability and Latency constraints

+-----------+----------+-------------+-----------------+
|  Service  |  Latency | Reliability |Packet Loss Rate |
+-----------+----------+-------------+-----------------+
| Analogue  |          |             |                 |
| measure   |   16 ms  |    99.99%   |   < 10-6     |
+-----------+----------+-------------+-----------------+
|  Status   |          |             |                 |
|information|   16 ms  |    99.99%   |   < 10-6     |
+-----------+----------+-------------+-----------------+
|Protection |          |             |                 |
|  traffic  |    4 ms  |   100.00%   |   < 10-9     |
+-----------+----------+-------------+-----------------+
| Reporting |          |             |                 |
|and logging|    1 s   |    99.99%   |   < 10-6     |
+-----------+----------+-------------+-----------------+
|  Video    |          |             |   no specific   |
| surveill. |    1 s   |    99.00%   |   requirement   |
+-----------+----------+-------------+-----------------+
| Internet  |          |             |   no specific   |
|connection |   60 min |    99.00%   |   requirement   |
+-----------+----------+-------------+-----------------+
| Control   |          |             |                 |
| traffic   |   16 ms  |   100.00%   |   < 10-9     |
+-----------+----------+-------------+-----------------+
| Data      |          |             |                 |
| polling   |   16 ms  |    99.99%   |   < 10-6     |
+-----------+----------+-------------+-----------------+

5. Intra-Domain network considerations

A Wind turbine is composed of a large set of subsystems (sensors, actuators) that require time critical operation. The time-criticallity of different actions is shwon in Table 3. These subsystems are connected to an intra-domain network used to monitor and control the operation of the turbine and connect it to the SCADA subsystems. The different components are inter-connected using fiber optics, industrial buses, industrial ethernet, EtherCat or a combination of them. Industrial signaling and control protocols such as Modbus, Profibus, Profinet and EtherCat are used directly on top of the L2 or encapsulated over TCP/IP.

The Data collected from sensors or condition monitoring systems is multiplexed into fiber cables for transmission to the base of the tower and to remote control centers. The turbine controller continuously monitors the condition of the wind turbine and collects statistics on its operation. As the name implies, the controller also manages a large number of switches, hydraulic pumps, valves, and motors within the wind turbine.

There is usually a controller both at the bottom of the tower and in the nacelle. The communication between these two controllers usually takes place using fiber optics instead of copper links. Sometimes, a third controller is installed in the hub of the rotor and manages the pitch of the blades. That unit usually communicates with the nacelle unit using serial communications.

6. Inter-Domain network considerations

As mentioned in the introduction, a remote control center that belongs to a grid operator, regulates the power output, enables remote actuation and monitors the health of one or more Wind Parks in tandem. It connects to the local control center in a Wind Park over the Internet (Figure 2), via firewalls at both ends. The AS path between the local control center and the Wind Park typically involves several ISPs at different tiers. For example, a remote control center in Denmark can regulate a Wind Park in Greece over the normal, public AS path between the two locations.

The remote control center is part of the SCADA system, setting the desired power output to the Wind Park and reading off the result once the new power output level has been set. Traffic between the remote control center and the Wind Park typically consists of protocols like IEC 104 [IEC104], OPC XML-DA [OPCXML], Modbus [MODBUS], and SNMP [RFC3411]. Usually, QoS requirements are not strict, so no SLAs or service provisioning mechanisms (e.g., VPN) are employed. Traffic flow across the domains is best effort. In case of events like equipment failure, tolerance for alarm delay is in the order of minutes, due to redundant systems already in place.

+--------------+
|              |
|              |
| Wind Park #1 +----+
|              |    |      XXXXXX
|              |    |      X    XXXXXXXX               +----------------+
+--------------+    |   XXXX    X      XXXXX           |                |
                    +---+                  XXX         | Remote Control |
                        XXX    Internet      +---------+     Center     |
                    +----+X                XXX         |                |
+--------------+    |    XXXXXXX            XX         |                |
|              |    |          XX     XXXXXXX          +----------------+
|              |    |            XXXXX
| Wind Park #2 +----+
|              |
|              |
+--------------+

7. Security Considerations

On top of the classical requirements for protection of control signaling, it must be noted that Wind Farm networks operate on critical infrastructures with heterogeneous devices and networks. This includes heterogeneous L2 technologies that must be secured in a per link model. Control and signaling occur at the transport layer and therefore end to end security mechanism must be installed.

8. Wind Park Networks Future

In the future we expect cloud-based SCADA systems controlling, storing and monitoring the critical and non-critical subsystems of the windfarm. We foresee an increase in the number of sensing devices and technologies, combining wireless and wired capillars. We foresee heterogeneous L2 technologies, homogenized by common IPv6 frameworks such as those developed by 6TiSCH, 6lo, LPWAN and 6MAN. We expect windfarm networks to be operated by standardized and common management planes, enabling the orchestration of the different building blocks and underlaying technologies and being able to Internet-connect enabling service gurantees and remote operation with quality of service. Therefore protocols for network management, flow control, increased reliability and security are mandatory in order to improve the operation of critical infrastructures, including in this case Wind Farms.

9. Wind Park Network Wish List

The community would like to see a set of protocols that enable the inter-domain and the intra-domain operation of a wind park infrastructure satisfying the timing, security, availability and QoS constraints described above, such that the resulting converged network can replace the heterogeneous, sometimes propieatary field networks. Ideally this connectivity should extend to the open Internet. This would imply an architecture that can guarantee

• Low communication delays from <4 ms to 1000ms in the inter-domain network
• Low communication delays from <150 ms to 5000 ms in the intra-domain network
• Low jitter (< 1 ms)
• Tight feedback intervals (4ms - 10ms) in the intra-domain
• High network availability (up to 99.9999% )

10. Acknowledgements

This requirements have been extracted from the study of Wind Farms conducted within the 5GPPP Virtuwind Project. The project is funded by tge European Union's Horizon 2020 research and innovation programme under grant agreement No 671648 (VirtuWind).

11. References

11.1. Normative References

11.2. External Informative References

Authors' Addresses