THE ROLE OF RTUs IN SUBSTATION

RTUs IN SUBSTATION

The electric utility has been divided into several departments to reliably and securely provide energy to their customers. These departments typically encompass a separate Supervisory, Control and Data Acquisition (SCADA) branch and a Protection & Control (P&C) branch, each with its own focus. SCADA gives a manned control center with real-time records that allows you to reveal and function their system, including strength management, outage restoration, safety, and reliability/availability. P&C affords automated protection of number one equipment. The SCADA and P&C departments every have their person obligations with clean lines of functionality and separate hardware.

Scada master

The number one gadget is one place that the SCADA and P&C hardware have connections in common. However, the acceptance of IEDs and specially microprocessor based totally relays for protection, metering and control functions has created a want to study how to reliably and economically provide strength to end users by means of optimizing the capability of the SCADA RTUs (Remote Terminal Unit) and microprocessor primarily based protective relays or IEDs inside the substation. Typical concerns of a SCADA technician are: who (SCADA or P&C staff) is liable for configuring the facts maps in the microprocessor based totally IEDs?, and when new defensive relay settings are downloaded to the microprocessor primarily based IEDs.

Analog Inputs

A conventional hardwired SCADA structure includes discrete transducers mounted for the duration of the substation measuring currents from contemporary transformers (CTs), voltages from ability transformers (PTs) and station batteries, and power (MW and MVARs) from the combination of those transducers. These DCmA indicators are then wired to analog enter terminations which might be linked to the substation RTUs (see Figure 2). The drawback of this structure is the quantity and fee of wire connections, the value and reliability of discrete transducer hardware and the necessary transducer calibration to guarantee dimension accuracy.

In a substation that fully makes use of microprocessor based relays and meters as IEDs, their metered statistics may be used to offer this identical functionality and communicate it digitally to the RTUs. IEDs include serial (RS232 and/or RS485) and/or Ethernet ports with various verbal exchange protocols, consisting of DNP3 (Distributed Network Protocol), Modbus RTUs, ASCII (American Standard Code for Information Interchange) and IEC61850 for example. In addition, those new devices generally have configurable statistics maps with settable scaling elements and offer a couple of simultaneous connections to the gadgets.

Transmitting the analog alerts from the substation number one gadget is best accomplished by way of direct substation SCADA RTUs communications to the IEDs through serial or Ethernet communications as proven in Figure 3. This might cast off the discrete transducer hardware and get rid of related installation (much less copper wire), working and renovation costs, and enhance reliability by way of reducing the amount of hardware to manage. When communicating the analog indicators thru a protocol, the scaling of the signals might be configured in the IED. One may additionally consider conversation between the substation SCADA RTUs and IED using the IED’s local protocol, together with Modbus RTUs or ASCII, which would not require change of the prevailing microprocessor based relay configuration for example.

Digital Inputs

A traditional hardwired SCADA architecture consists of discrete reputation indicators from number one system positioned at some point of the substation. These repute signals are then wired to digital repute terminations on the substation RTUs (see Figure 2). This method is often preferred by using substation engineers in that each reputation point has a clear indication light of a given factor’s repute. The drawback of this architecture is costly and congested wiring, regularly to redundant gadgets. Most IEDs have configurable binary enter maps and offer multiple simultaneous conversation connections. Transmitting the digital reputation alerts from the substation number one gadget is excellent achieved through direct substation RTUs virtual communications to IEDs, which includes microprocessor based totally shielding relays, as shown in Figure 3. This could take away duplicated wiring and related operating and preservation costs. As stated in the analog input consideration section, one may also consider conversation among the substation SCADA RTUs and IED the use of the IED’s local protocol, consisting of Modbus RTUs or ASCII, which would not require amendment of present microprocessor based relay configuration for example.

Control Considerations

A traditional hardwired SCADA structure includes discrete control relays related to primary gadget located at some stage in the substation. These manage relays are wired from RTU terminations with non permanent or latching manage relays (see Figure 2). The drawback of this architecture is the amount of wires and connections and discrete control relay hardware. The advantages of this conventional hardwired SCADA structure include the potential to provide a vital manipulate inside the occasion of a relay or IED failure and the safety of being capable of provide a visible air hole on substation controls whilst working beneath a clearance. Microprocessor based relays have configurable output controls with short-term or latching states and offer a couple of simultaneous communications connections.

The manage of substation primary equipment can satisfactory be executed with the aid of:

• Direct substation SCADA RTU communications to IEDs such as microprocessor based totally shielding relays as shown in Figure 3. This method could function the discrete output contacts of the protecting relay IEDs and enable/disable internal shielding features like reclosing, ground overcurrent trip, and/or synchrocheck. As stated inside the analog enter consideration section, one may additionally consider communique between the substation SCADA RTU and defensive relay IED the usage of the IED’s native protocol, which would no longer require modification of existing microprocessor based relay configuration.

• Using manage termination boards with control relays which can be linked to breaker ride and close circuits. This technique should use a special journey or close coil at the breaker or transfer than the microprocessor based shielding relays. The manage termination boards would be related to the substation SCADA RTU.

For distribution substations that best have one microprocessor based totally protective relay on a breaker, a aggregate of Method A and Method B is might be used. In this configuration, the substation SCADA RTU could be configured to function both Method A and Method B for any particular manipulate factor. This might provide operators the approach to remotely manage the substation in the case that the microprocessor based totally relay has a damaged output touch, or hassle circumstance on microprocessor relay, communications failure or manage termination board trouble. Redundant I/O could be finished using a mixture termination board with analogs, statuses and controls. As discussed earlier, redundant manage may be configured within the RTU to manipulate and offer manipulate redundancy between the redundant hardware used.

Communication Considerations

As mentioned previously, microprocessor based totally relays encompass RS232, RS485 and Ethernet ports with various communique protocols, which include DNP3, Modbus RTU, ASCII and IEC61850. It is recommended to connect the gadgets to the substation SCADA RTU by using direct serial or Ethernet communications.

The SCADA RTU serves as an essential part of the SCADA machine. On small systems it could be viable to ballot the IED directly from the SCADA Master. As the gadget will become larger, however, this creates a configuration mission at each substation in that every tool on a gadget must have a separate cope with (proscribing the quantity of devices) and that SCADA masters are frequently licensed based totally on the quantity of remotes. This assignment is maximum easily remedied through allowing the SCADA RTU to function a statistics concentrator. In this way, the SCADA RTU parses the records to be served up to the SCADA grasp and only makes use of one remote cope with for many devices.

Further, there may be a communications bandwidth hindrance to the substation. The more devices that the grasp has to ballot at one region may impact communications to that web page and/or different locations, supplying slower response times (longer waits among polls), dropped polls, or the want for larger bandwidth with more cost. It should additionally be mentioned that most grasp stations simplest talk the usage of one protocol. The use of two protocols could double the bandwidth requirement. A problem if the utility community is carried over public network that local network access is not used for Internet downloads so as to affect bandwidth. Use of a SCADA RTU inside the substation also allows for protocol conversion to house this requirement regardless of the protocol used by the substation IED and permitting legacy devices to be used. Modern master stations provide IP (Internet Protocol) communications to its slave devices. IP allows multiple devices to speak on a unmarried network at extra speeds and without losing messages. In fact, many exclusive protocols can be sent over the same Ethernet connection. IP sincerely puts all communications into packets (like envelopes being mailed to a specific address). It doesn’t count what these packets have in them so long as the packets maintain the frequent format. These packets are reassembled at the vacation spot and provided simply as they were obtained at the origin. On a serial connection, only one protocol may be used at one speed. The downside of the use of IP is that its use imposes many more security concerns and several NERC (North American Electric Reliability Corporation) CIP (Critical Infrastructure Protection) necessities to insure protection. Firewalls, encryption, protection servers, Virtual Private Networks (VPN), logs, and dynamically changing passwords are possible approaches of making sure security, however come with greater system and software to keep.

Protocol Conversion, Data Concentration, HMI Data Consideration

A key function of the substation SCADA RTU over the years has been protocol conversion among substation gadget and the SCADA grasp. Different communications protocols have existed for the SCADA grasp, together with DNP3, Landis+Gyr 8979, Harris 5000/6000, Tejas V, CDC, etc. Such that the SCADA RTU plays the protocol conversion between the substation IEDs and the SCADA master. This requirement is still crucial to electric utilities because there may be a major cost in upgrading the SCADA grasp to aid new protocols together with DNP3 or Ethernet communications (i.E. DNP3 / IP). The substation SCADA RTU also functions as a records concentrator, polling connected termination forums and IEDs for records, responding to information requests from the SCADA master or masters, and performing manage requests from the SCADA grasp.

The substation SCADA RTU ought to provide information without delay to a substation HMI. The substation HMI would communicate DNP3 / IP or IEC61850 to the substation SCADA RTU and poll and manage the exact identical points because the SCADA master. Thus, the substation HMI could act as a back-up SCADA master. In general, it is not encouraged to have two devices running the identical controls due to protection worries. Control have to require authorization from the manage center and be befell to the HMI in some manner, consisting of a SCADA on/off 43 manage transfer for example. This is one of the motives to use SBO. Once a touch is selected, handiest the selecting device can trouble commands.

With the HMI having connection to the microprocessor based protection system thru Ethernet commutations as shown in Figure 4 permits redundancy to provide local manipulate of substation shielding relays inside the event of SCADA RTU being off-line due to protection or failure. A disadvantage is the complexity of the control and status configuration in the HMI. In addition, the HMI might be securely accessed remotely to retrieve fault records, event information, waveforms and settings from substation microprocessor based defensive relays.

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