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Transmission Automated Control - Emergency Control Baseline Function

Contents

• Narrative
• Steps

Narrative

The purpose of the Wide Area Monitoring and Control Systems - Emergency Operations function is to provide communications services permitting an operator to take the following actions in response to a fault in the power system:

• Locate the fault
• Verify that protection has operated correctly to clear the fault
• Shed load to ensure that the fault does not cause an overload of unaffected lines
• Manually re-route power to restore service to subscribers
• Dispatch crews and emergency teams to fix the fault
• Capture fault recordings so engineers can later analyze the cause of the fault
• This function also addresses handling of environmental and security alarms.

A complete narrative of the Function from a Domain Expert’s point of view, describing what occurs when, why, how, and under what conditions. This will be a separate document, but will act as the basis for identifying the Steps in Section 2.

Emergency operations are organizational sequences of activities that involve multiple integrated actors exchanging information when a fault is detected on a power system. These activities are integrated through the use of Wide Area Control and Monitoring Systems (WAMACS) that provide operational control over the distributed network of actors that comprise the SCADA system. Each utility maintains their own WAMACS but in the future these systems must be linked to provide overall control and monitoring across multiple organizations to meet the future demands of the suppliers and users of electrical power.

The remainder of this narrative describes an example scenario illustrating the characteristics and sequence of activities that occur on the power system during Emergency Operations. The example is based on a typical substation with a SCADA system.

Initial State

When a typical substation is operating in the normal state, called initial state for the purposes of this discussion, there are at least two incoming lines connected to two transformers that feed two separate buses that supply the source side of the feeder circuits. In the initial state both lines and transformers would be energized and the main breakers would be closed to allow both buses to be energized. The bus tie breaker between the two buses is closed so both transformers are sharing the load from all four feeders.

The lines, transformers, buses and feeders each are monitored by separate protection relays that can sense abnormalities in the zone of protection that they are responsible for and isolate faults in the zone by opening the appropriate breaker. Adjacent protection relay zones overlap to ensure that there is protection at every point in the system. Incoming line relays are responsible for a zone of protection that extends out of the substation and down the line a certain distance. The other end of the line is usually owned by the power transmitter and there is similar distance protection on that end of the line. If a fault occurs on the line both relays report the distance and direction to the fault. There should be an overlap in the two fault reports and this is the portion of the line that the dispatched maintenance crew will check first for the fault.

In the initial state the SCADA system, shown in Figure 1, would have no outstanding alarms or abnormal readings that would require the operator to take action. The SCADA system, which is made up of these protection IEDs, monitor the critical areas of the substation and report data to the Data Concentrator.  The protection IEDs are most often located within a substation, but may also be located at remote sites or on pole-tops. 

Figure 1:  SCADA System

GUI – Graphical User Interface

L1/L2 – Line Distance relays

T1/T2 – Transformer relays

M1/M2 – Main Breaker relays

B1/B2 – Bus relays

F1/F2/F3/F4 – Feeder relays

The Data Concentrator  is located in the substation and is connected with a communication link to each IED. The substation is often equipped with a local GUI and an alarm logger. The Data Concentrator  is connected through a different communication link to one or more GUIs. GUIs can be local to the substation or in an operations center that monitors several substations.

The operator is located in close proximity to the GUI, which is the operator’s window into the substation. The operations center is usually equipped with a logging and trending system to store analog quantities on a regular interval or on a change to display the quantities on a graph.  There also must be a method of storing and accessing fault records that are used by engineers to analyze the faults. 

Data Concentrator s, GUIs, and the communications links between them and the IEDs are usually redundant, and perform switchover if one link fails.  Reliability is very important.

Fault Occurrence and Detection

There are several places where a fault can occur on this system and several different ways the equipment can be used to isolate different fault conditions. Some very simple fault scenarios are used in the discussion to help describe Emergency Operations. Each zone of protection is equipped with a protection IED, or relay, that monitors the state of the system and the values of the electrical quantities that are relevant to the zone of protection. If a fault occurs the relay is able to cause a breaker and sometimes also a switch to operate to isolate the fault.

If a fault occurs some distance down the incoming transmission line the line distance relay associated with that zone is responsible for detecting the fault, tripping the main breaker and disconnecting the incoming line disconnect switch. This isolates the substation from the fault so that the excessive current associated with the fault does not damage any equipment in the substation. This operation must occur within one cycle of the power waveform and this precise timing requires accurate time synchronization to be applied to the device so that the time the event occurred and the sequence of events can be determined later. The line distance relay also reports the system changes caused by the fault to the Data Concentrator  which in turn passes them to the GUI where the state changes are reported as system alarms to the operator.

A fault occurring in a different zone of protection would be handled in a similar way. For example the transformer relay is responsible for the closed zone of protection around the transformer. If a transformer fault occurs it opens the main breaker and then the disconnect switch.

Fault Record Generated

Often a fault recorder is monitoring the power system quantities at one or more strategic locations around the substation. The protection IEDs or the RTU often have built in fault recorders that can capture data before, during and after a fault. Engineers will use this information to determine the type, magnitude and profile of the fault. 

During normal operation the fault recorder is storing several cycles worth of data continuously. If a fault occurs it triggers the recorder to stop overwriting the pre trigger data and continue recording the actual fault data. In this way the engineer is able to observe the stable condition before the fault, (pre trigger), and exactly what happened at the instant the fault occurred and then the post fault profile which is the system reaction to the fault.

The fault data profile usually has separate channels for each of the three phase voltages and currents and can use other signals as well. The fault data is usually stored in a standard format in the IED or in the fault recorder that captured the data. A signal, in the form of an alarm, is sent to the data concentrator and subsequently to the GUI to notify the operator that a fault file is available to be analyzed.

Change of State

Faults cause the system to change state, usually because one of the protection IEDs has tripped a breaker. The state of the breaker is detected by a change in position in the detection device located in close proximity to the breaker. This change of state is generally detected by the IED and reported to the Data Concentrator  or detected directly by the Data Concentrator  in other situations. The Data Concentrator  determines if it is required to communicate the change of state to the GUI in which case it does. The Data Concentrator ’s function is to translate the protocol that contains the state change between the IED and the GUI.

Often there is a large number of devices in the substation, slow communication links, legacy protocols that do not have timestamps, sorting requirements or the requirement to send the change of state data to more than one GUI. All these are reasons why a Data Concentrator  is required.

Alarm

A fault causes a breaker to trip which causes a change of state in the system which is converted to an Alarm by the GUI to notify the operator of the system change. Usually the alarm shows up as a flashing text message at the bottom of the active screen on the operators GUI and is combined with an audible signal in case the operator is not looking at the screen. The flashing and audible signal stop when the alarm is acknowledged. Characteristics associated with each different alarm are preconfigured such as location, priority and description. 

The trip notification is a digital change of state, accompanied by a timestamp indicating when the event occurred.  The timestamp is important because it may be later used to reconstruct a “sequence of events” indicating when various devices and personnel within the utility took action to address the fault.  For this reason, time between devices within the SCADA system is typically synchronized to within 1 millisecond.  Sometimes this takes place over serial links or LANs, but most commonly is performed by connecting satellite time sources to each device.

Retrieve Fault Record

The Fault Record is stored in the IED and a notification that the record exists is provided and can be obtained. There is either a manual or automated process in place for retrieving the Fault Record so that it can be studied. The file is retrieved by the Data Concentrator  and passed to the GUI where it is stored and cataloged. The information in the record is analog and digital data that was captured before, during and after the fault.

There are several issues that can limit the systems ability to retrieve faults:

• Fault records contain large volumes of data that some devices cannot handle.
• Not all Data Concentrator s can forward file data.
• Legacy protocols may not support file transfer.
• Existing communication links may make the transfer of large files too slow.
• Some arrangements require personnel to be dispatched to the site to extract the Fault Record directly from the IED.

Change in Line Load

When a breaker trips there is no longer a path for electrical energy to flow from the live side of the breaker through to the other side. The breaker has isolated the downstream equipment by opening the circuit to stop the energy from getting to the equipment. This always causes a change to the entire load that the substation is connected to. Anything downstream of the breaker is not running because the power is removed unless there is a way to feed the load with energy from another line. The IED monitoring the changed load is responsible for detecting and reporting the change in the quantities, such as voltage and current.  The IED provides the information necessary to understand how the system has changed.

Analog Data Change

When Analog Data changes in the IED it is reported to the Data Concentrator  first. If the Data Concentrator  identifies the data as being of interest to one or more of the GUIs that are connected to it and the change in value is significant in that it exceeds a preconfigured deadband it translates the data from the IED to the GUI through the communications protocols.

Change in System Load

When the System Load changes and the GUI is notified of the change by the Data Concentrator  it must perform several activities for the operators to be able to investigate:

• Maps the point number of the protocol to the appropriate location in the database.
• Scales the data appropriately for accurate display in the correct units.
• Updates any values currently being displayed.
• Raises appropriate alarms associated with the quantities.
• Logs and trends data if required.

Shed Load or Restore Power

The System Operator addresses the alarm by viewing the system state displayed on the GUI.  IEDs pass load and switch state change information to the GUI through the Data Concentrator , permitting the System Operator to:

·         Determine the location of the fault

·         Verify that the protection equipment has operated properly to clear the fault (isolate the faulted section).

·         See the impact of the fault isolation on the rest of the power system.

The System Operator takes action to address the alarm.  This action may include any of the following items:

·         Reset any breakers that should not have tripped.

·         Disconnect the faulted section of line from the remainder of the power system, if protection equipment has not already done so.

·         Re-route power so the loss of the faulted line does not cause other portions of the system to be overloaded.

·         Re-route and restore power so the affected area is minimized.

·         Disconnect (shed) load from the power system if there is no other way to prevent an overload.

From the point of view of the communications network, the effect of all these actions is the same:  the System Operator sends control requests to various IEDs within the substation and elsewhere in the network, to either open or close switches or breakers.  The System Operator enters these controls at the GUI.  They are forwarded through the Data Concentrator  to the correct IED.  At any point in this path, the SCADA System  may reject the controls if they violate safety rules, e.g. closing a switch to run power back onto the faulted line.  This is called Interlocking and is discussed further in the Automated Controls use case.

Control

If the operator has sent a control the GUI must format the control into an appropriate request, usually a Select-Before-Operate service is used, for the Data Concentrator  or IED and sent through the protocol. The control can be digital such as changing the state of a breaker or an analog setpoint such as changing the position of a valve. The information contained in the control request includes: Point number, operation, duration and setpoint value.

Forward Control

If the operator has sent a control that is destined for a protection IED the Data Concentrator  must forward the request to the IED. The IED acknowledgement to the request is received by the Data Concentrator  first and then forwarded to the GUI. This is a protocol translation function performed by the Data Concentrator . The indication of the correct state of the control being achieved or the alarm to indicate that the control failed is also received first by the Data Concentrator  and then forwarded to the GUI for display on the screens the operator is looking at. When forwarding the control the Data Concentrator  uses the Select-Before-Operate service similar to the GUI. The Data Concentrator  can support multiple GUIs issuing controls if required.

Locate Fault

The protection IEDs that monitor the lines, L1 and L2, and the feeders, F1, F2, F3 and F4, usually have the capability of locating the fault by measuring the impedance of the line, and then comparing it to known impedances that exist when the fault is not present and preconfigured characteristics of the line. Usually the direction and distance to the fault are outputs of the IEDs calculations. This information is used to make operational and maintenance decisions by the Data Concentrator , the GUI and the operator.

Display Fault Location

The GUI is capable of interpreting the fault data and providing a meaningful display of the fault location in terms of the location and distance values that include the overlap provided by two different relays reporting the same fault from different locations. It also logs the distance and direction for future reference by engineers that need to study the characteristics of the fault.

Dispatch

The operator dispatches a maintenance crew to an area where it is believed that the fault exists based on the distance and direction that was logged and displayed by the GUI. This dispatching may take place through many types of media, including radio, telephone, a separate dispatcher, or a computerized system. Whatever mechanism the operator uses must be wireless at some point because the crews are mobile. The crew does not utilize the SCADA network on their own but receives orders directly from the operator. 

Notify

The operator is also responsible for notifying engineers that study the fault characteristics to determine why the faults occurred where they did. The priority of this step is increased if the fault also affected service in a region. The information that these personnel use is obtained from the Protective Relay, the Data Concentrator , the GUI and the operator. The personnel interested in the data are emergency personnel, protection engineers, system stability analysts and management. The process of notifying additional personnel may or may not be automated but typically does not use the SCADA network.

Steps

IntelliGrid Architecture
Copyright E2I and EPRI 2004