Main Rack
CPU 270 CPU Type 3
2. MiCOM C264 MANAGEMENT
2.4 Time management
The main purposes of the time management include:
• To synchronize the internal clock in the computer (referred to as the local clock) with:
− The external clock for all C264 computers
− SCADA for all C264 computers
− SBUS or LBUS for all C264 computers except C264 Standalone computers
− Operator for all C264 computers except C264 Standalone computers
• To update the local clock
• To synchronise other devices through the SBUS with the local clock
You can synchronize the local clock with an external time reference from four sources:
• External clock sends an IRIG-B signal1
• SCADA sends a clock message through the TBUS and through the gateway1
• The system master clock sends a clock message through the SBUS - SNTP2
• An operator who sets the time2
There is a priority rule for these four external time references. If the external clock operates, it locks out changes from the three other sources: you cannot make modifications to the local clock in these ways:
• Locked out: SCADA sends a clock message through the TBUS and through the gateway
• Locked out: The system master clock sends a clock message through the SBUS
• Locked out: An operator who sets the time
For the C264 Multirack system, the main 1 rack delivers time synchronization for the main 2 rack and for the extension racks.
In event an external clock disconnects or does not operate, there is a priority order for the three external time references that remain:
1. SCADA sends a clock message through the TBUS and through the gateway or
2. The system master clock sends a clock message through the SBUS have priority over
3. An operator who sets the time
Just after the local clock is synchronized, and if it is the master system clock of the LBUS, the computer synchronises the Intelligent Electronic Device (IED) in agreement with the synchronisation procedure of the protocol. If the local clock is not synchronised, the computer synchronises periodically the IED all the same.
When the local clock is synchronised, all events and measurements include a time tag with a synchronised attribute. If the synchronisation is lost, or was never achieved, the attributes indicate that the time tag is not synchronised.
For a substation, the PACiS architecture can synchronize as many as 120 devices, such as the C264, OI, gateways, IED Relays, IEC61850 Relays, and so on.
1 For all C264 computers
2 For all C264 computers except C264 Standalone computers
The organization of the time management follows:
FIGURE 6: TIME MANAGEMENT 2.4.1 External clock
The external clock receives the synchronisation signal through one of several protocols:
GPS, DCF77, and so on. Periodically, the external clock sends the synchronisation signal, that includes the hour and the date, to the dedicated IRIG-B input of the MiCOM C264 / C264C computer.
Each computer has its own local clock. The local clock synchronises with the signal from the external clock. In this system architecture, the local clock operates as the System Master Clock.
In event of the loss of the radio signal from the external clock, two events can occur:
1. Some external clocks include a very accurate oscillator. After a loss of radio signal, these external clocks can synchronise the computer for 8 hours. The external clock sends two messages thru the protocol. The text of the messages is in agreement with the manufacturer specifications. For example:
no radio received
loss radio signal since more 8 hours
The local clock remains synchronised with the external clock until the message loss radio signal since more 8 hours activates. The status of the local clock now shows as not-synchronised.
2. Some external clocks do not have the internal accuracy to support the radio signal. In this event, when the confirmation of loss radio signal occurs in a few minutes, the status of the local clock shows as not-synchronised
If the computer operates as the system master clock for the other devices in the substation, then it continues to send the clock synchronisation message to the other devices, even if the local clock, or system master clock, is not synchronised with an external time reference. A binary input is dedicated to monitor the status of the external clock.
2.4.2 Clock synchronisation message from a SCADA gateway
The acquisition of a SCADA clock synchronisation message is a SCADA gateway specification.
The SCADA clock synchronisation depends on the protocol. The clock synchronisation message goes directly thru the SCADA link to the C264 computer.
The clock synchronisation message from the SCADA gateway is in Universal Time Coordinated (UTC) time.
From the SCADA, the clock synchronisation message goes to the “local clock update”
function in the computer. The computer does a check of the frame of the clock synchronisation message, removes the control fields, and transmits the clock synchronisation message.
When the SCADA gateway operates as the external time reference in agreement with the priority, the computer receives the clock synchronisation messages from the SCADA. An interruption relates to the arrival of the frame: the computer can acquire the clock synchronisation message from the SCADA gateway. The delay in the transmission from the SCADA gateway is compensated.
The clock synchronisation message from the SCADA gateway must contain:
Day / month / year / hour / minutes / seconds / milliseconds
To update the computer local clock, please refer to 2.3.5 Local Clock Update.
2.4.3 SNTP Client and Servers
On an IEC61850 network, the clock synchronisation uses the Simple Network Time Protocol (SNTP). In a PACiS system, you can define as many as two computers as the System Master Clock: they become SNTP servers. All other devices installed on the IEC61850 become SNTP clients. In event that one SNTP server fails, for example a computer is unserviceable, or an external clock fails, the SNTP clients automatically connect to the second SNTP server.
Use the PACIS tool, the System Configuration Editor (SCE), to configure a C264 as an SNTP server, that could be redundant: you can define 2 IP addresses. These IP addresses must be in the same range as the C264 Ethernet addresses.
In this event, no C264 is the Master Clock: all C264 computers are SNTP clients.
The synchronization performance of the system depends on the accuracy of the third-party SNTP server.
You can connect one C264, that is configured as an SNTP server, to a second C264 SNTP server to make a C264 redundant system.
2.4.4 Time set by an operator
If an external time reference operates, such as the external clock, the SCADA clock or the system master clock, you the operator cannot set the time.
If the system master clock synchronises any other device in a PACiS system architecture, you cannot set the time for that device. However, in event the system master clock is unserviceable or the connection fails, you can set the time for the device.
If the computer does not receive the clock synchronisation messages from the external clock, from the SCADA gateway, or from the system master clock, you are allowed to set the time manually.
Normally, you must manually set only the system master clock. You must use the correct tool for the specific system master clock.
With the C264 local Operator Interface, with the Human Machine Interface (HMI) front panel, or with the maintenance laptop computer and the Computer Administration Tool (CAT), you can set the time and date.
You use two commands:
• One to modify the time: hour/minutes/seconds
• One to modify the date: year/month/day
You can reserve a third command to do a check on the date and hour values.
You must raise a manual time-set-indication. The computer makes a record of all operator actions.
2.4.5 Local clock update
Each computer has its own local clock. Each local clock has its own quartz drift: a deviation of the internal oscillator of the computer. You must synchronise the local clock periodically to agree with an external time reference.
The time data includes:
• Loss of the external clock; loss of SCADA gateway clock synchronisation message; or loss of the system master clock
• An indication that the local clock was roughly synchronised. This indication shows any time difference, between the local clock and the clock synchronisation message, that is greater than the fixed value: Δthreshold = 20 millisecond. The computer sends a message about the discrepancy, prints the message, and archives the event.
• Indication that the operator performed a manual time set. This “manual time set”
indication is required to time-tag the events that occur.
The internal time format for the local clock is Universal Time Coordinate (UTC). The time difference between UTC and the local time is set during configuration.
The local clock operates automatically:
• For all dates including leap years thru to 2037.
• For seasonal time changes: it uses the values for Daylight Savings Time (DST) as shown in the Time Zone Adjustment Table (TZTAB) file. The TZTAB file shows the differences between UTC and local time.
If the DST rules change, you can set the new rules in the computer.
The computer manages the milliseconds with its own quartz.
The Local Clock Update function manages the synchronisation status of the computer as follows:
• Not synchronized: this is the status at initialization, the computer has never been synchronized since booting or rebooting. Date and time are invalid.
• Synchronized: the computer receives periodically the date and time. Date and time are valid. Or the computer is master clock on the Ethernet network for other equipment.
At initialisation, the computer shows the external clock as not-synchronised with the local clock. This status remains the same until you send a control message to the initialisation driver to receive synchronisation frames from an external time reference. When the control is OK and at the reception of the first external clock message, the synchronisation upon external clock is declared valid and the external clock is considered connected.
This synchronisation with the external clock could fall back to invalid: for example, in event the computer did not receive an external synchronisation message for N minute(s). This can indicate an external clock failure, such as a disconnect or a message that contains invalid data. The value of this timing is 300 seconds.
In event of an external clock failure, the local clock maintains the frequency in effect before the external clock failed. The computer generates an internal signal or alarm and declares the system master clock invalid. When this occurs, the computer makes a non-synchronisation mark against each event, for as long as the non-synchronisation with the external clock remains invalid.
When a device on the SBUS does not receive a clock synchronisation message in 180 seconds, it raises an alarm. The events processed by this device are tagged with a special mark and with the time, and the device stops the transmission of the synchronization frame to the IEDs.
2.4.6 Management of loss of external time reference
As the local clock update function receives the external time reference, it manages a loss of the external time reference as follows:
• If the external clock is lost, then the local clock update shows the loss of the external clock
• If the clock message from the SCADA gateway is lost, then the local clock update shows the loss of the SCADA gateway clock message
• If the system master clock is lost, then the local clock update shows the loss of the system master clock
2.4.7 Update method
When the computer validates the external clock acquisition, the computer manages the synchronisation of the internal system master clock to the external time reference as follows:
If a difference between the local clock value and the external time reference value transmitted exists, [C Local – C External] ≠ 0:
If [C Local – C External] < 1 ms, the local clock is not modified If [C Local – C External] > 1ms, two different events are processed:
1. If [C Local – C External] ≤ Δ Threshold (20 ms), the value of the local clock is progressively corrected (reduced or increased). This compensation is performed in a set time (t1=60 s).
2. If [C Local – C External] ≥ Δ Threshold (20 ms), the hour in the local clock is updated roughly.
If an operator gets the external clock time, the operator must update the local clock.
2.4.8 Limits and performances
The computer local clock has a small drift per day. This quartz drift, that is the deviation of the internal oscillator of the computer, is less than 0,5 s/day (that is, ≤ 5,8 μs/s). This quartz drift is the same through the entire operating temperature range of the computer. The operator must set the time periodically or synchronize the local clock with a master clock.
3. COMMUNICATIONS
The C264 includes different types of communications:
• Telecontrol Bus (TBUS)
• Legacy Bus (LBUS)
• Station Bus (SBUS)
• Inter-rack Ethernet communications.
For the C264, the possible combinations of protocols follow:
Item For this number of Protocols on
the TBUS
You can have this number of Protocols
on the LBUS
Total number of protocols allowed
1. 2 0 or 1 or 2 4 or less
2. 1 0 or 1 or 2 or 3 4 or less
3. 0 0 or 1 or 2 or 3 or 4 4 or less
The basic communications follow: