207

Study Committee B5 Colloquium2005 September 14-16

Calgary, CANADA

AUTOMATIC FUNCTIONS IN INTEGRATED DISTRIBUTION SUBSTATIONS

A. MONTOYA

GENERAL ELECTRIC

During the last years the integration of automatic functions in electrical substations, especially in distribution levels, has grew due to the advantages offered by the digital technology now widely use in IED’s. The utilization of distributed elements (IED’s) in the field (Level 1), all of them connected within a communications F.O network to the main substation Level 2 controller (industrial PC or Master Relay) and moreover, to the remote dispatching centers, results in a high flexibility to get information from all elements in the substation. In general, the huge amount of available information of the different protection and control functions every moment makes simple the operation of substations and thus reducing the personnel dependency. The protection functions are fully independent to decide on whether situations to trip, without any action of the control personnel. In other hand the control functions until now operator controlled in the substation control room (Level 2), these days frequently are permitted to be executed by automatic functions specially those caused by system disturbances. This is the “why” of the rapidly changing of the upcoming automatic functions. The automatic functions are implemented in the substation Level 2 industrial PC or Master Relay and are responsible for the execution of high complexity functions (i.e operations affecting different S/E zones) In this way the automatic functions can be defined as: Those action involving the operation of different power elements in different zones of electrical substations, and complex functions to be executed during emergency contingencies in remote and unattended substations. In addition other monitoring, recording etc. are also achieved. More than the classical control functions, this paper presents how the common data base of all substation elements in the Level 2 controller allows to perform some other automatic functions needed for control, parameterization, loss of life, maintenance, etc. as follows:

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Automatic substation restoration Under voltage load shedding Under frequency load shedding Feeder breaker failure On Load Tap Changer – Reactive Compensation Thermal image of power transformers

The key element of the system to perform automatic functions is the ability to keep and remember past fast events, pickups, resets, and the steady-state values as well. Figure 1 shows a ring bus bar distribution substation where the different power elements are monitored and controlled by the automatic functions hereafter described. Figure 2 shows the architecture of system showing some basic levels:

Level 0 Substation field elements (instrument transformers, C. Breakers, power transformer, etc.)

Level 1 Intelligent electronic devices (IED) per substation zone (i.e. F650) Level 2 Substation control level – Master relay or PC (i.e. F600) Level 3 Remote or dispatching centers

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L2 L1 VL1 VL2

Busbar 3

D60_2

CB2 CB3

CB1

TP1 138/13.8 kV

F650

F650C

D60_1

CBB1Busbar 1 Busbar 2

F650

V2 V3V1

F650

CBPT

LAN

D60 Distance Relays (21) F650 Overcurrent Relays (50/51)+Level 1 bay controller (IED) CBxx Circuit Breakers

OLTC

F650 F650

CBB2 CBB3

F650 F650

F650 F650

CBn CBn CBn CBnCBn

CBPT

Master Relay (Level 2)

Automatic Functions (PLC)

SUBSTATION 138/13.8 Kv

CB Protectiontrip command CB Control command Ethernet LAN

igure 1

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F650

F

IEC 61850 INTEGRATED S/E Architecture (Figure 2)

Level 3 -IEC 870-5-101 -IEC 870-5-104 -Modbus RTU -Modbus TCP/IP-WISP+ -Indactic 2033

Substation Ethernet

-Logical Nodes Monitorization -Protection Settings -Reports -Logs -Digitals -Controls

SNTP Synchronization

OPC Server

IEC 61850 IED

F650 IED UR IED

GOOSE/GSSE Messages

-Analogs -Digitals -Controls -Events -Records -Counters -Date/Time Sync.

IRIG-B Clock

EWS

Level 1

Bay Level

IEC 61850 data

WANF600 PLC IEC1131-3 Inside Automatic Functions

Level 2

Data Conversion

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1. Automatic Substation Restoration Description The automatic restoration tries mainly to maintain closed the HV ring bus bar (Figure 1) after any disturbance by closing if needed the different circuit breakers as a function of: Exploitation criterion (blocking, maintenance, etc.) Remote sources Live-Dead condition(lines L1 and L2) Criterion about the circuits and CB’s exclusions (blocked since the beginning,

manual operation, reclosing blocked, etc). Criterion on circuits and CB’s inhibitions (50BF tripping on lines, 86 tripping

(latching device) on power transformer, etc.) Voltage drop and restoration criterion on lines L1 and L2 Priority (circuit breakers closing sequence depending on user load or system

restoration priority). The power transformers are not coupled at MV bus bars. Starting The function remains in the “Ready” condition (wait) and being the HV ring bus bar closed and voltage equal or near the rated value in both lines. The function starts (process status) on

• Any distance tripping (21) on both lines L1 or L2 • Any over current (50/51) tripping in the HV side of power transformer. • Voltage drop of both VL1 and VL2 without any circuit breaker change of status. • Voltage drop of both VL1 and VL2 without any tripping signal. • Voltage drop of VL1 or VL2.

The automatic restoration checks if any CB had trip or has been manually open, the actual voltage levels, CB’s conditions, and the operation conditions first to try to close the ring bus bar (restoring the load through the power transformer). Examples of Restoration and Inhibition Case 1 • If any voltage drops below the threshold, the function starts and goes to the

“Process” status analyzing the status and conditions of all circuit breakers and of each one of the circuits associated to the bus bar (in this case 3 circuits). If the voltage returns exceeding the threshold and there is an open CB for any reason, the automatism will close it and will verify sequentially the status of the three circuits involved in this ring bus bar and in MV side. All this process is done only if there is no inhibition condition for the CB. Once the circuit breakers have been closed, the automatism returns to the “Ready” (wait) status.

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Open CBPT-Cap Banks-Feeders-Lower OLTC Vmin < VL1-VL2 < Vset

CB1-CB2-CB3 Closed Checks-timing

VL1-VL2>VsetClose CBPT- Close MV feeders Acc. to priority No Self Blk

Ready

Case 2 If both line voltages drops (remote ends disconnected), the restoration will open circuit breakers CB3, CB1 and CB2, will lessen the On Load Tap Changer of power transformer to the lowest tap. After that it will open also the capacitor banks connected to the MV side bus bar (if available) and MV feeders under maintenance conditions. All these actions are taken to avoid over voltage conditions when rated voltage returns to the substation. If the voltage return then the automatism will close sequentially CB1 and CB3, CB2 and goes to the “Ready” (wait) status.

Open CB1-CB2, CB3-CBPT-Cap Banks-Feeders-Lower OLTC VL1-VL2 < Vmin

CB1-CB2-CB3 Closed Checks-timing

VL1 >VsetClose CB2-CBPT1-MV feeders1-CB1-MV feeders2-CB3 VL2 >VsetNo No

Close CB1-CBPT1,-MV feeders1-CBPT2- MV feeders2-CB3-CB2 Ready

Self Blk

Caso 3 If after a tripping of any distance function (21) the reclosing action is successful, the function will returns to the steady-state (wait) wait condition without doing any action on CB’s. If the reclosing shot cannot close the just opened pole or circuit breaker, the automatism will go to “Process” status and will check the tripped CB’s conditions. If no impediment or handicap (50BF condition, last reclosing shot, closing blocked, etc.), the automatism will close them following the pre-configured sequence (in this case Circuit 1, then Circuit 2 and finally Circuit 3), and will return to the “Ready” (wait) condition.

52 Checks 79 Success Any CB

block No No

Ready Self Blk

Line 21 trip Close tripped CB’s and complete ring Case 4 If operator (from level 2 or 3) issues a open command to any circuit breaker, the restoration automatism will not start and will go the “Block” status (see algorithm).

Self Blk

Operator command from Level 2, 3

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Case 5 If any 50/51 tripping signal of power transformer is present in the system, the restoration will move the On Load Tap Changer to the lowest tap. Then will open capacitor banks in the MV bus bar (if connected), and will open also the feeders in MV bus bar that might be in maintenance condition. After that will close the just tripped CB’s and will return to “Ready” status.

Open connected capacitor banks-Feeders-Lower OLTC Self Blk PT 50/51-87 Trip

And finally the general automatism is:

Cases programmed

Failed Action Non-Programmed Condition Communications Loss Exclusion

Ready (wait)

Manual Closing Restored Busbar

Voltage Drop Protection Tripping

Operator

Operator

Self Blocking

Block

Process

Automatic Substation Restoration (HV Side Restoration)

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2. Under voltage Load Shedding and Restoration Description This function works only to the medium voltage bus bar (in this case the 13.8 kV – See Fig. 1), and is dedicated to shed loads (feeders) on under voltage conditions (the rated voltage drops below some prefixed threshold). When some under voltage condition is detected, the automatism starts and proceeds to open the different feeders that might been configured by setting to be shed, after the different time delays set for each one have been expired. As the medium voltage bus bar (13.8 kV) may be fed by only one power transformer (the case where CB12, CB13 and CB14 are closed), or may be split in some point in such a way to be fed from two different power transformers not-coupled at MV bus bar. This automatic function determines every moment the actual topology the MV bus bar has. It also sees what is the part of the split bus bar affected by the under voltage situation. With the substation topology every moment refreshed (continuously check of MB CB’s), and some or all feeders previously configured the automatism starts the shedding process. The shedding process is started for all configured feeders at the same time if they were connected previous to the under voltage condition. The parallel coupling of power transformers can be permitted or not depending on the user system operation. If this coupled situation should not be permitted (the case of load change for maintenance), the automatism will go to the “Block” status during the time this situation extends. Once the configured feeders have been shed the automatism will go to the “Block” status providing events to indicate the end of the process. Once the shedding has been finished and after the recovering of the secondary voltage up to normal values, the operator can close manually the different feeders or live the restoration process to be done automatically according to a load restoration priority program. Starting Under voltage load shedding starts when the rated voltage drops below some prefixed value and remains above other minimum voltage level also set. If the voltage drops below the minimum voltage (zero voltage like fuse failure situation), then the automatism will remain unaltered and will not proceed to the “Process” status. If the voltage remains in this “permitted voltage range” (by setting) during certain time the function will go to the “Process” status. The feeders have been configured in groups with independent shedding time delays to grant the selectivity according to the different type of loads. The process starts the shedding for all groups at the same time opening the configured feeders at the end of their independent time delays. At the end of the process the automatism goes to the “Block” status.

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Under voltage Load Shedding Examples Because under voltage load shedding is a single criterion starting function and also because it has only on process to be executed, the number of starting cases will be in accordance quiet close to the different under voltage conditions of the system. Load rejection on generation power plants, voltage drop at the remote ends of incoming lines (remote sources dead or line faults), big loads connection etc. are the most common under voltage situations causes.

Operator

V > VmaxV<Vmin

Reset (wait)

Voltage Drop and Busbar Topology P

Vmin < Vactual < Vmaxt > tset

Restoration Program *

Block All configured feeders shed * Level 2 - According to feeder priority (Customer, typ

Undervoltage Load Shedding and Res

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CBnCC

rocess CBn Breaker Failure Event recording

e of load, etc.)

toration (MV Bus bar)

3. Under frequency Load Shedding and Restoration Description This function performs the shedding of MV feeders when any system disturbance varies the rated frequency below the rated value. In this case the under frequency situation is detected individually en the distributed IED’s of each feeder. The under frequency function in each IED is set as per the load priority criterion of the system. These 81U functions on Level 1 IED’s are primarily responsible to issue a trip only to the feeder where installed and on under frequency situation The second part of this automatic function is performed at Level 2 (main substation Level 2 controller), and is started when any 81U tripping signal arrives from any IED in Level 1. When the frequency at the MV bus bar (13.8 kV) tends to be normal, and depending on the restoration frequency of each feeder, this function will proceed to close one by one the tripped feeders if the actual frequency remains stable above the restoration limit during certain time. Starting The function starts when some 81U tripping signal given by any MV IED is received at Level 2. In the mean time each feeder trips as per their own under frequency and time delay settings. The automatic function goes to the “Process” status when at least one 81U tripping signal has been received al Level 2. Then the system waits for other possible 81U tripping during some prefixed time to avoid transient conditions. After some preset time delay the function starts to see if the actual frequency exceeds the restoration value of any feeder previously tripped (the feeder with the lower restoration frequency). If does and if the frequency remains stable then proceeds to close that feeder, continuing with others if the actual frequency exceeds their shed value. The priority to close any tripped feeder is determined by the lowest restoration frequency. Under frequency Load Shedding and Inhibition Examples • As in the under voltage load shedding the frequency drop in the electric systems is

originated by external and remote events. It should be note that it is also possible that under frequency and under voltage situations may occur at the same time, i.e during load rejection, stability loss on sustained faults in transmission lines, etc. If both conditions of under voltage and under frequency are present at the same time, the under voltage load shedding will go to the “Block” status while the under frequency load shedding takes priority and continue the process up to the end. The under frequency condition has higher priority than the under voltage one.

• A voltage supervision is implemented by setting in the IED’s as well as in the Level

2 automatism. All 81U trips will be disabled if the actual voltage level does not reach the minimum fixed.

• If any feeder circuit breaker fails to close during the restoration, the function will

provide an event-alarm message and will continue to restore other feeders if frequency allows to do it.

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81u trip in feeder n

V<VSup

All configured feeders tripped

Pr

Reset (wait)

Restoration Program *

fmin < factual < fmaxt > tset

Block

Underfrequency Load Shedding and Restoration

207 - 11

CBnCC

Operator

t ocess

(MV Bus bar)

5. Distribution Breaker Failure Description In distribution substations is not usual to install breaker failure protection for feeders because different considerations. However the breaker failure index in distribution systems can be equal or higher than the case of HV circuit breakers in transmission substations. This and the facilities of digital technology allows to restore the healthy feeders after the tripping of the secondary CB of power transformer. This automatism fully applicable on split bus bars (Figure 1) operates when after a tripping decision on someone MV feeder, the circuit breaker fails to open. In this situation the 51 protection upstream on the secondary side of power transformer will trip the upstream CB. Since the status of all elements connected to MV bus bar is continuously refreshed and the pickup/trip events of all feeders are kept at Level 2, the algorithm can define properly the circuit breaker condition in each feeder. The first action is to try to open the failed circuit breaker. If does then will proceed to close the power transformer secondary CB that has been tripped before. If not, the automatism will try to isolate the bus bar section where the failed circuit is connected by the giving a open command to the bus bar CB just near the section where the failed circuit is connected (see Figure 1) Starting The functions starts when general circuit breaker of secondary side of anyone power transformers is tripped. The automatism checks for the following conditions:

• If upstream power transformer CBPT was tripped by any 50/51 operation, and absence of tripping in some feeder.

• If CBPT was tripped by 50/51, and presence of some feeder tripping The first case is the typical situation of a bus bar fault and the automatism will not start. In the second case the automatism will start going to the “Process” state. The system knows where the trip signal comes from (or at least some pickup) without CB change of status (close to open). The system can therefore define properly the faulted CB. The actions hereafter will be:

Give trip command to the failed CB. If succeeded the power transformer CBPT will be closed after some checks and

time delay and thus restoring all MV bus bars. If not the system will open the closest bus bar circuit breaker CBBn to isolate the

faulted feeder. Then after will close the power transformer CBPT to restore service to the remaining bus bar sections.

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If both power transformers were not coupled at MV bus bar (MV bus bar open somewhere – See Figure 1), the automatism will start and work independently for each power transformer system, depending on the already open CBPT. If both power transformers are coupled at MV bus bar the automatism will go to the “Block” state the time this situation remain.

CBPT blocked

Reset (wait)

P

Block

Operator

Open closest CBBn

50/51n Pickup CBn Closed

CBPT Open 50/51PT Trip

d

Rigid busbar tied to CBPT

Distribution Breaker Failure

207 - 13

CBnOC

CBPT blocke

CBPTCC

ro

(

Unsuccessful command to CBn

cess

Omission)

6. Voltage Regulation (VR) – Reactive Compensation Description The voltage regulation automatic function in substations checks periodically (by setting) the MV and HV bus bar voltages (13.8 kV and 138 kV – see Figure 1), and connects/disconnects the capacitor banks and the On Load Tap Changer (OLTC) of power transformers. This is performed to fix the voltage level according to the actual load current values in MV bus bar. These load current values and the maximum/minimum permissible voltage deviations are user defined. The algorithm has basically three ways to operate:

Based in the HV bus bar voltage the automatism connects or disconnects the capacitor banks in the MV bus bar - Gross Voltage Adjustment.

If HV bus bar voltage is within the limits, then it checks the voltage at MV bus bar and if needed operates the OLTC to set the most properly voltage according to load current-voltage deviations table set by the user - Fine Voltage Adjustment.

Additionally by setting it can be choose a Reactive Load level Algorithm that checks also periodically the reactive power delivered by the power transformer.

The case of gross voltage adjustment, the automatism decides to connect or disconnect the available capacitor banks to the MV bus bar, but it primarily calculates the effect on the voltage level that might result from the capacitor bank connection/disconnection. If according to the actual load the resultant voltage might fall out the limits of the range then it will move the OLTC to a tap that after the connection/disconnection will maintain the voltage between limits. This new voltage level previously calculated is the voltage that will result after the connection/disconnection of the capacitor banks. In the case of fine adjustment (when the HV voltage is within the limits), the automatism performs according to the MV bus bar voltage. In this case will compensate the voltage by moving the OLTC until reach some voltage value within the limits according to the actual load current. If reactive load check has been chosen, the automatism calculates the actual reactive power and compares it with the capacitive reactive power of capacitor banks. Once has been established the need to connect or disconnect the capacitor banks, the automatism calculates first the effect on the resultant voltage and moves the OLTC first to the desired tap before to operate the capacitor banks. The reference voltage levels of the range are defined by setting and according to the actual load delivered by the power transformer. The verification of the voltage level at HV and MV bus bar is done each 10 seconds. In case to choose the reactive check algorithm, it is also possible to set the time interval for checking. The automatism performs the tap changing according to a inverse curve. This curve is proportional to the voltage deviation against the voltage reference.

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Starting The voltage regulation automatic function always is active once has been finished the warm-up process of the Level 2 PC and the operator has reset it to the “Ready” state. The VR function periodically (each 10 seconds) checks the reference voltages of HV and MV bus bars and operates accordingly if one, the other or both are within or without the reference ranges. This reference voltages are also defined depending on the actual load current also read from the main refreshed signals at Level 2. At the end of each check with or without capacitor banks and OLTC operations, the automatism returns to the beginning and starts again after timing has elapsed. If reactive power check is chosen the automatism will also perform it depending on the time setting. At the end of each check with or without capacitor banks and OLTC operations, the automatism returns to the beginning and starts again after timing has elapsed.

Reset (wait)

Process

Block

Periodic Sequence f(∆t)

Move tap of OLTC

VHV y VLV within range f(A)

Capacitor Bank

ON/OFF

Move tap of OLTC

Q>QBCQ<QBC

Capacitor Bank

ON/OFF

Operator

Voltage Regulation (VR) – Reactive Compe

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VLV <Vmax

VLV>Vmin

VHV <V max

VHV>V min

Move

∆t = Sett tap of OLTC

nsation

7. Thermal Image for Power Transformers Description This function is automatically started after the warm-up of the Level 2 controller (PC or Master Relay). It does not need operator action to be reset since has no “Block” state. Basically performs the following tasks:

To reed in discrete time steps the top oil temperature of the power transformer by the use of a analog signal set at some IED unit in Level 1. This analog signal is taken from a normal resistance temperature detector (RTD) placed in the transformer case.

During each time step ∆t the thermal image automatism calculates the

temperature increment of the oil due the power delivery during the actual time step.

During each time step it also calculates the difference between the top oil

temperature and the hottest point of winding as a function of the power delivered during the actual time step.

Finally, the actual top oil temperature plus oil temperature increment and the

temperature difference between oil and the winding hottest point during the last time step (due load during ∆t), gives the actual winding hottest point temperature. Depending on the settings provided and the winding hottest point temperature calculated the thermal image performs the following operations:

The switching ON/OFF of PT cooling equipment (fans) Generates event-alarms for predefined temperatures of the PT winding. Tripping output for circuit primary and secondary circuit breakers on

resulting winding temperatures above the predefined setting. Like the temperature increase due the load, if the power transformer cools down because load or ambient temperature decrease, or because the fan operation effect, the automatism will stop the cooling equipment at some temperature below its starting level (hysteresis). Starting The function is always started after the initialization mentioned in the first paragraph and has no “Block” state. Function States Starting – This is the initial state (not reset) where it starts to perform the cyclic check.

This is the departure and the final state of all cyclic temperature checks.

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Fail - This state is reached if the temperature difference between 2 consecutive checks

is higher or lower than prefixed thresholds. This is done to avoid abnormal function outputs due to any RTD failure, as well as during sustained faults in external lines that imposes large short circuit currents to the power transformer with the resultant drastic temperature increase. This state is left automatically and goes to “Starting” if transformer power reading returns to values closest to the rated power.

ALARMOTtwinding:

Fans - 2nd GroupStartup

Fans - 1st GroupStartup

ALARMTRIPtwinding:

Fans - 2nd GroupOFF

Fans - 2nd GroupShut off

Fans - 1st GroupOFF

Fans - 1st GroupShut off

Trip Power TransformerOpen CB1-CB2

STEP 1stw θθ ≥

TRIPw θθ >

Yes

NoOpen Trip Contact Open Alarm Contact

No

Trip 86T

Yes

Yes

Yes

No

No

Yes

No

Yes

No

FailureF(0 < K < 2 p.u)

Top oil temperature reading from

transducer (4-20 mA

θw =Winding temperature at instant kt.θo = Oil temperature at instant kt.∆θon = Oil temperarute increase at Prated.

∆θwn = Winding temperature increase at Prated.K = Actual Power/Rated Powerl.d = Factor: Losses at Prated/Losses no loadm= Temperature increase factor in the hottest point of the winding regarding the top oil temperature. Normally 0.8 for ONAN-ONAF ratedx = Total Losses/Rated Losses. Normally 0.9 for ONAN-ONAF rated τo = Oil thermal time constant at Prated. Normally 2 to 5 hours.t = Sampling rate in seconds.. Usually 18s.

STEP 2ndw θθ ≥

ALARMw θθ >

θwactual = 0

θwactual >θwprevious+40θwactual <θwprevious -40

θ

θ

)actual(o

o )1tK( −

No

Yes

Yes

No

( ) ( )

( ) [ ]d1K*d1

andwhere

)h51(oilofttanConsTimeThermalands18settingtfor

K*1*

2 x

onop)1kt(0)actual(0op

0

m2n0wn00op)1kt(00 e

t

++

θ∆=θ∆θ∆−θ∆=θ∆

−===

θ∆−θ∆+⎥⎦⎤

⎢⎣⎡

⎟⎠⎞⎜

⎝⎛ −θ∆−θ∆+θ=θ

−

−

τ

τ

.

power Transformer Thermal Image

207 - 17

8. Conclusions Installation and operation

Full control of substation from the remote dispatching centres (Level 3). The sequence, steps and final state of any automatic process are known remotely assuring the correct response of the operator including under very distorted system conditions. Easy restoration of service after system disturbances.

Control of unattended substations Reduction of the classic low voltage wiring and maintenance in the substations. Time tagging at Level 1 Possibility of other and complex automatic functions and monitoring as well

(statistics, etc.). Maintenance

Reduction of different platforms for protection and control functions (clear advantage in their field installation, lower failure index etc.).

Additional monitoring of S/E elements, i.e. life expectancy and maintenance of circuit breaker,CT output comparison, loss of life in power transformers, etc.

Dependable and secure operation by the use of single commands step-by-step, with fast periodically check of timings, incoming voltage levels, breaker status, function pickups, etc.

Reliability

Level 1 protection functions are independent form the communication system. Algorithms and trip decisions are taken at Level 1.

Large amount of substation data at Level 2 (events, oscillography records, alarms, etc.), make possible complex post fault analysis and monitoring.

Peer-to-peer communications allows adaptive protection functions and to improve automatic functions based in the fast detection and availability of signals in the LAN.

The use of a master relay (F600) instead of industrial PC’s allows a reliable and long life operation since has no moving parts (hard disks, fans, etc.).

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