Over current protection

We need relay coordination to minimize the disruption due to fault and operate the nearest relay first. We can not coordinate fuses because the melting time is fixed.

For Fuse:
total clearance time = pre arching time + arching time
The time current characteristic of a fuse has two curves - minimum melt curve and total clearing time

Sectionalizers  can not interrupt a fault. It counts the number of time it seen the fault and operate after preset number. Reclosers have limited fault interrupting capability.


Types of over current protection:

1.  Instantaneous relays - current only ( can not discriminate the fault currents If1 = If2)
2.  Definite time relays - time only ( Faults near to the source has higher currents and it also show higher time)
3. IDMT inverse definite minimum current relays - both current and time

Consideration of  coordination:

• Maximum and Minimum momentarily Short circuit current
• Maximum and minimum ground fault current
• Total time interval

Coordination parameters

• TAP value (pick up current in secondary CT) - Pick up current
• Time Dial (TD) - Time multiplier setting (MTS) or time dial setting (TDS)
• Instantaneous TAP (IT)
• Extremely inverse characteristic

We use extremely inverse relay characteristic in the industry. There should not be any crossing between fuse characteristics and relay characteristics ( we can adjust this using the correct TDS in the relay.when TD is increased it will become more inverse ). It means fuse characteristic always has to be lower than relay (0.20 s gap). If we coordinate two over current relays their characteristic should have 0.4 s gap.

With CT ratio 500:5  and TAP value 5 means.
Pick up current in CT secondary = 5A
Pick up current in CT primary = 500 A

Fault Calculation

To obtain more accurate results calculation has to be carried out in different time ranges

• Sub Transient- large current 50 ms
• Transient - after 0.5 s
• Steady state- after 1 s

When we analyze large systems with different voltage levels we use base quantities and per unit values .Decaying DC component is considered using the asymmetric factor. Understanding the positive, negative and zero sequence components are required in unbalanced faults.

• For transmission lines positive and negative impedance are same and equal to the per unit impedance of the line. Zero impedance depends on the grounding configuration. Therefor obtain it from manufacture data sheet. 
• Balanced generator only generate positive sequence voltage.
• For motors has same positive and negative impedance. Since most of the motors are ungrounded it doesn't has a zero impedance.

L-L-L or L-L-L-G  => positive sequence
L-G => positive, negative, zero in series
L-L => positive, negative in parallel
L-L-G => positive, negative, zero in parallel

Line to ground voltage = line to line voltage / root(3)

Aspects of Protection system

Reliability

• Dependability - protection should operate when it should operate
• Security - protection should not operate when it should not operate

Speed
We need fast protection to minimize the damage

Selectivity/Discrimination
Zones of protections are determined by the CTs
Understand the type of the fault and fault location

Cost
Cost/beneficial analysis
fast operation and duplication require additional cost
Back up protection

ANSI reference numbers
21 - Distance relay
50 - Instantaneous over current
51 - time delayed over current
52 - circuit breaker
67 - Directional over current
87 - Differential

Faults in power systems

There are two categories of faults:

• Active Faults : Current flow from one phase to another or phase to ground. Two sub categories solid faults(complete breakdown of insulation) and incipient faults(faults start from very small beginning).
• Passive Faults : These are not real faults, but stressing the system to its maximum capacity, and ultimately active fault occurs. (Overloading, over voltage, under frequency and power swings)

Transient Faults : do not damage the insulation permanently and allow the system to re energized after a short time period (lightning strike, momentary tree contact)
Permanent faults : does not disappear when the power is disconnected. equipment has to be repaired.

Symmetrical faults are balanced faults. Sinusoidal are equal about their axis. represent steady state operation.
Asymmetrical faults displays a dc offset and become a symmetrical fault after some time

Availability of hardware in substations

The software and hardware in the substation are designed in a way that they meet high availability requirements. The means high reliability ( Long mean-time-to fail MTTF) and short down time ( Short mean-time-to recover MTTR). MTTF means statistical time until component need a repair.Short down times can be achieved by extensive diagnostic functions, modular hardware designs, fast reconfiguration and automatic restart after a power supply failure.

The redundancy recomended:

• Repair faulty parts in process and bay level in less than two hours and less than 4 h for station level.
• Standby hardware exist physically connected and per-configured.
• Warm standby - standby HW constantly supervise active HW, automatically takes over in a failure. Time stamped events may be lost. commands are reusable after 10-30 sec
• Host standby -  standby HW constantly supervise active HW, it takes over in a failure. No Time stamped events  lost. commands are reusable after 1-5 sec. at bay level switch over time is less than 100ms.

Self supervision of IEDs

• Insensitive against EMI
• A/D conversion may subjected to aging should be supervised by a reference signal
• Watchdog should supervise the response time from processing algorithms
• Checksums are used detect failure in mamory
• Loss of power should be checked

Supervision of communication:

• All communication devices ( Star couplers, routers, switches) are subjected to self supervision
• Detection of errors, check the response time and counting lost messages

Communication requirements of Substation Automation architecture

Introduction of microprocessors in to substation allows process data in digital form. to convert analog data in to digital ADCs are used. These digital data is not distorted due to aging of the hardware and can easily exchanged  by serial communication. But these serial communication introduces additional delays.Also information processing hardware must withstand harsh environment in the substation, specially EMI.
The data is acquired at the process level by means of remote i/o units (RIO) and intelligent sensors (PISA = process interface for sensors and actuators). The process bus connects them to the bay level equipments.

Communication requirements:
Maximum allowed age - worse case response time can be tolerated. This means, that this time must be guaranteed in normal operation.
Data integrity - degree of communication safety in the case of disturbances. If data is directly influence the process those data has higher integrity.
Exchange method - Spontaneous mean communicated as soon as it happens. Request means communicated on request by some function or human.

Alarm - 1s - Medium - Spontaneous
Commands - 1s - High - Spontaneous
Process sate data -2s (binary) 5-10s (measured) - Medium - Spontaneous (gives overview of the process state)
Time stamped events - 10s - low - On request (used for later analysis)
Interlocking data - 5ms - high - Spontaneous (used to prevent dangerous commands)
Interlocking data (state info) - 100ms - high - On request
Trip from protection - 3ms - high - Spontaneous (used to clear faults)

The actual communication throughput capacity must be higher than needed for normal operation( at least 10% higher). When we design the communication system we should avoid the single point of failure.
No communication message failure shall lead to a unsafe action. This can be tackled using communication error detection mechanisms and making transmission media immune to disturbances( reduce number of bit errors). Today all process buses are typically a Hamming distance of at least 4 - 6, to detect transmission errors. This is sufficient for medium integrity. In substation error rate is higher than the telecommunication environment. There for we use glass fiber in the process bus and special communication procedures like "select before use" is introduced.
No lost or late message is allowed to lead to unsafe action.Messages could be lost due to buffer overflows or overloaded routers and switches. There for lost messages and loss of message source should be detected. In IEC 61850 topical flag is used to indicate data is up to date.Glass fiber can cover a distance up to 2000m with out loosing transmission speed. while plastic fiber is used for shorter length( tenth of meters). Also plastic fiber is aging sooner than glass.

Today we can place the microprocessor based relays close to the process. In new architecture physical signal marshaling is replaced by logical signal marshaling, which means complexity is the same. Electrical CAD systems are replaced by signal engineering tools.

For redundancy we duplicate the protection devices at least in HV substations.
To provide passive safety in logical nodes it sends at least two telegrams before a command is executed. This two step approach is called select before approach (SBO). HMI send the select command to CBC node. Then CBC sends selected command back to HMI. Then only HMI sends the operate command to the exact switch.

Substation Automation Structure

Business benifits of substation automation are :

• Better information, Higher productivity
• Intelligent automation, Higher productivity and higher availability

Substation automation structure include Station level, Bay level and process level.

Station Level provides Remote communication to Network control center (NCC) ,HMI, Station level automation, data evaluation and archiving, condition monitoring, events and alarms, station level protection and  Data exchange. Station level equipments are always separated in to two rooms - operation room and the communication equipment room.
Bay Level provides bay level automation, time synchronization, condition monitoring, bay level protection, bay level control, object protection and data acquisition.

Process Level includes GIS or AIS switchgear, instrument transformers, power transformers and surge arresters.Output of VT (100v or 200v) and CTs (1 or 5A)

Time synchronization has two general approches;
Separate synchronization pulse - separate wire for all the IEDs
Using communication buses- master clock broadcast time telegrams and slaves regularly ask for time