Protective Relaying in a TSS
Posted by Anup Mohan on Tuesday, April 24, 2012 Under: Electrical Substation
What could possibly prevent the mishap that would happen if the live overhead catenary - contact system comes in contact system comes into contact with earth or train roof ?? This was asked by a fellow passenger of mine whilst my recent train journey. The simplest, yet convoluted answer is "Protective Relaying".
A relay is the most important component in any protection system. During a fault or any abnormal conditions, one or more of the electrical quantities such as current, voltage, phase angle, frequency, rate of rise of current in the circuit of the relay changes. Relays can be categorized as Electro-mechanical relays, Static relays, Numeric relays and some special types of relays; namely Buchholz relay and Thermal relays.
Components of a Protection System :
1. Protective relays and its associated components in the Control Panel
2. Instrument Transformers ( CTs and PTs)
3. Trip Supply
4. Inter-tripping relays
5. Wiring
Electro-mechanical relays were used since the beginning and are now being replaced by static and numeric relays which are of high precision and requires lower maintenance.
Nature of Faults :
The faults occurring on the catenary system are due to;
Earth faults - caused by failure of overhead equipment insulators, flashover at arcing horns etc.
Overloads - caused by abnormal traffic conditions.
Incorrect Switching Operation - caused by incorrectly closing the bridging interrupter at Neutral section.
Relays used :
The Power Transformers have the following types of protection:
Over Current and Restricted Earth Fault Relay :
Over Current Protection :
The Transformer is protected for over current on both HV and LV side. Over Current Relays are set to a predetermined value such that they operate when the magnitude of current exceeds the pre-set value. The relay gets its input from the current transformers placed in the HV and LV side in the transformer circuit.
Restricted Earth Fault Protection :
The REF protection is provided as back up protection to the differential protection. Protection against earth faults is given by this relay. The term Restricted earth fault is because, the relay only protects for the earth faults in the restricted area (circuit). It works by measuring the actual current flowing to the earth and will operate if it exceeds a pre-set limit.
High Speed Differential Relay :
Differential Protection :
Differential relay is the reliable protection against internal faults for equipment like generators, transformers and busbar. Differential Protection operates for faults occurring in clearly defined region, i.e on two sides of the equipment that is to be protected. The differential protection relay is connected to bushing type CT of HV and LV sides of the transformer. The current entering and leaving the transformer is compared and the relay operates in case of any inequalities.
Buchholz Relay :
Buchholz Relay is a gas actuated relay that is used for the protection against internal faults of the transformer. It sts off an alarm in case of a slow developing fault or an incipient fault. The relay consists of two elements, a mercury switch connected to float on the upper end and at the lower end is a mercury switch hinged to a flap.
In case of a slow developing fault, the heat due to the fault causes some of the transformer oil to decompose thus producing hydrogen gas. The gas being light, tries to get into the conservator and in the process it gets blocked in the upper part of the relay chamber. When sufficient pressure if accumulated, the float tilts and closes the mercury switch attached to it. This completes the alarm circuit to sound the alarm.
In case of severe internal faults, the oil in main tank rushes towards the conservator through the lower part of Buchholz relay. By doing so, it closes the flap and hence its associated mercury switch to complete the alarm circuit.
In addition to the above mentioned protection schemes, excessive winding temperature and oil temperature alarm circuits with trip contacts are also provided.
The substation and its equipments are protected against over voltages by means of Lightning arresters, and against abnormal loads by feeder circuit breakers. The protective relays used for the protection against the faults in catenary system has its associated breakers whose functions are to detect all the short circuits on the catenary system and to operate with a minimum time delay.
The protective system for the catenary system should be such considered that it should be sufficient in case of an extension of the protection zone (if necessary the bridging interrupter shall be closed which extends the zone of protection).
Integrated Digital Traction Feeder Protection Relay
This relay is connected with the current transformer and potential transformers of type-1. It aids in the protection against catenary - earth fault, auto - reclosing of CB and wrong phase coupling and also over current.
Generally the zone of protection in case of normal scenario is from a TSS - FP to the next SP. This relay samples the current and voltage signals and when voltage drops and current raises it will calculate the values of R and X according to the RCA angle (parallelogram characteristics). When R and X are within set limit then relay will give the trip command to the connected breaker.
At a SP, there are possibilities for the bridging interrupters to be closed under normal conditions. This causes a short circuit of two different feeds from two different substations. Apart from this, wrong phase coupling can also occur when a train crossing an SP's Insulated Overlap, the pantograph is not lowered inspite of the signboards being placed. The wrong phase coupling occurs only as a result of mistakes. The integrated digital traction feeder trips the associated feeder circuit breaker in the event of wrong phase coupling.
This relay also provides protection against over current in the catenary system.
The voltage and current signals are sampled and in the event of any abnormal conditions the corresponding relay is actuated and the associated circuit breaker is tripped. Then after a pre-set time the circuit breakers are automatically closed. This is to avoid discontinuity of operation in case of any incipient or any short duration faults (which are not hazardous for the operation and/or people). If the CB is tripped again after the reclosure, it is locked indicating that the fault is persistant.
Panto - Flashover Relay :
When ever one section of the IOL is tripped on intermittent fault and the electric locomotive enters from live to dead section of FP, there will be a heavy flashover, particularlly when the panto leaves the IOL which may damage the panto. The extent of the damage is propotional to the intensity of the current drawn by the locomotive. The pantofalsh over relay is connected to the type-1 PT in the FP. It detects for any flashover and trips the CB connected to the live side of the overlap.
High Resistive Fault Relay :
The distance protective relay may fail to detect faults of high resistive nature. Protection against high resistive earth faults is provided by this type of relay which works on principle of vectorial difference between base and fault currents. If the difference is more than the pre-set limit, the trip command is initiated. This is used as a backup protection the main distance protection.
A relay is the most important component in any protection system. During a fault or any abnormal conditions, one or more of the electrical quantities such as current, voltage, phase angle, frequency, rate of rise of current in the circuit of the relay changes. Relays can be categorized as Electro-mechanical relays, Static relays, Numeric relays and some special types of relays; namely Buchholz relay and Thermal relays.
Components of a Protection System :
1. Protective relays and its associated components in the Control Panel
2. Instrument Transformers ( CTs and PTs)
3. Trip Supply
4. Inter-tripping relays
5. Wiring
Electro-mechanical relays were used since the beginning and are now being replaced by static and numeric relays which are of high precision and requires lower maintenance.
Nature of Faults :
The faults occurring on the catenary system are due to;
Earth faults - caused by failure of overhead equipment insulators, flashover at arcing horns etc.
Overloads - caused by abnormal traffic conditions.
Incorrect Switching Operation - caused by incorrectly closing the bridging interrupter at Neutral section.
Relays used :
The Power Transformers have the following types of protection:
Over Current and Restricted Earth Fault Relay :
Over Current Protection :
The Transformer is protected for over current on both HV and LV side. Over Current Relays are set to a predetermined value such that they operate when the magnitude of current exceeds the pre-set value. The relay gets its input from the current transformers placed in the HV and LV side in the transformer circuit.
Restricted Earth Fault Protection :
The REF protection is provided as back up protection to the differential protection. Protection against earth faults is given by this relay. The term Restricted earth fault is because, the relay only protects for the earth faults in the restricted area (circuit). It works by measuring the actual current flowing to the earth and will operate if it exceeds a pre-set limit.
High Speed Differential Relay :
Differential Protection :
Differential relay is the reliable protection against internal faults for equipment like generators, transformers and busbar. Differential Protection operates for faults occurring in clearly defined region, i.e on two sides of the equipment that is to be protected. The differential protection relay is connected to bushing type CT of HV and LV sides of the transformer. The current entering and leaving the transformer is compared and the relay operates in case of any inequalities.
Buchholz Relay :
Buchholz Relay is a gas actuated relay that is used for the protection against internal faults of the transformer. It sts off an alarm in case of a slow developing fault or an incipient fault. The relay consists of two elements, a mercury switch connected to float on the upper end and at the lower end is a mercury switch hinged to a flap.
In case of a slow developing fault, the heat due to the fault causes some of the transformer oil to decompose thus producing hydrogen gas. The gas being light, tries to get into the conservator and in the process it gets blocked in the upper part of the relay chamber. When sufficient pressure if accumulated, the float tilts and closes the mercury switch attached to it. This completes the alarm circuit to sound the alarm.
In case of severe internal faults, the oil in main tank rushes towards the conservator through the lower part of Buchholz relay. By doing so, it closes the flap and hence its associated mercury switch to complete the alarm circuit.
In addition to the above mentioned protection schemes, excessive winding temperature and oil temperature alarm circuits with trip contacts are also provided.
The substation and its equipments are protected against over voltages by means of Lightning arresters, and against abnormal loads by feeder circuit breakers. The protective relays used for the protection against the faults in catenary system has its associated breakers whose functions are to detect all the short circuits on the catenary system and to operate with a minimum time delay.
The protective system for the catenary system should be such considered that it should be sufficient in case of an extension of the protection zone (if necessary the bridging interrupter shall be closed which extends the zone of protection).
Integrated Digital Traction Feeder Protection Relay
This relay is connected with the current transformer and potential transformers of type-1. It aids in the protection against catenary - earth fault, auto - reclosing of CB and wrong phase coupling and also over current.
Generally the zone of protection in case of normal scenario is from a TSS - FP to the next SP. This relay samples the current and voltage signals and when voltage drops and current raises it will calculate the values of R and X according to the RCA angle (parallelogram characteristics). When R and X are within set limit then relay will give the trip command to the connected breaker.
At a SP, there are possibilities for the bridging interrupters to be closed under normal conditions. This causes a short circuit of two different feeds from two different substations. Apart from this, wrong phase coupling can also occur when a train crossing an SP's Insulated Overlap, the pantograph is not lowered inspite of the signboards being placed. The wrong phase coupling occurs only as a result of mistakes. The integrated digital traction feeder trips the associated feeder circuit breaker in the event of wrong phase coupling.
This relay also provides protection against over current in the catenary system.
The voltage and current signals are sampled and in the event of any abnormal conditions the corresponding relay is actuated and the associated circuit breaker is tripped. Then after a pre-set time the circuit breakers are automatically closed. This is to avoid discontinuity of operation in case of any incipient or any short duration faults (which are not hazardous for the operation and/or people). If the CB is tripped again after the reclosure, it is locked indicating that the fault is persistant.
Panto - Flashover Relay :
When ever one section of the IOL is tripped on intermittent fault and the electric locomotive enters from live to dead section of FP, there will be a heavy flashover, particularlly when the panto leaves the IOL which may damage the panto. The extent of the damage is propotional to the intensity of the current drawn by the locomotive. The pantofalsh over relay is connected to the type-1 PT in the FP. It detects for any flashover and trips the CB connected to the live side of the overlap.
High Resistive Fault Relay :
The distance protective relay may fail to detect faults of high resistive nature. Protection against high resistive earth faults is provided by this type of relay which works on principle of vectorial difference between base and fault currents. If the difference is more than the pre-set limit, the trip command is initiated. This is used as a backup protection the main distance protection.
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