Protection Systems Applied in Substation and Power Networks


Protection Relay | Source: Siemens 

The task of protection and control in substations and in power grids is the provision of all the technical means and facilities necessary for the optimal supervision, protection, control and management of all system components and equipment in high and medium-voltage power systems. 


"The task of the control system begins with the position indication of the HV circuit breaker and ends in complex systems for substation automation, network, and load management as well as for failure- and time-based maintenance". 


The purpose of these devices is to detect faults and to switch off and isolate these selectively and quickly from the network as a whole so that the consequences of the fault are limited as much as possible. With today’s high fault current levels and highly integrated networks, faults have far-reaching consequences, both direct (damaged equipment) and indirect (loss of production). Protection relays must therefore act very fast with the greatest possible reliability and availability, however also very selectively, to not switch off parts where it is not needed.


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Limit Protection 

  • Overcurrent and Time-Overcurrent protection 
  • Overload protection
  • Frequency protection
  • Voltage protection
  • Other limit protections


Overcurrent and Time-Overcurrent protection 

Single- or three-phase currents above a set limit will be detected and switched off after an also set time delay. The tripping time is independent how much the limit has been exceeded. This protection is called Definite Time Lag (DTL) Relay. 


Time-Overcurrent relays are used in radial networks with single infeed. The relays are connected via a current transformer. With a direction-sensing function measuring current and voltage and considering changing phase relations in case of fault, the relay is extended to directional time-overcurrent protection. Such protective devices are preferably used for parallel lines and for the undervoltage sides of parallel operating transformers.


Overload Protection

The temperature conditions at the protected object are simulated with the same time constant in the relays. Any load bias is taken into account by this thermal replica in the relay in accordance with the heating and cooling curves. Alarm signals or trip commands are issued if a set temperature limit is exceeded. The relays are built as secondary relays and operate usually in two or more stages. Overload relays are used for objects that can overheat such as transformers and motors, but less commonly for cables. The quality of this protection depends strongly on the accuracy of the thermal replica.


Frequency Protection

If the frequency (f) goes above or below set limits or decays at an unacceptable rate (df/dt), this is detected and results in load shedding or disconnection of network parts (islanding). The deviation from the rated frequency is a good indication for an imbalance between produced and consumed active power (P). If e.g. the frequency decays because of the loss of generating group, a corresponding load has to be shed as soon as possible.


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Voltage Protection

Voltage deviations are reported, allowing the system load to be reduced as necessary. There are both over-and undervoltages.


Other Limit Protection

Other protective devices used for dedicated objects in the substation include e.g. interturn-fault, negative sequence, reverse-power protection for generators. Buchholz protection, temperature monitors, oil level indicators, oil and air flow indicators are used for power transformers. Insulation monitoring is special for conductors.


Testing of Protection Relays | Source: Megger



Comparisson Protection

  • Differential protection 
  • Busbar protection  
  • Comparative protection


Diffrential Protection

The currents measured at the beginning and end of the protected object (line, transformer, generator, etc.) are matched in phase angle and magnitude and compared. If a set ratio of difference current to through current is exceeded, the relay issues a trip command. 


The differential protection is switching off very fast and selectively the faulted object between the measuring points. For correct protection operation, the communication link has to be supervised properly. To cope with the loss of communication, the differential protection is complemented by distance or overcurrent protection.


Busbar Protection

The busbar in a substation is a node in the power grid. According to Kirchhoff’s law the sum of all incoming and outgoing currents has to be zero. The busbar protection acquires and sums up all these currents. If the current exceeds a set value near zero, all connected feeders are tripped.


Very commonly, busbar protection consists of one centralized unit to calculate the current difference and make the trip decision, and one decentralized unit per feeder for data acquisition and trip execution. In the decentralized unit, all line protection functions may be integrated also, at least for back-up protection.


Comparative Protection

The variables measured at the beginning and end of the protected object are not compared per sample but as averages in a certain time window (e.g. for a half-wave of sinusoidal values) checked for coincidence (phase comparison protection) or for equal signal direction (signal comparison). These protection devices require only low communication bandwidth and are very insensitive to interference. Since not the raw data but calculated data are used this protection is slower than the differential and busbar protection.


Directional Protection

  • Distance protection
  • Directional earth-fault relays
  • Object protection with directional comparison relays


Distance protection 

The distance of a fault from the relay is calculated by comparing the fault impedance with the known line impedance. Therefore, voltage and current are measured and a tripping range (protection zone) is assigned. A device for distance protection comprises normally some forward zones and one backward zone. The tripping characteristic is represented in the impedance plane as complex polygons or circles. In accordance with adjustable distance-time parameters, the distance protection trips the allocated circuit breaker directly or with some delay as some kind of back-up protection. 


Directional earth-fault relays

An indication of direction is obtained from the sign of the angular difference between the phasors of neutral current and neutral voltage. The side of the fault is identified by comparing the values measured in the network. Other methods of measurement are possible.


Object protection with directional comparison relays

Are direction protection relays at the boundary of a power grid zone or of a busbar bay comparing all acquired directions, it may be decided if the fault is inside or outside these extended objects. A comprehensive communication network is needed, but with a low bandwidth only because of the limited information to be exchanged. Compared with differential protection, this simple solution is slower because of the time needed for fault direction. 


Autoreclosing

In case of faults on overhead lines the line protection (e.g. time-overcurrent relay or distance relay) interrupts one or all three phases to cut off the power infeed into the fault. Assuming a transient fault the line or the power respectively should be switched on as soon as possible. For this purpose, the protection-related function auto reclosure is used. 


This function provides normally a closing sequence of one fast step and two slow ones. If the closing step is successful, the auto reclosure function is reset. If the fault persists, the protection will trip again and the next autoreclosing step is initiated. An unsuccessful closing sequence ends with a final trip of the breaker. After the first unsuccessful step, tripping and autoreclosing is done normally for all three phases independent from the first step. 


Autoreclosing assumes an appropriate communication between the protective device(s) and the autorecloser device e.g. by serial communication according to IEC 61850.



Reference

  • Protection and Control in Substation and Power Networks | Download
  • Publisher: ABB

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