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ABSTRACT
Transformers are static devices which are completely sealed and immersed in oil, as a rule. Therefore, the chances of errors on them are very rare. “But the consequences of even a rare error can be very serious if the transformer is disconnected from the network quickly. This requires adequate protection for the auto-transformer of possible errors. Small distribution transformers are usually connected to the network through series fuses instead of circuit breakers. However, the probability of failures of transformers and certainly more so the protection is absolutely necessary, the transformer is large and very important institutions in power. It requires reliable protection devices.
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Transformer protection using numerical relays is the modem version of protection. Numerical relays have got several advantages as compared to ordinary electromagnetic relays. Even though the chances of faults occurring on them are very rare, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system.
INTRODUCTION
Protective relays work in concert with sensing and control devices to accomplish their function. Under normal power system operation, a protective relay remains idle and serves no active function. But when fault or undesirable condition arrives Relay must be operated and function correctly.
A Power System consists of various electrical components like Generator, transformers, transmission lines, isolators, circuit breakers, bus bars, cables, relays, instrument transformers, distribution feeders, and various types of loads. Faults may occur in any part of power system as a short circuit & earth fault. Fault may be Single Line to Ground, Double Line to Ground, Line to Line, three phase short circuit etc.
The protection system operates and isolates the faulty section. The operation of the protection system should be fast and selective i.e. it should isolate only the faulty section in the shortest possible time causing minimum disturbance to the system. Also, if main protection fails to operate, there should be a backup protection for which proper relay co-ordination is necessary. Failure of a protective relay can result in devastating equipment damage and prolonged downtime.
Objective: The main aim of this topic is to analyze and study the existing transformer protection relays used in different types of protection employ different types of relays. Here we have studied about functioning of transformer protection using numerical relays.
REQUIREMENT ANALYSIS
It is mainly consist of hard components and software components used as follows:
2.1 HARDWARE REQUIRMENTS:
• Current transformer
• I to V converter
• Precision rectifier
• Analog to digital converter
2.2 SOFTWARE REQUIREMENTS:
• Multiplexer
TRANSFORMER FAULTS AND PROTECTION SCHME
3.1 COMMON TRANSFORMER FAULTS:
Transformer may suffer only from:
• Open circuit
• Over heating
• Winding short circuits (eg: earth- faults, phase to phase faults & inter-turn
faults)
3.1.1 OPEN CIRCUIT FAULTS:
An open circuit in one phase of a three-phase transformer may cause undesirable heating. In practice, relay protection is not provided against open circuits because this condition is relatively harmless. On the occurrence of such a fault, the transformer can be disconnected manually from the system
3.1.2 OVER HEATING FAULTS:
Over heating of the transformer is usually caused by sustained overloads or short circuits & very occasionally by the failure of the cooling system. The relay protection is :also not provided against this contingency and thermal accessories are generally used to sound an alarm or control the banks of fans.
3.1.3 WINDING SHORT CIRCUIT FAULTS:
Winding short-circuits (also called internal faults) on the transformer arise from deterioration of winding insulation due to over heating or mechanical injury. When an [internal fault occurs, the transformer must be disconnected quickly from the system [because a prolonged arc in the transformer may cause oil fire. Therefore, relay protection is absolutely necessary for internal faults.
PROTECTIVE RELAYS:
"Protective relay is a device that detects the faults and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system."
High-performance protective relaying comes into its own when it's a question of minimizing power system operating costs. Uncomplicated operation, convenient commissioning tools and flexible communication are all important elements when service and maintenance costs have to be reduced.
The relays detect the abnormal conditions in the electric circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening breaker and disconnection of faulty circuit.
FUNDAMENTAL REQUIREMENS OF PROTECTIVE RELAYING:
• Speed
• Sensitivity
• Reliability
• Simplicity
• Economy
• Protection relays can be classified in various ways depending on their construction, functions and are discussed Selectivity below.
3.2.1CLASSIFICAION OF PROTECTIVE RELAYS BASED ON TECHNOLOGY
Protective relays can be broadly classified into the following categories depending on the technology used for their construction and operation.
• Electromagnetic relays
• Static relays
• Microprocessor-based relays
WORKING AND OPERATION OF NUMERICAL RELAY:
Numerical relays are micro processor based relays and having the features of recording of parameter used as disturbance recorder flexibility of setting & alarms & can be used one relay for all type of protections of one equipment hence less area is required. Wide Range of setting, more accurate, Low burden hence low VA of CT is required which minimize the cost. Numeric relays take the input analog quantities and convert them to numeric values. All of the relaying functions are performed on these numeric values.
The following sections cover relay hardware, relay software, multiple protection characteristics, adaptive protection characteristics, data storage, instrumentation feature, self-check feature, communication capability, additional functions, size and cost-effectiveness. Numerical protection devices are available for generation, transmission and distribution systems. Modern power system protection devices are built with integrated functions. Multi-functions like protection, control, monitoring and measuring are available today in numeric power system protection devices. Also, the communication capability of these devices facilitates remote control, monitoring and data transfer.
These relays provide great precision and convenience in application in the sophisticated electronic products. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays. The disadvantages of a conventional electromechanical relay are overcome by using micro controller for realizing the operation of the relays. Micro controller based relays perform very well and their cost is relatively low Also, the communication capability of these devices facilitates remote control, monitoring and data transfer. Traditionally, electromechanical and static protection relays offered single-function, single characteristics, whereas modern numeric protection offers multifunction and multiple characteristics. Some protections also offer adaptable characteristics, which dynamically change the protection characteristic under different system conditions by monitoring the input parameters.
OPERATION OF RELAY:
A current signal from CT is converted into proportional voltage signal using I to V converter. The ac voltage proportional to load current is converted into dc using precision rectifier and is given to multiplexer (MUX) which accepts more than one input and gives one output.Microprocessor sends command signal to the multiplexer to switch on desired channel to accept rectified voltage proportional to current in a desired circuit.
Block diagram of numerical relay
Output of Multiplexer is fed to analog to digital converter (ADC) to obtain signal in digital form. Microprocessor then sends a signal ADC for start of conversion (SOC), examines whether the conversion is completed and on receipt of end of conversion (EOC) from ADC, receives the data in digital form. The microprocessor then compares the data with pick-up value. If the input is greater than pick-up value the microprocessor send a trip signal to circuit breaker of the desired circuit.
In case of instantaneous over current relay there is no intentional time delay and circuit breaker trips instantly. In case of normal inverse, very inverse, extremely inverse and long inverse over current relay the inverse current-time characteristics are stored in the memory of microprocessor in tabular form called as look-up table.
4.2 ADVANTAGES OF NUMERICAL RELAY:
Compact Size: Electromechanical Relay makes use of mechanical comparison devices, which cause the main reason for the bulky size of relays. It uses a flag system for the indication purpose whether the relay has been activated or not. While Numerical Relay is in Compact Size and use Indication on LCD for Relay activation.
Digital protection can be physically smaller, and almost always requires less panel wiring than equivalent functions implemented using analog technology.
Flexibility: A variety of protection functions can be accomplished with suitable modifications in the software only either with the same hardware or with slight modifications in the hardware.
Reliability: A significant improvement in the relay reliability is obtained because the use of fewer components results in less interconnections and reduced component failures.
Multi Function Capability: Traditional electromechanical and static protection relays offers single-function and single characteristics. Range of operation of electromechanical relays is narrow as compared to numerical relay.
Different types of relay characteristics: It is possible to provide better matching of protection characteristics since these characteristics are stored in the memory of the microprocessor.
Digital communication capabilities: The microprocessor based relay furnishes easy interface with digital communication equipments. Fibre optical communication with substation LAN
Modular frame: The relay hardware consists of standard modules resulting in ease of service.
Low burden: The microprocessor based relays have minimum burden on the instrument transformers.
Sensitivity: Greater sensitivity and high pickup ratio.
Speed: With static relays, tripping time of ½ cycle or even less can be obtained.
Fast Resetting: Resetting is less.
Data History: Availability of fault data and disturbance record. Helps analysis of faults by recording details of
• Nature of fault
• Magnitude of fault level
• Breaker problem
• C.T. saturation
• Duration of fault
4.3 LIMITATION OF NUMERICAL RELAY:
• Numerical Relay offers more functionality, and greater precision. Unfortunately, that does not necessarily translate into better protection.
• Numerical Relay can make faster decisions. However, in the real world, faster protection itself is of no value because circuit breakers are still required to interrupt at the direction of the protective equipment, and the ability to make circuit breakers interrupt faster is very limited.
• Numerical Relay protection often relies on non-proprietary software, exposing the system to potential risk of hacking. Numerical Relay protection sometimes has exposure to externally-sourced transient interference that would not affect conventional technology.
• Numerical Relay protection shares common functions. This means that there are common failure modes that can affect multiple elements of protection. For example, failure of a power supply or an input signal processor may disable an entire protective device that provides many different protection functions.
CONCLUSION:
Transformer protection using numerical relays is the modem version of protection. Numerical relays has got several advantages as compared to ordinary electromagnetic relays. the chances of faults occurring on them are very rare, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessitated providing adequate automatic protection for transformer against possible faults.
5.2 FUTURE SCOPE:
It is almost impossible to have a relaying scheme that a suitable for all types of faults and abnormal conditions. therefore , it is necessary to consider some of the situation for which further research is required to improve the performance of a digital relaying scheme is suggested and different digital distance relaying schemes are proposed for two types of simultaneous faults , namely ,inter circuit faults and simultaneous open conductor and ground faults .