This article discusses the modeling of overcurrent relay in simulink MATLAB. Which Blocks are used for this project. How to setup different blocks. Test bed and verification of modeled relay will be tested on the testing circuit.
What is over current relay?
It is an electrical protective device which senses the overcurrent condition and trips the circuit breaker if the current increase from certain preset value. In simple words it protects the electrical system by tripping the circuit breaker when the current exceeds certain value. There are basically three main types of over current relay.
- Instantaneous over current relay: It gives the trip command to the circuit breaker instantaneously without any delay when the current exceeds certain limit.
- Definite time over current relay: This over current relay trip the circuit breaker after a preset delay when current exceeds preset limit.
- Inverse time over current relay: This relay sends trip command to the breaker after a time delay which is inversely proportional to the current magnitude. When the magnitude is very high the delay is less. When the magnitude of current is comparatively low, the time delay is more. So the delay in the trip signal has inverse relation with the magnitude of current.
MATLAB is one of the most extensively used software by the engineers. It has a rich library enabling user to perform a number of tasks. However not every equipment is given the MATLAB library. So the rich library of MATLAB is used to construct the unavailable devices. Overcurrent relay which is already described above is not available in MATLAB library. In this regard this article will explain the step by step process of constructing, testing and modeling of overcurrent relay in simulink MATLAB.
Blocks which are needed for modeling the overcurrent relay in simulink MATLAB are as follows
- De-multiplexer (Demux)
- Relational operator
- RS flip-flop
- AND gate
- Data type conversion
- Transport delay
- Detect Change
Function of each block along with connection diagram is shown in the figure.
DEMUX: The three phase measurement block (labeled as Bus) gives current and voltage of three phases collectively. In order to separate the phase currents and voltage we use Demux. The position and number of inputs, output can be seen in the figure.
Relational operator: This block play key role in modelling of overcurrent relay in MATLAB simulink. It compares the current in the circuit (particular bus) with the predetermined relay settings. In the given case, if the quantity at input 1 which is current in bus 4 (load current) is greater than quantity at its input 2 which is preset settings, the output of this block will becomes high. So when the load current is less than relay setting, the output is low.
Constant (Settings): A constant block is named settings in our circuit. It is used to set pre-determined pick up value of the relay. This block is connected to relational operator which compares the current of circuit with preset settings.
S-R Flip Flop and AND gate: Flip flop has two inputs S set and R reset. Output Q gives reflects the input S when input R is low. It means what appears on S also appears on Q. The output !Q is inverts the input S. In this setup output !Q is used. In normal condition when load current is less than relay settings, relation operator gives low output. So S input is low and output !Q is high. Since all input at AND gate is high output of AND gate is also high (1). High input to breaker keeps it close and continues circuit operation. When the load current exceeds relay settings, high input (S) makes output (!Q) low. Consequently AND gate gives low output.
Data Type conversion (Double): The output of the AND gate is Boolean data type. Such data type is only readable by digital electronic devices. Since the output of AND gate needs to be given to the Circuit breaker which works with double data type values. So the output of AND gate is converted from Boolean to double by data type conversion block. This makes the AND gate compatible with Circuit breaker.
Transport Delay: In every practical scenario it takes some time for the relay to sense the faulty condition and gives tripping signal to circuit breaker. To include this practical aspect, a transport delay bock is added which sends trips command to circuit breaker with 0.001 s delay.
Re-closure of Circuit breaker (Transport delay + detect change + Data conversion): During fault, the circuit breaker opens thus isolating faulty part of the power system from the rest of the system. However after the fault is cleared, the circuit breaker has to re-close in order to reinstate the detached part of system. For this purpose a re-closing signal needs to be given to the circuit breaker by the relay. To model this facility, transport delay + detect change + data conversion blocks are used. It gives the reset signal to the flip flop after every 5 sec when circuit breaker is tripped. Detect change is such a block which gives high output whenever a change occurs in its initial state. The initial state of relay is high (1). When fault occurs, relay changes its output state to low (0) to give trip signal to breaker. Detect change block senses the change and reset the relay by giving reset signal to flip flop after every 10 sec.
To validate the modeling of overcurrent relay in simulink MATLAB, a simple power system is modeled in Simulink. It consists of a 40MVA grid, whose power is transferred to 15 MW industrial and 20MW domestic load through 50Km transmission line. Transmission voltage is 33KV which is step down to 440V using transformers. A measurement block is added to show the voltages and currents of each bus. To test relay, fault is introduced at domestic load. Position of overcurrent relay and breaker is shown in the following figure. The details of power system are also shown. Three phase symmetrical Fault is introduced at 1.2 sec and cleared at 2.5 sec.
Following figure shows the voltage and current of bus 4 without using the relay. It can be seen that very high current flows through the system during fault while voltage is very low during fault.
The following figure shows the voltage and current of bus 4 when overcurrent relay in MATLAB Simulink is installed.
Upper graph shows the voltage. It can be seen that during fault, small voltage dip occurs as compared to condition when relay was not installed.
Middle graph shows the relay setting and current through bus 4. It can be observed that during normal condition, system current is less than relay settings. At 1.2 sec, fault occurs and system current exceeds relay settings. In 0.05 sec, the breaker is tripped and fault current is reduced.
So the given results our exercise of modeling of overcurrent relay in simulink MATLAB.