This article will explain the modeling of overvoltage relay in MATLAB Simulink. Which blocks are required to create over voltage relay in MATLAB. How to combine those blocks. Testing and simulation results of modeling of overvoltage relay in MATLAB Simulink are also included in this article.
What is overvoltage relay and why it is used?
It is a protective device used in electrical system which provides protection from over voltage condition. Power system can encounter over voltage condition which can be due to sudden drop of load, addition of power source and due to switching transients. If the voltage increases from certain limit, it can damage insulation and can lead to insulation puncture. Therefore it is necessary to monitor the voltage of system continuously. If high voltage is encountered in any part of power system, that faulty portion of network should be isolated. Over voltage relay continuously monitors the system voltage and if over voltage condition occurs, it sends tripping command to the breaker.
Need to model overvoltage relay in MATLAB
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 component is available in the MATLAB library among which overvoltage relay is also included. So Engineers use the available components in library of MATLAB is used to model the unavailable devices. The purpose of this article is to explain the process of modeling of over voltage relay in MATLAB Simulink using the readily available components. Please note that this presents the modeling in MATLAB simulink however actual construction of over voltage relay will be different. The overview of construction of overvoltage relay is presented in following article
Following blocks are used to model over voltage relay in MATLAB.
- 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) is used to take three phase measurements collectively. In order to separate the phase currents and voltage, Demux is used. The position and number of inputs & outputs can be seen in the connection diagram.
Relational operator: This block play key role in modelling of overvoltage relay. It compares the current in the circuit (particular bus) with the predetermined relay settings. In the given case, if the quantity at input 1 is greater than quantity at its input 2, the output of this block will becomes high. So when the load current is less than relay setting, low output is achieved. When the fault occurs making the voltage high, the system voltage will exceed the relay setting, the relational operator gives high output.
Constant (Settings): A constant block which is named settings in our circuit is used to specify pre-determined settings of the relay. This block is connected to the input of the relational operator
S-R Flip Flop and AND gate: Flip flop has two inputs S set and R reset. Output Q 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 inputs at AND gate are high, output of AND gate is also high (1). High input to breaker keeps it close ensuring continues operation. When the load current exceeds relay settings, high input S makes output !Q low, resulting in low output from AND gate.
Data Type conversion (Double): The output of the AND gate is Boolean data type. Such data type is only readable by digital electronic devices. 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 that 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 reclose in order to reinstate the detached part of system. For this purpose a reclosing signal needs to be given to the circuit breaker by the relay. To add 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. Transport delay counts 5 sec. 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 alteration and reset the relay by giving reset signal to flip flop after every 10 sec.
Designed overvoltage relay in MATLAB Simulink is shown in the above figure.
Testing of Overvoltage relay on Power system.
After completing the modeling of overvoltage relay in MATLAB Simulink, a simple power system is modeled in Simulink. It consists of a 40MVA grid, whose power is transferred to 15 MW industrial and 20 MW domestic loads 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 the modeled relay, fault is introduced at domestic load. Position of overvoltage relay and breaker are shown in the following figure. The details of power system are also shown. Over voltage fault is introduced by adding a voltage source at load end. In real condition, over voltage can occur due to several reasons including sudden drop of load, switching transients or any un-symmetrical faults.
Following figure shows the voltage at bus 4 without using the relay. It can be seen that at instant of 1.5 sec, over voltage fault is introduced in the system by adding a voltage source at load end and it is cleared at 2.5 sec. It can be seen that voltage level is increased to 900V which can be dangerous for electrical as well as home appliances.
The following figure shows the voltage and relay output signal when over voltage relay is installed.
Lower graph shows the bus 4 voltage. At 1.5 sec, over voltage fault is introduced as explained above and its value exceeds relay setting. Soon after the fault, the voltage becomes normal because the over voltage relay tripped the breaker isolating domestic load from system. Near 2.1 sec, some voltage spikes can be seen which is due to momentary closing of breaker by relay.
Upper graph shows the relay output signal and preset relay settings. During normal condition its output is one. But when voltage of system exceeds the preset voltage settings; its output signal becomes zero, tripping the breaker. At 2.1 sec, the relay sends reclosing signal to the breaker however the over voltage condition still exists, so the breaker is tripped again. At 2.5 sec the fault is cleared and breaker is reclosed at 2.55 sec shown by relay output signal.
NOTE: ignore the initial tripping signal it is due to high initial voltage. Such false tripping can be avoided in practical situations.