Auto-reclose Relay Simulation for Research and Education

I. INTRODUCTION

Auto-reclose relay is one of main relay used in transmission system to reclose back transmission line after the line was tripped by main protection relay because of a fault [1]. It gets the trip signal from main relay (distance or line current differential relays) and starts the timer to reclose the line. After set time is elapsed, it will send a signal to circuit breaker to reclose back the line after all set conditions are fulfilled and supply of that line will be back to normal. If the fault still exists during reclosing time, main relay will send a trip signal again but auto-reclose relay will be locked out [2]. The conditions where auto-reclose relay will be locked out from operation is when a fault is permanent [3] such as due to tower collapse to ground, crane or tree permanently touching the line etc and also when there is evolving fault and fault during reclaim time [4] which is normally due to lightning.

Auto-reclose relay is normally installed at both local and remote connected substations for interconnected transmission grid. Auto-reclose scheme is a bit different for different levels of voltage [5]. It depends on how the line supposed to be reclosed. For lower transmission voltage level, normally all the three phases of a line will be tripped and reclose altogether at the same time and type of circuit breaker used is three poles circuit breaker. However, for higher voltage level, any affected phase of a line can be tripped and reclosed individually and type of circuit breaker used is one pole breaker where each phase has its own mechanism to trip and reclose the breaker. The set time for auto-reclose relay to send reclose signal to breaker is different for local and remote substations where for "dead line charging" substation, no synchronization is required to reclose circuit breaker and for the other end substation, synchronization is required where line and busbar at that time already energized. Because of that, the location/terminal where synchronization is required has a little extra reclosing time compared to dead line charging location for the synchronization to take place. Dead line charging location is where power is flowing out from the substation to remote substation which is the synchronization checking location (power flowing in) [6].

There are several conditions that must be met before reclosing signal will be sent to circuit breaker. The conditions are breaker must be in open status (after the tripping), signal received from main protection relay (such as distance or current differential relays) to start the timer, closing mechanism of circuit breaker must be ready and signal from synchronization check relay which confirmed that synchronization is successful.

There is many research have been done which modeled protection relays using Matlab Simulink whether for engineering research or for education such as distance relay [7]- [9], differential relay [10]- [13] and overcurrent relay [14]- [17]. This paper presents modelling and simulation of auto-reclose protection scheme using Matlab Simulink software which can be used for education or research by students/researchers in universities and other institutions. Modelling process was done with several assumptions to reduce the complexity of the model. Results are shown for the operation of line protection and auto-reclose relays starting from fault initiation, detection, tripping and finally reclosing.

II. RESEARCH SCOPES

Protection scheme is a complex and difficult scheme to be modeled using software. It is quite difficult to find research software which has built in models for protection relays. Specific protection relay models normally can be found in software used by electrical power industries and the cost to buy it is very high. Currently, there are no specific protection relay models or toolbox inside Matlab or Simulink package. The research scopes for this project are listed in following subsections.

A. Single Line to Ground Fault

Only single line to ground fault was considered in the model. Fault at any phase of the line will make all three phases to be tripped and reclosed simultaneously at the same time.

B. Definite Time Overcurrent Relay

Main protection relay used to protect transmission line normally distance or current differential relays. However, to reduce the complexity of model, the relay developed and used in model is definite time overcurrent relay which will send the trip signal to circuit breaker and to start auto-reclose relay timer after a set delay time.

C. Fault inside the Range of Protected Line

Normal operation is auto-reclose relay will be blocked for fault outside protected line although main protection relay detects the fault. For this model, blocking feature of auto-reclose relay was not modeled so faults only simulated to occur inside the range of protected line.

D. Single Shot Auto-reclose

In this project, only single shot auto-reclose was modeled meaning that only one attempt will be made by auto-reclose relay to reclose the line. Trip and lockout signal will be sent to circuit breaker for permanent fault and fault during reclaim time.

E. Three Poles Auto-reclose

As already mentioned in introduction, auto-reclose can be single pole or three poles schemes where single pole and three poles circuit breakers are used respectively. In this model, the breaker used is three poles type meaning all the phases will be tripped and reclosed simultaneously at the same time.

F. Synchronization Checking

Synchronization between line and busbar voltage is one of input to auto-reclose relay. Synchronization must be fulfilled before auto-reclose signal can be sent. The setting of synchronization relay can be either Dead Line Live Bus (DLLB) or Live Line Live Bus (LLLB). To simplify the model, synchronization requirements which are voltage magnitude difference, voltage phase angle difference, frequency deviation range and phase rotation are assumed to be fulfilled. Table I shows parameters and settings used in the model and simulation. The parameters are for three phase sources which represent local and remote substations, three phase transmission line and three phase fault while the settings are for over-current and auto-reclose relays. Figure 1 shows substations, transmission line, load and measurements subsystem. Transmission line is separated into two parts to simulate single phase-to-ground fault at red phase conductor 10 km from local substation. Figure 2 shows subsystem for definite time over-current relay. The function of low pass filter subsystem is to filter any harmonic component which may appear during fault occurrence. Rms value of fault current will be compared with pickup current setting. If fault current is more than pickup current, operation time of over-current relay will be started and after it has been elapsed, two signals will be sent which are signal to initiate/start auto-reclose relay and signal to trip the circuit breaker. There are two definite time over-current relays installed which are one at local substation and the other one at remote substation. Both relays have same operation time which is 0.5 s. First relay to detect and send the trip signal is the relay which is nearer to fault point because fault current is higher. Figure 3 shows auto-reclose relay subsystem. For this subsystem, the inputs or requirements are circuit breaker status, circuit breaker in ready condition and synchronization status. After auto-reclose initiation/start signal is received from over-current relay, dead time will be started. Similar to over-current relay, auto-reclose relay is installed at both substations but with different dead times. The last autoreclose relay to reclose (weak point) has slightly longer dead time than auto-reclose relay at strong point. This is to ensure sufficient time for synchronism checking relay to detect synchronism between bus and line voltages at weak point substation. In the case of no synchronism achieved, after the dead time of weak point auto-reclose relay has been elapsed, auto-reclose relay will lock out and no reclose attempt will be made.

Figure 1

Substations, transmission line, load and measurements subsystem Auto-reclose relay subsystem

Figure 2

Figure 3

III. MODELLING AND SIMULATION

IV. SIMULATION RESULTS

This section presents the results of auto-reclose scheme after a transient fault was initiated at red phase line. Figure 5, 6, 7 and 8 show waveforms of local red phase current, local red phase current in rms, local red phase-to-ground voltage and local red phase-to-ground voltage in rms respectively. A fault was initiated from 0.06 s until 0.12 s at 10 km from local substation.

Figure 5

Local red phase current (A) Fig. 6. Local rms red phase current (A) Fig. 7. Local red phase-to-ground voltage (V) Local rms red phase-to-ground voltage (V) V. CONCLUSION

From all figures, after normal condition, fault was detected at around 0.071 s when fault current was higher than pickup current (Ipickup = 345.6 A) but breaker was not trip yet. It can be seen that current became very high from nominal current while voltage dropped from nominal value during fault condition. After fault was detected, timer of 0.5 s of local definite time over-current relay was started and trip signal was sent to local circuit breaker at around 0.582 s and line current became zero due to open circuit. Voltage became zero as circuit breakers at both substations were tripped at the same time. At the same time of trip signal sent, a signal was sent to auto-reclose relay to initiate/start auto-reclose relay. The dead time of 1.0 s for local auto-reclose relay was started and running until around 1.58 s and local circuit breaker was reclosed. The current still zero because remote circuit breaker didn't reclose yet.

After local breaker reclosed, voltage at local substation back to normal but slightly higher than nominal voltage (equal to busbar voltage). At around 2.076 s, remote breaker was reclosed after remote auto-reclose relay dead time of 1.5 s had been elapsed. Remote breaker successfully reclosed after all conditions for remote auto-reclose relay such as breaker in open condition, closing coil was ready, initiate/start signal from over-current relay received and synchronism between line and busbar voltage were achieved. It can be seen that after both circuit breakers from local and remote substations successfully reclosed, current and voltage back to nominal value. Auto-reclose scheme was successfully modelled and simulated using Matlab/Simulink software. From the result, the waveforms produced show the capability of Matlab/Simulink software in modelling and simulating power system protection scheme even for complex protection scheme such as auto-reclose scheme. In the designed Simulink model, some limitations and assumptions were applied to ease modelling process because the aim of this research is for educational and research purpose and the important thing is the waveforms produced are as expected and similar to results produced by dedicated protection system software. However, the limitations and scopes as stated earlier can be further included in the model to simulate actual scheme applied in transmission substations.