Increased reliability over current relay (ocr) as a transformer protection with non-cascade coordination patterns

Protection using Over Current Relays (OCR) with cascade coordination patterns is commonly used in 20 kV primary distribution networks to protect transformers from phase to phase fault currents. To anticipate a large fault current, the OCR feeder is equipped with an instant time setting. The problem occurs when the OCR feeder or PMT feeder fails to work then the transformer will be at risk because it is burdened with the fault current for a relatively long time. This study will discuss the use of a non-cascade pattern for the coordination of OCR feeders and incoming OCR in a primary distribution system supplied by a 60 MVA transformer, 150/20 kV from the substation by the calculation method. OCR feeders use very inverse (VI) characteristics and OCR inverses use standard inverse (SI) characteristics. A large feeder fault current is obtained by determining the 3 phase fault current upstream of the feeder as 7 x the nominal current (In) of the transformer. The result is that when the PMT feeder fails to work, the incoming OCR cascade coordination pattern will work with a time of 1.05 seconds. Meanwhile, if we use a non-cascade pattern the incoming OCR will work in the time setting of 0.3 seconds. Work time difference of (1.05 - 0.3 = 0.75 s) shows an increase in protection reliability and reduces the risk of damage to the transformer.


Introduction
Outages in the distribution power system are expected to have a localized effect. Fault Statistics show that distribution power systems contribute the most to customer interruptions and failure events [1], [2]. Among all faults, a balanced 3-phase fault current is the largest fault current. However, it is one of the least to occur [3], [4]. Over-current relay (OCR) is a relay in the distribution power to protect the transformer from phase to phase fault current, include 3-phase fault current.
This relay works by comparing the input current and the relay current setting. If it exceeds the relay current setting, the relay will send a trip signal to the Circuit Breaker (CB) after a time delay applied to the settings [5], [6]. Three-phase faults can occur at the feeder or 20 kV bus. If the fault occurs at the feeder, the OCR feeder must work first and OCR incoming will work as a backup. If the fault occurs at a 20 kV bus, the OCR incoming must work. So OCR feeder and OCR incoming must be coordinated to obtain selective, sensitive, and reliable working [7], [8]. Proper coordination between OCR feeder and OCR incoming is needed to improve the reliability of the power distribution system. Reliability of a power distribution system is defined as the ability to deliver uninterrupted service to the customer. The closer the circuit breaker to the source works, the less reliable because of more customer outage from the distribution power system [9]. Generally, the coordination pattern used in the radial distribution system is cascade coordination. This coordination pattern is to be less reliable in protecting power transformers when a large fault current occurs, for example, 3 phase fault very close to the bus 20 kV. OCR feeder will detect fault current and send a signal trip to the Circuit Breaker (CB) feeder, but if the OCR feeder or CB fails to work, the incoming OCR will take over to protect the transformer by sending the signal trip to the incoming CB. With a cascade pattern, the OCR incoming will work using a delay time. It will be a risk for the transformer because the transformer receives a very large current in an amount of time [10], [11].
This study compares the work reliability of feeder OCR and incoming OCR using the cascade coordination pattern and the non-cascade coordination pattern. The study was conducted on a 60 MVA, 150/20 kV power transformer that supplies a 20 kV primary distribution system.

Methodology
The method in this study is a calculation based on

Fault current at 20kV bus
Fault current calculation requires a positive sequence fault impedance on the 20 kV bus (Z1) that consists of network impedance (Z N ), transformer impedance Z T ), cable impedance (Z L ).

Cable impedance calculation (ZL)
The type of cable used is IEC 6502-2, 630 mm 2 with impedance 0,1259 Ω/km. The cable length from the transformation to the 20 kV bus is 90 m. So the cable impedance (Z L ): pu 10 x 1.275 = 10 49 . 8 20 Positive sequence fault impedance at 20 kV bus incoming (Z1) is:

Fault current calculation at 20kV bus
The largest fault current is the 3 phase fault which is calculated using the following equation:

Tms OCR feeder
The relay work time in the feeder must be faster than the relay work time installed on incoming. The value taken for the time setting is not less than 0.3 seconds with the aim that the relay does not trip due to the inrush current of the distribution transformer connected to the distribution network. In this calculation, the selected value is 0.4 seconds. Relay characteristics use Inverse Standards (SI). So the Tms value that will be set up in the feeder is as follows.  Table 1 shows the characteristics of OCR incoming working time to fault currents 3 x In until 7 x In.

Cascade pattern OCR coordination
The OCR setting calculated in 3.1 -3.5 is a cascade protection pattern. To speed up the handling of very large fault currents the cascade pattern protection is equipped with instantaneous settings. In the cascade pattern, instantaneous settings are only applied to the feeder. While the working time for incoming is the same as the initial settings. Instantaneous settings in the feeder that are applied to a cascade pattern at least 0.8 x 4 x In transformer : For example, setting I >> is 7 with instant time (t >> = 0 seconds). The 7 x In fault current occurs in the upstream feeder because the 3 phase fault current on the 20 kV bus is 7.286 x In. The application of this cascade setting is only effective if the feeder OCR and CB conditions are in normal working conditions. But when OCR and CB in the feeder fail to work, incoming OCR and CB will trip following the initial set of incoming OCR work time. Therefore, the relay to be less reliable when When the feeder OCR and CB are in a normal condition, the feeder OCR will work instantly to give trip signal to the feeder CB. However, if the condition of the feeder OCR and CB fails to work, incoming OCR must work to secure the system. The problem is that setting the incoming OCR work time very slow to secure the system. In the calculation as follows: So incoming OCR will work in 1.05 seconds when fault current 12124 A occurred. This is very risky for the safety of the transformer from a large fault current

Noncascade patern OCR coordinatione
The difference between the cascade pattern and the non-cascade pattern lies in the application of its instantaneous settings. For the cascade pattern, instantaneous settings are applied only to the feeder OCR. while for the non-cascade pattern, instantaneous settings are applied to the feeder and incoming OCR.
3.9.1. Instantaneous setting incoming OCR. The instantaneous incoming OCR setting is 7 x In the 20 kV side transformer with a time setting of 0.3 seconds. So with non-cascade pattern when there is a fault current as 12124 A incoming OCR will work within 0.3 seconds while the cascade pattern works within 1.05 seconds.
3.9.2. Blocking system OCR. This blocking system is implemented to prevent incoming OCR and feeder OCR from working at the same time. The blocking system works when a very large disturbance, according to instantaneous settings, occurs in the feeder which is also felt by incoming OCR.

A large fault current when feeder OCR and CB normally work condition.
When a large fault current occurs in the feeder and meets the instantaneous setting, the feeder OCR and the incoming OCR will sense the current. If the condition of feeder OCR and CB are normal, the instantaneous setting on the feeder OCR feeder will work and give an order signal for CB to instantaneously work. Even though the incoming OCR felt a disturbance, it did not directly trip the incoming CB. This is because when the feeder OCR senses the fault current, the feeder OCR will send a signal to the feeder CB to work instantaneously and send a signal to the incoming OCR to block the instantaneous trip of incoming CB.

A large fault current when feeder OCR and CB abnormally work condition
If the feeder OCR and CB fail to work when a large fault current occurs in the feeder that appropriates with the feeder OCR instantaneous setting, the incoming OCR will continue to feel the fault current and the incoming OCR instantaneous setting will work to tripping incoming CB with the instantaneous working time 0.3 seconds.