Over current and earth fault protection

Over current protection and earth fault is simple protection. Being low cost protection it is widely used across all levels. For lower voltage levels it is main protection, and for higher voltage levels it is used as backup protection. The reason being this protection does not have clear boundaries of protection.

Over current protection is for phase current and earth fault is for neutral current (residual current). The characteristics for both protections are same.

This protection is of two types: definite time (DT) or inverse definite minimum time (IDMT).

Definite Time (DT): In this type, protection will start when current reaches a set value, and protection will operate after a set time. For example, we for a transformer having maximum through fault current of 100A we can set DT protection 110A with time delay 100ms. In this way protection will operate in case of faults inside transformer only. For faults on downstream current will be less than 100A and protection will not operate.

Inverse Definite Minimum Time (IDMT): In this type, protection will start when current reaches a set value, and operating time of protection will depend on the level of current. If current is very high compared to set value, protection will operate fast. If current is slightly high from set value, relay will take more time to operate. In this way, a curve can be drawn between current and time for each relay. There are standard curves for which relays are designed. User can select from these curves as per requirement. For mechanical relays these curves are fixed as per design. However, in case of static / numerical relay curve can be selected form settings of relay.

For example, we have set relay in IDMT standard inverse for a transformer. Full load current of the transformer is 100A, and we have set IDMT start at 120A. The relay will start when current increases beyond 120A. If the current is 1200A, which means 10 times of set value. For 10 times current, the operating time is 2s (from curve). There is another setting called Time multiplier setting (TMS), say it is set at 0.4 for this transformer. The operating time of relay will be 0.8s.

If fault current is 240A, which means double of set value. For double current the operating time is 10s. The operating time of relay will be 4s.

In this way, relay will operate in 800ms for fault current of 1200A and in 4 sec for fault current of 240A

The operating time can be calculated as per below (IEC):

Standard inverse: t = TMS x 0.14 / [(Ir^0.02) – 1]

Very inverse: t = TMS x 13.5 / [Ir – 1]

Extremely inverse: t = TMS x 80 / [(Ir^2) – 1]

Time synchronization

Introduction: After any disturbance in power system, analysis for cause of disturbance requires sequential events from relevant protection and control devices. Therefore, all numerical IEDs (relays and controllers) needs to be time synchronized for proper sequence of events in chronological order. 

Generally IEDs are time synchronized by one of the following method:

1. Pulse Per Minute (PPM)
2. Inter-range instrumentation group timecode-B (IRIG-B)
3. Simple Network Time Protocol (SNTP)
4. Precision Time Protocol (PTP)

PPM: Time of the relay has to be set once manually, only seconds and milliseconds will be reset to 00.000 after receiving pulse from time synchronization equipment. It is simplest way of time synchronization. Large number of relays can be synchronized by connecting in parallel. Its accuracy depends on accuracy of internal clock as after synchronization it has to run on internal clock for one minute untill next pulse is received. Being obsolete system it is rarely used these days.

IRIG-B: It is standard format for transferring time information. Information can contains data for day of year, Hours, Minutes, Seconds and milliseconds.Data is transmitted by moduling on 1KHz sine wave through co-axial cable. Accuracy of microsecond level can be achieved. 

SNTP: It is time synchronization using Local Area Network (LAN) system. GPS equipment works as server and sends time information, which contains complete data Year, Month, day, Hours, Minutes, Seconds and milliseconds. All the IEDs get time information from server and time is synchronized. Its accuracy is low as delay in network components is not compensated. Accuracy of millisecond level can be achieved.

PTP: It is used where more accuracy is required than provided by SNTP. In PTP delay in network components is calculated by system and compensated by modifying time stamps. In this way, accuracy of nanosecond level can be achieved.