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Location

This example model can be found in the software under the Protection > Directional_Overcurrent.ecf.

Model Description

Protection systems help in the safe operation of a power network by isolating the faulted section through breakers. The relays within that section respond in a coordinated fashion to achieve high selectivity. While the majority of the relays in the existing network are non-directional, there are certain cases where direction-based discrimination is required to determine if a relay should respond or not. The co-ordinated response of relays along with directionality ensures the safe operation of the network and minimizes service interruptions. Directional overcurrent protection relays are very useful in this regard.

Directional overcurrent protection is used to protect the system when fault currents could circulate in either direction along the network line. With the increasing penetration of Distributed Generation (DG) sources at the distribution network, it becomes even more important to implement directional protection to provide correct fault detection and ensure protection co-ordination. 

This example demonstrates the operation of the Directional Overcurrent (505167) relay.

 

Some assumptions:

In this example, the fuses, reclosers, and sectionalisers are not implemented. The lateral lines are protected by the main-line directional overcurrent relays. 

DG protection is also not discussed in this example. The focus is on feeder protection.

Network Description

A 25 kV, 60Hz distribution feeder is used as an example for demonstrating the directional overcurrent protection relay operation. The network has two generating sources: an upstream grid modeled as a voltage source and a 5 MVA, 2.4 kV  DG (modeled as a synchronous machine) connected at the other end of the feeder through a 2.4 kV / 25 kV (Delta lead- Star grounded) transformer. The DG is operating at 1.8 MW and is operating in the PV mode. The network comprises of a diversity of loads such as dynamic loads, unbalanced loads, induction motors, etc. The schematic of the system is shown in the figure above.

The Current Transformer (CT) and Voltage Transformer (VT) are emulated by the current and voltage measurement sensors respectively. CTs and VTs ratios are selected as 100 A / 1 A and 25 kV / 110 V respectively. The figure below shows the page 2 of the model with the sensor measurements connected to the directional overcurrent protection (505167) relays.

The complete network is divided into three sections 1,2 and 3. The directional overcurrent (505167) relays in the different sections are configured as shown in the figure and table below.

Relay Parameters

Directional Settings

 In relays 2, 3, 5 and 6 the directional element (67P) is enabled by checking the 'Enable phase directional 67P'. Element Characteristic Angle (ECA) is selected as 30 deg as shown below:

Whereas in relays 1 and 4, the directional element (67P) is disabled.

Coordination of the Relays

Coordination of the Downstream Relays

The table below shows the parameters of the downstream relays, and the methodology of time dial calculation is followed:

In general, the pickup current for the relay is selected such that:  1.2 I_{full_load} < I_pickup < 1/3 I_{min_Fault}

Operating time using U.S. Inverse (U2) as per IEEE C37.112-2018 is given as:

 

where T.D. is the Time Dial setting

Calculation of Time Dial setting

Step 1: In this step, the Time Dial for relay 3 is set.
Choose the lowest value of Time Dial setting for relay 3 (0.04 s). No intentional time delay is provided because it does not have backup responsibility.

For close-in fault (three-phase-to-ground fault close to relay) at relay 3 ( 

 )* at the relay terminals, the operation time of relay 3 is:

Step 2: In this step, Time Dial for relay 2 is set.

We co-ordinate relay 2 with relay 3 for close-in fault for relay 3.

For close-in fault at relay 3, the relay 2 will act as a backup protection, the time of operation of relay 2 will be: 

Expected operating time for relay 2 = Operating time of relay 3+ coordination time interval (example: 0.05 s)

                                                          = 0.008 + 0.05 s

                                                          = 0.058 s

Step 3 : In a similar way relay 1 is coordinated with relay 2.

 

The characteristic curves of the downstream relays with the parameters from the table above are shown in the figure below:

*  Fault studies were performed and the fault currents were noticed. 

Coordination of the Upstream Relays