Applying wireless LAN for IEC 61850 based smart LV/MV distribution substations

Control & monitoring application using WLAN

Fast Distribution Bus Protection Scheme

A dedicated bus protection may not be economical for medium/low voltage distribution substation. Therefore, conventional method to protect distribution bus is using overcurrent relays, where the loads on the bus are fed radially (unidirectional power flow). Normally, the upstream protection IED (IED-1) is delayed by downstream feeder IEDs by coordination interval, i.e. approx. 400 to 500 ms or more.

This coordination interval is inserted in order to allow downstream feeder IED before bus IED in case of a feeder fault. However, if there is a fault on bus, bus overcurrent protection is delayed by this coordination delay (400 ms to 500 ms). This time delay can be drastically reduced by deploying low cost WLAN communication as explained below:

In this scheme, peer-to-peer IEC 61850 GOOSE message can be used to send signal from feeder protection IEDs to bus overcurrent IED.

The feeder IED will immediately send BLOCK command using IEC 61850 GOOSE message, and feeder IED will isolate the fault.

On the other hand, if there is a fault on the distribution substation bus, none of the feeder IEDs will detect a fault, and this way, bus IED will not receive any BLOCK message, and will operate normally after a delay of approx. 60 ms (normally fault is detected in 4 ms, and average WLAN communication delay is approx. 5-10 ms).

This way coordination delay can be reduced from 400-500 ms to 60 ms. This scheme improves the life of components installed on upstream of distribution bus, i.e. distribution transformer, since the time of fault current passing through these components reduces.

Furthermore, this scheme further can be enhanced to simultaneous faults. For example, if there is a simultaneous fault on bus and a feeder, this scheme can utilize IEC 61850 GOOSE message communication to not only send BLOCK command, but also incorporates information like, types of faults. This way, if feeder IED detects L-G fault, however, bus IED detects multiple phase faults, bus IED can immediately override BLOCK command, and operates.

The concept of fast distribution bus protection scheme is shown in Figure 1 above.

LTC Control and Monitoring for Enhanced Transformer Protection

(LTC – on-load tap changer)

Figure 2 shows the IEC 61850 based wireless LAN to enhance the distribution substation protection and automation. A technical report from IEEE PSRC has presented protection and control applications using spread spectrum radio, some of the reported intra-substation applications which include an enhanced transformer differential protection.

The wireless LAN channel can be installed to facilitate communication between LTC control/monitoring field device and transformer protection and control IED.

The transformer IED can control LTC to maintain the bus voltages within the predefined range. The IEC 61850 GOOSE message can be used to communicate RAISE/DROP the transformer tap settings. The performance requirement for LTC control is 0.25 second, which can be well within WLAN average delay.

However, this higher protection slope affects the sensitivity and dependability of the protection. The IEC 61850 GOOSE messages with current tap value can be communicated to the conventional transformer IED over the WLAN network. And, this way slope of the differential protection IED can be adjusted adaptively (according to the tap status).

This way, with this wireless LTC sensor can enhance transformer differential protection by online monitoring of LTC position, as shown in Figure 2 above.