Grid Connection Options - Single Feed

There is an increasing demand for connection of renewable energy parks to existing power systems infrastructure.  In many cases these are by a single connection to the grid which of course represents a single point of failure that may put the ability of the generator to produce.  However some options may still provide increased flexibility and increased availability and maintainability if considered as below.

One option is of course a direct connection at the nearest utility substation.

However quite often these renewable energy projects require connection to an existing transmission line somewhere between the utility substations and so connection options must be considered carefully.

Tee Connection

The Teed connection is arguably the most simplest connection simply tapping on to the existing transmission line at some point between the two existing substations.

Operationally however this means that the entire line between the three substations must now be treated as a single protection zone.

If Distance protection has been used on the two-substation line, due to the difficulty in setting distance relays for teed line impedances, these will invariably need to be replaced with three-ended current differential protection relays.  Three ended current differential will require communication links between the three substations.  If a fault occurs anywhere in the shaded zone, all three line ends must be tripped, and hence there is an increased exposure to the existing substations not being able to transmit power as they previously did.

Tees are therefore generally not a main stream option.

Three CB In-Out Connection

This arrangement involves three Circuit Breakers (and associated plant) in a "Y" type arrangement. 
This is achieved typically at two of the existing poles/towers in the line where they are re-diverted in and out of the new substation.

Operationally we can then spilt the system into three separate zones thereby aiding in discrimination and isolation of the faulted section of the system.  This means that  the generation connection can still generate to one of the substations in case of loss of one of the existing lines.

The existing line protection at each end does not necessarily need replacement as distance or differential protection, but clearly must be matched on each end in the new substation.

However the problem with this arrangement is that it relies on all three circuit breakers being in service.  Any one CB out of service will disrupt the normal operation of the three substations.

Three CB Mesh Connection

A simple re-arrangement of the three Circuit Breakers used in the In-Out arrangement produces a small three Circuit Breaker mesh - sometimes drawn as a triangle with a CB on each side of the triangle.

In this mesh arrangement, we only need any two of the three Circuit Breakers to be closed for full operation between all three substations.  This allows for maintenance on the circuit breakers plant without disruption to the ability to generate power to the grid.