AUTOMATIC CHANGEOVER SYSTEMS

AUTOMATIC CHANGEOVER SYSTEMS

Induction motors are typically used to drive critical loads. In some plants, such as those involving the pumping of fluids and gases, this has led to the need for a power supply control configuration in which motor and other loads are automatically transferred on loss of the normal supply to an optional supply. A fast changeover, allowing the motor load to be re-accelerated, decreases the possibility of a process trip happening. Such configurations are typically used for big generating units to transfer unit loads from the unit transformer to the station supply/start-up transformer. When the normal supply fails, induction motors that stay connected to the busbar slow down and the trapped rotor flux creates a residual voltage that exponentially decays. All motors connected to a busbar will start to decelerate at the same rate when the supply is lost if they stay connected to the busbar. This happens because the motors will exchange energy between themselves, so that they tend to stay ‘synchronized’ to each other.

Consequently, the residual voltages of all the motors decay at nearly the same rate. The magnitude of this voltage and its phase displacement with respect to the healthy alternative supply voltage is a function of time and the motor speed. The angular displacement between the residual motor voltage and the incoming voltage will be 180° at some instant. If the healthy back-up supply is switched on to motors which are running down under these conditions, very high inrush currents may happen, generating stresses which could be of adequate magnitude to cause mechanical stress, as well as a severe dip in the back-up supply voltage.

Two automatic transfer methods are applied:

- in-phase transfer system

- residual voltage system

The in-phase transfer method is presented in Figure below. Normal and standby feeders from the same power source are used.

Auto-transfer systems (a) In phase transfer approach

Phase angle measurement is applied to discover the relative phase angle between the standby feeder voltage and the motor busbar voltage. When the voltages are in phase or just prior to this condition, a high speed circuit breaker is used to complete the transfer. This approach is restricted to big high inertia drives where the gradual run down characteristic upon loss of normal feeder supply can be accurately anticipated.

Below figure presents the residual voltage method, which is more typical, particularly in the petrochemical industry. Two feeders are used, feeding two busbar sections connected by a normally open bus section breaker. Each line is capable of transferring the total busbar load. Each bus section voltage is monitored and loss of supply on either section causes the relevant incomer CB to open. Given there are no protection operations to show the presence of a busbar fault, the bus section breaker is automatically closed to restore the supply to the unpowered busbar section of after the residual voltage created by the motors running down on that section has decreased to a an acceptable level.

Auto-transfer systems (b) Residual voltage approach

This is between 25% and 40%, of nominal voltage, dependent on the power system characteristics. The residual voltage setting selection will affect the re-acceleration current after the bus section breaker closes. For example, a setting of 25% may be anticipated to result in an inrush current of around 125% of the starting current at full voltage. Optionally, a time delay could be used as a residual voltage measurement substitute, which would be set with knowledge of the plant to make sure that the residual voltage would have sufficiently decreased before transfer is started. The protection relay settings for the switchboard must take account of the total load current and the voltage dip during the re-acceleration period in order to avert spurious tripping during this time. This time can be few seconds where big inertia HV drives are used.

Reference: CED Engineering (Continuing Education and Development, Inc.), Industrial System Protection paper by Velimir Lackovic