XYLENE POWER LTD.

DG TRANSMISSION/DISTRIBUTION

By Charles Rhodes, Xylene Power Ltd.

The system reliability with Distributed Generation (DG) depends on the existence of an integrated transmission/distribution system that can simultaneously regulate voltage and efficiently convey energy from locations that have surplus power to locations that at that need additional power.

SWITCHGEAR LIMITATION:
A significant constraint on distributed generation network architecture is the ability of the switchgear to clear short circuit faults under worst case conditions. If there are multiple generators connected to a circuit and bidirectional power flows measures must be taken to ensure that the short circuit disconnect capacity rating of every isolation safety switch will not exceeded. Switchgear failures can cause large costs due to protracted power outages, damaged equipment and personnel injury/deaths.

Generally, if the load can be met by generation located at either end of a main distribution circuit, the short circuit disconnect rating of a switch isolating a branch circuit from a main distribution circuit must be doubled as compared to a network with just central generation.

VOLTAGE DROP:
Another constraint on transmission/distribution for intermittent distributed generation is voltage drop along a long distribution feeder. Let the nominal feeder voltage be Vn. In order to meet the input requirements of electrical appliances, at all points and at all times along the feeder the voltage V must be in the range:
0.94 Vn < V < 1.06 Vn.
The voltage at a generator that is operating at its maximum output but is located far from a substation on a lightly loaded feeder must always be less than 1.06 Vn. The voltage at a load that is located far from a substation on a heavily loaded feeder with little operating distributed generation must always be greater than 0.94 Vn. These constraints limit the amount of intermittent generation and intermittent load that can be connected near the far end of of distribution feeder.

VOLTAGE REGULATION:
Assume that each distributed generator has an output power versus voltage characteristic such that the power generation is maximum when the feeder voltage at the generator is .94 Vn and the power generation is zero when the feeder voltage at the generator is 1.06 Vn. Hence the generator's output power versus voltage curve has a negative slope. Implementation of this control algorithm is easily done but may involve generator control hardware that is not presently contemplaed by the parties.

The voltage at a substation transformer secondary is controlled by an automatic transformer tap changer. The controller for this tap changer should be programmed to maintain a negative slope power to feeder versus voltage curve. This curve can be remotely offset by the IESO. The offset is chosen so that under average distributed generation conditions the feeder voltage measured at the substation takes its nominal value Vn. If maximum power is to be fed from the transmission system to the feeder the control curve is offset so that this voltage rises to 1.06 Vn. If maximum power is to be fed from the feeder to the transmission system this offset is reversed so that the feeder voltage measured at the substation falls to 0.94 Vn. Note that the substation transformer secondary tap changer control algorithm should have a dead band sufficient to prevent the tap changer short cycling.

POWER CONTROL:
Power control involves giving the Independent Electricity System Operator (IESO) control of the tap changer contol algorithm offset. By changing this offset the IESO can indirectly change the voltage on the feeder and hence the amount of operating distributed generation connected to the feeder. Note that if the distributed generation is out of service the optimum value of this control offset changes. Hence, there should be feedback to the IESO to warn the system operator of prolonged out-of-normal voltage or power as measured at the substation transformer secondary.

GENERATION CONSTRAINT:
As intermittent generation is added to an electricity system, corresponding sheddable load, energy storage or balancing generation must also be added to the electricity system. If all of the generation is distributed and there is no energy storage or sheddable load, then in order to achieve voltage control and system reliability almost all of the distributed generation must be constrained, reducing the average unconstrained generator annual output by about 40%.

This generation constraint requirement can be met by requiring all distributed generators to operate on a specified negative slope power versus voltage curve. However, implementation of this generation constraint methodology requires acceptance by the parties. This generator constraint is not presently contemplated by the Ontario Power Authority (OPA) Feed-In Tariff.

Operation of unconstrained generators leads to loss of revenue by load following generators. This is a financially intolerable state of affairs that has yet to be adequately addressed by the OPA.

NETWORK STABILITY:
The availability of sufficient constrained generation is key to network stability when an major energy source drops out either due to a generator trip or due to a transmission line fault. The OPA should be including an element of voltage dependent generator constraint in all renewable energy supply contracts.

GENERATOR INDEPENDENCE:
Another issue that has not been adequately addressed by the OPA is requiring distributed generators that export energy to the grid to be self excited to reduce their dependence on other generation for voltage regulation, reactive power and black start.

FAULT ISOLATION:
With Distributed Generation (DG) the process of locating and isolating faults is more complex than with Central Generation (CG).

ISLANDING:
When a feeder with distributed generation is disconnected from a substation transformer secondary, power islanding can occur. The substation control system must be enhanced to so that it reconnects to the power island only when generation within the power island is locked off or when the power island is synchronized to the grid.

This web page last updated June 12, 2010.