Generac Grid Services Blog

The Case for Distribution Voltage Management

Posted by Malcolm Metcalfe on May 24, 2018 10:25:23 AM
Find me on:

The California Duck Curve reveals a potential costly issue for utilities and their customers. The annual peak load appears to be continuing to grow -- because it occurs after dark when there is no solar power being generated -- yet energy sales may be declining with the growth of distributed solar generation during the day. This results in the need to continue to expand the grid, but without the sales revenue to support the added capital expense, presenting a Catch-22 that utilities are struggling to overcome.

Several attempts have been made to correct this issue, including potential demand charges on users and peak demand management. While demand charges can help with the revenue issues, they can also damage the economics of privately owned rooftop solar, especially when considering the added expense of battery storage.

Peak demand management, on the other hand, simply avoids the peak capacity of distribution substations by cutting load during the peak periods. While this is effective, it also results in potential loss of revenue for utilities and may also inconvenience customers.

There may be a better way.

A Better Approach: Voltage Management

One method that can be useful in helping to achieve greater efficiency is voltage management or VAR compensation.  By better managing distribution system voltages, utilities cannot only reduce peaks and losses, but can also improve efficiencies, achieve energy savings and reduce demand.

The limit on capacity is the current being delivered. In transformers, where there are multiple voltages, and the windings are designed for each. The rating is in kVA or MVA, but this is simply a proxy for current. The actual limit in most devices is the dissipation of heat that is caused by I2R loss in the conductors. Transformers can increase their MVA limits by adding fans for cooling or by leveraging cold winter ambient air temperatures.  If current can be reduced, without limiting power, substations will operate with less heating, and the overall loss in delivery will be reduced.

Peeling the Onion

It can be shown that the current carried to deliver single phase power is:

Screen Shot 2018-05-23 at 12.07.23 PM

A balanced 3-phase system would carry a total of 3x the watts and VARs that are shown in the per-phase formula shown. Watts are the portion of the electric power that deliver useful work, while VARs are just watts that flow to the load for half of every cycle and are returned in the other half cycle. There is no energy over time in a VAR to produce useful work, but from the formula shown above, VARs do increase the current in the lines to deliver power, and thus they also increase losses, which in most cases appear as heat.

Again, it is the current that limits the capacity of the substation, and in some cases, it can also limit the distribution line capacity to deliver real power (watts) to users. Most devices are inductive in nature and are said to “consume” VARs. Even the distribution line itself consumes VARs. Capacitive devices, which store and return energy at exactly the opposite times, are said to supply VARs.

A Deeper Dive

Watts, which are often referred to as real power, MUST ultimately come from a generator, while VARs, which are known as reactive power, are actually energy that is stored and returned in every cycle. They can be injected or removed at almost any place on the grid and do NOT have to come from a generator.

Both watts and VARs provide important support in the operation of the grid. Operators balance watts generated and used to maintain frequency. VARs balance is needed to maintain voltage. The watt balance must be done once for an entire grid, while the VAR balance is maintained in all locations. Physics can show that watts tend to flow toward a location that has a lagging voltage angle, while VARs flow toward a lower voltage magnitude.

For example, if a distribution system is drawing 20 MW at a power factor of 90%, then there would be a total draw of 22.222 MVA; 20 MW and 9.69 MVAR. If capacitive VARs were installed downstream from the transformer that would deliver 9.69 MVAR, then the load would fall from 22.22 MVA to 20 MVA and with no VARs. That is a decrease of over 10% in the current drawn, without any rejection of load, and that reduction would reduce heat generated by about 19% (P = I2R – if I drops from 1 to 0.9 the I2R drops to 81% of what it was.)

What’s the Difference?

This reduction in reactive load would make a significant difference. The current in the substation transformer would drop by 10%, reducing heating by almost 20%, and if the capacitors were installed at the load, the same savings would occur for loss in the distribution line and transformers.

The benefit of this method is that it can be left running 24/7/365, so in addition to reducing peak demand, it would reduce line loss on a continuous basis. The concept of interrupting load would apply ONLY to any additional reduction needed after the reactive power was reduced.

Many utilities penalize companies for low power factor, and the common solution to this is NOT the same as what’s recommended here. Static capacitors are added to the circuit by the customer to bring the AVERAGE PF to 90% or better. This means that at times the PF may still fall well below the 90% level or alternately may become a leading PF for a period of light loads. An Enbala VPP would manage the PF to be close to unity in order to continually deliver the best possible results.

That’s a lot to think about.  I’d welcome your comments.


Topics: peak load management, demand response, voltage management, distributed energy, VPPs, Thanksgiving, VAR compensation

Subscribe to Blog Updates

Recent Posts

Posts by Topic

see all