Demand controller: Know what it is

A demand controller is a device dedicated to monitoring and controlling the level of demand of a Consumer Unit (CU). It acts by removing and inserting loads, according to the projected power behavior, so that the contracted limits are not exceeded, thus avoiding fines.

The demand controller measures quantities through the user output available on the utility meter and provided for by NBR 14522. This output is available via an optical coupler, which allows pulses to be sent from the utility meter to the controller.

The Controller, in turn, projects future values ​​in a defined time window and acts if necessary. In some cases, it also offers global power factor control and online monitoring of billable quantities, allowing actions to be taken prior to billing.

What is electrical demand?

Demand is the power required by an installation, given in kW (kilowatts), and measured, for billing purposes, in 15-minute windows. The customer will pay the utility company the highest demand value in the month, that is, the exact 15-minute period that had the highest power demand. Yes, 15 minutes is enough to get a fine or a very high amount on the bill.

Demand is different from consumption, which in turn is a value used over time, given in kWh (kilowatt-hour).

These are important definitions:

Contracted Demand: 

This is the amount that the Dealer is obliged to provide continuously for the period of validity of the contract with the Consumer. Amount agreed between the dealer and the customer and explicitly stated on the invoice.

Active Demand Measure: 

It is the average of the Three-Phase Electrical Power in the 15-minute interval, expressed in kW. Value shown on the monthly bill.

Reactive Demand:  

It is the average of the Three-Phase Reactive Electrical Power in the 15-minute interval expressed in kVArh. Value shown on the monthly invoice.

What is a demand controller for?

To understand “How does a demand controller work?” and “What is it for?”, it is first important to understand the Tariff Structure in force in Brazil, relating to the consumption of electrical energy.

Tariff Structure

Consumers are divided into 2 large groups:

Group A – High-capacity consumers with high consumption levels. They are connected to voltages equal to or greater than 2.3 kV and are subdivided into 5 subgroups:

• Subgroup A1 : 230kV or more;

• Subgroup A2: 88kV to 138kV;

• Subgroup A3: 69kV;

• Subgroup A4: 2.3kV to 25kV;

• Subgroup AS: Underground Electrical Networks.

Group B – These are all consumers who have their electrical connection in Low Voltage (Here are the residential ones), which are subdivided into 4 subgroups:

• Subgroup B1: Residential and Low-Income Residential;

• Subgroup B2: Rural, Rural Cooperative and Public Irrigation Service;

• Subgroup B3: Other Classes;

• Subgroup B4: Public Lighting.

From this, it is important to highlight that “Demand Control” will be indicated for consumers in Group A.

Within the Group A tariff structure, tariffs are adopted for each subgroup, and there is also a division of time slots, which are defined periods throughout the 24 hours of the day.

These periods, or rather, time stations, are important, because at certain times of the day we have different levels of consumption. And, in an attempt to better distribute consumption throughout the day, thus avoiding large peaks, and also reducing the need for large infrastructures for small moments of the day, a different type of price per kWh and kW is adopted for each moment. This encourages use during idle periods, and discourages use during periods of high demand. Below are the definitions for each station.

Timetables

Peak Hour: This is the period of 3 consecutive daily hours, valid only from Monday to Friday, when consumption is highest. This time is defined by each concessionaire in the country. In most cases, it starts around 6 pm, varying by more or less 1 hour, disregarding time zones. During peak hours, the price of kWh is more expensive, as is peak demand, if there is a differentiation provided for in the contract.

Off-Peak Time: This is the period corresponding to the hours that are not part of peak time. Off-Peak time always has a lower consumption value. This time is divided between Inductive time and Capacitive time (early morning). During these times, the consumer must respect the power factor limit.

What are the rates?

We know the groups and the time slots, we just need to understand the fares. There are two possible fares for Group A:

Green Tariff – Subgroups A3, A4 and AS.

Tariff where the consumer unit has a single Contracted Demand value, regardless of the Time Zone (Peak or Off-Peak). In this case, the amount that will be charged is the highest value between the Contracted Demand and the Measured Demand. 

	Example A	Example B
Contracted Demand	1500 kW	1500 kW
Demand Measure	1200 kW	1700 kW
Demand Charged	1500kW	1700 kW

For example B, there is a surplus in the contract of 200 kW and this will generate a heavy fine for the consumer, this being the biggest motivator for the use of demand controllers.

Blue Tariff – Subgroups A1 and A2

Tariff where the consumer unit has 2 Contracted Demand values, one for each Time Station (Peak and Off-Peak). In this case, the amount that will be charged at Peak is around 3 times the Off-Peak amount.

The amount that will be charged is the highest amount between the Contracted Demand and the Measured Demand, but at both Time Stations.

	Example A Out tip	Example A Tip	Example B Out Tip	Example B Tip
Contracted Demand	1500 kW	500 kW	1500 kW	500 kW
Demand Measure	1200 kW	400 kW	1700 kW	657 kW
Demand Charged	1500kW	500 kW	1700 kW	657 kW

In the example, there is no fine for the consumer in example A. On the other hand, the consumer in example B will be fined for overtaking at both stations.

How does a demand controller work?

Based on the tariff structure, it is clear that maintaining the Measured Active Demand within the Contracted Demand is not always easy, and in this case, many companies choose to keep certain priority loads active, while others, less priority, can be turned off so that the Demand remains within the contracted level. Knowing that any 15-minute window can result in a fine. How can this be controlled automatically? 

Using a Demand Controller!         

The demand controller receives data directly from the utility's energy meter, via an optical cable that is connected directly to the meter. It receives information with second-to-second synchronization via the ABNT-CODI Protocol.

The ABNT-CODI protocol has 3 variations: normal, extended and mixed.

The normal type sends only Active and Reactive Energy data in the consumed energy quadrants.

The extended type sends Active and Reactive Energy data in the four quadrants, that is, for both generated and consumed energy.

The mixed protocol, in addition to sending all this information, also sends the 3 voltages and 3 currents of the system.

After the Demand Controller receives this data, it will start controlling the loads that can be inserted or removed from the system automatically, so that demand always remains within the contracted value.

There are basically 3 control methods that are currently used: load line, moving window and the adaptive predictive method. One of the most used today is the Load Line method, as shown below:

The algorithm used for control is a demand projection algorithm. The 15-minute period is divided into configurable time windows, where the controller analyzes the current load of the installation. At the end of the window, the controller makes a projection of the current power for the end of the interval, checking the consumption still available until the end of the 15-minute period and calculating how much load should be removed or released for operation.

The size of the window should be selected according to the characteristics of the installation. Smaller windows lead to faster control actuation, but may generate unnecessary actuations on the loads; larger windows make the control more spaced out. It is important to talk to a specialist consultant and always perform an energy analysis to correctly size the system according to the characteristics of the consumer unit. For each system, one or another control strategy may be more appropriate.

In some cases, for example, it may be recommended to start a generator rather than turn off a load. To determine the best strategy, we recommend analyzing the load through an energy analysis service and the invoice to assess feasibility and cost optimization, aiming for the lowest possible payback.

Below, a demand control controller, together with an energy management system (GDE4000 integrated with POWER5000) that allows real-time assessment of demand levels:

Link to a Demand Controller

Real-time management system link integrated with demand control