Control of KVA and Power Factor Using Q Pulses

Introduction

The 300C may be used to control both KVA and power factor for applications where metering includes both a kWh pulse and a Q pulse. The use of Q pulses permits the phase angle of the current relative to the voltage to range from 30 degrees leading to 150 degrees lagging before the pulse frequency drops to zero. Accuracy is difficult to obtain when the pulse frequency approaches zero.

The use of a Q meter is defined in the Power Input Setup window. If a Q meter is used the Q pulse Input Number must be set to 2. When this is done the 300C will control KVA instead of KW. The main demand will also be graphed as KVA. Enter kVa in the Data Units as a reminder. The pulse constant for the Q pulses is the same as for the kWh pulses.

When using Q pulses the 300C also controls the power factor. When using the Q pulse input, relays K21, K22, and K23 are reserved for control of capacitors to correct the power factor.

The power factor is calculated every second for use in the Real Time Graph and every minute the power factor status is checked to see if a capacitor bank should be switched. If the power factor is below the pf Setpoint, Inductive% a capacitor will be switched in. If the power factor is more leading than the pf Setpoint, Capacitive% a capacitor will be switched out. If the power factor is between the two set points no capacitor switching will occur. If leading power factors are not allowed, set the pf Setpoint, Capacitive to 100.

It is important to select the power factor set points and capacitor sizes so a single capacitor cannot switch the power factor from being more lagging than the inductive set point to more leading than the capacitive set point. If this happens the capacitor will continually be switched in and out.

Special Considerations

The 300C uses six buffers to store data for later downloading into a PC. The size of these buffers are defined on the Data Partitions window. Buffer 5 is used to store the KYZ pulse counts. Buffer 3 is used to store the Q pulse counts. (This is an arbitrary selection by the system.) Buffer 3 stores the Q pulse counts so they may be downloaded for a permanent record. An example of one possible buffer assignment is given in Figure 4. In the PC the kW and Q pulse counts are used to calculate the power factor (as a percentage) so it may be plotted on the Day Graph. When using Q pulses:

1. A Q pulse Input Number other than 0 on the Power Input Setup window will enable the the Q pulse. Digital Input 2 is normally used.

2. In the Data Partitions window, Buffer 3-Analog Input 3 must have a 1-Minute Average specified for storing the Q pulse counts. Since Buffer 3 is used to store the pulse counts it does not store an analog signal. If Analog Input 3 is used for an analog signal, such as temperature, the result can be viewed on the Real Time Graph only since the signal is not placed in one of the primary buffers for storage and viewing on the Day Graph. However, if Analog Input 3 is used for the Auxiliary Demand, the data will be stored in Buffer 6 so it is available for the Day Graph. When controlling a generator where the power is monitored by a 5-20 ma input signal, using Analog Input 3 is a good choice because it leaves Analog Input 4 available for any additional signal needed. In this case the Analog Input 3 box should not be checked on the Real Time Graph Setup window as it would generate a confusing line on the graph. The data from Analog Input 3 is represented by the Auxiliary Demand.

3. The Day Graph Setup window must have the Display Line for Analog Input 3 clicked and a color chosen before the power factor will be displayed on the Day Graph. The settings on the Analog Input Setup window for the Analog Input 3 Graph Axis will determine which axis the power factor will use for plotting. The Analog axis should be chosen as shown in Figure 5.

4. To display kW as well as KVA on the RTG display, click the small box at the bottom of the RTG Setup window. Only the current value of kW is saved by the 300C for transmitting to the PC. When a connection is made to the 300C the value of kW at the time of connection is plotted as a straight line for the prior hour. While connected, the kW will be plotted. If kW is to be plotted the box must be checked again every time the PC makes a new connection with the 300C. In some cases the kW plot changes rapidly and may create a wide line on the plot. In this case it is best to monitor the kW on the DAY graph.

Figure 4 makes it apparent that more data items are downloaded for plotting on the Real Time Graph than the Day Graph. This is one reason only one hour of past history data for Real Time Graph data is usually kept in the 300C whereas several days of past data are stored in the buffers used for the Day Graph and the database.

Figure 5 shows Analog Input 4 set up for a CT input. This is submetering a portion of the load. For many applications it will be useful to submeter different loads on a rotating basis until the load characteristics are well known.

The 300C will store a history of the operation of relays k1 through k7 if specified in the Analog Input Setup window. The relay data may be stored in Buffer 3 or Buffer 4. If Q pulses are used Buffer 3 contains the Q pulse counts so Buffer 3 cannot be used for relay data. Thus, Buffer 4 must be used if relay data is important. A choice must be made between saving submetering data or other analog data using Buffer 4 and relay data.

When a Q pulse input is specified the DAY graph will show both the kVA plot and the kW plot using the same color. The DAY Graph Setup window is used to specify whether the kVA plot will use a step graph or a bar graph (or both as in the example shown in Figure 6). The kW will always be plotted as a line graph.

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