Picking a good KYZ Pulse Value
We’re often asked “What is the proper pulse value to set for a customer?” or “How do you know what to set the pulse output value for?” The answer is -- it depends. It depends primarily on the kW size of the customer and what they want to do with the pulses. Most pulse metering applications these days are related nearly exclusively to peak demand applications -- that is, either demand monitoring, demand control, demand response programs, etc. – and less for kWh energy monitoring. Recall that there are two basic purposes for pulses. You can count them for energy consumption, usually kWh’s, or you can time them for demand, usually kW.
Because most applications are demand-related, it means that we are timing the pulses to derive instantaneous demand. Further, it means that we need to make sure that the pulses are not too fast at full-scale demand, but offer good resolution for the piece of equipment that is receiving the pulses, usually a customer-owned energy management system. My rule of thumb is 1 pulse per second at full-scale demand. This is a good compromise between not having pulses too fast and more resolution than you will ever need in 99.9% of all demand control applications. In addition, you’ll never have to worry about overflowing or “saturating” a memory register in a system that is using pulses with pulse rate of one pulse per second.
For the purposes of this discussion assume that we are dealing with a standard Form C (3-Wire) pulse output. The customer may use either all three wires at the calculated value or may use 2 of the 3 wires (K-Y or K-Z) and double the calculated pulse value.
To calculate the proper pulse value, the first piece of information you need is the customer’s highest annual demand for at least the last year. If you can find or get the customer’s last 5 years of annual peak demands, that will be better. If you can’t get this information, use 80% of the electric service size and do the math for maximum kW depending on whether it is single-phase or three-phase. For our example, let’s assume the customer has an average annual peak demand of 186.9kW. Round this number up to the next highest reasonable whole ten’s or hundred’s number. Here, because 186.9 is so close to 190, it won’t make much difference so let’s go ahead and round it up to 200 (kW).
Kilowatts (kW) are mathematically the same as kilowatt-hours per hour (kWh/h). The hours divide out and you are left with kW. At a peak demand of 200 kW, the maximum kilowatt-hours that can be used per hour are 200 kWh.
Let’s convert kilowatts to watts by multiplying by 1000: 200 kW X 1000 = 200,000 watts. This is the same as 200,000 watt-hours per hour.
Next, let’s divide wh/h by 3600 (since there are 3600 seconds in an hour) to convert to watt-hours per second (wh/s): 200,000 / 3600 = 55.55 watt-hours per second. This is the Pke or the Primary Ke value, which represents the actual energy that goes by the meter in 1 second at a 200kW demand. So a number close to 55.5 watt-hours per pulse is what we need to set the meter’s pulse output value for the customer in this application. Before we do that however, let’s consider two other factors.
Future Customer Growth: This Pke value is adequate if the customer has had many years of steady peak demands without any growth. If it is likely that the customer will be increasing in size electrically, then take our 200 and divide by .75 to give a good “headroom” factor for future growth. In this case 200 / .75 = 266.666 kW. Round to 267. 267 x 1000 = 267,000. 267,000/3600=74.16 wh/s. Round up to 75 wh/s. At this point, because we want the number of watt-hours per second and one pulse per second to be the same, then 75 becomes our watt-hour per pulse (wh/p) or Pke value.
Meter Multiplier: If this application happens to be a self-contained meter with a KYZ pulse initiator, then the multiplier is 1. The Ke value (Secondary Pulse Constant) is the same as the Pke (Primary Pulse Constant) and the meter is programmed with that watt-hour or kilowatt-hour value.
However, in the majority of pulse metering applications, you’ll likely be working with an Instrument-Rated metering installation, which means there are Current Transformers (CT’s) and in some cases Potential or Voltage (PT’s or VT’s) involved in the metering configuration. The meter multiplier must be taken into account when calculating the correct pulse value. Let’s assume this customer has 800 Amp CT’s and no PT’s. Thus, our 800A:5A CT’s give us a meter multiplier of 160.
Next, take our PKe value of 75 wh/p number and divide by 160 to derive the Ke value, the programmable value in the meter. Depending on your meter brand, this may be expressed in watt-hours or kilowatt-hours and you’ll need to know this before moving forward. Let’s assume it is in watt-hours. Thus, 75 / 160 = .46875 watt-hours per pulse. Round up to .50000 wh/p. If, however, this is expressed in kilowatt-hours, the number will be .0005 kwh/p. Rounding our Ke up now makes our PKe value 80 watt-hours per pulse or .080 kWh per pulse, FORM C (3-Wire).
If the customer’s energy management system only uses two wires AND it only counts a pulse upon the closure of the KY or KZ contacts, then the PKe value is 160 wh/p. There are some energy management systems available on the market that use two wires but count both the closure and the subsequent opening. In this case, the true pulse value is 80, since this scheme emulates a 3-Wire pulse output.
Write on the customer’s meter enclosure something like the following:
KYZ Pulse Values:
This will help the customer or the energy management contractor to understand the value correctly.
While this short summary may not be applicable to all pulse metering situations, it is generally applicable to most pulse metering applications.
Please call Solid State Instruments at (970)461-9600 at any time for technical support on KYZ Pulse Metering application.