Bulk power transfer metering
Those involved in the business of buying and selling electricity know that the performance and quality of meters and metering systems is the barometer of their bottom line, and also of the level of customer confidence they can command.
The demand pull created by the ever-increasing deregulation and restructuring of electricity supply, coupled with the supply push created by modern day electronic and computation technologies, have resulted in an increasing emphasis on high precision metering. As a result, many commercially available tariff metering systems perform as well as – and often better than – reference standards used for testing only a decade ago.
In the modern electricity supply regime, bulk power exchanges take place from the generating company to the transmitting and distributing companies. Metering assumes great significance at each of these bulk transfer points. In addition to being highly accurate and reliable, these metering systems must collect, compute, collate, communicate and present data in a form suitable for analysis and usage. Independent power producers, generating and transmitting companies should take great care when selecting, installing, commissioning and maintaining their metering systems. Some key features to be considered when selecting systems are discussed here.
One of the crucial requirements for metering at bulk power points is the accuracy of measurement. Since the revenue involved is enormous, even small measurement errors can contribute to significant differences in billing. Class 0.2s meters with Class 0.2s potential transformers and Class 0.2s current transformers are traditionally used for these applications. The meter and instrument transformer inaccuracies, together with the losses on the potential transformer cables, result in overall system accuracy of about 0.5% at full load. In many poorly selected and installed metering systems, the total accuracy can be as bad as 1%.
This can contribute to significant loss of revenue for bulk power transfer points – as an example, the revenue loss for a 100 MW load could amount to $750 000. While it is not economically feasible at the present level of technology to use CTs of higher accuracy for commercial applications in the field, advanced metering technologies can offer a cost-effective solution. Present day meters can offer compensation for the errors of the external instrument transformers (CT and VT) thereby significantly enhancing the system accuracy. Such meters actually store and compensate for the linear and non-linear error characteristics of the external current transformers.
Many installations have the meter located far from the actual VT measurement points. This results in voltage drops on the VT cables. It also introduces significant errors in the voltage measurement, particularly when the voltage circuit burden (VA rating) is high. We recommend installing the meters as close to the measurement points as practicable. Meters with low voltage circuit VA burdens are always useful in this respect.
FOUR QUADRANT METERING AND HIGH REACTIVE ACCURACY
It is important to carry out full four quadrant metering for bulk power exchange points. In many inter-utility bulk power tie lines, conditions of line float are encountered when the active power transfer (import or export) is very low and yet the reactive power transfer (lag or lead) is significantly high. New tariffs based on reactive drawls are becoming common.
The metering system should be able to measure accurately in all four quadrants, so that a just tariff regime can be agreed and applied. Conventional static metering technologies were only able to measure reactive energies with a limited degree of accuracy. Today, however, state-of-the-art technologies allow a great degree of accuracy to be achieved in both active and reactive measurements.
The need to have a high up time for grid metering systems can not be over-emphasised. The equipment used for such applications is exposed to the harshest environments in the electricity industry, and must be designed to withstand the high levels of surge and switching transients that will probably occur.
Despite all the reliability considerations in the design of such meters, dual redundancy is almost always provided by means of a check metering arrangement. In the event of an incident it should be easy to remove and replace the meters; a module construction with ‘hot plugability’ is the order of the day. Such meters are capable of being pulled out and replaced without interrupting the power. Arrangements are provided to short the external CTs automatically by means of a ‘make-before-break’ type contact.
We recommend using all grid metering equipment with auxiliary power. This has at least two distinct advantages over powering up the metering system from the voltage transformers. First it reduces the VA burden, thereby improving accuracy and allowing usage of cheaper VTs. Secondly it allows meter reading and verification even under conditions of feeder shutdown. A reliable metering system should employ at least two auxiliaries, one AC and the other from the station back-up DC supply. The system should be capable of monitoring the status of these lines and automatically switch to the back-up system whenever necessary.
DATA COMMUNICATION AND SECURITY
Reliable data storage is a must for managing complex energy flows across power networks. All data pertaining to metering, tariff, billing, demand and load profiling should be passed on for further analysis and use. The new trend is towards storing all this data on the memory available within the meter, rather than the complex pulsing data loggers used up to now. The choice of the right medium for communication is almost always specific to a particular application. In addition to local ports for data communication, remote meter reading using PSTN modems, cellular or Packet data network connectivity is very important. The central and local load dispatch centres require access to this data, either directly or through the Internet.
While data transfer, assimilation and analysis have become popular in metering systems, the more important issues of data integrity and data security are often ignored. Insecure methods of time synchronisation, energy pulse accumulation and so on must be avoided altogether; all control and data transfer functions should be handled through reliable digital data communication methods. Present day systems employ several levels of security keys for controlling data access, and use some form of authentication algorithms to ensure data integrity. With the advent of the information age the significance of data security can hardly be ignored, particularly for high revenue applications like bulk power transfer.
Metering systems today are highly customised applications. Different utilities and power producers have different power purchase agreements and tariff structures. As the process of deregulation is in a fluid state in most parts of the world, the application of tariffs and use of meters is evolving over time. The metering system chosen must be amenable to configuration of the definitions of the energy register types – time-of-day (TOD) registers, rate registers, billing parameters, load surveys, event logging, alarm annunciation and many more.
SELF DIAGNOSIS AND TESTING
Because it is the utility’s cash register, the metering equipment needs continuous monitoring. Self diagnostic alarm annunciation should be employed. Such alarms can include watchdogs, auxiliary power failure, phase imbalance and so on. A good technique to check overall system accuracy is to monitor the instantaneous summation of energy registers of all the incoming and outgoing feeder meters, configured to give an ideal summation of zero if all registers are being measured accurately.
The meters should allow for testing in situ. The most commonly employed technique is the use of pulsing LEDs. However, more advanced systems provide a means of testing all the energy registers simultaneously using digital data transfer techniques from the local ports. This is substantially easier to use, as well as being time efficient, particularly with large numbers of energy registers used with bulk power transfer applications.
LOW CUBICLE SPACE
Retrofitting high accuracy metering equipment requires cost effective use of space and should also reduce new cubicle costs. The metering equipment for present day systems should be compact and easy to handle. Nineteen-inch rack mounted products with removable metering modules are frequently used.
Bulk power transfer metering has reached a new level of sophistication and maturity, giving independent power producers, generating and transmission companies unsurpassed accuracy and functionality in a single product.