Microprocessor controlled communication line level meter

Abstract

ESKOM control a massive power grid in a vast geographical area in the R.S.A.. This power originates at the power stations, from where it is distributed to the users. All the power generated is pumped into a National Power Grid. The backbone of the network consists of the following supply voltages: • 765 kV • 400 kV • 132 kV These voltages are stepped down locally at substations to lower voltages for the customers. Bigger customers (e.g. Municipalities, Mines, etc.) are bulk users and use high voltages. Lower voltages range from 220V up to 66kV. In order to ensure a good service to all power user customers ESKOM must be able to identify power failures and other abnormal conditions as quickly as possible and react fast to restore power again. When supervising a power grid good communication systems are essential. Communications systems serve as links between the following functional systems: • Contacting personnel with radio (Mobile or Handheld) • Contacting personnel \\ith pagers (Digital or Analogue) • Receiving up to date information on the SCADA network • Protection on power lines and transformers (Fault conditions) Without good telecommunication ESKOM will not be able to control the national grid efficiently. The Telecommunication Department fulfil a vital role ensuring that the National Grid functions at its optimum. It is normally impossible to do an accurate measurement of the power level in dBm or dBv on a communication line while an RTU is communicating to the MASTER. This is mainly because the duration of the data burst on the communication line is less than the sample time required by the level meters available. The time duration on a TELKOR PUTU general poll is 250ms. With the available digital meters (e.g. W & G SPM33A) it is totally impossible to get a power level reading because the sampling time of the instrument is I second. With the analogue meters available (e.g. W&G SPM09, SPM31) it is possible to get a reading, but this normally between 2 dBm and 4 dBm Iow, because of the dynamic behaviour of the moving coil. Thus before the pointer of the meter has reached the correct level, the burst of data has stopped This is characterised by three quantities: 1. The inertia (1) of the moving coil about its axis of rotation. 2. The opposing torque (S) developed by the coil suspension 3. The damping constant (D) A solution is to sample the receive and transmit levels during polling with an electronic circuit and feed it into an ADC connected to a Microcontroller (e.g. 8031 family). The Microcontroller will do all the converting and mathematical functions and will output a value through a DAC. This output value will be a current (mA) value directly proportional to the input level (e.g. -20dBm to OdBm = 0 to 5mA). These RX and IX level values can be fed into analogue inputs of the RTU. This realtime measurement of the levels on communication lines will be available at the SCADA master. These values can then be trended and if a downward trend is observed, maintenance can be done on the line before a failure. This should result in higher availability of the SCADA network.