There are an escalating number of increasingly complex and non-linear loads and generators being connected to the transmission network. To ensure that the network remains secure, background harmonic levels must be checked before a new generator is connected. At present measurements are usually taken up to the 50th harmonic, but recently it has not been unusual for measurements to be required up to the 100th harmonic. A lot of work is being carried out to find suitable and reliable monitors to measure and record the data.
When a portable monitor is taken to a substation to measure harmonic data, it is connected to a Voltage Transformer (VT). At present, there are three types of VT, a Wound VT (WVT), a Capacitor VT (CVT) and a Resistor Capacitor Divider VT (RCD VT). It is commonly accepted that the RCD VT is the most suited to this purpose, as it specially tuned to each application; unfortunately this also makes it very expensive. A WVT is also more expensive due the amount of copper required and although it is more expensive it is not particularly suited to measuring harmonics as it has a low bandwidth, only allowing measurements up to around the 12th with any good accuracy. The third option is to use a CVT that has been fitted with a PQ Sensor. This is a simple technology that increases the accuracy and the bandwidth of the CVT, which is otherwise limited in accuracy to measurements at fundamental frequency.
At present there are only six RCD VTs on the network and a very limited number of CVTs that have been retro fitted with a PQ Sensor (PQCVT). This means most of the Quality of Supply (QoS) measurements are carried out using a WVT. It is proposed that National Grid install a PQ Sensor at a site and do a comparison of the three VTs, to confirm or refute the hypothesis surrounding Voltage Transformers with regards to QoS. Once this study has been carried out it will provide a sound understanding of VTs and aid informed decisions when reviewing any policy surrounding the use of VTs for QoS and dynamic system monitoring applications.
A difficulty with this trial has been finding a site with the three types of VT on the same circuit. One such location is Singlewell 400 kV Substation. The next problem was finding an outage on the correct circuit to provide us with an opportunity to retrofit the CVT with a PQ Sensor. The next outage that National Grid could do this during is 31/10/11 – 04/11/11. If this outage is missed a comparison will not be possible for another two years (2013).
To make this VT comparison fair three identical monitors are to be used. Although National Grid have two types of power quality monitor, of which National Grid have three units, these monitors are not able to measure over the 50th harmonic. For the purposes of this trial National Grid would like to measure as high as possible to give us a true indication of the accuracy and capability of each VT. It is also proposed to purchase two power quality monitors to carry out this comparison.
Objectives
To evaluate the performance of the three types of Voltage Transformer used in National Grid with respect to QoS and system monitoring capabilities.
