Kehui International

The DC system in a substation provides power to a variety of equipment involved in the control and protection of the electrical power system. It is therefore very  
important that this system is working correctly. As the DC system is a floating ungrounded system, a ground fault on either the positive or negative terminal will  
not cause a disturbance and the system will continue to run normally. However, should a ground fault occur simultaneously on the other terminal, it will result in  
a virtual short-circuit between positive and negative through the ground, causing the failure of the DC supply in the substation. To prevent this, it is important to
identify the faulty feeder and to trace the fault location quickly and precisely when the initial fault occurs. This process does not disturb the normal operation of
the system.

The ZDT-300 DC Ground Fault Locator has been developed for the fast identification and location of ground faults on the DC system to ensure the damage which would occur in the event of a second fault, is avoided. The system quickly and accurately finds the fault point, while overcoming the influence of the distributed capacitance of the system. It provides effective identification of high resistance faults, low resistance faults, faults creating system loops, on systems with single or multiple faults. It can also identify the presence of AC interference on the DC system.

The XC-2100E will allow the user to identify and locate events on AC or DC power systems. It can be applied to overhead, underground or submarine lines, or hybrid combinations. The most urgent issue on the line is a fault, but its integrity and operation may also be affected by the effects of lightning and deterioration in the performance of the circuit breaker, both of which can be monitored by this equipment.

When a fault does occur, it can be difficult to locate, particularly on long remote lines. Conventional fault locators work on the principle of measuring the impedance of the loop through which the fault current flows. In most cases, this includes a return path through the ground for which knowledge of the actual impedance is hard to obtain and is often estimated, leading to inaccuracies in the measurement of the distance to the fault. By utilising traveling waves, created when the fault occurs, the XC-2100E system can measure the fault position independently of impedance and identify the fault point much more accurately.

The system provides accurate fault location solution for all transmission and distribution power lines, with each unit monitoring up to 8 lines. The fault distance measurement error is less than ±150m and is independent of line length and other factors which affect the fault location accuracy of traditional impedance measurement methods, including; line and earth return loop impedance, instrument transformer accuracy and uncertainty of zero sequence impedance due to variance of soil resistivity along the line corridor. When the measurement is made it can be automatically plotted on to a Google Map of the network, or an offline map if the internet is not available, to allow the fault to be quickly accessed.

Connection is normally made using clip-on CTs around the protection CT wiring in the substation, this allows ready retrofitting of the system as the wiring is not disturbed. Alternatively, the equipment can be directly connected to the line CT or PT, or to a clip-on CT around the ground wire of a CVT. For HVDC systems the traveling wave is obtained through a clip-on CT placed around the ground wire of the harmonic suppression filter. The system is synchronised by a GPS signal and the results from each end can be transmitted to a master station pc throough the utility's SCADA system or other internal communication network.