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Medium Voltage Networks

Status of the neutral


To identify ground faults in a network and therefore carry out effective protection, it is necessary to know in detail how the neutral is run. Identification of ground faults is made by means of voltage and/or homopolar current measurements and therefore knowing the existence and order of these parameters is fundamental in being able to select and set the protection system. Unlike the protections against overload or polyphase short-circuit, no signal (voltage or current) normally comes to the protections which have to identify ground faults, but, on the other hand, only comes when there is a ground fault in the network. This condition makes the protection system to be provided very simple, generally only requiring one threshold (voltage and/or current) with relatively short trip times. By analysing the various types of status of the neutral, the types of protections which can be associated can be defined.






Isolated neutral 


In networks with isolated neutral, no circulation of homopolar current is generated deliberately (by means of grounding systems) in the case of a fault between a phase and ground. However, there is a circulation of homopolar current in the plant linked to the phase ground capacities of the machines and cables (for what regards the transformers, the phase to ground capacities are very small and they can be overlooked). 

The difficulty (in any set-up the network may be found to run in) of being able to identify ground faults using selective protections which measure the fault current can be deduced from this. The only way to be able to ensure identification of the fault is measurement of the homopolar voltage (voltage normally equal to zero in the absence of a fault and different from zero only in the presence of a phase to ground fault). Unfortunately the voltage homopolar protection (like all voltage protections for that matter) is not of the selective type, i.e. it is not able to identify the position of the fault, but is only able to indicate that there is a fault in the network without specifying its position.

Homopolar current, homopolar voltage and angle between voltage and homopolar current in a network are: 

– homopolar current only from capacitive contribution (operation of the metallically interconnected network) of variable value in any case and, in general, not guaranteed for all the conditions the network can be run in. Identification of the faults is not always certain by means of homopolar current measurements; 

– homopolar voltage always present in the case of a ground fault. It is therefore definite identification but with uncertainty linked to the position of the fault since the voltmetric signal is practically the same for the whole network and does not allow selective identification;

 – angle between voltage and homopolar current: the current is in advance by 90° compared with the voltage (capacitive type of network).


Solidly grounded neutral 


With solidly grounded neutral, the single-phase to ground fault current is in the same order of size as the short-circuit current for polyphase faults. Consequently simple and selective identification of the faults by means of protections which measure the homopolar current is possible (or the homopolar protection could even be omitted and only the phase protection used). Homopolar current, homopolar voltage and angle between voltage and homopolar current in the network are: 

– homopolar current of high value. Therefore identification of the faults by means of measuring the current is always certain and of selective type (the part of the network seat of the fault can be identified correctly); 

– homopolar voltage: if this voltage is measured between star point and ground, the voltage is nil, whereas, if the vectorial sum of the three phase voltages is measured, this is different from zero and gives indication of a fault in the network (but not of selective type); 

– angle between voltage and homopolar current: the current is late (typical values 75-85°) compared to the voltage (inductive type of network source). 



Neutral grounded by means of resistance 


Grounding the neutral by means of resistance allows a definite current to be obtained in the case of a fault and consequently to be able to carry out selective protection of the network. 

Depending on the value of the resistance installed, fault current values which are higher or less high are obtained, but:

 – the lower the fault current is, the small the damage to the machines is; 

– the higher the fault current is, the more easily the fault is identified (and the protection with lower sensitivity is required). Homopolar current, homopolar voltage and angle between voltage and homopolar current in the network are: 

– homopolar current of known value. Identification of the faults is possible by measuring the homopolar current. The protection is therefore of the selective type; 

– homopolar voltage: if this voltage is measured between the star point and ground, the voltage varies according to the value of the grounding resistance (for grounding resistances of high value one falls back into the situation of isolated neutral, for grounding resistances of very small value, one falls back into the situation of solidly grounded neutral). If the vectorial sum of the three phase voltages is measured, it is different from zero and gives indication of a fault in the network (but not of the selective type); 

– angle between voltage and homopolar current: theoretically equal to zero (in phase). In reality, the angle is in any case capacitive for the contribution of the to ground capacity of the network. There are various methods to create network grounding according to the availability or lack thereof of the star point as shown in the figure.





Neutral grounded by means of impedance (Petersen coil) 


Grounding the neutral by means of impedance allows the network capacitive currents to be compensated and therefore to reduce the current to relatively small values in the case of a fault (in Italy, utilities limit the fault current to 40-50 A) and with a fault angle about equal to zero (compensated network). Homopolar current, homopolar voltage and angle between voltage and homopolar current in network are: 

– homopolar current of known value. Identification of the faults by means of homopolar current measurement is possible. The protection is therefore of the selective type; 

– homopolar voltage: the measurement of the vectorial sum of the three phase voltages is different from zero and gives indication of a fault in the network (but not of the selective type). 

– angle between homopolar voltage and current: theoretically equal to zero (network tuned). In actual fact, the angle can in any case diverge slightly both in advance and delayed according to the setting of the compensation reactance and to changes in the network set-up. 


Measurement of the ground fault current and identification of the faulted phase 


Since the advent, first of electronic and then of digital protections which have low absorption on the current circuit, the use of ring type CTs has been possible (able to generally give very small performances), which allows the vectorial sum of flux to be measured instead of the vectorial sum of the three currents (residual connection). When a homopolar overcurrent protection is connected to the residual connection of the phase CTs (Holmgreen connection) it performs a vectorial sum of the currents and the resultant is therefore affected by the aperiodic components linked to magnetisation of the transformers or to motor starting. In this case, very conservative settings of the protections are required and the stability of these is not normally guaranteed (risk of unwanted trips). It is therefore suggested to systematically use (obviously where possible) CTs of ring type associated with the homopolar overcurrent protection. In the case where it is necessary to identify which of the phases is the seat of the ground fault, identification is possible using undervoltage protections with measurement for each independent phase connected between the phase to ground (obviously to the VT secondary).



This article has been extracted from Protection criteria for medium voltage networks by ABB

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