Converter grounding - Page 1
Forum section: electronics / general

author: mike buba
author_rank: Contributor
timestamp: March 11, 2022, 12:18:30 am
content: How should I ground my converter?The converter is supplied from a single-phase home electrical sockets (TN-S system). The converter consists from a step-up PFC converter and a single-phase inverter.I have seen both DC-bus mid-point grounded (case A) and negative DC-rail grounded (case B). What is the 'correct' way/benefits?I am certain I should not ground the output negative (case C) as the same example is given in BS 7671 "Requirements for Electrical Installations" showing whatnotto do.

author: zeke.
author_rank: Contributor
timestamp: March 11, 2022, 02:49:56 pm
content: Cant talk for the British Standards but I don’t usually see a full bridge with more than 1 cap on the DC link. For half bridges you will see the neutral point of the DC link caps grounded. In this situation for a full bridge, case B should be grounded but the neutral point on the dc link for this circuit throws me off.

author: penfold
author_rank: Frequent Contributor
timestamp: March 11, 2022, 03:00:44 pm
content: By "ground' do you mean "earth" as in "mains earth"? Without isolation, you probably want to avoid connecting any point directly to earth as it will trip the RCD.

author: Ian.M
author_rank: Super Contributor
timestamp: March 11, 2022, 03:19:03 pm
content: None of the above!Consider the DC bus relative to incoming Neutral. On positive half cycles (of Line) the bridge clamps the negative rail to Neutral as soon as the PFC converter starts drawing current. On negative half cycles the negative rail follows Line, peaking at the same (negative) voltage as the incoming mains. Therefore the whole DC bus including the capacitor midpoint is 'bouncing' relative to Neutral, following a negative going half-wave rectified sinusoid While doing so, the positive rail remains whatever DC voltage the PFC circuit provides above the negative rail, so relative to Neutral, its the same waveform, offset positive by the DC bus voltage.As Neutral isn't fully floating with respect to Ground (PE), connecting any of the three labelled points to Ground (PE) is dangerous, and will cause a significant fault current to flow, and most likely is destructive.

author: mike buba
author_rank: Contributor
timestamp: March 11, 2022, 04:33:04 pm
content: Quote from: Ian.M on March 11, 2022, 03:19:03 pmIf I connect the main supply via a single-phase transformer, will this make a neutral floating (neutral ungrounded)?For example, the connection below.[ Specified attachment is not available ]Can I then connect the earth to DC(-) or mid-point?This article gives different combinations of earthing with a neutral point of AC side transformer ungrounded (Table 1 Page 7 and Figure 9 Page 9).Fardapaper-Protection-and-grounding-methods-in-DC-microgrids-Comprehensive-review-and-analysis.pdf(3047.11 kB - downloaded 39 times.)

author: Ian.M
author_rank: Super Contributor
timestamp: March 11, 2022, 04:44:49 pm
content: Once you've added an isolation transformer in the feed, there is no Neutral, just two antiphase floating Lines. You can then ground any point on the DC side that doesn't carry a switching waveform, and actually has a DC path to the DC bus, which rules out points A (no DC path) and C (Switched by H-bridge).A is no good, because as there is no (or only low current) balancing circuit for the capacitors, any leakage current could over-voltage one of the series capacitors, and C is no good because the fast edges of the switching waveform could result in significant current through the transformer's inter-winding capacitance, or secondary to core capacitance, possibly resulting in insulation breakdown.That only leaves you with B (grounding the negative side of the DC bus) as a viable option.It would be different if the bridge was replaced with a voltage doubling rectifier, with the secondary connected to the midpoint of the capacitors, as that will tend to keep the capacitor voltages more or less balanced, so A also becomes an option.

author: penfold
author_rank: Frequent Contributor
timestamp: March 11, 2022, 05:00:33 pm
content: Quote from: Ian.M on March 11, 2022, 04:44:49 pmWould ground at B then make the output somewhat dangerous? with the output -ve jumping up and down between the DC link and earth?

author: Ian.M
author_rank: Super Contributor
timestamp: March 11, 2022, 06:48:37 pm
content: The output's dangerous anyway as its fed from a >340V DC bus with significant reservoir capacitance. To make one side of it safely groundable, or make it center grounded would incur significant extra cost, either in filtering to remove the fast edges, or an output transformer, or a change in topology with symmetrical positive and negative DC bus and a grounded 0V rail.

author: Faringdon
author_rank: Super Contributor
timestamp: March 11, 2022, 06:51:21 pm
content: Primary side of any offline converter (ie the non isolated bit) is never directly connected to earth.The earth connection should be made by Y capacitors from live/neutral to earth.It looks like a water boiler element heater. Is it you want to earth it for safety?...its not necessary....think of your element in your boiler now....its not connected to earth...just has line and neutral connection.If you want to earth the cctry for EMC...then this is worth reading...https://www.eevblog.com/forum/renewable-energy/earthing-a-metal-smps-enclosure-and-the-secondary/...especially post #4

author: mike buba
author_rank: Contributor
timestamp: March 11, 2022, 07:26:58 pm
content: Quote from: Faringdon on March 11, 2022, 06:51:21 pmNo, it is my 'small' project; getting a controlled AC from a single-phase grid source; 1st stage: step-up active power factor correction circuit, 2nd stage: DC/AC inverter

author: mike buba
author_rank: Contributor
timestamp: March 12, 2022, 12:55:29 pm
content: Quote from: Ian.M on March 11, 2022, 04:44:49 pmSo I can ground only DC(-) if I have an isolation transformer?Hereit says:"Therefore, a snubber circuit, consisting of RSNUBand CSNUBfrom the switch nodeto power groundmay be required. A snubber is an energy-absorbing circuit used to eliminate voltage spikes caused by circuit parasitic inductance when a switch opens. By providingan alternate path to groundfor the current flowing through the circuit's parasitic inductance, the snubber reduces the voltage transient and damps the subsequent ringing with the parasitic capacitance that occurs when the switch opens.Does ground mean earth (PE)?In a high-voltage low-currentdevices(e.g., 1 kV, 3-4 A) they use single-phase voltage supply and no need for isolation transformer and the output DC(-) is not grounded (connected to PE).