Identifying/Isolating Sources of Voltage Sags Quickly

Recently I’ve attended a Power Quality (PQ) Seminar organised by Fluke in Manchester, UK as part of their Fluke 430series PQ Analyzer promotion campaign. During this seminar, they talked about power quality problems, their experiences in identifying those using Fluke monitors, how they helped customers solve these problems, gave us an hand-on experience in using PQ monitor equipment.

The following are the two quick tips recommended during this seminar on how to identify the source of voltage sags.

• If voltage sags coincides with depression in current flow into the customer’s feeder then the source of voltage sag source is external to customer’s facility;
• However if the voltage sag coincides with increase in load current flow into the customer’s supply feeder then the source of voltage sags problem is within the customer’s facility.

Principle:

Typical causes of voltage sags due to external utility are faults and switching on/off of large loads (induction motors etc.) at the utility network or at the neighbouring customer’s facility connected to the point of connection (or supply) as our customer. In this case, the cause of voltage sags is due to large voltage drops across circuits caused by large currents drawn during system events (faults, sudden connection of large load etc), and our customer’s facility as a result will receive reduced current. Therefore for external causes of voltage sags, the voltage sags coincide with depression current consumption in to our customer’s feeders.

On the other hand, the same system events (faults etc.) when arise within our customer’s facility, then large currents are drawn by customer’s supply feeders causing voltage to depress, and thereby having increased current consumption coinciding with voltage sags.

The above voltage sag source detection logic is simple, yet has hugely practical implication in isolating the problem quickly.

There is also considerable amount online resource available on various power monitoring and application aspects at the Fluke UK Application Notes section. Hope you will learn something new here having thoughts like: ‘Hey! That’s interest and useful, I didn’t know that before.’, or feel good at heart thinking ‘Ah! I knew that and I could teach some folks perhaps’. However you feel, hope you will have a good time perusing through them.

Have fun!

Alex McEahern: Use all Senses Except Taste to Identify PQ Problems

I have had an opportunity to meet Alex McEahern and talk to him during his visit and meeting with my PhD academic supervisor Prof. J V Milanovic. It was a pleasure talking and listening to his ideas, work and his company ‘Power Standards Lab’.

He developed an  interactive software teaching toy for young engineers like to me to understand and identify various  power quality problems. Links: Power Quality Teaching Toy; Online PQ tutorials; and Power Standard Lab.

Online Overhead Line High Voltage Inspection

This is a video showing the inspection of a live overhead electric transmission line.

For those power system engineers like me who spend most of their time at their work desk with ocassional field trips, this is one darn video bringing in the good blood rush up their veins. Pretty impressive and highly motivational!

Have Fun!

Power System Harmonics, True Power Factor & DPF Measure

Displacement Power Factor (DPF) is the power factor as we know at fundamental system frequency (50Hz in UK). However, True Power Factor (PF) or just Power Factor is the product of the distortion power factor and DPF. Check out the Wikipedia article on this  topic. The following equation related components:

Where,  is the total current harmonic distortion at the point of measurement, and are fundamental and total harmonic RMS currents, and  is the distortion power factor (in other words distortion factor associated with power factor).

The above equation leads to the following conclusions:

• PF≤DPF, True Power Factor is always less than or equal to Displacement Power Factor.
• PF = DPF, True Power Factor equals Displacement Power Factor when there are current harmonics at the point of measurement;
• PF<DPF, suggests presence of harmonics, take it easy: awareness is good.
• PF<<DPF, means its time to take action.

The above observations, comparing DPF and PF will give you a quick assessment of harmonic severity, however if detail assessment is required then you will need to monitor both and .

As I understand, most meters or monitoring equipment that display PF and DFC also may have the ability to calculate both current and voltage total harmonic distortion factors: and , sometimes including individual harmonics levels as numbers and/or as a harmonic spectrum bar chart. Now if have measured these values, i.e. both THD for current and voltage, and individual harmonics levels in %, then compare them against the harmonics standards that govern your electric network, and you will know the severity of the harmonic problem.

In UK, DNOs are required to comply with EN50160 Std. and consumers with G5/4-1 Std.

Overhead Line Lightning Strike Severity and Probability

Lightning Phenomenon:

Lightning Strike is the discharge of electric charge accumulated in the clouds to the ground. Clouds accumulate typically the negative charge, and in response the ground produces the counter charge: the positive charge. Electric discharge happens between the two, i.e. clouds and the ground, under certain electric discharge conducive conditions, effectively creating the shortest electrical path.  These conditions include accumulated charge density, humidity in the air (enabling faster dielectric break down), ground elevation (buildings, mountains, tall living things) etc.

Physics behind this phenomenon is much elaborated in several books and articles found in both libraries and internet, and their perusal is recommended.

Lightning Strike Frequency:

Although the frequency of lightning strikes might vary year to year, long term (usually many years to decades) maintained records give a statistical approximate number of expected strikes each year. This is usually referred as Isokeraunic Level [1]. Reference [2] and [3] give typical Isokeraunic Levels around the world, and gives a geography dependent empirical formulae to calculate Ground-Flash Density (or – Strike Density). The empirical relationship between Isokeraunic Level and Ground-Flash Density for UK is given as:

Where, GFD is the Ground-Flash Density in ,  is a factor that varies between , b is a factor that varies between , and  is the Isokeraunic Level in . Isokeraunic Level for Ireland, north UK, mid-west UK  and southern-west UK is typically between 5-10, while for middle-east UK and southern-east UK is between 10-20.

Overhead Hit Frequency:

Strike radius is the scope of the ground elevated structures or ground itself that will be exposed to lightning. This, usually calculated in meters, is very much dependent of lightning strike peak current magnitude.

Lightning strike distance in meters [3],

Where, is the Lightning Strike peak magnitude in kA.

Now, the scope of Lightning Strike area that will hit the OHL will depend on the tower structure. For a un-shielded wood pole structure (assuming all conductors are same height from the ground level), this is given as following:

Strike Area,

Where,  is the sum of strike radius of left most conductor, distance between the farthest conductors, and strike radius of the right most conductor respectively. $latex L$ is the length of the exposed overhead line.

Total estimated OHL Flashes per Year if every flash was as chosen Strike Current, , is given as,

For a given strike current, Cumulative Lightning Strike Probability is given as [3],

For a given strike current, Estimated Annual OHL Lightning Strikes in (Flashes/Yr) =

Now plot a bar chart between various strike current versus expected annual lightning strike frequency, and you see typical decline of Flashes/Yr with increasing strike current magnitude. And if you invert the Flashes/Yr you get expected number of years before you get see a lightning strike for a given strike current peak.

References:

1. Lightningtech Website. [online], Available: http://www.lightningtech.com/d~ta/faq2.html, Accessed: Oct 2009.
2. Chowddhuri, P., Electromagnetic Transients in Power Systems, Exeter: Research Strudies Press Ltd., 1996.
3. Grigsby, L. L., Power Systems, Boca Raton: CRC Press, 2007.

Per Unit Formulae and Conversions

Even the best of the power systems Geeks sometime stumble at the basics. Here are some per unit formulae to brush up if you are one of them or among those willing to learn like me.

A per unit value is a scaled factor of the measured system parameter to the chosen system base value. In other words, the chosen value becomes by which all other values for consider system parameter will be compared. It’s a ratio and is a unit less quantity.

Most common approach, for most studies, is to choose a system apparent power base (typically 100MVA) and system local network voltage (line to line voltage) base values and base values for all other parameters can be estimated there after.

Base Value Calculation:

         (1)

        (2)

        (3)

Per Unit Calculation:

        (4)

        (5)

         (6)

Per Unit Conversion:

        (7)

References:

1. Wikipedia, Per Unit System, Wikipedia. [online]. Available: http://en.wikipedia.org/wiki/Per-unit_system. [Accessed: Sep. 19, 2009].
2. W. D. Stevenson, Jr., Elements of Power System Analysis, 3rd ed. New York: McGraw-Hill, 1994.

Power Quality Working Groups

Power systems research and application engineers, depending on their ‘point-of-view’, are broadly divided into four categories:

• Those that take a utility’s point of view;
• Those that take end-user’s point of view;
• Equipment manufacturer’s and third party solutions and services (consultants, experts etc.) provider’s point of view;
• And finally those (e.g. PQ related standards, working groups, regulators, university PQ projects that are funded by independent bodies or institutes etc.) who see the big picture and bring equilibrium among the above three perspectives.

Each of first three groups, i.e. utility (Group 1), customer (Group 2) and equipment manufacturer/consultant (Group 3), have valuable insight, expertise, and know how in their respective representing group. These three representing groups usually have biased view on various PQ issues. Some of these include: ‘who is responsible for PQ problems?’ and ‘who should and how much each should contribute towards problem solution?’ However, it is this biased view that makes them a vital constituent in maintaining a fair share of involved party’s (utility, end-user etc.) PQ improvement responsibility. The final or the fourth group, although seeks to see the big picture, taking a collective view of all three groups preceding it, may lack in depth knowledge of particular PQ problems each of other groups are handling.

When PQ standards are written, a working group is set up, which at best tries to include at least one delegate or more, representing each of these groups to review and to push forward a new standard that is fairly acceptable for all concerned parties.

A comprehensive study, nevertheless, should aim to develop a methodology that includes tools/modules to bring customized voltage disturbance assessment for both utilities and industrial customers, such that results obtained from the tool could be well beneficial for equipment manufacturers to access PQ market potential and to establish specification range of their equipment to meet a specific customer’s or utility’s voltage disturbance immunity criteria. The objective should be to present a tool that brings interaction and integration of data consensus from utilities, customers and equipment manufacturers, thus enabling faster optimization of PQ improvement through iterative interactions of these parties (utilities, customers and equipment manufacturers).

Power Factor Capacitor Discharge Resistor Design

When a Power Factor Capacitor (PFC) step, a single unit in a bank of several capacitors, is disconnected or switched off, the discharging resistor connected across the capacitor will discharge it to designed retained voltage value in discharge time seconds. Typical discharge resistor ratings for a given power factor capacitor  in μF or MVAR include: maximum normal operation system voltage  in kV and required retained discharge voltage  in Volts (around 50V) at discharge time  in seconds (usually <60s). The discharge resistor  in kΩ  is given as,

         (1)

Depending whether the PFC bank step is star or delta connected (usually star connected), for given system frequency  and capacitor step’s rated reactive power output in MVAR, the capacitor step’s capacitance  in μF is given as:

• If star connected,
• If delta connected,

Explanation:

Capacitor voltage decay across the resistor is given as, . Rearrange this for Discharge Resistor ‘R’ and you get (1). Simple!

Ireland Minimum Tranmission Line Sags Limits

Typical transmission line mandatory minimum safe heights from ground level:

110kV: 7m over ground, 8m over roads;

220kV: 8m over ground, 9m over roads;

400kV: 9m over ground, 10m over roads.

This take into account the worst sag possible.

The following are mandatory transmission line safe height limits [1] in Ireland:

• 110kV: 7m over ground, 8m over roads;
• 220kV: 8m over ground, 9m over roads;
• 400kV: 9m over ground, 10m over roads.

Compliance to these limits are required irrespective transmission line loading and environmental operating conditions. Please feel free to post for other countries or transmission networks if you have. Thank you! ^__^

References and Reading:

1. O. M. Armstrong, “Transmission Line Conductor Sag Measurement with LIDAR Survey and Non-contact Temperature Determination,” in 2007 Proc. RSPSOC. [Online]. Available: http://www.ceg.ncl.ac.uk/rspsoc2007/papers/122.pdf, [Accessed Aug. 27th, 2009].

H-Type Wood Pole Transmission Tower Transient Model