Power Quality Troubleshooting
Introduction Table of Contents Page Safety ............................................................ 2 Getting Started ............................................... 3 First Steps ..................................................... 3 Part I: Facility Distribution System Section 1: Receptacle Branch Circuit ................... 4 Section 2: Service Panels ...................................... 8 Section 3: Transformers .......................................
Getting Started Lighting Panel Lighting Load Utility XFMR MV/480Y Switch Gear Motor M.C.C. PF Correction Capacitor ASD Induction Motor Recept. XFMR 480/208Y Recept. L.C. Receptacle Load Simplified electrical distribution system typical of commercial and industrial facilities. Start at the scene of the crime To troubleshoot PQ problems, one approach is to start as close to the “victim load” as possible. The “victim load” is the sensitive load, typically electronic, that is somehow malfunctioning.
Part I: Facility Distribution System Section 1 Receptacle Branch Circuit Many PQ problems show up at the branch circuit level. There’s a simple reason for this: that’s where most of the sensitive loads (and sensitive employees) are located. It’s also the “end of the line” of the electrical system, and the place where shortcomings can’t be hidden. Let’s assume you’ve been called in to solve the problem.
Flat-topped voltage The flat-topped waveform is typical of the voltage in a commercial building with computer loads. What causes flat-topping? The utility supplies ac power, but electronic equipment runs on dc power. The conversion of ac into dc is done by a power supply. The PS has a diode bridge which turns ac into pulsating dc, which then charges a capacitor. As the load draws the cap down, the cap recharges.
Ø1 Ø2 Ø3 Ø1 Ø2 Receptacle N-G Voltage Measurement Notes 1. A rule-of-thumb used by many in the industry is that N-G voltage of 2V or less at the receptacle is okay, while a few volts or more indicates overloading; 5V is seen as the upper limit. There’s obviously some room for judgment in this measurement. 2. A high reading could indicate a shared branch neutral, i.e., a neutral shared between more than one branch circuit.
Solutions Performance Wiring vs. Code Minimum Any experienced PQ troubleshooter will tell you that the first place to look for most problems is in the building wiring system (including its grounding system). Quality power depends on quality wiring; the term the industry uses is performance wiring (See Table 1.2). The basic intent of performance wiring is to maintain or restore L-N voltage to the load. There is a distinction between “performance wiring” and “code minimum” wiring.
Table 2.1 Service panel measurements. Section 2 Service Panels Check-out the service panel as follows: • Visual inspection • Feeder conductor current test • Neutral conductor current test (feeder and branch) • Phase-to-neutral voltage test (feeder and branch) • Neutral-to-ground voltage test (feeder) • Circuit breaker voltage drop and current on branch phase conductors The service panel is where the effects of single-phase harmonic loads are easy to measure.
Measurements 1. Feeder phase current Check each phase to make sure it is not overloaded. Also check for excessive unbalance. 2. Feeder neutral current Measure the feeder neutral conductor for cumulative neutral current. Third harmonic currents from all three phases will add arithmetically in the neutral. 3. Feeder neutral-to-ground voltage test As at the receptacle, excessive N-G voltage indicates overloading. A N-G voltage at or very near zero indicates the existence of an illegal N-G bond in a subpanel.
Section 3 Transformers Transformers are subject to overheating from harmonic currents. Transformers supplying non-linear loads should be checked periodically to verify operation within acceptable limits. Transformers are also critical to the integrity of the grounding system. Measurements 1. Transformer loading (kVA) If the transformer has a fourwire wye secondary, which is the standard configuration for commercial single-phase loads, actual kVA can be easily determined. (See Figure 3.
Table 3.1 Measurements at the distribution transformer. 3. Total Harmonic Distortion Check for THD of both voltage 1. kVA Transformer loading. If loading 43, 41B and current: exceeds 50%, check for harmonics • For voltage, THD should and possible need for derating. not exceed 5% 2. Harmonic spectrum Harmonic orders/amplitudes present: Same • 3rd harmonic (single-phase loads) • For current, THD should not 5th, 7th (primarily three-phase loads) exceed 5-20% (Table 3.
Table 3.2 IEEE 519 limits for harmonic currents at the point of common coupling. (All percentages are % of I L, maximum demand load current.) Odd Harmonics SCR=Isc/IL <11 11-17 17-23 23-35 >35 <20 4.0% 2.0% 1.5% 0.6% 0.3% TDD 5.0% 20-50 7.0% 3.5% 2.5% 1.0% 0.5% 8.0% 50-100 10.0% 4.5% 4.0% 1.5% 0.7% 12.0% 100-1000 12.0% 5.5% 5.0% 2.0% 1.0% 15.0% >1000 15.0% 7.0% 6.0% 2.5% 1.4% 20.
A final word on measuring THD: the one place not to apply the specs is at the individual harmonic-generating load. This will always be a worst-case distortion and a misleading reading. This is because as harmonics travel upstream, a certain amount of cancellation takes place (due to phase relationships which, for practical purposes, are unpredictable). Measure at a PCC, or at the source transformer. transformer supplying office loads.
Solutions There are a number of solutions for transformer-related PQ problems: • Install additional distribution transformers (Separately Derived Systems) • Derate transformers • Install K-rated transformers • Used forced air cooling 1. Separately Derived System (SDS) The distribution transformer is the supply for a Separately Derived System (SDS), a term which is defined in the NEC (Article 100).
value, %HD (harmonic distortion) of the harmonic currents, and the square of the harmonic order (number). It is not necessary to actually perform the calculation because a harmonic analyzer will do that for you. The important thing to understand is that the harmonic order is squared in the equation and that is precisely where the high- frequency heating effects, like eddy current losses, are taken into account. K-rated transformers are designed to minimize and accommodate the heating effects of harmonics.
• Section 4 Electrical Noise and Transients Electrical noise is the result of more or less random electrical signals getting coupled into circuits where they are unwanted, i.e., where they disrupt information-carrying signals. Noise occurs on both power and signal circuits, but generally speaking, it becomes a problem when it gets on signal circuits. Signal and data circuits are particularly vulnerable to noise because they operate at fast speeds and with low voltage levels.
ØA • • ØB Ø-Ø N ØC Noise Coupling Ø-G N-G Ground • Figure 4.2 Noise coupling. Ground noise measured as ø -G or N-G noise. • A transient surge, especially if it occurs on a high-energy circuit, causes a very fast change in current which can couple into an adjacent conductor. Lightning surges are a worst case, but common switching transients or arcing can do the same thing.
RFI noise reduction employs a number of strategies: • Fiber optic cable, of course, is immune to electrical noise. • Shielded cabling (such as coax cables) attempts to break the coupling between the noise and signal. • Balanced circuits (such as twisted pair) don’t break the coupling, but instead take advantage of the fact that the RFI will be coupled into both conductors (signal and return). This noise (called Common Mode noise) is then subtracted, while the signal is retained.
Transients Cursor moves to display peak Min/Max values. Real-time stamp. Date:hr:min:sec Figure 4.3 Fluke 43 can capture and save up to 40 transients. Transients should be distinguished from surges. Surges are a special case of high-energy transient which result from lightning strikes (see section 5, “PQ Troubleshooting of Lightning Protection Systems”). Voltage transients are lower energy events, typically caused by equipment switching.
Voltage susceptibility profile The new ITIC profile (Information Technology Industry Council) is based on extensive research and updates the CBEMA curve. The CBEMA curve (Computer Business Equipment Manufacturers Association, now ITIC) was the original voltage susceptibility profile for manufacturers of computers and other sensitive equipment. Similar curves are being developed for 230V/50Hz equipment and for adjustable speed drives. Sensitive equipment should be able to survive events inside the curve.
Table 5.1 Inspection of lightning protection system. Section 5 Lightning Protection Lightning protection plays a vital part in the overall power quality of an installation. Lightning occurrence varies by geography, with Florida being the lightning capital of the U.S. Lightning does not have to score a direct hit to be disruptive. It has so much energy that it couples surges into conductors, both those exposed to air and those buried in the ground. Basic lightning protection has two main requirements: 1.
Part II: Three-Phase Loads Section 6 Voltage unbalance can be caused by severe load unbalance but it could just as easily be caused by loose connections and worn contacts. Polyphase Induction Motors About two-thirds of the electric power in the U.S. is consumed by motors, with industrial three-phase motors above 5 HP (7 kW) being by far the bulk of that load. They are linear loads and therefore don’t contribute to harmonics.
Flattopping Effects of inrush current 1. Inrush causes voltage sags if the source voltage is not stiff enough: • Relays and contactor coils might drop out (typically, the sag would have to get as bad as about 70% of Total harmonic distortion < 5% normal line voltage); or, if they hold in, their contacts Figure 6.1. Voltage distortion. might chatter (especially if the additional load causes 4. Loading 5. Inrush a long-term undervoltage).
Measuring Displacement Power Factor on 3-phase Induction Motors Select which of the two methods to use based on the transformer configuration supplying the motor. Either method will give the same results. Method #1 is for the grounded-Y source. It is simple and can be applied in most situations, since virtually all of the low voltage motors in commercial and light industrial facilities are fed from a grounded-Y source. Method #2 is for floating sources sometimes found in heavy industrial facilities.
Section 7 PQ Troubleshooting of Adjustable Speed Drives AC ASDs can be both a source and a victim of poor PQ (see “Measurement of Adjustable Speed Drives with Fluke Meters,” document number G0416UEN, for more information on drive troubleshooting). Voltage distortion If high-voltage distortion shows up as excessive flat-topping, it will prevent dc link capacitors from charging fully and will diminish the ride-through capability of the drive.
Figure 7.2 Voltage Notching. For the SCR converter, there are three main issues that affect line-side PQ: • Commutation notches. SCR switching or commutation is such that there are brief moments when two phases will both be “ON.” This causes what is in effect a momentary short circuit that tends to collapse the line voltage. This shows up as “notches” on the voltage waveform. These notches cause both high V-THD and transients.
12-pulse converter If the delta-wye/delta-delta are packaged together (delta primary, delta and wye secondary) and each secondary feeds one of two paralleled six-pulse converters, a 12-pulse front-end is created with all the benefits mentioned above. 18-pulse designs are also available. Because of the extra cost, this type of solution tends to only get used on high HP loads.
Power system resonance Hot vibes can result when harmonics and capacitors get together Is it possible to install “Power Factor Correction Capacitors” and have PF get worse? It certainly is, and a starting place to understanding this puzzle lies in the distinction between Displacement PF (DPF) and Total Power Factor (PF). The penalty for not understanding the difference can be blown capacitors and wasted investment.
Power System Resonance which are many times greater than the exciting current. This In a worst-case scenario, the so-called “tank circuit” can inductive reactance (XL) of the severely damage equipment, transformer and the capacitive and it will also cause a drop reactance (XC) of the PF correcin power factor.
Section 8 Troubleshooting Commercial Lighting Loads Lighting loads are a major load for many large facilities. Evaluating these circuits is important for both energy conservation and power quality. Keep in mind that commercial lighting loads are wired single phase, with the loads connected from phase to neutral. Typically, the phase-to-phase voltage is 480V, with the phase-to-neutral voltage at 277V.
A Lineup of Power Quality Culprits From utility source to receptacle Lightning Can be extremely destructive if proper surge protection is not installed. It also causes sags and undervoltages on the utility line if far away. If close by, it causes swells and overvoltages. But in the final analysis, lightning is an act of nature and not in the same category as the damage man does to himself.
A word on test tools The minimum requirement for test tools used in PQ troubleshooting are: • True-rms for accurate measurement with harmonics and distorted waveforms. • In addition, instruments with recording capability, waveform display and specialized measurements (such as harmonics, sags and swells, transient capture, high frequency noise, etc.) are needed.
