POWER SYSTEMS QUALITY Topic 5: Principles for Controlling Harmonics

Transcription

1 POWER SYSTEMS QUALITY Topic 5: Principles for Controlling Harmonics EE589-Power System Quality & Harmonics Electrical Engineering Department School of Engineering University of Jordan 1

2 Control of Harmonics Two common causes of harmonic problems Nonlinear loads injecting excessive harmonic currents Interaction between harmonic currents and the system frequency. 2

3 Principles for Controlling Harmonics When a problem occurs, the basic options for controlling harmonics are: 1. Reduce the harmonic currents produced by the load. 2. Add filters to either siphon the harmonic currents off the system, block the currents from entering the system, or supply the harmonic currents locally. 3. Modify the frequency response of the system by filters, inductors, or capacitors. 3

4 1. Reducing harmonic Currents in Loads 1. Adding a line reactor or transformer in series will significantly reduce harmonics, as well as provide transient protection benefits. 2. Transformer connections can be employed to reduce harmonic currents in three-phase systems. I. Phase-shifting half of the 6-pulse power converters in a plant load by 30 o can approximate the benefits of 12-pulse loads by dramatically reducing the fifth and seventh harmonics. II. III. Delta-connected transformers can block the flow of zerosequence harmonics (typically triplens) from the line. Zigzag and grounding transformers can shunt the triplens off the line. 4

5 Three-Phase Transformer Connection Zig-zag transformer application as third harmonic filter. Cancellation of fifth and seventh harmonic currents by using 30 phaseshifted transformer connections. 12-Pulse Converter 5

6 2. Filtering 1. The shunt filter works by short-circuiting harmonic currents as close to the source of distortion as practical. I. This keeps the currents out of the supply system. II. The most common type of filtering applied because of economics and because it also tends to correct the load power factor as well as remove the harmonic current. 2. Another approach is to apply a series filter that blocks the harmonic currents. I. This is a parallel-tuned circuit that offers a high impedance to the harmonic current. 3. Active filters work by electronically supplying the harmonic component of the current into a nonlinear load. 6

7 Harmonics Filters Shunt Passive Filter Series Passive Filter Active Filter 7

8 3. Modifying the System Frequency Response There are a number of methods to modify adverse system responses to harmonics: Adding or removing capacitor banks Changing the size of the capacitor banks Adding shunt filters Adding reactors to detune the system away from harmful resonance 8

9 3. Modifying the System Frequency Response 1. Add a shunt filter. Not only does this shunt a troublesome harmonic current off the system, but it completely changes the system response, most often, but not always, for the better. 2. Add a reactor to detune the system. Harmful resonances generally occur between the system inductance and shunt power factor correction capacitors. I. The reactor must be added between the capacitor and the supply system source. II. Add reactor in series with the capacitor to move the system resonance without actually tuning the capacitor to create a filter. h r X reactor X c X source 9

10 3. Modifying the System Frequency Response 3. Change the capacitor size. This is often one of the least expensive options for both utilities and industrial customers. h r MVA Q C sc X X c SC 4. Move a capacitor to a point on the system with a different short-circuit impedance or higher losses. 5. Remove the capacitor and simply accept the higher losses, lower voltage, and power factor penalty. If technically feasible, this is occasionally the best economic choice. 10

11 Where to Control Harmonics? The strategies for mitigating harmonic distortion problems differ somewhat by location. The following techniques are ways for controlling harmonic distortion on both the I. Utility distribution feeder II. End- user power system 11

12 Solution On Utility Distribution Feeders The X/R ratio of a utility distribution feeder is generally low. Therefore, the magnification of harmonics by resonance with feeder banks is usually minor in comparison to what might be found inside an industrial facility. Utility distribution engineers are accustomed to placing feeder banks where they are needed without concern about harmonics. However, voltage distortion from the resonance of feeder banks may exceed limits in a few cases and require mitigation. When problems do occur, the usual strategy is to first attempt a solution by moving the offending bank or changing the capacitor size or neutral connection. 12

13 Solution On Utility Distribution Feeders Some harmonic problems associated with feeder capacitor banks are due to increasing the triplen harmonics in the neutral circuit of the feeder. To change the flow of zero-sequence harmonic currents, changes are made to the neutral connection of Y-connected banks. To block the flow, the neutral is allowed to float (Ungrounded). In other cases, it is more advantageous to aid the flow by putting a reactor in the neutral to convert the bank into a tuned resonant shunt for a zero-sequence harmonic. 13

14 Solution On Utility Distribution Feeders Harmonic problems on distribution feeders often exist only at light load. The voltage rises, causing the distribution transformers to produce more harmonic currents and there is less load to damp out resonance. Switching the capacitors off at this time frequently solves the problem. 14

15 Solution In end-user Facilities When harmonic problems arise in an end-user facility, the first step is to determine if the main cause is resonance with power factor capacitors in the facility. When it is, first attempt a simple solution by using a different capacitor size. With automatic power factor controllers, it may be possible to select a control scheme that avoids the configuration that causes problems. In other cases, there will be so many capacitors switched at random with loads that it will be impossible to avoid resonant conditions. Filtering will be necessary. 15

16 Solution In end-user Facilities Installation of filters on end-user low-voltage systems is generally more practical and economical than on utility distribution systems. The criteria for filter installation are more easily met, and filtering equipment is more readily available on the market. When the magnitude of harmonic currents injected by loads is excessive, industrial users should also investigate means of reducing harmonics by using different transformer connections and line chokes. In office buildings, zigzag transformers and triplen harmonic filters can reduce the impact of triplen harmonic currents on neutral circuits. 16

17 Solution In end-user Facilities Installation of filters on end-user low-voltage systems is generally more practical and economical than on utility distribution systems. The criteria for filter installation are more easily met, and filtering equipment is more readily available on the market. When the magnitude of harmonic currents injected by loads is excessive, industrial users should also investigate means of reducing harmonics by using different transformer connections and line chokes. In office buildings, zigzag transformers and triplen harmonic filters can reduce the impact of triplen harmonic currents on neutral circuits. 17

18 In-line Reactors or Chokes A simple, but often successful, method to control harmonic distortion generated by adjustable-speed drives involves a relatively small reactor, or choke, inserted at the line input side of the drive. This is particularly effective for PWM-type drives. A typical 3% input choke can reduce the harmonic current distortion for a PWM-type drive from approximately 80% to 40% percent. The choke size is computed on the drive kva base. Isolation transformers can provide the same benefit as a choke but may be more costly. However, isolation transformers with multiple drives offer the advantage of creating effective 12-pulse operation. 18

19 In-line Reactors or Chokes A typical 3% input choke can reduce the harmonic current distortion for a PWM-type drive from approximately 80% to 40% percent. Typical line chokes used in 480-V ASD applications 19

20 In-line Reactors or Chokes Comparison between the effectiveness of a 3% choke in reducing harmonic current distortion to the condition without a choke for various ASD sizes (ASD sizes are normalized to the service transformer kva). The larger waveform is without the choke. A substantial improvement is achieved by inserting a choke in the ASD line. 20

21 12-Pulse Configuration A 12-pulse configuration can be achieved by supplying one drive through a -Y connected transformer, and another drive through a - connected transformer. Current waveforms for two separate six-pulse ASDs. 21

22 12-Pulse Configuration When the two waveforms are added together on the primary, the resulting waveform injected onto the utility system has much lower distortion, primarily because the fifth and seventh harmonics are cancelled out. These two harmonics are responsible for most of the distortion for six-pulse drives. 22

23 Zigzag Transformers Zigzag transformers are often applied in commercial facilities to control zero-sequence harmonic components. A zigzag transformer acts like a filter to the zero-sequence current by offering a low-impedance path to neutral. This reduces the amount of current that flows in the neutral back toward the supply by providing a shorter path for the current. To be effective, the transformer must be located near the load on the circuit that is being protected drives. 23

24 Zigzag Transformers The two most important problems in commercial facilities are overloaded neutral conductors and transformer heating. Both of these problems can be solved with proper zigzag transformer placement. Some new commercial buildings include zigzag transformers on the 480/208- V supply transformer secondaries to prevent transformer overheating. 24

25 Zigzag Transformers Typical results with a zigzag transformer show that it can shunt about 50% of the 3 rd -harmonic current away from the main circuit neutral conductors. Thus, the zigzag transformer can almost always reduce neutral currents due to zero-sequence harmonics to acceptable levels. The largest zero-sequence harmonic will nearly always be the third harmonic in office buildings with many computers and related equipment. X=5% V V L H = kv = 480V ~ i(t) 1 A 500 PCs 25