Tips on Hooking up a Generator (genset) to a Uninterruptible Power System (UPS)

by Dexter Hansen

header



Index  

Introduction

Sizing Considerations

Start Up Load / In Rush Current

Other Generator Tips

UPS Tips

Other Alternatives

UPS's for Sale

Other Links


Introduction   

There are many people in the generator and UPS industries that have tried to operate engine driven alternating current (A.C.) generator sets (gen-sets) and uninterruptible power systems (UPS) together. Although some systems are successful, there are problems that can occur. This article should provide insight into the problems of using a gen-set to power a UPS as well as provide enough information to possibly save the reader from a problem installation.

Achieving and maintaining adequate control of frequency and line noise is a common issue in generator design and use. Unless the gen-set is sized 2 to 3 times the size of the UPS, and the UPS is about 75 percent loaded, the UPS may never switch from the inverter (battery power) to generator power. Another factor is that there is a great deal of line noise from small generators compared to utility line power. This can also cause the UPS to stay on inverter until the batteries go dead. To address these problems, Best Power Technology, Inc., a fairly well known U.S. UPS supplier, started a DC generator product line. They have since dropped that product line due to a take over by a UPS competitor who did not want to be in the generator business.

When preparing to connect a gen-set to a UPS, first check with the UPS manufacturer for recommendations on using their UPS with a generator. Sizing and compatibility issues should be addressed before even considering hooking a gen-set to the UPS. Loading and the ability of the generator to regulate the load, possibly the hardest issues to deal with, should be considered.

In addition, the type of power plant and alternator used in the gen-set can effect whether the equipment will work together or not. For instance, Diesels/turbine engines work better than gasoline/liquid propane (LP) and Natural gas (NG) engines. It has been noted on gen-sets that single-phase alternators work better than three phase alternators. Turbo-charged engines work better than normally aspirated engines, as well as being better at higher altitudes. Fuel injected engines work better than engines with carburetors (Aspirated engines use carburetors). See Figure 1 below for a block diagram of a typical gen-set/UPS installation.

figure 1

Figure 1. A Typical Generator - UPS Installation

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Sizing Considerations

Tables 1 and 2 provide generator sizing and loading recommendations for various sizes of UPSs. These recommendations are based on UPSs and gen-sets from several manufacturers, and are more rules of thumb based on experience with connecting gen-set and UPSs together than absolute rules. The two tables differ in the fuel type of the gen-sets engines.

The reason that a background load is shown in the tables is two fold. First, the additional load helps the generator regulate better. Second, in an emergency situation, it is always a wise move to put in emergency lighting, and is an electrical code requirement. Caution is advised when operating fluorescent lights from the UPS for emergency lighting. Preferably, power the fluorescent lights from the generator and have some incandescent emergency lights running from the UPS. Fluorescent lights will sometimes flicker, especially if the UPS is one that is a double conversion unit that uses a Ferro-resonant transformer. As improvements in UPS and generator technologies take place, these recommendations may become less relevant.

Gasoline/LP/NG Fuel


UPS

Gasoline/LP/NG
Single Phase
Generator

Background
Load

Gasoline/LP/NG
Three Phase
Generator

Background
Load

KVA

KW

KVA

KW

KW

KVA

KW

0.5 to 3.5

10

12

2

10

12

2

4

10

12

2

10

15

2

5

13

16

2

15

19

2

7

16

20

3

19

24

2

10

24

30

4

29

36

4

12

30

38

5

36

45

6

18

46

57

7

55

68

8

Table 1. Gasoline/LP/NG Fuel Gen-sets

Diesel Fuel

UPS

Diesel
Single Phase
Generator

Background
Load

Diesel
Three Phase
Generator

Background
Load

KVA

KW

KVA

KW

KW

KVA

KW

0.5 to 2

5

7

1.5

5

7

1.5

2.5 to 3.5

5

7

1.5

6

8

1.5

4

6

8

1.5

8

10

1.5

5

8

10

2

10

13

2

7

10

12

2

13

16

2

10

14

18

2.5

19

24

3

12

18

23

3

24

30

4

18

28

34

4

37

46

6

Table 2. Diesel Fuel Gen-sets

From Tables 1 and 2, note that the size of the gen-set is significantly larger than the UPS. There are a number of reasons. Besides the obvious losses due to line loss, there are inefficiencies involved every time another piece of equipment is brought into the equation. One of the biggest factors is that the generator needs to be sized to handle the start up of the largest load, not just the continuous load. Based on calculations and manufacturers specifications, the gen-set recommendations look like they are larger than needed. However, if the generators engine can't get up to speed and running, for any reason, the system will fail.


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Start Up Load / In Rush Current

When a gen-set is being used to put power directly into the battery bank of the UPS, as shown in Figure 3, the load can exceed the gen-sets ability to supply power. The UPS, in holding the load, starts to deplete battery power immediately, at the start of the outage. By the time the gen-set is up and running, and prepared to go on-line, the battery bank has lost a considerable amount of power. At the instant power is provided to the battery charger, the charger is trying to power a battery bank that acts like a very large capacitor and the in rush of current into the batteries takes all the power available from the battery charger. This has been known to stop the engine on some gen-sets. This has also been known to start a "stop-restart cycle" where the gen-set never gets running to power the load and the UPS batteries finally run down and the UPS shuts down.

A work-around for the current in-rush problem is to limit the amount of current the batteries can draw from the charger. Putting a Current Shunt in-line with one of the battery leads can do this. Another option is, if the gen-set has the capability of auto-start from sensing the UPS battery bank float voltage, set the float voltage on the battery bank at a higher voltage to get the gen-set to start sooner also may help.


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Other Generator Tips

Other gen-set related caveats that allow the gen-set to work with the UPS are as follows:

1. If installing a three-phase generator, install the UPS across the generator phase that is monitored by the generator's voltage regulator.

2. Adjust the governor of the generator so that the line frequency doesn't deviate by more than plus or minus 3 hertz.

3. Install additional ballast loads such as emergency lighting that are equal to 15% of the KW rating of the gen-set. The ballast loads share the generator with the UPS and stabilize the generator so that the frequency is less erratic. The loads should be stable and continuous such as lighting or electric heaters. In other words, do not use air conditioners or air compressors for a background load. See Figure 2 below.

figure 2

Figure 2. Adding Background Load as a Ballast to Improve Regulation

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UPS Tips

In hooking a UPS to a gen-set, the following should be reviewed and considered:

1. When connecting a generator to the UPS, make sure that there is a disconnect switch between the UPS and generator. A disconnect switch is usually required by code as well as a serviceability and safety factor. In most cases, a means of disconnect is required. If soft wired, you may be able to get away with a plug and socket (i.e. Anderson Connectors); however, check with the authority having jurisdiction.

2. Ensure that the UPS has enough power to operate any ventilation fans or other required peripheral emergency equipment connected to the UPS during an outage. It is common for the gen-set ventilation fans and/or powered intake louvers to be powered from the UPS.

3. When using an existing generator with a UPS, the UPS may not run on the generator's power because the UPS's software is telling the UPS that it has poor input power. Gen-set power compared to most commercial utility power, is very erratic. Before giving up, look into buying a battery charger and hook the battery charger directly into the UPS's battery bank. See item #4 below.

4. When hooking power from a battery charger directly into the UPS battery bank, make sure to check with the UPS manufacturer to find out if the UPS's inverter is rated for continuous duty. There is nothing worse than burning up the UPS inverter because it was only designed to run as long as the manufacturers factory installed internal batteries would carry it. Always verify with the UPS manufacturer that their units have been rated for continuous duty. At this time, I know that the UPPI and Best Power Ferrups units have been rated for continuous duty, however, it is always a good idea to confirm that your particular unit is rated for it. Due diligence has saved a number of people a lot of time, money and warranty head aches from their power equipment suppliers. See Figure 2 below.

figure 3

Figure 3. Using a Battery Charger to Avoid Generator Regulation Issues.

5. If using a battery charger that is powered from a generator, ensure that the battery charger provides the correct voltage for the UPS battery bank, and is large enough to handle the load. This means that the battery charger has to produce enough power out to handle the UPS input requirements as well as to compensate for any other inefficiency.

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Other Alternatives

If a UPS and gen-set still do not operate correctly, there may still be a way to salvage the project, depending upon whether the generator has sufficient power to hold the load and whether the problem is a regulation issue. The first alternative is to simply add additional (background) load to the gen-set. This will help to tighten regulation, though this is not always effective.

The second alternative is more expensive. This alternative requires some retrofitting expertise to replace the governor on the generator with a better grade governor or an electronic controller such as a Barber-ColmanTM. The generator supplier may have an alternative governor available, though most small gen-sets typically do not. Some mechanical engineering work may also be required to fit an electronic controller onto the gen-set if the supplier doesn't have an off-the-shelf solution.

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UPS's for Sale  

For those looking for a UPS or Power Conditioner: Click here  for your  Uninterruptible Power Products needs from Amazon.com.


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Other Links

Generic Generator Installation Manual




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Copyright Dexter A. Hansen