UPS power sharing battery pack solution

1. Introduction

In order to improve system reliability, most computer rooms now use N+X parallel redundant or dual bus UPS configuration solutions. In previous solutions, the number of UPS hosts increased, and the number of batteries often increased proportionally, which increased the investment in money, space, load-bearing, maintenance, etc. for batteries, sometimes even several times that of the UPS host. So is there a solution to use fewer batteries when the system backup time is not affected or less affected? Is there a solution that does not require the battery pack to be redundant when the UPS host is redundant?

Yes, shared battery packs are a good solution.

2. Theoretical basis and superiority of shared battery pack solutions

The so-called shared battery pack solution refers to a solution in which two or more UPS hosts use one or more battery packs at the same time. When the mains power is normal, each UPS charges the battery pack at the same time. When the mains power is abnormal or interrupted, each UPS uses the energy of the battery pack to invert into AC power to supply the load.

The system architecture diagram of the shared battery pack solution is as follows:
In the N+X parallel redundant or dual-bus configuration system, the UPS host must have redundancy, such as: in the 2+1 parallel system, the redundancy accounts for 33% of the total capacity, in the 1+1 parallel system, the redundancy accounts for 50% of the total capacity, in the 1+2 parallel system, the redundancy accounts for 67% of the total capacity, and in the dual-bus system, the redundancy accounts for at least 50% of the total capacity, etc. According to the conventional battery configuration method, each UPS host is equipped with its own battery pack. If the UPS host cannot be inverted for some reason, the battery pack it carries will also be invalidated, even if the battery is not faulty. Therefore, if the UPS host is redundant, the battery must also be redundant. The host redundancy accounts for a few percent of the total UPS capacity, and the battery redundancy also accounts for a few percent of the total battery capacity. Only in this way can the system backup time be unaffected and a truly redundant effect be achieved. Considering another way of thinking, when a UPS host fails, if its equipped battery is transferred to other normal UPS for use, then the battery configured in the system does not need to be redundant? Isn't the backup time of the entire system also unaffected? This is the theoretical basis of the shared battery pack solution.

The shared battery pack solution has the following advantages:

1. Save the investment in purchasing batteries

The system redundancy accounts for a few percent of the total system capacity, which can save a few percent of the total battery investment. With the soaring battery prices today, this amount of money can be saved. At the same time, the number of batteries is reduced, and the corresponding investment in transportation and installation will also be reduced.

2. Save the investment in installation space

The installation space occupied by a large number of batteries is also very large. Reducing the number of batteries will proportionally reduce the investment in installation space. At the same time, it will also reduce the investment in rent, decoration costs, air conditioning configuration, etc.

3. Save the investment in load-bearing

The battery pack is very heavy. In order to solve the problem of floor load-bearing, the battery placement area is generally expanded or a load-bearing bracket is made. If the number of batteries is reduced, the investment in this aspect will be saved accordingly.

4. Save operating cost investment

With fewer batteries, the system itself and the room air conditioner consume less power, and the maintenance cost required is also reduced, which will also be more environmentally friendly.

5. It is more convenient to expand the system

For UPS systems with shared battery packs, you don’t need to add batteries when expanding in the future. If the backup time of the existing battery pack is still sufficient, you can directly add a UPS host. Expansion will become very simple, convenient and save money.

6. Maximize the efficiency of batteries and improve battery utilization

Batteries need to be maintained. If they are not discharged for a long time, they will lose their activity. For traditional battery configuration solutions, due to the large number of batteries, the batteries will discharge with a small current after a power outage, and the battery capacity may be restored before much of the city power is discharged. This shallow discharge with a small current is not good for the battery. Over time, the battery performance will decline. Once a UPS breaks down, the backup time of other UPS batteries will not meet the requirements. For the shared battery pack solution, due to the relatively small number of batteries, the discharge current of the battery will be relatively large after a power outage, and the battery capacity can also be discharged more, which is conducive to improving the activity of the battery and extending the battery life. Once a UPS fails, the backup time of the system will not be affected, because the battery will not fail with the UPS.

From the above analysis, it can be seen that the shared battery pack solution has many advantages, especially when the conditions such as investment, load bearing, and installation space are limited, this solution shows its superiority.

3. Several problems that need to be solved in the shared battery pack solution

Since the shared battery pack solution has many advantages, why has it not been widely used? Why do many manufacturers usually promote their UPS with shared battery pack functions, but dare not recommend customers to use it in actual application? This is because not every UPS can share battery packs, and shared battery packs require certain technical support.

For the shared battery pack solution, the main issues that everyone is more concerned about are the following:

1. If the DC busbars are directly connected, will the rectifiers of each UPS be affected? Can the voltage stability of the DC busbar be guaranteed? Is the voltage ripple of the DC busbar small enough?

2. How do each UPS handle the floating charge of the battery?

3. Can the "battery online test" function still be used when sharing the battery pack?

4. If the rectifier of one UPS fails, how will the other UPS operate?

5. If the inverter of one UPS fails, how will the other UPS operate?

6. If the shared battery pack fails, such as short circuit, how will each UPS protect itself?

These issues are directly related to the safety and stability of the entire system and are very important. Different manufacturers have different ways of handling them. Therefore, when choosing a shared battery pack solution, you must choose a UPS manufacturer with strong technical strength and rich manufacturing experience.

4. Shared battery pack solution of Delta NT series UPS

Delta NT series UPS can share battery packs with multiple machines, and can be used in series, parallel, and dual busbar architectures. At present, there are two 30KVA UPS shared battery packs in normal operation in the dispatching and control center of Shanghai Railway Bureau; two 320KVA UPS shared battery packs in normal operation in Jiujiang Telecom; two 40KVA UPS shared battery packs in normal operation in the Confidential Bureau of Shandong Provincial Party Committee; two 30KVA UPS shared battery packs in normal operation in Shandong TV Station; three 40KVA UPS shared battery packs in normal operation in Yingtan Telecom, etc. These cases have never failed due to shared battery packs. Delta's UPS production line also uses shared battery packs to test parallel machines. Therefore, Delta UPS's technology in shared battery packs is still quite reliable and its experience is very rich.

Delta NT series UPS mainly handles the above problems in this way:

For problem 1: After the DC busbars of each UPS are connected, the work of the rectifier is actually basically unaffected, because the SCR opening of each rectifier is controlled by its own drive signal, which has nothing to do with whether the DC busbar is connected. As long as the feedback signal of the DC voltage is accurate and timely, and the drive signal is correct, the work of the SCR will not be affected. Therefore, 12-pulse rectifiers and 18-pulse rectifiers can operate safely and stably, and even dozens of rectifier modules of the switching power supply can be directly connected in parallel for safe operation.

Looking deeper, if the control is good, parallel connection of rectifiers is not only harmless but also beneficial, because they can offset some low-order harmonics, making the DC voltage ripple smaller and the DC voltage more stable.

Each rectifier of Delta NT series UPS is controlled by a dedicated CPU, which collects signals accurately and timely, drives signals correctly, and has strong anti-interference ability. UPS also has an automatic current balancing function, which not only ensures that the three-phase input current of each UPS is consistent, but also regulates the current difference between the rectifiers of each parallel UPS not to be too large, otherwise the UPS with large current will limit current protection to improve the safety of the entire system. Whether the Delta NT series UPS is running alone or sharing a battery pack, its DC voltage stability can reach <±1%.

For question 2: Delta NT series UPS can set parameters such as battery capacity, equalization/floating charge voltage, charging current, battery low voltage alarm point, battery cut-off voltage point, etc. on the operation panel according to the battery capacity and load requirements. If the battery pack is shared, set "Shared Battery Pack" on the operation panel to "YES", and set the battery capacity and charging current parameters to 1/N of the battery required values ​​(N is the number of hosts).

After the city power outage, each UPS starts to discharge. When the power comes back, if the battery voltage is lower than the equalization voltage point, each UPS will start the equalization function at the same time. When the cut-off voltage point or cut-off time is reached, each UPS will switch to floating charge at the same time.

In normal operation, if the equalization function of a single UPS is manually started, the UPS will be charged at the maximum set charging current, and will automatically switch to floating charge after the time is reached, which will not affect the system.

For question 3: When Delta NT series UPS share a battery pack, the "battery online test" function is invalid, because it is meaningless to detect the "battery" of a single UPS at this time. If needed, the software can also be modified to allow each UPS to act simultaneously when the "battery online test" function of one of the UPS is started.

For question 4: During operation, if the rectifier of a UPS fails, the faulty machine will alarm and switch to battery working state until the battery pack voltage drops to 330V, and then the machine will be locked for protection (the battery protection lock voltage of a normal UPS is generally 300V). Other UPS will always work in normal working state, charging the battery at the maximum set charging current.

If the rectifier is seriously damaged or even short-circuited, the fuse connected between the rectifier of the faulty machine and the DC busbar will quickly blow, preventing a single fault point from affecting the entire system.

For question 5: During operation, if the inverter of a UPS fails, the faulty machine will alarm. For a parallel system, the faulty UPS will lock the output, and the load will be borne by another UPS. If the other UPS is overloaded, all UPS will switch to bypass working state at the same time and continue to supply power to the load. For a series or dual-bus system, the faulty UPS will directly switch to bypass working state and continue to supply power to the load.

If the inverter is seriously damaged or even short-circuited, the fuse connected between the faulty inverter and the DC busbar will quickly blow, preventing a single fault point from affecting the entire system.

For Question 6: Delta's NT series UPS has electromagnetic contactors and fuses between the battery pack and the DC busbar. When the UPS detects battery leakage, reverse polarity, low battery voltage, etc., the electromagnetic contactor will not be attracted. When the battery pack is short-circuited, the fuse will quickly blow to protect the entire system.

V. Issues to note when implementing a shared battery pack solution

When using a shared battery pack solution, the following two points should be noted:

1. Try to avoid sharing only one set of batteries

Because there is a single point of failure, once this set of batteries fails, all UPSs will have no batteries available. If the city power fails at this time, the entire system will stop.

According to experience, it is best to use 2-5 sets of batteries for sharing, because this can not only avoid single-point failures of the battery pack, but also protect the battery pack.