Phase imbalance: causes and protection

Recommendation points

• An excursion into the theory of electrical engineering
• Causes and consequences of phase imbalance
• Neutral wire restoration
• Inverter phase stabilizers
• Balancing transformers
• Overvoltage protection

Neither household appliances nor production equipment can operate without a stable power supply. Load and voltage imbalance or phase imbalance is the main cause of failures and breakdowns. This phenomenon can and should be fought, which requires a comprehensive understanding of the rules for the operation of a three-phase electrical network.

An excursion into the theory of electrical engineering

The three-phase AC system was introduced into industry more than a century ago, practically in the form in which it has survived to this day. The main developer of the three-phase network is considered Mikhail Osipovich Dolivo-Dobrovolsky, a domestic scientist who took the ideas of Nikola Tesla as the basis for his developments.

The advantages of a three-phase network are obvious: if, during the rotation of the magnetic field, a current appears symmetrically and consistently on the three-pole winding of the generator, its shape can be easily used to reverse the conversion of electrical energy into rotation. In the era of developing scientific and technological progress, the ability to freely use electric machines was extremely important, and it remains so..

Guaranteed power supply unit AGM-7,5

However, the three-phase power supply system is not without its drawbacks. The voltages in each of the phases are interconnected by the symmetry coefficient. In a three-phase network, two types of electrical voltages are distinguished: linear, acting between phases, and phase, which is measured between the phase and the neutral wire. If the load on each phase is the same (symmetrical), the line voltage is v3 times higher than the phase voltage. Given that the reversal of the voltage polarity on each phase alternates with the rest and partially overlaps in time, significant unevenness in the distribution of loads leads to unstable operation of the entire system.

Causes and consequences of phase imbalance

When the load asymmetry appears, a phase voltage loss is observed in one of the phases, while the line voltage remains constant. The circuit according to which three-phase loads are connected can be considered as a voltage divider: its drop in the most loaded phase will be maximum due to low resistance, while at the least loaded phases, the voltage will rise and tend to linear. In other words, the voltage across the phases is distributed in proportion to the connected load.

We observe this in household power grids: all consumers are connected to different phases, but there is no guarantee that with a strict individuality of the operating modes and power of electrical equipment, the load will be evenly distributed. Therefore, the most common scheme for connecting loads in a three-phase network, called a “star”, is supplemented with a neutral wire connected to a central point and electrically connected to the grounding system. Thanks to this addition, the effect of unbalanced loads on phase voltages is significantly reduced, while the equalization efficiency is highly dependent on the conductivity of the neutral conductor..

If the conductivity is insufficient or the neutral conductor is cut off, the load imbalance increases again and causes an uneven distribution of phase voltages. This mode of operation of the power grid is fraught with serious consequences: with an increase in voltage in each active consumer, the current strength increases up to the limit values, capacitive filters of power conversion devices fail, the probability of insulation breakdown increases, overheating and an increase in parasitic currents are observed in three-phase motors. A zero break in the city network will certainly cause damage to electrical appliances connected to an unprotected branch, even if they are not working at the moment. Often, damage to equipment is irreversible, in addition, the likelihood of a fire increases significantly. Phase imbalance also negatively affects three-phase power supplies – step-down power transformers and three-phase generators..

Neutral wire restoration

To transmit electricity over long distances, colossal voltages are used, due to which it is possible to reduce the cross-section of conductors to reasonable values. As you approach the consumer, there is a stepwise decrease in voltage using power transformers and a gradual branching of the power grid. There is no need to connect the transformers with a neutral wire, such a wonderful conductor as the earth’s crust perfectly copes with this task. Therefore, a zero break can occur only at the final stage of transformation: a 6-0.4 kV step-down substation or at any point of the low-voltage distribution network.

To figure out where a break in the neutral wire is possible, let us turn to a classic example – a three-phase power supply network of an apartment building. A three-core cable and a common grounding bus can be laid in the technical channel connecting the floor areas. It is also possible to connect the neutral bus to the substation grounding loop using the fourth core of the cable. In almost all cases, it is quite simple to determine the location of the break, it is enough just to measure the electrical potential between the zero bus and the ground with a voltmeter. If the device shows values ​​close to the deviation of the phase voltage from the norm, then the place of damage must be looked for earlier according to the scheme, moving towards the substation.

The situation is different with overhead power lines. The neutral wire follows together with the phase ones along the entire length of the distribution network, starting from the substation or transformer. Naturally, no one will independently measure the voltage between the neutral conductor and the ground on each pole of the overhead transmission line. The break can be determined only visually, and even better – by the forces of emergency service workers. Additionally, we note that it makes no sense to independently ground the neutral conductor in your area of ​​responsibility, because in this case, the unloading of the entire network will occur along the consumer’s conductor, which means that the current will flow through the meter.

Inverter phase stabilizers

Not only consumers with a single-phase connection, but also three-phase subscriber networks, including industrial ones, suffer from the asymmetry of voltages and currents. One of the most effective ways to solve the phase imbalance problem is to install a phase stabilizer. Unlike conventional household voltage stabilizers, phase stabilizers eliminate asymmetry by amplifying or redistributing the load..

In fact, the function of a polyphase balancing stabilizer can be performed by an assembly of three single-phase voltage stabilizers. However, if three devices are combined into one, this can promise significant benefits. The principle of operation of a three-phase device lies in the fact that it has one storage and energy conversion device, in the role of which is a pulse transformer. In short: a single-phase stabilizer, installed in the most sagging phase, is forced to compensate for the increase in voltage by increasing the power consumption, which is accompanied by a strong decrease in the efficiency of the converter.

In turn, three-phase stabilizers draw the energy necessary for equalization from phases where the voltage is higher than the nominal, due to which the amount of conversion losses is much lower. In this case, an additional load is carried out on the unloaded phases, that is, not only the consumer, but also partly the supply network is stabilized. The presence of a common inverter also allows you to maintain a three-phase network with a temporary lack of voltage on one of the power phases.

Three-phase voltage stabilizer FNEX SBW 100

Not without flaws. First of all, these are the complexity of the device and the high cost of three-phase stabilization devices. For the most part, phase stabilizers are used in the power supply of small enterprises equipped with electrical equipment with a total power consumption of up to 80–100 kVA: boiler houses, mobile base stations, furniture shops. For more powerful consumers, other stabilization methods are provided.

Balancing transformers

Another type of device for stabilizing currents and voltages is balun transformers. They have a wider power range. For networks with a power consumption of up to 400 kVA, it is recommended to install low-voltage transformers of the TST type, for more powerful – balancing transformers 6 / 0.4 kV of the TMGSU type.

Both types of transformers differ from conventional power transformers in that they have an additional winding. It is located in parallel with the primary windings and is connected between the working zero and the ground loop of the transformer midpoint. The principle of operation is simple: when an asymmetry of the loads appears in the neutral wire, a current arises, which is transmitted to the magnetic core of the transformer, and then pulls up the most loaded phase. Compensation is carried out automatically due to the difference in the periods of oscillation of different phases.

TMGSU transformers practically do not differ from low-voltage baluns. Placing the phase balancing device on the step-down transformation stage simply makes it possible to exclude an additional transformation chain and, accordingly, to avoid additional losses in the magnetic circuit. Simplicity, reliability and low cost make balun transformers the best solution for networks with low requirements for sinusoidal purity. However, transformers do not have such a wide range of protection and stabilization functions that inverter-type devices have..

Overvoltage protection

Well, what about consumers with a single-phase connection? Unfortunately, it is not possible to somehow influence the likelihood of imbalance and the resulting increase in voltage. Such phenomena periodically happen, the fault is in the insufficient equipment of the main networks, the lack of work on forecasting the loads and the deplorable technical condition of electrification systems.

However, you can still protect your own electrical facilities. The simplest way is to install a voltage relay, which will turn off the supply of the object when the maximum operating parameters appear in the network. If even a temporary lack of power supply at the facility is unacceptable, there are two ways to protect against phase imbalance: installing a single-phase stabilizer or equipping the input-distribution group of automatic transfer switches with an autonomous power source.