Automatic Voltage Control Scheme Principle

3.4. Automatic Voltage Control Scheme Principle

The main purpose of the AVC scheme is to maintain the desired voltage level at the substation busbars under varying load conditions. When a voltage level outside the tolerable range is detected by the AVC relay, a raise or lower signal is sent to the OLTC in order to correct the voltage level.

A simple AVC scheme arrangement is shown in figure 3.4.1. In this arrangement only voltage measurement is used. The AVC relay compares the voltage measured at the secondary side of the transformer (V_VT ) with the target voltage (V_TAR) in order to determine whether any actions are required.

Figure 3.4.1 Simple AVC scheme arrangement

In order to ensure appropriate and efficient performance of the AVC scheme, basic settings such as voltage target, bandwidth, and time delay are essential, however, additional settings such as LDC, substation firm capacity, transformer impedance etc.

can be also required.

3.4.1. Voltage Target, Bandwidth and Time Delay Settings

The voltage target setting specifies the desired voltage level at the substation busbars which an AVC relay is intended to maintain. The voltage target varies from DNO to DNO and from network to network, but it is common for the 132/33 kV substations to run at a fixed voltage target at 101.5%. That gives the voltage level of 33.5 kV. For 11 kV networks the voltage target is typically set at 102.5% which gives the voltage level of 11.275 kV.

control relay is called the bandwidth setting, normally represented by a ± value from the voltage target. The bandwidth is defined as the voltage deviation from the voltage target below which no action is required by the voltage control relay. When the voltage deviation exceeds the bandwidth setting, the tap changer mechanism is activated in order to bring voltage back to a desirable level. The minimum bandwidth setting is determined by the voltage step of the tap changer. To prevent the AVC relay from hunting, the total bandwidth should be set greater than one step. The greater the bandwidth (BW) setting, the fewer the number of tap changer operations. On the other hand it should not to be set too high as the voltage precision will be compromised.

Typically, the bandwidth is set between 1.5 and 2 tap steps [31].

Moreover, in order to avoid excessive tap changing due to short-term voltage fluctuations, a time delay must be introduced. The time delay setting is also used to coordinate operation of the AVCs between higher and lower voltage levels. It is desirable for a voltage change at 132 kV system to be corrected by the 132/33 kV transformer before correction is made by 33/11 kV transformers OLTC. Thus, time delay setting of the AVC scheme at the grid substations is usually between 30-60 seconds while at the primary substations is typically between 60-120 seconds.

The performance of the voltage control relay in time domain with the voltage target,

VTAR, measured voltage ,V_VT, bandwidth and time delay settings is illustrated in figure 3.4.2.

Firure 3.4.2 Performance of the AVC scheme

The voltage control relay monitors the voltage at the substation busbars and compares it with the voltage target and bandwidth settings. When the voltage exceeds the allowable voltage range, the relay starts timing. If the voltage level persists outside the range for longer then time delay setting, the relay takes actions to correct the voltage i.e. sends raise or lower signal to the OLTC.

3.4.2. Automatic Voltage Control with Load Drop Compensation

Another feature commonly implemented in the AVC relay is the LDC technique. In this method, in addition to voltage measurement, transformer current measurement is also used in order to determine the voltage level at a remote point on the network. An AVC relay with LDC therefore does not maintain the voltage at the substation, but at a remote point on the feeder under varying load conditions.

Figure 3.4.3 AVC scheme with LDC

There are two different approaches for the LDC scheme. They have similar purposes, but different implementation. The first approach is known as ‘line drop compensation’.

In this method feeder current, resistance and reactance of the line are used to estimate the voltage level at a remote point of the line. The line drop compensation bias is applied in proportion to measured current and R and X settings of the voltage control relay and calculated as follows:

VLDC =ICT ⋅

(

)

The second method is called ‘load drop compensation’ and is intended for substations with multiple feeders. Voltage boost at the substation busbars, R_%, is applied in proportion to the ratio of the actual current measurement to the current at full load. The LDC voltage bias is calculated as follows [32]:

TXMAX TX

LDC I

R I

V = % ⋅ (5)

This allows compensation for voltage drop along the feeders caused by load flow under various load conditions.

The main advantage of the load drop compensation technique over line drop compensation is that the former is much easier to set up as the maximum load of the substation is known, and transformer current is measured. On the contrary the latter technique requires knowledge of resistance and reactance of the feeder which are not clearly defined and feeder current which is not usually available. Additionally most substations have multiple feeders and R and X settings only apply to a single feeder with an end load. More details about the LDC schemes are presented in [25] and [33].

At substations where an LDC scheme is used the voltage target setting of the AVC scheme represents the voltage target at no load and the LDC setting represents the required voltage boost above the target voltage at maximum load conditions. Once again, various settings can be used for an AVC scheme with LDC, but typical settings for 33 kV busbar is 97% voltage target at no load plus 5% LDC boost at full load giving the voltage range from 32.01 kV up to 33.66 kV. The voltage target for 11 kV system is usually set up at 99% with 5% LDC voltage bias provide voltage range from 10.89 kV at no load and 11.44 kV at full load.